Variable Frequency Drives: Applications and Power Quality
Transcript of Variable Frequency Drives: Applications and Power Quality
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Variable Frequency Drive and
Power Quality Workshop
Robin Priestley
Power Control Manager
Rockwell Automation
52ND ANNUAL RURAL
ENERGY
CONFERENCE &
WORKSHOP
FEBRUARY 12-14, 2014
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RULES OF
ENGAGEMENT
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Be BRUTALLY Honest!
The goal is to make MREC seminars & Robin BETTER!
Please fill out each section while we’re there
CRITIQUE SHEET
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ROBIN’S DISCLAIMER
• Based on today…
• 100% Accurate?
• Compiled by “Drives Guy”
• Balanced with Resources
• List of Web Sites
• Discussion is technical
• Not brand specific
• Not “correct”, “sensitive” or “inclusive”
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RESOURCES
• http://www.ab.com/drives/energy_savings/index.html
• http://www.angelfire.com/pa/baconbacon/page2.html
• http://www.pupman.com/listarchives/2001/June/msg00679.html
• http://www.energysafe.com.au/products.html
• http://www.iserv.net/~alexx/lib/general.htm
• http://www.myronzuckerinc.com/docs/Specification%20-
%20Trap%20Filter.pdf
• http://www.transcoil.com/
• http://www.et-sales.com/K_Factor.html
• http://www.ab.com/drives/energy_savings/index.html
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TOOLS
• Engineering Assistant
• Break/Regeneration Calculator
• Harmonics Estimator
• Energy Savings Calculator
• Allen-Bradley Team
• Seminars
• Harmonics/Power Quality, Etc.
• Application Expertise
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How many can you think of?
DRIVES IN
RURAL
APPLICATIONS
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• Irrigation
• Hammer, flow, energy, aquifer management, saves
piping
• Dairy
• Agitation, vacuum pumps
• Three Phase Conversion
• Animal Health
• Air Quality, Temperature control
HMMMMM?
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AC Input Rectified to DC Waveform Smoothed Chopped into AC
1. Alternating Current is brought into the drive.
2. The AC voltage is rectified to DC using bridge rectifiers or an
SCR circuit.
3. The DC voltage has it’s ripple removed by a capacitor bank.
4. Transistors “switch” the DC voltage on and off.
Using Pulse Width Modulation, AC is seen by the motor.
1 2 3 4
How Do AC Drives Work?
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WHAT IS PWM?
Pulse Width Modulation
Pulse Width Modulation is a technique that involves turning an output ON for a period of time, and then OFF
for the balance of the time. This is done without varying the voltage.
When all Pulses have same Width, the output is a square wave.
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WHAT IS PWM?
Short ON times with
Long OFF times result in
lower average voltage.
Long ON times with
Short OFF times result
in Higher average
Voltage.
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WHAT IS PWM?
Changing the Pulse Width in a dynamic way will result in a simulated
AC sine wave.
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PARAMETERS AND
PROGRAMMING
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Local Control (HIM)
Remote Potentiometer (0-10V)
Analog (0-10V, 4-20ma)
Preset Speeds MOPs
Network Control EtherNet and 30 others
Speed Control
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PROGRAMMABLE
INPUTS
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Control Wiring
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CONTROL WIRING
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Control Wiring
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CONTROL WIRING
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Stop Modes
Ramp to Stop
Coast To Stop
Ramp to Hold
DC Brake
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MOTOR CONTROL- WHAT IS
IMPORTANT?
• What dynamics does the application require?
• Ultimately, A motor shaft should:
• 1) Start to spin when commanded, i.e..; suitable “Breakaway Torque”
• 2) Accelerate as quickly as the application demands: “Speed Response”
• 3) Maintain operating speed without drift in motor speed
(not frequency): “Speed Regulation”
• 4) Produce torque quickly enough satisfy the speed regulator
needs “Dynamic Response”
• 5) Provide the Greatest Efficiency as a combined motor/VFD system,
• i.e.: what is the wire to shaft efficiency ! !
• VFD should not be susceptible to, or create problems with other electrical
equipment operated on the same power distribution circuit.
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4
BASIC CONTROL TYPES
Volts/Hertz Control
(V/Hz)
Sensorless Vector Control
(SVC)
Flux Vector Control
(FVC)
Field Oriented Control
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BASIC CONTROL CLASSES
Volts/Hertz
Control
(V/Hz)
Sensorless Vector
Control
Encoderless
Field Oriented
Control
Field Oriented
Control
w/ Encoder Fdbk
Basic Volts/Hertz Enhanced V/Hz Vector Control
V/Hz with current
limiting
V/Hz with slip
comp.
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7
VOLTS / HERTZ CONTROL
Voltage
Control
Inverter
V/Hz Control
V/Hz
M Ref V mag
Slip Frequency
Slip
Estimator
Current
Limit
Current Fdbk
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• Ratio exists between voltage and frequency “volts per hertz” (V/HZ)
• 460/60 = 7.7 V/HZ (Voltage at 10hz =77v, voltage at 30hz=230v)
VOLTS/HERTZ CURVE
0 Frequency
Voltage
Base Voltage
Base Frequency 460
60
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9
• Allows complete motor/drive optimization to produce maximum performance with minimum
current.
• Used on applications with tougher starting, acceleration or running torque requirements.
AC Drive Functionality
CUSTOM VOLTS-PER-HERTZ
Break Voltage
Break Frequency Start
Boost
Base Voltage
Base Frequency
Maximum Voltage
Maximum Frequency Voltage
Frequency 0
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0
• Allows drive and motor to adapt to various starting conditions.
• Provides optimum motor performance while controlling current.
AC Drive Functionality
AUTO DC BOOST IN
VOLTS/HZ
0 Frequency
Voltage
Base Voltage
Base Frequency
Automatic Selection
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1
0.0
50.0
100.0
150.0
200.0
250.0
300.0
350.0
400.0
450.0
500.0
0 5 914
19
23
28
33
38
42
47
52
56
61
66
70
75
Output Voltage
Output Frequency
Allen Bradley Speed Torque Curve1336S PLUS Drive w/ Custom Volt/Hertz Curve
Volts/Hertz Ratio
0.0
10.0
20.0
30.0
40.0
1.4
4.2
7.0
9.8
12
.7
15
.5
18
.3
21
.1
23
.9
26
.7
29
.5
32
.3
35
.2
38
.0
40
.8
43
.6
46
.4
49
.2
52
.0
54
.8
57
.7
60
.5
63
.3
66
.1
68
.9
71
.7
74
.5
77
.3
80
.2
83
.0
85
.8
88
.6
FrequencyVo
lta
ge
VOLTS / HERTZ MOTOR CONTROL
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3
2
VOLTS/HERTZ CONTROL-
TORQUE VS SPEED
Per Unit
Torque
Speed in Hertz
3.0
2.5
2.0
1.5
1.0
.5
0 16.7 33.3 50 66.7 83.3
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3
3
SENSORLESS VECTOR
CONTROL
“V Angle” controls the amount of total motor
current that goes into motor flux
Current
Limit
Voltage
Control
Inverter
V/Hz Control
Volt
Vector
M Ref
V mag
Current Fdbk
Slip Frequency
Slip
Estimator
V ang
Autotune Parameters
Torque Cur
Estimator
Torque Cur
Estimator
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WHAT IS THE DIFFERENCE BETWEEN “OUT OF THE BOX”,
WIZARD STARTUP & A STARTUP WITH TUNING?
Constant Torque Speed Range
0%
50%
100%
150%
200%
250%
0
1
2
3
4
5
6
7
8
9
10
Output Hertz
Torq
ue
Optimum (Detailed StartUp) = 120:1
MotorNameplate DataEntered = 40 :1
Out of the box = 20 :1
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3
5
FLUX VECTOR CONTROL -
TORQUE VS SPEED
2
1
Torq
ue
Speed (Hz)
1 2 5 10 20 30 40 50 60
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STARTING TORQUE
• Out of the Box = 150%
• W/ Motor NP Values = 200%
• Optimum Tuning = 250%
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SPEED TORQUE CURVES
Per Unit
Torque
Speed in Hertz
3.0
2.5
2.0
1.5
1.0
.5
0 10 20 30 40 50 60 70 80 90 100
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3
8
DC DRIVE CONTROL WITH
FEEDBACK
Field
Cur. Reg. Speed
Reg.
SCR
Control
Field
Bridge
High Bandwidth Current regulator
Armature
Cur. Reg.
Arm
Bridge
M
PG
Ref
Voltage Fdbk
Current Fdbk
Current Fdbk
Speed Fdbk
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3
9
FIELD ORIENTED CONTROL
W/ FEEDBACK
Torque
Ref
Flux
Reg. Speed
Reg.
Voltage
Control
High Bandwidth Current regulator
Current
Reg.
