Motor Control and Protection
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Transcript of Motor Control and Protection
Motors and Motor Control
Oladokun Sulaiman
Understand working principles of motor starters and various protection devices
Objective
• At the end of the lecture students will be able to describe the working principles of motor starters and various protection devices
3
Motors• A motor is basically a generator running in
reverse.
• A current is passed through the coil, producing a torque and causing the coil to rotate in the magnetic field.
• Once turning, the coil of the motor generates a back emf, just as does the coil of a generator.
• The back emf cancels some of the applied emf, and limits the current through the coil.
4
Motors and Back emf
• The phrase back emf is used for an emf that tends to reduce the applied current
• When a motor is turned on, there is no back emf initially
• The current is very large because it is limited only by the resistance of the coil
Motor• DC
• AC
AC Motor
• Induction motor
• Synchronous motor
• Wound rotor motor
Operate based on Speed (S)= 120f/p
• F-Frequency
• P-Number of poles
6
Control
What you want to control = what you can control
For DC motors:
speed voltage
N
S
N SV
V e back emf
R
windings’ resistance
e is a voltage generated by the rotor windings cutting the magnetic field
emf: electromagnetic force
Control: getting motors to do what you want them to
Needs for Motor Control1. Induction motor – drawn 5-8x full-load current
(FLC) when starting2. Due to maximum flux cutting rate (s = 100%) in
rotor- creating large induced rotor currents3. Supply power factor very low i.e. 0.2 lagging at
starting, 0.5 lagging on no-load & 0.85 lagging on full-load
4. This starting surge current reduces as motor accelerates up to rated speed
5. Operating at light loads with low power factor - inefficient as supply current higher causing higher I²R (copper) losses
6. To improve - reduce supply voltage for light loads motor
7. Achieved with electronic voltage controller i.e. soft-starter and/or energy manager - match supply voltage to start-up & load conditions
8. This will maintain operating power factor as high as possible - minimise supply current & power losses
9. Most induction motors have Direct-on-Line (DOL) - inexpensive & simple to operate & maintain provided current surge not cause heating damage to motor
10. When larger motors started by DOL – can cause voltage dip due to large starting current
11. May result in malfunction of others - lighting dip & flickering effects
12. To limit, motors started at reduced voltage- full supply reconnected when accelerated close to rated speed - star-delta, auto transformer & electronic "soft" starter
1000
100
10
1
0.1
0.0110 100 10001
Tim
e in
S
econ
ds
Current in Amperes
Motor Characteristics
Inrush Current
Normal Operating Current
Motor Inrush Curve
300 %
Overlo
ad
1000
100
10
1
0.1
0.0110 100 10001
Tim
e in
S
econ
ds
Current in Amperes
Sh
ort C
ircu
it1000
100
10
1
0.1
0.0110 100 10001
Tim
e in
S
econ
ds
Current in Amperes
0100
200
400
600
Cu
rren
t %
0Slip %Auto transformer
on 60%
STAR DELTA STARTING
AUTO TRANSFORMER STARTING
DIRECT ON LINE STARTING
COMPARISON OF STARTERS
Contactor
Performing switching action to connect/disconnect power supply to motor. Electromagnetically operated 3-pole switch initiated from local, remote stop/start push buttons. If current above rated, contactor will tripped out automatically by OCR, disconnecting motor from supply.
Direct on line• Simple arrangement, used for majority induction motor• Motor directly switched onto 3 phase AC power supply
lines• Further circuit additions – remote control & reversing
(required extra contactor)• Short duration but large starting current • Acceptable provided voltage dip < 10~15% during
starting • For larger motor - unacceptable voltage dip at bus-bars -
malfunctions of other consumers & possible drop out of supply contactors
• If prolonged – cause supply line & generator protection to trip
Power circuit operation Control circuit operation
Manual closing of fused isolator Q1
Control circuit voltage available (e.g. 110V from control transformer)
Closing of line contactor KM1 Press start button “I” (local or remote)
KM1 contactor ‘holds-in” Auxiliary contact on KM1 ‘latches’ contactor Remote indicator lamp ‘on’
KM1 contactor drops out, motor stops
Press stop button ‘O’ (local or remote) on overload the OCR trips out the stop buttonOCR must be manually reset (after thermal time delay)
Star delta
• If motor stator winding is star connected, only 1/3 of starting current required if motor start with delta connected
• For small motors – operated by manual c/o switch• For large motors - phase windings automatically
switched using timing controlled contactors• At initial starting, motor won’t rotate, thus no
mechanical output produced• Therefore, current taken by the motor will determine
by supply voltage & impedance of motor phase windings
Power circuit operation Control circuit operation
Manual closing of fused isolator Q1 Control circuit voltage available(e.g. 110V from control transformer)
Closing contact of KM1: star connection
Press start button S2 to close KM1
Closing of KM2: motor supply KM1 closes KM2
Opening of KM1: star connection opens
“hold in” of KM1 – KM2 by KM2 auxiliary
Closing of KM3: delta connection Opening of KM1 by KM2 auxiliaryClosing of KM3 by KM1 auxiliary
KM2 & KM3 contactors drop out, motor stops
Stop by S1 button or OCR trip F1
Note: KM2 has a pair of auxiliary contacts with a time delay action (typically 40ms) between the operating of the N/C and the closing of the N/O contacts.
