ELG4139: DC Drives Types of DC Motors: Shunt; Series; Compound;

Separately; excited; Switched reluctance motors.

Motor Selection: Cost; Thermal capacity; Efficiency;

Torque-speed profile; Acceleration; Power density,

volume of the motor; Ripple, cogging torque; Peak

torque capability.

Motor Drives

Speed is determined by Armature Voltage

Torque is determined by Armature Current

Output - AC

- DC

Input

Source - AC

- DC

- unregulated

Reference

sensors

DC Motor Example

• V = 12 volts

• Max current = 4 A

• Max power out = 25 W

• Max efficiency = 74%

• Max speed = 3500 rpm

• Max torque = 1.4 N-m

• Weight = 1.4 lbs

• Forward or reverse (brushed)

Torque Equation: Motor Drives

dt

dJTTor

dt

dJTT LeLe

0 Le TT Acceleration

0 Le TT Deceleration

0 Le TT Constant speed

Te : motor torque (Nm) TL : Load torque (Nm)

Equivalent Circuit of DC Motor

at ikTe Electromagnetic torque

Ea ke Armature back e.m.f.

Lf Rf

if

aa

aat edt

diLiRv

+

ea

_

La Ra

ia +

Vt

_

+

Vf

_

dt

di L i R v f

f f f

Torque-Speed Characteristics

aaaa EIRV In steady state,

T

ae

T

a

k

VT

k

R

2

Therefore speed is given by,

Three possible methods of speed control:

Armature resistance Ra

Field flux F

Armature voltage Va

aa

aaa edt

diLiRV

Armature circuit:

DC Motor Drives

Methods of speed control

Armature voltage control

Variable voltage source

Phase-controlled Rectifier

Switch-mode converter (Chopper)

1Q-Converter

2Q-Converter

4Q-Converter

• DC Drives: Electric drives that use DC motors as the prime movers.

• DC motor: industry workhorse for decades.

• Will AC drive replaces DC drive ?

– Predicted many years ago

– AC will eventually replace DC; at a slow rate.

– DC strong presence; easy control; huge numbers

DC Drives

• Advantage: simple torque and speed control without sophisticated electronics.

• Limitations: Maintenance; expensive and heavy;

the problem of sparking!

Power Electronic Converters in Electrical Drive Systems

DC Drives AC Drives

DC Source AC Source

AC-DC-DC AC-DC

AC Source

Const.

DC

Variable

DC

AC-DC-AC AC-AC

NCC FCC

DC Source

DC-AC DC-DC-AC

DC-DC DC-AC-DC

AC-DC Drive

Shunt

Field

M

Field Supply

3 Phase AC

Power

SCR

Gate

DC Armature

T1 T3 T5

T2 T4 T6

L1 L2 L3

+

-

Firing Circuit

Regulato

r

Operator

Control Station

Current

Feedback

Voltage

Feedback

AC-DC-DC Drives

control

AC

DC-DC

Q1 Q2

Q3 Q4

T

+

Vt

-

T1

Switch Mode Converter: 1Q Converter or Class A

• When chopper is ON, supply voltage V is connected across the load. • When chopper is OFF, vO = 0 and the load current continues to flow in the same

direction through the freewheeling diode. • The average values of output voltage and current are always positive.

• It is used to control the speed of dc motor.

Vs

Is Ia

Va

Va= D Is Po=Va Ia Is = D Ia Ri = Vs / Is

+

Vt

-

T1

D1

T2

D2

Q1 Q2

Q3 Q4

T

Q1 T1 and D2

Q2 D1 and T2

Switch Mode Converter: 2Q Converter

Output current

Thyristorgate pulse

Output voltage

ig

i0

v0

t

t

ttON

T

CH ON

FWD Conducts

T1 conducts va = Vdc

Q1 Q2

Va

Ia

T1

T2

D1

+

Va

-

D2

ia

+

Vdc

DC-DC: Two-quadrant Converter

Q1 Q2

Va

Ia

T1

T2

D1

+

Va

-

D2

ia

+

Vs

D2 conducts va = 0

Va Eb

T1 conducts va = Vdc

Quadrant 1: The average voltage is made larger than the back EMF

DC-DC: Two-quadrant Converter

Is Ia

Q1 Q2

Va

Ia

T1

T2

D1

+

Va

-

D2

ia

+

Vdc

D1 conducts va = Vdc

DC-DC: Two-quadrant Converter

Q1 Q2

Va

Ia

T1

T2

D1

+

Va

-

D2

ia

+

Vdc

T2 conducts va = 0

Va Eb

D1 conducts va = Vdc

Quadrant 2: The average voltage is made smaller than the back EMF,

forcing the current to flow in the reverse direction.

DC-DC: Two-quadrant Converter

Q1 Q2

Q3 Q4

T

+ Vt - T1

D1

T2 D2

D3

D4

T3

T4

Switch-Mode Converter: 4Q Converter