M
PG
Ref
Inverter
V mag
V ang
Autotune Para
Adaptive
Controller
Slip Frequency
Current Fdbk
Voltage Fdbk
Speed Fdbk
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4
0
TORQUE VS SPEED
Per Unit
Torque
2.5
2.0
1.0
0.0 1 3 10 20 30 40 50 60 70 80 90
Flux Vector Drive, 1000:1 motor, Tuned
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CONSTANT TORQUE LOADS
Heat generated is same at all speeds
Cooling system deteriorates at reduced speed
unless equipped with blower
Example: Conveyer
Winch
Auger
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CONSTANT TORQUE 4:1
0
10
20
30
40
50
60
70
80
90
100
0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90
Torque
Torque
Acceptable Region
for Continuous Operation)
HZ
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CONSTANT TORQUE TO ZERO
SPEED
Torque Horsepower
Constant Torque Range Constant Horsepower Range
100% Percent of Base Speed
Torque &
Horsepower
0
100%
200%
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CONSTANT TORQUE LOADS
Typical of conveyors and machine tools
Torque demand remains constant
throughout speed range
Operation at low speed may need
consideration
Motor enclosure a possible issue
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VARIABLE TORQUE LOADS
Typical of Centrifugal Pumps and Fans
Torque drops as square of speed reduction
Operation at low speed not a problem due
to low torque requirement
Motor enclosure not an issue
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Variable Torque Loads
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WHAT’S MY LINE VOLTAGE?
• Some common ratings are:
• 120v, 1phase
• 200-230v, 1 phase
• 200-230v, 3 phase
• 380-480v, 3 phase
• 500-600v, 3 phase
• Medium Voltage (2300, 4160, 6600, 13800)
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WHAT IS THE MOTOR VOLTAGE?
• Some common motor voltages are:
• 200-230v, 3 phase
• 380-480v, 3 phase
• 500-600v, 3 phase
• (Even if the incoming line voltage is single phase, the drive output will always be 3 phase. Single phase motors and Variable Frequency Drives are not designed to work together.)
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WHAT HORSEPOWER?
• Three phase ¼ hp to 80,000 hp
• Single phase conversion to 40,000 HP
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WHAT IS THE FULL LOAD AMP RATING
OF THE MOTOR?
• Even though motor and drive sizes
are commonly referred to in Horse
Power, it is best to make certain that
the drive can supply the current that
the motor requires.
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WHAT VOLTAGE DO THE DIGITAL
INPUTS NEED TO CONFORM TO?
• Will there be external controls such
as Start, Stop, or Jog pushbuttons?
And if so how will they be wired. Some
drives have Contact Closure Only
inputs while other drives have options
of 120v,or 24v
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ARE SPECIFIC ANALOG INPUTS
AND OUTPUTS NEEDED?
• Will there be remote speed control and if so what type? Is there a need for analog outputs and how many?
• Some common options are:
• Ohms, 10K potentiometer
• Voltage, 0-10v or -10v/+10v
• Current, 4-20ma or 0-20ma
Drives usually come standard with all of the analog inputs above. 0-10v is usually a standard analog output. Sometimes additional analog option boards are required when multiple analog inputs and outputs are required, or when a current outputs are required.
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ENCLOSURE RATING • Panel Mount IP 20(NEMA Type 1)
• Flange Mount IP 20(NEMA Type 1)
• NEMA 4
• NEMA 12
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ARE COMMUNICATIONS OPTIONS
NEEDED?
Some communication networks are:
• DeviceNet
• ControlNet
• EtherNetIP
• Modbus, Profibus, P1, Metasys…
Additional options may be needed when drives are
communicating on a network.
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WILL DYNAMIC BRAKING BE
REQUIRED?
• If the motor needs to be
stopped abruptly and/or
moves a high inertia load,
dynamic braking options may
be required.
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• Full Text LCD with Multiple
language support
• Multiple control options
(or no control)
HUMAN INTERFACE MODULE
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
HOW FAR APART WILL THE DRIVE
AND MOTOR BE MOUNTED?
• Long motor leads can create conditions such as
Reflected Wave Phenomena and Capacitive
Coupling.
• A good “rule of thumb” is to check the lead length
recommendations in the drive user manual any
time the lead lengths approach 50ft or more.
• There are many different corrective measures and
devices to correct long lead conditions.
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TROUBLESHOOTING
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
HELPFUL MANUALS:
• User Manuals
• Troubleshooting Guides
• Installation Instructions
• Spare Parts Lists
• Manufacturer’s Website
![Page 59: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/59.jpg)
COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
TROUBLESHOOTING TIPS
• Is the problem with the Motor or with the Drive??
• Before assuming the drive is defective, try
disconnecting the motor leads from the drive.
Then run the drive as you normally would, you
should see the drive status on the HIM (Drive
Running, Stopped) If the drive still faults without
the motor connected there is most-likely a
problem with the drive.
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
TROUBLESHOOTING TIPS
• Is the problem with the Motor or with the Drive??
• In the case of reoccurring phase faults, sometimes it is helpful to rotate all three of the motor leads. (i.e. U to V, V to W, and W to U) If the re-occurring fault was a UV short, a bad drive component would have the same fault. If the UV short changes to a VW short, it would appear that the motor has bad windings.
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
TROUBLESHOOTING TIPS
• Is there a problem with the Control Wiring or
with the Drive??
• Before assuming that drive inputs are bad, try
using Jumper wires to manually connect the
appropriate connections on the drive terminal
block. This will ensure that the correct
connections are made in the correct sequence.
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
TROUBLESHOOTING TIPS
• Is there a problem with Parameter Settings??
• In some cases the drive parameter settings can
prevent a drive from running as expected. If incorrect
parameters are suspected, sometimes the easiest solution
is to reset the drive to Factory Defaults and go through the
Drive Startup routine. During the procedure you will run
start the drive from the HIM and ensure that the drive runs
appropriately.
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
TROUBLESHOOTING TIPS
Overvolt Faults
• Overvolt faults occur when the Drive’s DC bus
exceeds safe limits. There are a number of reasons this
can happen including:
• Voltage spikes on the incoming line.
• Excess regenerative braking.
• Reciprocating loads that cause regeneration.
• The first step in determining the cause of an Overvoltage
fault is to determine when the fault is occurring.
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
TROUBLESHOOTING TIPS
Overvolt Faults caused by Voltage spikes on the
incoming line
• If the Overvoltage fault occurs when the drive is not
running and the motor is not spinning.
• If the Overvoltage faults occur at random times
• Check incoming line voltage and verify it is not too high
• 5 to 6 in the morning?
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
TROUBLESHOOTING TIPS
Overvolt Faults caused by Excess Regenerative Braking
• The fault only occurs when the drive is decelerating.
• As the drive decelerates the motor becomes a “generator” and the
power is absorbed by the DC bus. If the load is decelerated too
quickly, it may create too much voltage for the DC bus.
• Try increasing the Decel time and see if the fault still occurs. If
the Decel time must be increased to an unacceptable level, then
Dynamic Braking must be used to achieve the required
deceleration time.
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
TROUBLESHOOTING TIPS
Overvolt Faults caused by
Reciprocating Loads
• If you are turning a load with uneven weight.
• Sometimes the load regenerates on every “downward” swing of the rotation.
• If your load moves back and forth repetitiously.
• Sometimes these loads reach a resonant frequency that causes
line regeneration.
These are both cases where Dynamic Braking may be required.
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Why are we using Drives?
• Process Improvement
• Increased Reliability
• Energy Savings
• Extending Service Life of Existing
Systems
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30 Second Answers for
the Most Frequently
Asked Questions
• Multiple Motors on Single Drives?
– Independent Overload Protection
– Oversize Drive + 20%
• Over Speeding Motors?
• Minimum Speeds on Pumps?
• Are Energy Savings Real?
• Reactors?
– Up Stream? Downstream?
![Page 69: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/69.jpg)
Biggest “Missed”
Opportunities
• Process Trim
• Flying Start
• Velocity Profiling
![Page 70: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/70.jpg)
Radiate, Emitted & Conducted
Noise
![Page 71: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/71.jpg)
• Ideal Variable Torque Load Phenomenon
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Potential
Energy
Savings
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Frequency Control
• Cycle Converter 1950s
• Six Step SCR / Drive 1960s
• PWM GTO Drive 1970s
• PWM Bipolar Transistor 1980s
• PWM IGBT Drive 1990s
• PWM 4th Generation IGBT 2000
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The Reflected Wave
Phenomenon
• First identified in 1900 with power distribution
lines.
• Also known as Standing Wave or
Transmission Line Effect.
• Well documented in digital communications.
• Coming to the forefront in IGBT based drives.
• Can cause voltage peaks at the motor.
• Presents the possibility for insulation
breakdown.
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0
-1
+1
+2
Typical PWM VLL Output
Pulse at the Motor Terminal
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IGBT vs. Bipolar
Transistor Current
1336 @ 60HZ NO LOAD
SWITCHING FREQUENCY
1.26KHZ
1336 PLUS @ 60HZ
NO LOAD SWITCHING
FREQUENCY 9KHZ
7.5HP MOTOR
Bi-Polar
IGBT
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• The cable between the drive and motor represents a substantial impedance to the
PWM voltage pulses of the drive
• Cable impedance is proportional to length
Inductance / unit length
Capacitance / unit length
• If the cable surge impedance does not match
the motor surge impedance----
– Voltage reflection WILL occur !!