Comparison if star & delta connection
3
1
.3
.3
)(
)(
ZV
ZV
I
I
L
L
L
YLRatio of
Current surge from star to delta
• Motors generate back emf against power supply when running
• When supply removed, magnetic field does not collapse immediately
• Motor will slow down but still generate emf• When supply reconnected, supply voltage & motor
emf are not in phase• Thus each time the starter is operated, different
current surge will produced• To overcome – auto transformer is used where the
supply is eventually never disconnected during starting period
Auto transformer• Starting large motor with prolong run-up period demand very
high current surge from supply generator even for few seconds
• Will causes severe voltage dip - affects other loads• Reduced voltage starting will limit starting surge current• One method – step it down using transformer• When motor accelerated up to almost rated speed, then
“reduced” voltage will resume to normal • Special transformer – uses one winding for input & output• Thus, cheaper, smaller & lighter than equivalent double-
wound transformer• Meant for operation of short starting period only• Only applicable to large motor drives due to initial cost
Power circuit operation Control circuit operation
Manual closing of fused isolator Q1 Control circuit voltage available(e.g. 110V from control transformer)
Closing KM1: star connection of transformer
Press start button S2 to close KM1Interlocking of KM3 by KM1Closing KA1 by KM1
Closing KM2: motor supply via transformer
Closing of KM2 by KA1Hold in of KM2
Opening KM1: star connection opens Opening KM1 by KA1 (after time delay)
Closing KM3: direct supply to motor Closing KM3 by KM1Interlocking KM1 by KM3
(Note the mechanical interlock of KM1-KM3)
Hold in of KM3Opening of KM2 by KA1
KM3 contactors drop out, motor stop Stop by S1 button or OCR trip F1
• Supply voltage connected across complete winding & motor connected to reduced voltage tapping
• Number of tapping available - giving output voltage ranging from 50% ~ 80% of main supply
• If 60% tap supplied at 440 V, output will be 60% x 440 = 264 V• Multiple tapping - to match motor current demand to supply
capability• Autotransformer can be use in both open & closed transition
switching sequence between start & run conditions• Star delta - reduced voltage initially supplied, disconnected & then
full supply voltage rapidly reconnected to motor – open transition• Danger with open-transition - very large surge current can flow
after transition from reduced to full voltage
Auto transformer - operation
Soft starter (additional)
Conclusion
• DOL starter - simple & cheap but causes large starting surge
• Star delta starting reduces surge but more complex – require 3 contactors & timer
• Auto transformer - can arranged to match motor surge current & run-up period with suitable voltage but the most expensive one
Controlling speed with voltage
DC motor model
V e
R
• The back emf depends only on the motor speed.
• The motor’s torque depends only on the current, I.
e = ke
= k I
kke
Controlling speed with voltage
DC motor model
V e
R
• The back emf depends only on the motor speed.
• The motor’s torque depends only on the current, I.
e = ke
= k I
• Consider this circuit’s V: V = IR + eIstall = V/Rcurrent when
motor is stalledspeed = 0
torque = max
How is V related to
V = + ke R k
- or -
= - + R ke V
Speed is proportional to voltage.
speed vs. torque
torque
speed
ke V
at a fixed voltage
R kV
max torque when stalled
no torque at max speed
speed vs. torque
torque
speed
ke V
at a fixed voltage
R kV stall torque
no torque at max speed
Linear mechanical power Pm = F v
Rotational version of Pm =
speed vs. torque
torque
speed
ke V
at a fixed voltage
R kV stall torque
max speed
Linear mechanical power Pm = F v
Rotational version of Pm =
power output
speed vs. torque
Motor specs
Electrical Specifications (@22°C)For motor type 1624 003S 006S 012S 024
-------------------------- -------- -------- -------- --------- -------nominal supply voltage (Volts) 3 6 12 24armature resistance (Ohms) 1.6 8.6 24 75maximum power output (Watts) 1.41 1.05 1.50 1.92maximum efficiency (%) 76 72 74 74no-load speed (rpm) 12,000 10,600 13,000 14,400no-load current (mA) 30 16 10 6friction torque (oz-in) .010 .011 .013 .013stall torque (oz-in) .613 .510 .600 .694velocity constant (rpm/v) 4065 1808 1105 611back EMF constant (mV/rpm) .246 .553 .905 1.635torque constant (oz-in/A) .333 .748 1.223 2.212armature inductance (mH) .085 .200 .750 3.00
ke
k
Back to control
Basic input / output relationship:
How to change the voltage?
