ECE 442 Power Electronics 1

Controlled-Rectifier Fed Drive

Armature Field

ECE 442 Power Electronics 2

Chopper-Fed Drive

Armature Field

ECE 442 Power Electronics 3

Equivalent Circuit of a Separately-Excited DC Motor

ECE 442 Power Electronics 4

ff f f f

aa a a a g

g v f

d t f a

d L

div R i L

dtdi

v R i L edt

e K i

T K i i

dT J B T

dt

ECE 442 Power Electronics 5

0

f f f

g v f

a a a g

a a a v f

d t f a

d L

d d

Steady state

d

dtV R I

E K I

V R I E

V R I K I

T K I I

T B T

P T

ECE 442 Power Electronics 6

a a a v f

a a a a a a

v f fv

f

V R I K I

V I R V I R

K I VK

R

Solve for the motor speed

ECE 442 Power Electronics 7

Control of the motor speed

• Control armature voltage, Va

– Voltage control

• Control the field current, If

– Current control

• Control the armature current, Ia

a a a

fv

f

V I R

VK

R

ECE 442 Power Electronics 8

Magnetization Characteristic

ECE 442 Power Electronics 9

Characteristics of Separately-Excited Motors

Rated speed

Use field-current control

Use armature voltage control

ECE 442 Power Electronics 10

Equivalent circuit of a DC Series Motor

ECE 442 Power Electronics 11

( )

( )

( )

g v a

a a f a g

a a f a v f

d t a f

d L

a a f a

v f

E K I

V R R I E

V R R I K I

T K I I

T B T

V R R I

K I

ECE 442 Power Electronics 12

Controlling the Motor Speed

• Control the armature voltage, Va

• Control the armature current, Ia

ECE 442 Power Electronics 13

Characteristics of DC Series motors

ECE 442 Power Electronics 14

Operating Modes

• In variable-speed applications, a dc motor may be operating in one or more of the following “Modes”

• Motoring

• Regenerative braking

• Dynamic braking

• Plugging

ECE 442 Power Electronics 15

Motoring Mode

“Back emf”, Eg is < supply voltage Va

Both Ia and If are positive

Developed torque meets load demand

ECE 442 Power Electronics 16

Regenerative Braking Mode

Motor acts as a Generator

Eg becomes > supply Voltage Va

Ia becomes negative

Kinetic energy of the motor is returned to the supply

ECE 442 Power Electronics 17

Dynamic Braking

Replace the supply voltage with a resistor

Power dissipated in the resistor rather than given back to the source

ECE 442 Power Electronics 18

Plugging

Reverse the armature terminals while running

Va and Eg act in the same direction

Ia is reversed, producing “braking” torque

ECE 442 Power Electronics 19

Four-Quadrant Operation

ECE 442 Power Electronics 20

Single-Phase DC Drive

Change Va by changing firing angle Inductor Lm is a “smoothing” Inductor to prevent

discontinuous current

ECE 442 Power Electronics 21

Armature Reversal

ECE 442 Power Electronics 22

Field Reversal

ECE 442 Power Electronics 23

Single-Phase Half-Wave Converter Drive

(1 cos )2

0

(1 cos )

0

ma a

a

mf f

f

VV

VV

ECE 442 Power Electronics 24

Quadrant of Operation

ECE 442 Power Electronics 25

Waveform Summary

ECE 442 Power Electronics 26

Single-Phase Semiconverter Drives

(1 cos )

0

(1 cos )

0

ma a

a

mf f

f

VV

VV

ECE 442 Power Electronics 27

Quadrant(s) of Operation

Semiconverter – one quadrant converter

One polarity of output voltage and current

ECE 442 Power Electronics 28

Waveform Summary

ECE 442 Power Electronics 29

Single-Phase Full-Converter Drives

ECE 442 Power Electronics 30

Quadrant(s) of Operation

Full converter – two quadrant converter

Output voltage polarity can be positive or negative

Output current has one polarity

ECE 442 Power Electronics 31

Waveform Summary

ECE 442 Power Electronics 32

Single-Phase Dual-Converter Drives

Dual converter can operate in all four quadrants

Output voltage and current can be positive or negative