Z 0 =
The Physics of it All
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1
1.2
1.4
1.6
1.8
2
2.2
Mo
tor
Over
vo
ltag
e /
V
dc
1 10 100 1000 10000
Cable Distance [ft]
4 us
2 us
1 us
600 ns
400 ns
200 ns
100 ns
50 ns
Semiconductor Risetime
IGBT
BJT
GTO
Predicted Motor Overvoltage
for IGBT’s, BJT’s & GTO’s
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• NEMA MG1 Part 31 1600V Motor is Inadequate
• Reduction of 1000 Vpk Motor Insulation Life Accelerated
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Time ( m s)
Pea
k L
ine-
Lin
e M
oto
r V
olt
ag
e (V
PK
)
480 Volt System V LL /V DC = 3 Per Unit
Corona Susceptible Areas 1600 Volts
1000 Volts
480v Reflected Wave Stress - Long
Cable
![Page 81: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/81.jpg)
• Begin Corona
• Not Harmful
• Extreme Corona
• Damaging > 5 - 10pc
Corona Testing
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• White Residue
• Phase to Phase W/O Separator
• Turn to Turn
• Drive Typically OL Trip
Effect of Corona
![Page 83: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/83.jpg)
Protect the Motor
• Output Reactor between drive & motor
– Slopes off the waveform (lengthens rise time)
– Reduces destructive force for same
amplitude
– Allows longer lead lengths
– Does create Voltage drop
• May cause reduction in torque
• Output Filters
– 1204-RWR2
• LR filter
– KLC filters
![Page 84: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/84.jpg)
The Terminator
• Highly Cost Effective
• Smaller
• No Voltage Drop
• Works @ any cable distance
• Maintains current waveform
• 2 - 3 choices fit all applications
• Most effective solution
• Solves multi-motor installations concerns
• Works on all A-B IGBT & BJT drives
![Page 85: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/85.jpg)
Solutions AC Drive
Allen-Bradley 1329 Inverter Duty
Motor AC
Motor
AC Drive
1204-RWR2 Reactor
KLC filter @drive or
AC Motor
Terminator 1204-TFA1 1204-TFB2
@Motor
Non Inverter Duty Motor
![Page 86: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/86.jpg)
After addition of
1204-RWR2
720Vpk @ Motor
660Vpk @ Inverter
Before addition of
1204-RWR2
1180Vpk @ Motor
660Vpk @ Inverter
Plot 2
Before and after the addition of a 1204-RWR2 & 3.0mhy output reactor
1305 3HP 460V 60HZ No-load
300ft shielded cable
After addition of
1321 output reactor
1140Vpk @ Motor
14ms/rise time
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Custom Eliminator
![Page 88: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/88.jpg)
• PVC Cable failures on IGBT drives, the conditions were: 12 awg
- wet , steam, Water based lubricant used,
- PVC cold flow problem
• Noise problems are reduced when shielded cable used.
QUESTIONS:
• What insulation type is the best for IGBT drives ?
• What insulation thickness withstands reflected wave
voltage spikes for 20 year cable life?
CABLE OBSERVATIONS
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Scale Comparison of Single #12 AWG Conductors
0.045”
0.030”
0.019”
XLPE,
RHW-2
Focus Cable
XLPE,
XHHW-2
Standard Wire
Thickness
PVC 15 mil -
Nylon 4 mil,
THHN
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Measured CIV vs. Insulation Thickness
for 600 V Un - Aged PVC & XLPE Insulation
20 00
30 00
40 00
50 00
60 00
70 00
80 00
Pea
k S
inew
ave V
olt
ag
e [
Vpk
]
10 10 0
XLPE & PVC Insulation Thickness [mils]
y = 902 x0.408
y = 2526 x0.290
y = 1255 x0.403
XLPE Extreme Corona
XLPE Begin Corona
PVC Begin Corona
PVC
BEGIN CIV
XLPE EXTREME CIV
XLPE
BEGIN CIV
BEGIN CIV ~ 10 % failures
EXTREME CIV ~ 90 % failures
XLPE is 1.4x better than PVC
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Degradation of PVC & XLPE 600V
Insulation under Hi pot and BIL Testing
.0.1 1 100 10
5,000
4,000
3,000
2,000
1,000
0
Measured BEGIN Corona,new & un-aged
Insulation Service Life [Years]
20 mil
XLPE 15 mil PVC
• Hypot Testing: [ 2 * VRATED( eg. 600V) + 1,000 VRMS ] ~ 3,110 VPK
•Basic Impulse Level Testing: [ 1.25 *VPK ] ~ 1.25 * (600V * 1.414) ~ 3,889 VPK
•UL 1569 Recommended test value
• 30 mil XLPE XHHW 3 kVRMS ( 4,240 VPK)
• 15 mil PVC THHN 2 kVRMS ( 2,820 VPK)
Hipot Testing @ 600V
BIL Testing @ 600V
UL 1569 XLPE
UL 1569 PVC
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Other Cable Issues Require
Attention.
• Capacitive Coupling.
• Cable charging current.
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Problems Identified With
Common Mode Noise
• Non operational
– Control Interface(4-20ma, 0-10V)
• PLC communication errors
• Radiated noise
• Conducted Noise
– Ultra Sonic Sensors
– Temperature Sensors
– Bar Code
– Vision System
– Metal Detectors
![Page 94: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/94.jpg)
L LINK
PHASE A
MOTOR C MOD
Cable Without Shield
Triangular 3 Phase
Power Cable
I SG1 SG C
I SG2
I SG
CHASSIS GND
C MOD
GROUND
WIRE ALL CURRENTS I MUST
RETURN HERE OR HERE G
I G RETURN
Problem: Customer Ground Noise
* Return Path in Ground Through Stray Capacitive Divider (i.e.. Unknown Paths)
* I GND Can Find Its Way Into CNC, PLC, And Computer Grounds
* Conducted Ground Current Customer EMI Noise Problem
Existing Condition: dv/dt
“Noise” Current
![Page 95: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/95.jpg)
70 ns
V LL
Inverter output voltage
6 MHz
I PEAK
Common Mode
Current
I Cdv
dt
Fundamental Problem
![Page 96: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/96.jpg)
MOTOR
CHASSIS
ATTENUATES NOISE WITH
COMMON MODE CHOKE
LEM
LEM
SHIELD CAPTURES NOISE
RETURNING TO DRIVE L LINK
L LINK
+
+
+
GND
COMMON MODE CAPS
Capture and Return Noise
to Source
![Page 97: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/97.jpg)
70 ns
V LL Inverter output voltage
6 MHz
I PEAK
Common Mode
Current
1.5 to 50 us
I PEAK 1/3
200 kHz to 63 kHz SPECTRUM Current With
Common Mode
Chokes
V Ldi
dt Vground Lground
di
dt
What Do Common Mode Chokes Do?
![Page 98: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/98.jpg)
Common Mode Chokes
• Common Mode Chokes Reduce High
Frequency Current To Ground.
• Reducing High Frequency Ground
Potential Difference.
• Reducing PLC Errors & Other
Problems.
i.e.. 20 amps Peak Current with 100 nano
second Rise Time is reduced to typically less
than 5 amps with 5 micro second Rise Time.
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Cable Impact on
Conducted & Radiated Emissions
![Page 100: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/100.jpg)
AC Drive
Common Mode
Current Path
PE
EARTH
GROUND
Potential #3 Potential #1
Motor
Potential #2
X O
R U
V
W
S
T
PE
I lg
Motor Frame
I lg
I lg
I lg
I lg
I lg
C lg-c
C lg-m
A
B
C
Vdc
bus
(-)
(+)
Logic
True Earth Ground (TE)
Interface Electronics
0-10V, communication,
4-20ma,sensor, interface, etc
Potential # 4
Tach
Input Transformer
Non - Recommended VFD Wiring Practice
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AC Drive
Common Mode Current Path
PE
EARTH GROUND
Potential 4
Potential #3 Potential #1
Motor
Motor PE
GND wire
Potential #2
X O
R U
V
W
S
T
PE
PE
I lg
Motor Frame
I lg
I lg I
lg
I lg
I lg
I lg
C lg-m
A
B
C
Vdc
bus
(-)
(+)
Accidental
Contact of
conduit
Conduit
s
t
r
a
p
Input Transformer
GOOD VFD Wiring Practice
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AC Drive
Common Mode Current Path
PE
EARTH GROUND
Potential 4
Potential #3 Potential #1
Motor
Additional
Motor PE
GND wire
Potential #2
X O
R U
V
W
S
T
PE
PE
Motor Frame
I lg
I lg
I lg
I lg
I lg
I lg
C lg-m
A
B
C
Vdc
bus
(-)
(+)
Shielded Cable / Armor
with PVC Jacket
PVC
Input Transformer
BETTER VFD Wiring Practice
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AC Drive
Common Mode
Current Path
PE
EARTH GROUND
Potential 4
Potential #3 Potential #1
Motor
Additional
Motor PE
GND wire
Potential #2
X O
R U
V
W
S
T PE
PE
Motor Frame
I lg
I lg
I lg
I lg C
lg-m
A
B
C
Cable / Armor
& PVC Jacket
PVC PVC
Cable / Armor
& PVC Jacket
Transformer
Cabinet Frame
HRG or
SOLID GND
BEST VFD Wiring Practice
• Shielded Input & output keep noise out of ground grid.