We want a particular motor speed .
We can control the voltage applied V.
V = + ke R k
V is usually controlled via PWM -- “pulse width modulation”
PWM
• PWM -- “pulse width modulation
• Duty cycle:– The ratio of the “On time” and the “Off time” in one cycle– Determines the fractional amount of full power delivered to
the motor
Open-loop vs. Close-loop Control
Open-loop Control:
actual speed
desired dV
Motor
a
actual speed a
- compute V from the current error
d a
Closed-loop Control: using feedback
desired speed Controller solving for V(t)
V(t)
Motor
If desired speed d actual speed a,
So what?
PID controller
Speed control:
• Stator voltage control
• Supply frequency control
• Rotor resistance control
• Pole changing
VSD• Conventional control of supply frequency and terminal
change of phase to minimize losses – counter current /pluging+ regenerative +dynamic
• Development in speed and torque control
• From ward leornard system -> thyristor controlled DC drive ->PWM AC variable voltage regulation ->variable frequency converter-> AC VSD or inverter
• Cost effective method of speed control+ application to high power+relibaility+maintainability+save energy+ improve efficiency+ match speed and torque of drive with process drive
Backdrop- complexity
Component-• Motor
• drive control unit-power source to motor, increase and decrease motor set point at operator panel+ feed back loop give the driv the actual speed+Power modulation – control the speed , torque and power along with direction of motor and machine- i.e converter, inverter, cycloconverter.
• +sensing unit
• +operator unit
Different Categories of Overload
ProtectionMotor enclosure
• Totally enclosed , non ventilation
• Splash –proof type
• Totally enclosed fan cooled
• Drip proof type
Name plate- rating, supply , connection ,frame type and size,permisible temperature,rpm, enclosure type,# of pole.
Motor Protection1. Short-circuit protection of stator windings
2. Stator-overheating protection
3. Rotor-overheating protection
4. Under voltage protection
Protection
Measurement• Temperature• Voltage and current-• Insulation resistance
winding resistance• Vibration• Speed•
Testing:
No load test
Full load test k
Failures:• Insulation failure• Rotor bar failure• Mechanical problem
Maintenance
Periodic inspection-
Accurate shaft alignment or belt tension
Check motor heating@ heating- check and clean air filter
Keep motor clean and free from dirt
Keep motor dry - Check for dampness around and inside motor
Check bearing regularly- lubrication at right quantity
Vibration analysis- of motor and coupling
Check noise
Circuit Breaker• Safely & interrupt prospective short circuit fault
current expected in circuit• Will trips but can be reset & reused • Link mechanism provided, closes main contacts
under spring pressure & wipes the surface of fixed contact points - ensuring good electrical contact
• Main contact open rapidly with snap action• Resulting arc transferred to special arcing
contacts above the main contact• Arc chutes with arc ‘splitter’ quickly stretch &
cool the arc till it ‘snaps’• Circuit breaker is ‘open’ when the arc quenched
The Magnetic Trip Block
Fuse • Protect circuit from damage – faults & over
current• Designed to blow rapidly before circuit damage
takes place• Many types and sizes, marked with size of steady
current can be carried without blowing - fuse rating Transparent casing
Brass cap
Tinned wire copper
Fuse Rating • Important – correct rating for normal current
flowing in circuit it protects • Lower rating - every time switch on, fuse will blow • Higher rating – promoting positive dangerous
circuit with over current flowing without blowing fuse - overheat & can cause fire
• If fuses blow, must replaced by same type & same rating
• Position - between supply and the circuit – fuses removal means total isolation for the circuit
• Two main types:– Cartridge fuse– High rupturing capacity (HRC) fuse
Checking Fuses: Visual inspection
Relays are amazingly simple devices. There are four parts in every relay: 1. Electromagnet 2. Armature that can be attracted by the
electromagnet 3. Spring 4. Set of electrical contacts
Case study 1: How a relay works?
