DC MOTORS

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# DC MOTORS - PowerPoint PPT Presentation

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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Presentation Transcript

### DC MOTORS

SEE 3433

ELECTRICAL MACHINES

### DC MOTOR

- Shunt motors

- Separately excited

- Starter

+

-

+

-

### DC MOTOR

DC motor

Load torque opposing the motor torque

+

Va

-

+

Vf

-

+

-

+

-

Tm

DC motor

+

V

-

+

V

-

+

-

+

-

Tm

F = mg

DC motor

Hoist

### DC MOTOR

- 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

If

It

+

Vt

Rcf

Ia

Ra

Rcw

Te

Mechanical

Shunt motor

Vt = IaRa + Ea

### DC MOTOR

It = Ia + If

Ea = k

Te = kIa

k = Vt - IaRa

Three possible methods of speed control:

Field flux

Armature voltage Vt

Armature resistance Ra

Varying Vt

TL

Vt↓

Te

Requires variable DC supply

Varying Ra

Ra↑

TL

Te

Simple control

Losses in external resistor

Varying 

↓

TL

Te

Not possible for PM motor

Maximum torque capability reduces

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

Te

Maximum

Torque capability

base

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/

If

It

+

Vt

Rcf

Ia

Ra

Rcw

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

BUT there are problems !

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

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 ?

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

Starter in DC Motor

• At stand-still, w = 0  Ea = 0

Ra

 Ia

+

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

Ra

 Ia

+

Vt

+

Ea

When Ea = 0

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

4

3

2

1

Imax

Imin

1

2

3

4

t (s)

speed

t (s)

Starter in DC Motor

Practical Starter circuit