![Page 104: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/104.jpg)
6 Apk CM current of 3 output phases
3.6 Apk Current in Braided Shield & Foil
1.6 Apk Current in Insulated PE wire
0.8 Apk Net Ground Current outside of Cable
All traces: 2 Amps / Div 10 microseconds / Div
Measured VFD Currents Showing Cable Effectiveness
in Substantially Reducing Ground Grid Noise.
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System Grounding
PE - Power Earth Ground
TE - True Earth Ground
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1305 1305
Logic
PE Logic
PE TE
PE Bus PE Bus
TE Bus
1336 Plus
Logic
PE
1336 Plus
Logic
PE TE
1336 Impact 1336 Force
ao
Ground Potential #1 Ground Potential #2
Common Mode Current I ao I
Common Mode Voltage V 1-2
Logic
PE
Logic
PE
PE Bus
System Grounding Scheme
![Page 107: Variable Frequency Drives: Applications and Power Quality](https://reader031.fdocuments.us/reader031/viewer/2022022418/589ee2fa1a28abc2498c1478/html5/thumbnails/107.jpg)
PE Copper Bus
PLC
PE
Drive 4
PE
Drive 3
PE
Drive 1
PE
Drive 2
Cab
inet
Ba
ck P
lan
e
Output Conduit / Armor
M1, M2, M3, PE
W V U
Conduit or
Armor Bond
To System Ground
Noise Current
Return Path
PE
Output Conduit / Armor
L1, L2, L3
Conduit or
Armor Bond
S R T
Improper Cabinet Grounding w/Drives &
Susceptible Equipment.
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PE Copper Bus
PLC
PE
Drive 4
PE
Drive 3
PE
Drive 1
PE
Drive 2
Cab
inet
Back
Pla
ne
SR TU V W
PE PE
Common Mode
Current on Armor
Co
mm
on
Mo
de
Cu
rren
t o
n G
reen
Wir
e
Optional PE to Structure
Steel if Required
Output Conduit or Armor
Bond to Cabinet
All Drives
Input Conduit / Armor
L1, L2, L3, GND
Proper Cabinet Grounding w/Drives &
Susceptible Equipment.
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STRANDED
NEUTRAL
Cable Construction Can Affect Current Balance In V/Hz Drives
Greater Than 125HP
A
B C
A
B C
CONTINUOS WELDED
ALUMINUM ARMOR
A
B C A
B C
INTERLOCKED
ARMORED
STANDARD
TRAY CABLE EUROPEAN
UTILITY
PVC PVC ARMOR
PVC
A B C A B C
TRAY TRAY ARMOR
Effect of Cable Construction
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What is Capactive Coupling?
• In any given motor cable there will be a
certain amount of distributed stray
capacitance.
• Every time the drives’ DC bus voltage
switches at the carrier (or PWM)
frequency it causes current to conduct
through this capacitance.
• These capacitive current spikes then get
reflected back to the drive and measured
by it’s current feedback circuitry.
• THIS IS ALSO REFERRED TO AS
“CABLE CHARGING CURRENT”.
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MOTOR WINDINGS
MOTOR FRAME
MOTOR WINDINGS
MOTOR FRAME
CONDUIT
INCIDENTAL CONTACT OF CONDUIT TOBUILDING STEEL
X
X
CMODULE
DRIVE FRAME
LOGIC CMODULE
LOGIC
CMODULE
CMODULE
PE
DRIVE FRAME
Cable Charging Current
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Cable Charging Current
• This phenomenon exists for all drives
• 460 volt drives will exhibit this phenomenon to a
greater degree then will 230 volt drives.
• One of the ways to mitigate this effect is by
reducing the carrier (or PWM) frequency to 2
KHz.
• Another mitigation technique is adding a 3 phase
inductor on the output.
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• 6 Pulse
• 12 Pulse
• 18 Pulse
• 24 Pulse
• Active Front End
• Liquid Cooled
• Air Cooled
DRIVE TOPOLOGIES
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
WHAT’S HAPPENING
IN DRIVE DESIGN
• Better
• Faster
• Cheaper
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Past
Motor
MARKET ISSUES
& DRIVERS
DC/AC AC/DC
SMPS
dc/dc
EMI
FILTER
Gate drive
PowerFlex 700
Cm
core
• Motor Insulation Failure
Issue
• Motor Bearing Failure Issue
• Common Mode Noise Makes
System User Unfriendly
• Customer Sensor
Misoperation
Line
Reactor Line
Reactor
AB
1204 Insulation Type
• 1000 Vpk
• 1200 Vpk
RWR
1-5
hp
• Noise Issue reduced
• Reflected Wave Issue
addressed With Various
solutions in the Drive.
EMI
Filter
Lower $
DC/AC
AC/DC
new pre
charge
Lower $
SMPS
dc/dc
EMI
FILTER
Gate drive Dynamic
Brake
Insulation Type
• 1000 Vpk
• 1200 Vpk
• 1600Vpk
• 1850 Vpk
Motor RWR
1-5 hp
(Maxima)
Dynamic
Brake
Today’s Hardware is Being Replaced by Software & Improved Packaging
Size Reductions With Increased
Packaging
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CE Filter Built In Added Noise Reduction
Built in 7th IGBT Internal Brake Resistor (option)
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SIZE REDUCTION - CONTROL
BOARD DENSITY
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Smaller Packages with More Functionality
Transistors &
2nd Gen IGBT’s
Duals
3rd Gen IGBT’s
6 Pack
7th IGBT
3rd Gen IGBT’s
6 diodes
6 Pack
7th IGBT
Integrated drivers
4th Gen IGBT’s
6 diodes
6 Pack
7th IGBT
Integrated drivers
Inverter Control
SIZE REDUCTION - POWER
MODULE INTEGRATION
1992 1995 1998 2000+
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• Conclusion: Integrated module substantial reduction in radiated noise
Radiated emission reduction [ in dBuV/Mhz ] is a LOG [ Anew / Aold ]
Anew
Aold
Rectifier Diodes,
IGBT’s, brake IGBT
Reducing Radiated Emissions
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Radiated emission reduction [ in dBuV/Mhz ] is a LOG [ Anew / Aold ]
Anew
Aold
•Conclusion: smaller board substantial reduction in radiated emissions
lower SMPS frequency and slower FET risetime also helps, no heatsink
Reducing Radiated Emissions
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122
Now that they’re saving money,
Can I still sleep at night?
RETROFIT
CONSIDERATIONS
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123
RETROFIT SEQUENCE
• Analysis of Existing and Future Customer
Needs
• Analysis of Existing System
• Decision on Existing Hardware
• Obtaining all Parts to Complete System;
Hardware, Software, Communication
Scheme, Service and Training
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124
ANALYSIS OF CURRENT
AND FUTURE NEEDS
• Speed and or Torque Regulation Levels
• Speed Range
• Environment
• Communication (Now and Tomorrow)
• Support (Parts, Expertise, Training)
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125
ANALYSIS OF FUTURE
NEEDS
• Desired Communications Capability
• General Life Cycle Considerations
• Parts
• Training
• Service
• Obsolescence
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126
ANALYSIS OF CURRENT
SYSTEM TYPE
• Mechanical
• Hydraulic
• Eddy Current
• Rotating DC / MG Sets
• DC
• AC
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127
REASONS FOR CHANGING
FROM A MECHANICAL DRIVE • Increasing Efficiency(approximately 40%)
• Enhanced Speed Control
• Better Flexibility
• Better Regulation
• Reduced Space Requirements
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128
CHARACTERISTICS OF
TYPICAL MECHANICAL DRIVE
• Constant Torque from Low Speed to Midpoint
Speed
• Constant HP from Midpoint Speed to Full
Speed
• Internal Gearbox is the Norm
• Typical AC / DC drive is 1-5 times the HP of a
typical Mechanical Drive
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129
HYDRAULIC SYSTEM
• Must Define Speed / Torque Needs
• Hydraulic can and usually does
regulate Torque DC torque regulation
standard but AC requires use of Force
Technology
• Need to Define Physical Constraints
• Potential Energy Savings
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130
EDDY CURRENT SYSTEM
• Must Define Speed / Torque Needs
• Eddy Current Torque is outstanding
• Duty Cycle can be critical since Eddy
Current is basically a mechanical not
an electrical system
• Energy Savings must be considered
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131
REASONS FOR
CHANGING A MG SET • Inability to Get Spare Parts Economically
• Reduced Operating Cost under Load
• MG Efficiency (Less DC Motor) 72-81%
• DC Drive Efficiency is 98.6%
• Reduced costs at No Load
• MG Losses 10-12%
• DC Losses .6-.7 %
• AC Losses 2.5%
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132
MOTOR CONCERNS
• Obtain Nameplate Data
• Frame Size, HP, Armature
Voltage Armature Current, Field
Voltage / Amps, Base Speed /
Max. Speed, Tach Volts /
1000RPM & Blower HP / FLA
• Determine if Regeneration is
Needed
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133
DRIVE CONCERNS
• Torque that is really required
• Older Motors could accept greater overload
conditions for longer periods of time
• System may be over powered for Application
• Is Field Voltage Adequate
• Is Speed above Base Speed Required
• Will Motor Commutate Properly
• Is a Generator Involved
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134
OPERATOR CONCERNS
•Sequencing
•Reference
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135
DC BENEFITS AND
LIMITATIONS
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136
Speed - RPM
300
250
200
150
100
50
0
BREAKDOWN TORQUE
FULL VOLTAGE
PULLUP TORQUE
FULL LOAD
TORQUE
1800
1740
rpm
P
E
R
C
E
N
T
T
O
R
Q
U
E
STARTING TORQUE
0
DC ASSESSMENT OF
CURRENT HARDWARE
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137
PLS® Bearing
Lubrication System
Cast Iron
Construction
Stator & Rotor Epoxy
Coating
Stainless Steel
T-Drains
Neoprene Lead
Separator
Cast Iron
Conduit Box
V-Ring Slinger
Anti-static
Polypropylene
Corrosion
Resistant Fan
AC BENEFITS AND
LIMITATIONS
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A Practical Discussion for the Real
World
Capacitors & Drives
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Power Factor Improvements
The Pros & Cons of:
–Power Factor Correction Capacitors
–Variable Speed Drives
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You Pay for Power Factor
• Power Factor
–Charge or credits (depending on the power factor)
•Charged if power factor is below 85%
•Credited if power factor is above 95%
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Power Factor
• What is Power Factor?
