19222 14 Dc Machine Control
-
Upload
daniel-ihsan -
Category
Documents
-
view
231 -
download
0
Transcript of 19222 14 Dc Machine Control
-
8/3/2019 19222 14 Dc Machine Control
1/18
1
DC Machine ControlDr Andrew Cruden
Electronic & Electrical Engineering
University of Strathclyde
R: R3.08H
T: 0141-548-2991
19222 Electrical Machines & Control
-
8/3/2019 19222 14 Dc Machine Control
2/18
219222 DC Machine Control
Overview of Presentation
Instill an understanding of the typicalmethods and equipment used to control(start, stop and regulate) DC motors,
including relays and thyristorsSummary
See accompanying notes entitled 19222DC Machine Control
-
8/3/2019 19222 14 Dc Machine Control
3/18
319222 DC Machine Control
Quick Review
There are 2 main types of DC motor: series and
shunt motors
Shunt motors have been shown to be virtually
constant speed motors w.r.t. the applied loadSeries motors have been shown to be variable
speed, high starting torque motors
Typical applications for both motors have beengiven
-
8/3/2019 19222 14 Dc Machine Control
4/18419222 DC Machine Control
DC Machine Control
The DC motor can be accelerated and decelerated
quickly and easily, and the speed can also be accuratelycontrolled over a wide range by, amongst othermethods, by varying the applied voltage to the motor
However most industrial factories and premises aresupplied with AC power therefore conversion to DC isnecessary to utilise DC machines
Additionally DC motors are generally more expensivethan AC machines and these two factors have promotedincreased use of AC motors and variable speed drives
rather than use of a DC
-
8/3/2019 19222 14 Dc Machine Control
5/18519222 DC Machine Control
Basic Motor Control -
Starting!
The first aspect of DC motor control is to successfully start them!
Remember from previous that at starting ( = 0rad/s) that theback emf, E, will equal zero volts (there can only be an emfwhen the motor armature is turning)
If E = 0, then the armature current will be very high (limitedonly by the armature resistance which is typically quite small)then precautions must be taken to prevent motor damage
Usually small motors, say up to 1.5kW, can be direct-on-line(DOL) started, whereas large motors will typically employ eithervariable resistance or reduced voltage starting
-
8/3/2019 19222 14 Dc Machine Control
6/18619222 DC Machine Control
Variable Resistance
The armature resistance can be varied manually, howevergenerally this is not viable
More appropriate automatic regulation of armature resistance canbe achieved using various relays, operated either by the back emfitself or on a definite time basis
1CR11M1+
OL
Resistor
2CR1
1M2
Start Stop
2CR
ARM __field
series
1M
1CR
OL
normally closed
normally open
Relay
1M
1M2
1M1
Symbols
-
8/3/2019 19222 14 Dc Machine Control
7/18719222 DC Machine Control
Relay Operation
When the relay drive coil (symbol
1M) experiences current flow, amagnetic field is establishedattracting the relay contact tomove.
This moving contact (either 1M1or 1M2) will either open or closedepending on its initial condition
The control circuit (supplying thedrive coil) may be completelyseparate from the contact circuit,
hence isolation can be achieved
normally closed
normally open
Relay
1M
1M2
1M1
Symbols
Drive Coil
Contact
-
8/3/2019 19222 14 Dc Machine Control
8/18819222 DC Machine Control
Relay Operation - example
Start switch pressed
Current flows to 1M
1M1 closes current flows to
resistor and motor armature
1M2 closes start switch canbe released and is bypassed by1M2
As back emf, E, increases as
speed increases, 1CR operates
1CR1 closes and half of resistoris bypassed, therefore motorsupply voltage increases andmotor speeds up until 2CR
operates 2CR1 closes, resistor is
completely bypassed andmaximum supply voltagereaches motor. Maximum
motor speed achieved andmotor has started in controlledfashion
1CR1
shunt field
1M1+
OL
Resistor
2CR1
1M2
Start Stop
2CR
ARM
fieldseries
1M
1CR
OL
-
8/3/2019 19222 14 Dc Machine Control
9/18919222 DC Machine Control
Voltage Controlled Relays
(1CR & 2CR)
Relays 1CR and 2CR are specially
controlled relays in the previous exampleThe contacts (1CR1 and 2CR1) only
operate when the voltage across theirdrive coils (1CR & 2CR) are above userdefinable threshold values
Contacts 1CR1 & 2CR1 behave likevoltage controlled switches
ar a e es s ance
-
8/3/2019 19222 14 Dc Machine Control
10/181019222 DC Machine Control
ar a e es s ance -contd
The extra resistance added into the armature circuit at start up
(marked Resistor) is at a maximum at start up (i.e. zero speed) As the motor speeds up the two voltage sensitive relays (1CR &
2CR) progressively operate and switch out part, then all, of theextra resistance out of the armature circuit
These relays work as the back emf across the motor builds up,first 1CR operates and shorts out part of the resistor, then asthe speed and emf increase further, 2CR operates and shorts
out the complete resistor. Relay 1M operates as a latching relay and a thermal overload
(O/L) relay is included
Definite time relays are essentially timers - the relay changesstate after a definite period of time
Th l O l d R l
-
8/3/2019 19222 14 Dc Machine Control
11/181119222 DC Machine Control
Thermal Overload Relay
(bimetallic switch)
A bimetallic strip commonly acts as a thermal overload protection on a motor.
