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DC MOTORS . SEE 3433 ELECTRICAL MACHINES. DC MOTOR. - Shunt motors - Separately excited - Starter . DC MOTORS. +. -. +. -. DC MOTOR. DC motor. Load torque opposing the motor torque. + V a -. + V f -. +. -. +. -. T m. T load.

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dc motors

DC MOTORS

SEE 3433

ELECTRICAL MACHINES

dc motor

DC MOTOR

- Shunt motors

- Separately excited

- Starter

dc motor1

+

-

+

-

DC MOTOR

DC motor

dc motor2

Load torque opposing the motor torque

+

Va

-

+

Vf

-

+

-

+

-

Tm

Tload

Mechanical Load: fans, blowers, Compressors,

DC MOTOR

DC motor

dc motor3

+

V

-

+

V

-

+

-

+

-

Tm

Tload

F = mg

DC MOTOR

DC motor

dc motor5

- Some applications require the control the speed

DC MOTOR

- 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 motor6

If

It

+

Vt

Rcf

Ia

Ra

Rcw

Te

Tload

Mechanical

load

Shunt motor

Vt = IaRa + Ea

DC MOTOR

It = Ia + If

Ea = k

Te = kIa

k = Vt - IaRa

slide10

Three possible methods of speed control:

Field flux

Armature voltage Vt

Armature resistance Ra

slide11

Varying Vt

TL

Vt↓

Te

Requires variable DC supply

slide12

Varying Ra

Ra↑

TL

Te

Simple control

Losses in external resistor

slide13

Varying 

↓

TL

Te

Not possible for PM motor

Maximum torque capability reduces

slide14

Armature voltage control

Field flux control

Te

Maximum

Torque capability

base

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

slide15

Te

Maximum

Torque capability

base

slide16

Pmax

Constant torque

Constant power

base

P

Te

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/

slide18

If

It

+

Vt

Rcf

Ia

Ra

Rcw

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

BUT there are problems !

slide19

If

It

+

Vt

Rcf

Ia

Ra

Rcw

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

Controlling Vt will also affect If

Controlling If via Rcf caused losses  I2R

slide20

DC supply

for armature

DC supply

for field

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

Separately Excited DC motor

What if we have an AC supply ?

slide21

3-phase AC

source

Armature voltage control

Field voltage control

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

Separately Excited DC motor

AC to DC

converter

+

Vdc

-

+

Vdc

-

AC to DC

converter

slide22

Starter in DC Motor

  • At stand-still, w = 0  Ea = 0

Ra

 Ia

+

Vt

eg, Vt = 100, Ra = 0.1  Ia = 1000 A !

slide23

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

Ra

 Ia

+

Vt

+

Ea

When Ea = 0

Rst

  • As speed builds up (so too Ea), Rst is gradually reduced
slide24

Starter in DC Motor

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

Ia

Starter circuit

4

3

2

1

Imax

Imin

1

2

3

4

t (s)

speed

t (s)

slide25

Starter in DC Motor

Practical Starter circuit