–Power factor is the ratio between active power (KW) and total power (KVA)
•Active power does work
•Reactive power produces an electro-magnetic field for inductive loads.
PF(%) = KW ÷ KVA x 100
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Power Factor as a Cost
(In phase with line voltage)
Your Total True Cost
Real Work Power Factor
-.5
-.6
-.7
-.8
-.95
Unity
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Power Factor as a Cost
(In phase with line voltage)
Your Total True Cost
Real Work Power Factor
-.5
-.6
-.7
-.8
-.95
Unity
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Power Factor as a Cost
(In phase with line voltage)
Your Total True Cost
Real Work Power Factor
-.5
-.6
-.7
-.8
-.95
Unity
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Total PF = PF (Displacement) * PF (Distortion)
• Displacement power factor
– PF (displacement) = Ireal / I fundamental
– involves only the fundamental quantities
– includes the real and reactive currents
• Distortion power factor
– PF(distortion) = I fundamental / I total
– includes the fundamental and harmonic currents
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Displacement Power Factor
(In phase with line voltage)
Fundamental Current
Real Current
Reactive Current
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Distortion Power Factor
(In phase with line voltage)
Total Current
Fundamental Currents
Harmonic Currents
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What are the benefits of Power Factor Improvement?
• Less KVA (apparent) same KW (real work)
• More KW same KVA demand
• Better Voltage Regulation
• (K Factor)
–Reduction in size of transformers, cables and switchgear in new installations
• Reduced Losses in Distribution System
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K Factor
• An IEEE method of Rating I2R losses and Survivability
– >K=> I2R
– Nonlinear loads increase Eddy Current (Apparent) losses in Transformers
• Magnetic Structure is Enhanced
• De-Rating Transformers
– Increases Available Fault Currents
– Decrease Effective Load
– High Cost
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What can change our Power Factor?
• Capacitance
• Load Characteristics
• LRC Tank Circuits
– Inductive, Reactive (resistance), Capacitive Circuit
•Tuned Filter Traps
•Harmonic Filters
•Active or Passive Filters
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Remember “Reactive” Power
• Electro-magnetic field for inductive loads –Capacitors, rated in (KVAR)
•Reduces the amount of Reactive Power the Utility must supply for Inductive Loads
– Inductive/Capacitive Relationship
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Changing Your Power Factor
• Capacitance
–Flexible Configuration
–Proven & Simple
–Automatic “Switching” Banks
•Can create disturbances
–Location
•Capacitors effect the Upstream…
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Capacitor Location?
Utility
Distribution
Infrastructure
Metering Service
Entrance
MCC
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Changing Your Power Factor
• Load Characteristics
– Lightly Loaded Inductive Loads
• Transformers
–Remember K Factor Concerns
•Motors
–Increase % Load
–Decrease Inductive Value
–Use a Variable Speed Drive
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Changing Your Power Factor
• Hybrid Tank Circuits
–Location Sensitive
•Typically Service Entrance
–Power Factor Correction is Side Effect
•Generally Applied for Multiple Symptoms
–New, Advanced Technology
•Dynamic Marketplace
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Variable Speed Drives
• Located at Motor
–The VFD is a Capacitor
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VFD as the Big Payoff
• Exponential Reduction in Consumed kW
–Variable Torque
•Fans & Pumps
–“Variable Torque” Characteristics
•Systems that Cycle
–Throttled Loads
•Restricted Flows
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POWER QUALITY
CONSIDERATIONS
Harmonics and Noise for
those on a “low geek” diet
A Functional Understanding
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
REMOVING THE
MYSTERY
• Harmonics
• Defining the common terms
• The biggest hazard
• Financially prudent mitigation
• Displaced Neutral Voltage*
• AKA….
• Easy ways to stop the damage
• Common Mode Noise*
• The “real” harmonic issue
• Creation, Symptoms and Mitigation
• Grounding
• The copper connection rule..
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
HARMONICS: DEFINITIONS
• IEEE 519
• This guide applies to all types of static power converters used in industrial and commercial power systems. The
problems involved in the harmonic control and reactive compensation of such converters are addressed, and an
application guide is provided. Limits of disturbances to the ac power distribution system that affect other
equipment and communications are recommended. This guide is not intended to cover the effect of radio
frequency interference.
• Date of Publication : April 9 1993 Status : Active Page(s): 1 - 112 E-ISBN : 978-0-7381-0915-2 Sponsored by :
IEEE Industry Applications Society INSPEC Accession Number: 4441390 Digital Object Identifier :
10.1109/IEEESTD.1993.114370 Persistent Link: http://ieeexplore.ieee.org/servlet/opac?punumber=2227
More »
Year : 1993 Date of Current Version : 06 August 2002 Issue Date : April 9 1993 Related Information : An Errata is
available
Revision of ANSI/IEEE Std 519-1981
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Harmonic Estimator Report - One Line
Project Name WTF Consolidation Project Prepared by Robin Priestley
End User City of Iowa City Date prepared 1/22/2014
Customer ESCO email of preparer [email protected]
notes Stanley Consulting SP1 w DG1
version 050519P Notes:
PCC is a Point of Common Coupling
A "buffered drive" is one that has a DC Link Choke
A "xfmr" is a transformer
All of the values are recalculated when cell data is entered
Source
60 Hz Linear Load 1 on utility xfmr (hp+kW+A) 0 set Ifund load
0 total hp motor loads on utility xfmr to
PCC1 PCC2 PCC3 + 0 total kW resistive loads this % of Irated 1
PCC at utility xfmr PCC at user xfmr PCC at distribution panel + 0 additional Amp loads
Linear Load 2 on user xfmr (hp+kW+A) 0 set Ifund load
100 feet 1 50 feet 2 51.75 total hp motor loads on user xfmr to
Utility feet between User feet between Distribution + 0 total kW resistive loads this % of Irated 2
Transformer utility xfmr Transformer user xfmr and Panel + 90 additional Amp loads
or Generator and user xfmr distribution panel 6 pulse unbuffered drive without line reactor
750 kVA 1 300 kVA 2 total hp
3.00 %Z 1 5.75 %Z 2 50 feet to panel 100.0 % load
12470 Vsec 1 480 Vsec 2 480 Vsec 3 6 pulse buffered drive without line reactor
- OR - 0 Isc 1 - OR - 0 Isc 2 0 total hp
at PCC1 at PCC2 at PCC3 100 feet to panel 100.0 % load
750 kVA 1 300 kVA 2 6 pulse buffered drive with 3% line reactor
35 Irated 1 361 Irated 2 49.5 total hp
1158 Isc 1 6276 Isc 2 6002 Isc 3 100 feet to panel 100.0 % load
16499.2 L, uH 1 117.1 L, uH 2 5.3 L, uH 3 6 pulse buffered drive with 5% line reactor
1.3 K-factor 1.3 K-factor 0 total hp
22.2 % Irms total to Irated 55.5 % Irms total to Irated 100 feet to panel 100.0 % load
22.3 % thermal rating 55.8 % thermal rating 6 pulse buffered drive with basic harmonic filter
0.6 Irms harmonics 15.7 Irms harmonics 15.7 Irms harmonics 0 total hp
7.7 Irms fundamental 199.5 Irms fundamental 199.5 Irms fundamental 100 feet to panel 100.0 % load
7.7 Irms total 200.1 Irms total 200.1 Irms total 12 pulse buffered drive with auto xfmr
0 total hp
150.7 Isc/Iload 31.5 Isc/Iload 30.1 Isc/Iload 0 feet to panel 100.0 % load
0.3 % V(THD) Limit 2.0 % V(THD) Limit 2.1 % V(THD) Limit 12 pulse buffered drive with iso xfmr
7.9 % I(TDD) 15.0 7.9 % I(TDD) 8.0 7.9 % I(TDD) 8.0 0 total hp
YES IEEE special (3% Vthd) YES IEEE special (3% Vthd) YES IEEE special (3% Vthd) 0 feet to panel 100.0 % load
YES IEEE general (5%Vthd) YES IEEE general (5%Vthd) YES IEEE general (5%Vthd) 18 pulse buffered drive with auto xfmr
YES IEEE dedicated (10% Vthd) YES IEEE dedicated (10% Vthd) YES IEEE dedicated (10% Vthd) 0 total hp
0 feet to panel 100.0 % load
18 pulse buffered drive with iso xfmr
Design Checks: (blank if no issues) Cell Key: 0 total hp
data entry 0 feet to panel 100.0 % load
intermediate calc Custom Ifund at FL
100.0 % load
harmonic results
Before using this for the first time, go to the worksheet tab labeled "Tutorial"
For help and additional information, go to the worksheet tab labeled "Notes & Tools"
M
M
M
M
Mfilter
MIso
MAuto
MIso
MAuto
M
M
No
n-L
inear
Dri
ve L
oad
Please note that the information shown here is typical and does not constitute any guarantee of performance or measurement. Several outside factors can influence the harmonics measured on a power system, including other equipment within the plant and other equipment in neighboring plants. This can include, but is not limited to, drives, varying loads and/or other factory equipment.