Two strips of different metals are heated by current passing through them. The heat causes the
metals to expand and, due to the different metals, the expansion causes the fused strip to curve. This curvature draws the bimetallic strip away from a set of contacts and hence breaks the electrical
circuit.
Source: Wikipedia
C C
-
8/3/2019 19222 14 Dc Machine Control
12/18
1219222 DC Machine Control
Reduced Voltage AC to DC
power electronics
The voltage to a DC motor may be altered by a variety ofmeans - a potentiometer or by power electronic means
A typical DC shunt connected motor, and the AC/DC conversionequipment used to power it from the mains is shown below.
The armature voltage may be adjusted via controlling thyristor,
T
MV D
T
Gate
E
RL I
field
P El t i
-
8/3/2019 19222 14 Dc Machine Control
13/18
1319222 DC Machine Control
Power Electronics -
Thyristors A thyristor is a controllable diode - when a thyristor starts to conduct may becontrolled by means of a positive pulse to the gate relative to the cathode
Once the thyristor is turned on it will turn off only when the current through it
has reduced to zero (and remained at zero long enough for it to return to theblocking state) - similar to idea of a latching relay
By varying the firing angle, for an AC supply, the output DC voltage may bevaried
cathode
AC waveform
lower equivalent DC
signal due to thyristoaction at 90 degrees
Vpeak
2
Vpeak
anode
Gate
equivalent DC signal(the rms value)
0
degrees
180
Output for conduction
Vpeak
Vo
lta
g
0at 90
P El t i
-
8/3/2019 19222 14 Dc Machine Control
14/18
1419222 DC Machine Control
Power Electronics -
Thyristors (contd) One problem with thyristors is that they can not be
turned off by a control pulse This has led to the development of a GTO - Gate TurnOff thyristor which can be both turned on and off by
separate applied pulses There are alternative power electronic devices available
to facilitate AC to DC conversion - IGBT (Insulated Gate
Bipolar Transistor) as well as power MOSFET (MetalOxide Semiconductor Field Effect Transistor) that arebecoming increasingly popular as replacements for low -
to medium power thyristors
Red ced Voltage DC to DC
-
8/3/2019 19222 14 Dc Machine Control
15/18
1519222 DC Machine Control
Reduced Voltage - DC to DC
power electronics A chopper circuit is one where
the applied DC voltage isvaried by a simple powerelectronic switch
As the time the switch is on is
varied the resultant appliedvoltage is varied
Generally, for motor
applications, there is sufficientinductance in the circuit toenable current to continue toflow even during the off
periods
'Chopped' Voltage
Corresponding curre
'Chopped' Voltage
Corresponding curre
V
Power ElectronicSwitching Device
M
-
8/3/2019 19222 14 Dc Machine Control
16/18
1619222 DC Machine Control
Motor Control
Motor speed control can also be implemented through
the various techniques discussed in this lectureMost commonly field weakening for a shunt connected
DC motor can be achieved using a power electronic
device (e.g. a chopper) to allow speed controlMotor direction can be changed (reversed) by changing
the polarity of either the field or armature
Typically the current in either the field or armature isreduced to zero, a changeover contactor is operated,and then the current is increased again to operate the
motor in the reverse direction
M t C t l B ki
-
8/3/2019 19222 14 Dc Machine Control
17/18
1719222 DC Machine Control
Motor Control - Braking
Braking is a very important motor control!!
The motor may be left to coast to a stop, braked using mechanical
means or either employ dynamic braking or regenerative braking Dynamic braking dumps the motors energy via a resistor placed across
the armature as heat
Regenerative braking feeds the motors kinetic energy back into the
supply - it is generating
1M1
__
Stop
1M4
Start
Shunt Field
O/L
+
Dynamic Braking
Resistor
1M O/L
Variable Field resistance
1M3
M
1M2
-
8/3/2019 19222 14 Dc Machine Control
18/18
1819222 DC Machine Control
Summary of Presentation
Studied variable resistance starting usingrelays
Provided overview of relays
Studied reduced voltage starting usingboth ac/dc and dc/dc power electronics
Studied braking of motors using relays