# DC Motor Drives by ewghwehws

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```									   DC MOTORS

SEE 3433
ELECTRICAL MACHINES
DC MOTOR

- Shunt motors
- Separately excited
- Starter
DC MOTORS
DC MOTOR

+ -   + -

DC motor
DC MOTOR
+                 +                  the motor torque
Va                Vf
-                 -

+ -   + -

DC motor

blowers, Compressors,
DC MOTOR
+                +
V                V
-                -

+ -   + -

DC motor

F = mg
DC MOTOR

Hoist
DC MOTOR

- Some applications require the control the speed

- Some applications require the control the torque

- In order to control the torque or speed we need to know the T-
characteristics of the motor and the mechanical load

Intersections between the two characteristics will
determine the operating point
DC MOTOR
Shunt motor
Vt = IaRa + Ea
If             It
It = Ia + If
Rcf             Ia         +
Ra          Ea = k
Vt
Te = kIa
Rcw
              
Te                        k = Vt - IaRa

Vt  IaR a
k
Mechanical
          2
Te
k (k)
Three possible methods of speed control:

Field flux

Armature voltage Vt

Armature resistance Ra

Vt   Ra
          Te
k (k) 2
Vt   Ra
          2
Te
k (k)
Varying Vt

Vt
kT                                   TL

Vt ↓

Te

Requires variable DC supply
Vt   Ra
          2
Te
k (k)
Varying Ra


Vt                                    TL
kT

Ra ↑

Te

Simple control
Losses in external resistor
Vt   Ra
          2
Te
                  k (k)
Varying 

Vt                                   TL
kT

↓

Te

Not possible for PM motor
Maximum torque capability reduces
Method of speed control in DC motor drives

Armature voltage control : retain maximum torque capability
Field flux control (i.e. flux reduced) : reduce maximum torque capability
For wide range of speed control
0 to base  armature voltage, above base  field flux reduction

Armature voltage control
Field flux control
Te

Maximum
Torque capability

base                           
Te

Maximum
Torque capability

base              
P Te      Constant torque         Constant power

Pmax

base                          

0 to base  armature voltage,     above base  field flux reduction

P = EaIa,max = kaIa,max          Pmax = EaIa,max = kabaseIa,max
   1/
0 to base  armature voltage,   above base  field flux reduction
0 to base  armature voltage,    above base  field flux reduction

If           It

Rcf        Ia             +
Ra
Vt

Rcw


BUT there are problems !
0 to base  armature voltage,    above base  field flux reduction

If           It

Rcf        Ia             +
Ra
Vt

Rcw


Controlling Vt will also affect If

Controlling If via Rcf caused losses  I2R
0 to base  armature voltage,   above base  field flux reduction

Separately Excited DC motor

DC supply                                        DC supply
for armature                                       for field

What if we have an AC supply ?
0 to base  armature voltage,       above base  field flux reduction

Separately Excited DC motor
3-phase AC

AC to DC       +                               +
source

AC to DC
converter      Vdc                             Vdc      converter

-                               -

Armature voltage control            Field voltage control
Starter in DC Motor

• At stand-still,  = 0  Ea = 0

Ra
 Ia
+                                         Vt
Ia 
Ra
Vt

–                            eg, Vt = 100, Ra = 0.1  Ia = 1000 A !

Starter in DC Motor

• We can limit Ia at start-up by:

1) Controlling Vt using variable supply – e.g. using
power electronics converter

2) Adding external resistor  known as starter

 Ia     Ra
+                                            Vt
+              Ia                 When Ea = 0
Ea                    R a  R st
Vt
–
–       Rst

• As speed builds up (so too Ea), Rst is gradually reduced
Starter in DC Motor

• As speed builds up (so too Ea), Rst is gradually reduced

Ia
Starter circuit
Imax 1   2     3    4

Imin
1    2     3    4

t (s)

speed

t (s)
Starter in DC Motor

Practical Starter circuit

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