The calculated current and voltage harmonics shown in this report are for estimation purposes only.
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IEEE 519 INTENTIONS
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
HARMONICS: DEFINITIONS
• Fundamental Frequency
• Native frequency, cycles per second (CPS), Hertz (Hz)
• 60 Hz in America, 50Hz in Europe
Rfund.V =...
0
0
40.00m
40.00m
10.00m
10.00m
20.00m
20.00m
30.00m
30.00m
-150.0 -150.0
150.0
0 0
-100.0 -100.0
-50.0 -50.0
50.0 50.0
100.0 100.0
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
HARMONICS: DEFINITIONS
• Harmonics are multiples of the Fundamental Frequency
• 5th Harmonic is 5 x 60Hz = 300Hz
• 7th Harmonic is 7 x 60Hz = 420Hz
• Full spectrum to 127th harmonic
• Discontinuous Loads
• Solid State “switching” power supplies
• Lighting ballasts, computers, variable speed drives
• Greatest disturbance: One up/One down
• 6 Pulse drive
• 5th, 7th primary disturbance then 11th, 13th followed by 17th, 19th
• 12 Pulse drive
• 11th, 13th; then 23rd, 25th
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
HARMONICS: POP QUIZ
• An 18 Pulse drive’s greatest impact
can be measured at which
harmonics?
• _____th and _____th
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SO? WHAT ARE WE REALLY
TALKING ABOUT?
Rfund.V =...
0
0
40.00m
40.00m
10.00m
10.00m
20.00m
20.00m
30.00m
30.00m
-150.0 -150.0
150.0
0 0
-100.0 -100.0
-50.0 -50.0
50.0 50.0
100.0 100.0
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
START OFF CLEAN, ADD THE HARMONICS AND
PRESTO!
YOU NOW HAVE THE “CAMEL’S HUMP”
Rfund.V =...
0
0
40.00m
40.00m
10.00m
10.00m
20.00m
20.00m
30.00m
30.00m
-150.0 -150.0
150.0
0 0
-100.0 -100.0
-50.0 -50.0
50.0 50.0
100.0 100.0
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
OK, BIG HARMONICS =
CAMEL’S HUMP
• Heat is the big hazard
• 100 amp load (fundamental current 100 amps)
• 5% current (THD) total harmonic Distortion (5 amps)
• Current you didn’t pay for or use for work
• Current you didn’t measure
• Added heat and losses to your distribution system
• How much do you spend to eliminate the camel’s hump?
• When should you be really concerned?
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
MOST “HARMONIC”
PROBLEMS ARE NOT
HARMONICS!
• Displaced Neutral Voltage
• AKA “Shaft Bearing Currents” , “Common Mode Noise (CMN)
• Fluted Bearings
• “drifty” transducers
• “flakey” sensors
• “everything is fine until the drive is turned on”
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Before you take any measurements,
troubleshoot anything or spend any
money, (Except for PowerFlex Drives):
Understand your ground system!
There are many like it but
this one is yours!
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Installation Considerations for AC Drives
Line Transients/Sags
Grounding
&
Bonding
Common Mode &
Capacitive Coupling
Reflected Wave
The 99% of issues you
thought were harmonics
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Installation Considerations for AC Drives
Line Transients/Sags
Grounding
&
Bonding
Common Mode &
Capacitive Coupling
Reflected Wave
The 99% of issues you
thought were harmonics
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GROUNDING
CONSIDERATIONS
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TODAY’S RESEARCH
• What would an internet search reveal?
• “There is only one choice”
• High Resistance Ground
• Transients
• Locating Faults
• Fault Damage
• Personnel Safety
• Coordination
• First Fault
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ROBIN’S REBUTTAL
• Why is the information so consistent?
• If HRG increases safety exponentially, why
isn’t it mandated?
• Why do so many organizations still use:
• Ungrounded
• Solidly Grounded (Effectively Grounded)
• Why is low voltage safer in your home now?
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FAULT STATISTICS
• 98% of faults are phase to ground
• Detection and response is far more important than available current
• Phase to phase 1.5%, 3 phase 0.5%
• Arcing faults are discontinuous
• Strike, extinguish and strike again
• Provides time for protection to work
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THIS IS AN OPEN DISCUSSION
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ZIGZAG (WYE-DELTA) AKA
INTERCONNECTED STAR OR STAR
DELTA
• Unusual
• Requires short term transformers
• 10 to 60 second ratings
• Better suited to generator sets and
prehistoric MV systems
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UNGROUNDED
• Significant advantages do exist
• Safety, production, Limits damage
• Requires
• Discipline
• Consistency
• Modification for VFD and some other systems
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UNGROUNDED
• Principle Benefits
• Low value of current flow and reliability during a fault (<5 amps is industry expectation)
• Ensures production through first fault
• Low probability of line-to-ground arcing fault escalating to phase-to-phase or 3 phase fault
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UNGROUNDED
• Claims I can not substantiate
• Substantial Over Voltages
• Sputtering Faults
• “Produced in Laboratory Tests”
• Drives see no difference between Ungrounded and HRG. Modifications are required.
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HIGH RESISTANCE
GROUND
• Lowers incident energy levels
• Not enough in most cases
• Coordination still required and more effective
• Personnel injury will still occur unless PPE and other measures are in place and used.
• Reduces ground fault current
• Modification for VFD and some other systems
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HIGH RESISTANCE
GROUND
Clamps ground fault current at lower
level and may change time base
• May eliminate the ability for
Variable Frequency Drives to
detect ground faults.
• This can allow greater damage
to infrastructure. (example)
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HIGH RESISTANCE
GROUND
• Lowers incident energy levels
• Eliminates or marginalizes protective
systems
• Lightning arrestors
• TVSS
• Distribution
• VFD
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HIGH RESISTANCE
GROUND
• Lowers incident energy levels
• Eliminates low impedance path for noise
• Encoders
• VFD signal common and VCC
• Communication networks
• Displaced neutral voltage
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• High frequency
sources that can
cause Nuisance
tripping and
alarms:
• VFD
• Servo
• Encoders
• Ultrasonic
Measurement
• SCR
• DC Drives
• Heating
• Electron Beams
HRG & HIGH FREQUENCIES
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SOLIDLY (EFFECTIVELY)
GROUNDED
• Still preferred by utilities
• Eliminates transient voltages that cause intermittent ground faults
• Stabilizes the neutral voltage
• Prevents elevation of phase to ground voltage
• Faults are easily located
• Can supply line to neutral loads
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SOLIDLY (EFFECTIVELY)
GROUNDED
• Arc and stray path current
• Limited only by the impedances
which are small
• Result is a short term fault
• Current level is high enough to
permit ground fault protection to
function
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SOLIDLY (EFFECTIVELY)
GROUNDED
• Arc flash
• All shorts cause an arc
• All arcs release energy as heat and light
• Mitigation is mandated
• Coordination
• PPE
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Questions and discussion
WE HAVE ONLY SCRATCHED THE
SURFACE
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UTILITY POWER, THE BEST KEPT
SECRET CAUSING
MANUFACTURING DOWNTIME
John McWeeney Regional BDM
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19
4
DEFINITION OF A “POWER GRID”
Merriam-Webster
A network of electrical transmission lines connecting a
multiplicity of generating stations to loads over a wide area
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
GRID OVERVIEW
Source: Department of Energy
19
5
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POWER QUALITY VS.