Case study 2: Under voltage trip
UV relay coil
Fuse
Circuit breaker
Generator
3-ph 440V bus bars
Normal ConditionNormal Condition
M
1.4 A
1.4 A
1.4 A
208V 1/3 HP Motor 40 C
F.L.A. = 1.4 Amperes
M
0 A
2.4 A
2.4 A
(173%)
(173%)
What happened?What happened?
Case study 3: Single phasing
Bi-metallic Single-phasing Protection (differential action)
Single phasing
• Occurs when one of three back-up fuses blows or if one of contactor contacts is open-circuited
• Effect – current increase in two remaining lines• Cause noisy motor – uneven torque produced in rotor• Will detect by OCR – unequal heating of bi-metal strips
causes differential movement, initiate OCR to trip motor contactor
• For star connected motor – phase & line currents are equal, thus OCR has no problem in sensing correct winding current
• For delta connected – uneasy task, therefore, normally line current will divides phasorally between 2 phases of motor windings
LL
PH II
I 577.03
Single phasing
Healthy condition(balanced)
Single phasing fault condition(unbalanced)
% of rated FLC
% of rated FLC
IL2 and IL3 IA and IB IC
60 102 62 131
70 130 79 161
100 243 129 185
Facts of single phasing
• When one line open circuited, balanced condition will no longer exists
• Note that current C is higher than others• At 60% of full load, due to single-phasing, line currents are
102% of full-load value but current C is 131%• 102% may not activate OCR, thus motor remains connected• However, local overheating in winding C will quickly get
damage• Differential type relay used to protect motors against this
condition i.e. trips out with unbalanced currents• For most modern thermal OCR - protection against single-
phasing - normal feature
• If single-phasing occurs on light load, motor will keep running unless protection trips contactor
• If motor stopped, it won’t restart• When contactor closed, motor will take large starting
current but develop no rotating torque• OCR - set to allow starting current at prolong period
– sufficient for start up period • With no ventilation on stationary motor - time delay
will result rapid & severe overheating • Worse case - if operator makes several restart, motor
will burn out
Effect of single phasing
• If motor fails to start – investigate first• UV protection - disconnected consumers from supply if
total voltage loss / black-out, prevent restarting together resulting huge current surge, tripping generator again
• For LV motors – UV provided by spring loaded motor contactor
• For large HV motor - UV covered by relay separate from OCR function or part of special motor relay
• Motor won’t restart until contactor coil energised – require operator to reset manually
• For essential services – restart automatically after certain delay is utmost important
Effect of single phasing (cont/..)
61
Willas-Array Solution for Motor Control
Control Unit
With Motor Control Cell
Feedback Signal
(Speed / Positon)
Power Management
Power Stage = 6 x IGBT or MOSFET
IPM Module3 x Driver IC
Gate Driver
Gate Driver
Gate Driver
Block Diagram 1
Others
Hall Sensor
(Inside Motor)
BLDCMotor
PWM Signal
AC Input
Inspection & maintenance
• Moving contacts in control gear - ‘wipe’ phenomenon i.e. if fixed part need to removed, moving part would follow on
• Rolling / sliding action of contactor - to remove any oxide, ensure good metal-to-metal contact
• Frequently operate contact – subject to worn, bad contact, ‘wipe’ lost, reduction in contact pressure & overheating – regular inspection & cleaning
• Rough contact surface could lower contact resistance - file used sparingly & only on badly burned & pitted contacts
• Contact restorer - helps reduce mechanical wear, but excess oil / grease encourages burning & pitting
• Silver-faced & carbon contacts shouldn’t be lubricated
Inspection & maintenance (cont/…)
• Closed copper contacts with long periods tend to build oxide film - cause overheating
• Operated contact several times – to clean surfaces• Magnet faces - kept clean & free from grease/oil, rust
removed using fine emery• Moving parts – free, no undue wear at pivots, magnets
bedding properly & no filing on magnetic faces• Enclosure – dirt/rust accumulations, corroded parts,
starter fixing bolts & earth bonding connection • Contactors & relays – signs of overheating & loose
connections, dust/grease from insulating components
Inspection & maintenance (cont/…)
• Contacts – excessive pitting & roughness• NEVER file silver alloy contacts or remove silver oxide -
good conductor• If need to replace, always replace both fixed & moving
contacts in pairs • Connections – power & control connections for tightness,
overheating, fraying & brittleness flexible leads • OCR - proper size (relate to motor FLC),
dirt/grease/corrosion & freedom of movement• Control operation – sequence during start-up, control &
shut-down, excessive contact sparking, functioning of emergency stop & auto restart