POWER RELIABILITY
Power Quality: Related to fluctuations in electricity, such as momentary
interruptions, voltage sags or swells, flickering lights, transients, harmonic
distortion and electrical noise
Fewer such incidents indicate greater power quality
Events go mostly untracked by Utilities
Power Reliability: Continuity of electric delivery measured by the number and
duration of power outages (Zero voltage)
Outages are tracked by Utilities
Power can be as high as 99.999% reliable
Remaining 0.001% can take out a process as many as 20-30 times per year
19
6
The Grid is designed for Reliability, not Quality…
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
EPRI (ELECTRICAL POWER RESEARCH INSTITUTE)
Monitored 300 sites for 2+ years.
• 1993 Data Showed 92% of all events were voltage sags
under 2 seconds in duration
• Second study in 1995 verified initial study, but showed
that almost 96% of all events were power sags less than 2
seconds
• A typical site experienced 20-30 significant voltage sags
per year
• Today PQ monitoring shows that now 98% of all events are
sags of less than 2 seconds
How Often is it Only a Sag?
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IMPORTANCE OF POWER
QUALITY
Power quality events are mostly random
Utility side: Weather, animal / trees hitting
power lines, car accidents, construction,
equipment failure
Facility-side: Starting of large loads – motors,
poor electrical connections, Customer
equipment (arc welders)
Impact on production
Shut down equipment: voltage sags with as
little as 80% remaining can impact production
(lights may not blink)
Immediate or long-term damage to sensitive
electrical equipment
Consumer is responsible for power quality
Utility is responsible for power reliability Consumer is responsible for protecting their sensitive equipment
19
8
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
VOLTAGE SAG (DIP)
CHARACTERIZATION
• Sag - RMS voltage reduction between 1/2 cycle - 60 sec
• Magnitude and Duration
19
9
-1
-0.5
0
0.5
1
0 1 2 3 4 5 6 7 8
Duration: 4 Cycles
Magnitude: 60% Remaining
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-35%
-70% -25%
The depth of your sag is proportional
to the distance you are from the event
What is the result of fault?
1. When a short occurs, voltage sags until the re-closer trips
2. Voltage sag severity depends on user distance from fault and location of fault on grid
3. Control equipment does not like voltage sags!
Sag = Variation below nominal RMS voltage
of 10-90% with a duration ½ cycle to 1
minute
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
THE MOST COMMON
EVENT: VOLTAGE SAG
Sag - RMS voltage reduction between 1/2 cycle - 3 sec
Magnitude - % Remaining
20
1
*Source: EPRI
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20
2
Magnitude Duration
Power Quality Report Summary
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Variable Frequency
Drives Review
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Safety
• Don’t sue or call Human Resources
– Lock out tag out
• Never work on energized circuits
– Verify condition with Test Instrumentation
• Test the instrument prior to and after measurement
– Observe all applicable Safety Regulations
Including yet not limited to State, Federal,
Local guidelines
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Basic Drive Components
• Draw A Basic AC Drive
• Describe what the components are
responsible for
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PWM Drive Theory
Drive Rectifies the
Incoming AC voltage.
Section known as:
CONVERTER
RECTIFIER
“FRONT END”
Drive STORES
The DC Voltage.
Section known as:
STORAGE
BUS CAPACITORS
DC BUS
Drive INVERTS The DC
Voltage into a wave
The motor interprets to be a
sine Wave. Section known as:
INVERTER
Output Section
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AC Drive’s Output? Voltage is a DC Square Wave made up of
pulses of various widths
Motor INDUCTANCE causes the current
wave shape to be sinusoidal
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What does PWM Stand for?
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DV/DT Spike
• What causes it?
• Who does it affect?
• What are the concerns with cable
distance?
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AC Drive’s Output?
That is the dv/dt
spike.
D (delta) or change
in the Voltage
divided by the
change in Time
Shorter the time…
Bigger the spike
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dv/dt Spike
• All transistors (switching power
semiconductors) create noise
– Not brand specific
• When is the Wave Reflected
– How can that be solved
• Terminator
• Impedance matched motor
• Standing Wave….
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AC Line Reactor vs DC Link
Inductor
• Generally used to reduce Harmonic
Disturbances created by the drive
• Provides Impedance upstream of the drive
– Especially helpful with very large transformers
and small drives
• Magnetic structure rounds over (increases
time base) of disturbance
– Structure is developed by current
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Reactor Attenuation
Time Base Time Base
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AC Line Reactor vs DC Link
Inductor
• Basic difference in Magnetic Structure
– AC Line Reactors use current
– DC Link Inductors use Voltage
• Advantages of DC Link
– Magnetic Structure is fully developed anytime the
drive is energized.
– DC Link is effective regardless of load
• What does the DC Link not do
– Cannot protect a Drive from Line Disturbances
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DC Link’s effect on the Dv/Dt
Time Base Time Base
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Motor Theory
• List major differences between a Good
Inverter rated motor and Premium Efficient
Designs
• Multiple motor applications require what
additional components
• Insulation testing should be conducted at
what levels
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Regulators
• Describe the basic differences between
V/Hz and Vector Regulators
– Volts “per” Hertz is a fixed ratio of Voltage to
Frequency
– Vector regulators are dynamic. They change
based on load and motor charactoristics
• Can be tuned
• Too smart for multiple motor applications
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Common Mode Noise
• What creates CMN? – Anything with a Switch Mode Power Supply
• What are symptoms of CMN? – Devices connected to Neutral are affected when the
drive is energized
– Bearing Current
• How can CMN be detected? – Clamp on ammeter
• PowerFlex Drives combat CMN with what components or design features – CM Bus Caps, CM Magnetic Cores (CM Chokes)
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Parameters
• Describe the Use or Value of:
– Programmable relays
– Flying Start
– PID loops
– Sleep Mode
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Programmable Relays
• What basic functions are programable
• How many selections are available
• Describe 3 applications where the relays
could be employed
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“Typical” Failures
• Converter faults relate to?
• Inverter faults relate to?
• What is the simple way to
determine health of drive
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Energy Savings
• Drives have unity power factor
–Power factor is the ration of KW to KVA
• Remember Work (Torque) to Apparent (Magnetism)
–Drive can consume less current than the motor
• Affinity Laws
–Energy increases with the cube of Speed
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Power Factor as a Cost
(In phase with line voltage)
Your “Apparent” Consumption
IE: Required Generation Capacity
Real Work
“Billed KWh”
Power Factor
Lagging PF
+20%
+10%
Standard
-10%
-25%
Unity PF
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Power Factor as a Cost
(In phase with line voltage)
Power Factor
Lagging PF
+20%
+10%
Standard
-10%
-25%
Unity PF
Your “Apparent” Consumption
IE: Required Generation Capacity
Real Work
“Billed KWh”
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Power Factor as a Cost
(In phase with line voltage)
Power Factor
Lagging PF
+20%
+10%
Standard
-10%
-25%
Unity PF
Real Work
“Billed KWh”
Your “Apparent” Consumption
IE: Required Generation Capacity
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Energy
Consumed
Speed
Affinity Law
In Variable Torque “Ideal” Loads:
Energy Increases exponentially with speed
Energy Consumed = (speed)3
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Consumption reduction
=(Speed reduction
)3
100 kWh Motor @ 50% Speed:
What does it Cost?
Er=(1/2)3
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Now, how many rural
applications can you
think of?
List them on the critique sheet for
a “special” professional
development award
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Robin Priestley
Power Control Manager
563-343-8862 © 2014 Robin Priestley & Rockwell Automation
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COPY WRITE © 2013 ROBIN PRIESTLEY ROCKWELL AUTOMATION ALL RIGHT S RESERVED
MOTOR BEARING CURRENT
SOLUTIONS
• Background / Why is this an Issue?
• Bearing Damage / Failure Mechanisms
• Sine Wave Shaft and Bearing Currents
• Common Mode Voltages / Currents
• Solutions
• Measurements
• Conclusions
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BACKGROUND
Q: Since bearing currents in
rotating machinery have been
documented for at least 90
years, why is this a
contemporary issue?
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BACKGROUND
Q: Since bearing currents in rotating machinery have been documented for at least 90 years, why is this a contemporary issue?
A: Modern PWM inverters create both common mode voltages (CMV) and common mode currents (CMC) which provide new opportunities for current to flow through rotating bearings (along with couplings, gears, etc)
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BACKGROUND
While the 90 year old sources of bearing
currents are well understood and
solutions exist, it is important to keep
them in mind to avoid resurrecting them
in trying to solve the challenges brought
on by common mode voltages and
currents.
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BEARING DAMAGE /
FAILURE MECHANISMS
Fluting in outer race, from
prolonged operation after
damage from current flow
Individual arc damage spots
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BEARING DAMAGE /
FAILURE MECHANISMS
Fluting on inner race, from prolonged
operation after damage from current flow Fluting in outer race
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BEARING DAMAGE /
FAILURE MECHANISMS
• Interrupted current causes melting and
“re-hardening” of the race material,
creating untempered martensite, which is
brittle and prone to fatigue
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BEARING DAMAGE /
FAILURE MECHANISMS
• Interrupted current causes melting and “re-hardening”
of the race material, creating untempered martensite,
which is brittle and prone to fatigue
• The normal bearing loads are then capable of breaking off
small pieces of this brittle material
• Subsequent running on this brittle surface and in the presence
of the damage “trash” material creates the “fluting”
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BEARING DAMAGE /
FAILURE MECHANISMS
• If the damaged material does not progress to a fluted pattern
from subsequent running, two other patterns may be seen
• A “frosted” surface may appear, or
• A number of “pits” may be visible under
high magnification
• The verification of current flow as the root cause requires
more than visual inspection
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BEARING DAMAGE /
FAILURE MECHANISMS
False Brinnel Damage with
Appearance of Fluting
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BEARING DAMAGE /
FAILURE MECHANISMS
False Brinnel Damage
with Appearance of
Fluting
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SINE WAVE BEARING
CURRENTS
• “If it were possible to design a perfectly
balanced and symmetrical machine, both
theory and practice indicate that no
bearing current could exist” - C. T.
Pearce, Bearing Currents - Their Origin
and Prevention, The Electric Journal,
August 1927.
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SINE WAVE BEARING
CURRENTS
• Alternating flux “linking” the shaft …
• Net flux encircling the shaft is typically
due to asymmetric magnetic properties of
stator or rotor core
• Bearing current created by transformer
action in “single turn” secondary (shaft,
bearings, frame)
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SINE WAVE BEARING
CURRENTS
Boyd and Kaufman, 1959
Shaft
Flux path
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SINE WAVE BEARING
CURRENTS
• Currents flow thru shaft, bearings, endshields, and frame
• Axial voltage on shaft can be measured if a bearing is insulated (IEEE Std 112 - 1996)
• Small shaft voltage (500 mV) can lead to bearing currents above 20 amps
• Bearing damage is more likely to occur in larger machines
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COMMON MODE
VOLTAGE / CURRENT
• Modern PWM drives create switching patterns
where instantaneous average voltage to ground
is not zero.
• Voltage has a rapid change of magnitude with
respect to time (dV/dt)
• High dV/dt results in capacitively coupled
currents from motor windings to ground
through several paths
I = C x dV/dt
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COMMON MODE
VOLTAGE / CURRENT
PHASE
VOLTS
CMV
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COMMON MODE
VOLTAGE / CURRENT
PHASE
VOLTS
CMV
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COMMON MODE
VOLTAGE / CURRENT
PHASE
VOLTS
CMV
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COMMON MODE
VOLTAGE / CURRENT
HIGH FREQUENCY CURRENT PATHS
I = C X DV/DT
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COMMON MODE
VOLTAGE / CURRENT
BEARING CURRENT RELATIVE
MAGNITUDE
30
30.5
0
10
20
30
40
Stator Winding to
Frame/Shaft
Discharge dv/dt Charging
Peak A
mps T
hro
ugh B
earing
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COMMON MODE VOLTAGE
/ CURRENT
HIGH FREQUENCY CURRENT PATHS
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COMMON MODE
VOLTAGE / CURRENT
HIGH FREQUENCY END-END
CIRCULATION
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COMMON MODE
VOLTAGE / CURRENT
ROTOR DISCHARGE CURRENT
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COMMON MODE VOLTAGE / CURRENT
HIGH FREQUENCY CURRENT
PATHS –
CAPACITIVE CHARGING OF
ROTOR / BEARING
+
-
VCM
Bearing Frame
CSF CRF Cb
Stator Winding Rotor
+
-
Bearing Voltage : Vb = Csr
Csr + Cb + Crf VCM
CSR
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COMMON MODE
VOLTAGE / CURRENT
ROTOR DISCHARGE CURRENT
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COMMON MODE VOLTAGE /
CURRENT
TRANSIENT FRAME VOLTAGE
DISCHARGE
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COMMON MODE
VOLTAGE / CURRENT
BEARING CURRENT RELATIVE
MAGNITUDE
30
30.5
0
10
20
30
40
Stator Winding to
Frame/Shaft
Discharge dv/dt Charging
Peak A
mps T
hro
ugh B
earing
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BEARING CURRENT
SOLUTIONS
• Eliminate or reduce common mode voltage
/ current (Drive design issue)
• Create best high frequency ground paths
between drive, motor, and load
• Electrostatic shielded induction motor
• Insulated bearings
• Shaft grounding brush
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BEARING CURRENT SOLUTIONS
INSULATED OPPOSITE DRIVE-
END BEARING FOR
CIRCULATING TYPE CURRENTS
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BEARING CURRENT SOLUTIONS
INSULATED OPPOSITE DRIVE-END
BEARING AND DRIVE-END SHAFT
BRUSH
(BEARINGS IN COUPLED EQUIPMENT STILL AT PERIL)
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BEARING CURRENT SOLUTIONS
INSULATED OPPOSITE DRIVE-
END BEARING, DRIVE-END
SHAFT BRUSH, AND COUPLED
EQUIPMENT BOND STRAP
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BEARING CURRENT SOLUTIONS
TWO INSULATED BEARINGS,
DRIVE-END SHAFT BRUSH, AND
COUPLED EQUIPMENT BOND
STRAP
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BEARING CURRENT
SOLUTIONS – FARADAY
(ELECTROSTATIC) SHIELD
• Add grounded conductive layer between
stator and rotor
• Eliminates stator to rotor coupling
• Will not eliminate stator winding to frame
coupling
• Still need good high frequency ground current path from
motor to drive ground
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BEARING CURRENT SOLUTIONS
FARADAY SHIELD TO PREVENT ROTOR
CHARGING / DISCHARGING (BEARINGS
STILL AT PERIL FROM TRANSIENT FRAME
VOLTAGE DISCHARGE WHEN SHAFT IS
CONDUCTIVELY COUPLED TO GROUNDED
EQUIPMENT)
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BEARING CURRENT SOLUTIONS
FARADAY SHIELD
AND COUPLED EQUIPMENT
BOND STRAP
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BEARING CURRENT
SOLUTIONS
• Internal, end-end from magnetic
asymmetry
Insulate opposite drive-end
bearing
Insulate both bearings
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BEARING CURRENT
SOLUTIONS
• Shaft Extension Current (stray ground current)
Insulate coupling
Insulate bearings
Bond strap from motor to load
Better low impedance ground in cable from
inverter to motor
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BEARING CURRENT
SOLUTIONS
• Discharge of voltage on rotor
Faraday (electrostatic) shield
Shaft brush
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BEARING CURRENT
SOLUTIONS
• Precautions
NO opposite drive end shaft brush with
single opposite drive end insulated bearing
Beware of shaft brush option in opposite
drive end encoder
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MEASUREMENTS
(VOLTAGE)
Common
Mode
Voltage
Shaft
Voltage
250
V/div
12.5
V/div
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MEASUREMENTS
(CURRENT) INTERNAL END-END
CIRCULATION
2 A/div
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MEASUREMENTS
(CURRENT)
Common
Mode
Current
2A/div
(both).
Ground
Conductor
Current
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MEASUREMENTS
(CURRENT)
Shaft
Extension
Current (30 Amp Pulse)
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MEASUREMENTS
• Other than internally-sourced circulating
currents, all data is at high frequency
• Data tends to be non-repetitive
• Oscilloscope triggering technique
strongly influences perceived results
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CONCLUSIONS
• Current flow in rotating bearings is not
new
• Common mode voltages and currents
from modern inverters can cause current
flow through bearings (plus couplings,
gears, etc)
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CONCLUSIONS
• Corrective actions are dependent upon
the particular type of current flow
• Transient (high frequency) nature of the
voltages and currents imposes different
requirements than traditional 60 Hz
waveforms
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CONCLUSIONS
• Since the sources of the currents as well as
the paths are typically outside the machine
whose bearings are taking the hit, a thorough
understanding of the system is key
• Grounding is important, but more in the sense
of point to point (low impedance) “bonding”
rather than “earthing”
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COATED / INSULATED
BEARINGS
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“CONDUCTIVE” GREASE
While the notion of a conductive grease as a solution may sound
appealing, the electrical behavior of bearing lubricants is not as
simple as "insulating" versus "conducting." Both the behavior of
the grease in bulk as well as the behavior of the thin oil film
separating races from rolling elements is strongly dependent on
external influences, including the presence of a voltage. As a
result, the current and voltage characteristics seen in a rotating
bearing are not simply described by a resistive value. In fact, it is
not simply described by a combination of fixed resistors,
capacitors, and other circuit elements. It has a "memory" effect,
based on past applied voltages and current flow, as well as
behaviors that may best be described as "stochastic."
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“CONDUCTIVE” GREASE
• Different greases can have varying electrical
characteristics, based on their chemical composition, but
still would have the "inconsistent" behavior as described
above.
• Any proposed grease would obviously need to not
degrade the "normal" properties expected in a bearing.
• The conclusion of the points above is that a change to a
grease with different electrical properties is not a solution
to the basic problem of bearing currents (for neither VFD
nor line-fed motors).
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SHAFT VOLTAGE ?
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SHAFT VOLTAGE (NOT!)
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INVERTER DRIVEN
INDUCTION MOTOR
BEARING CURRENT
SOLUTIONS
QUESTIONS ?