ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

ECEN 5737: Adjustable Speed Drives

Lecture 8

•  Lecturers: Dr. Nikola Milivojevic Dr. Yusuf Gurkaynak •  Teaching Assistant: Joshua Traube

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

Lecture Outline:

•  Principle of Operation •  1 quadrant operation •  CCM vs. DCM •  2 quadrant operation •  4 quadrant operation •  Chopper types

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

Principle of Operation

•  What is chopper and why do we need it? •  How do we control speed/torque of DC machine?

Electric machine

+

- Va

Ia

T, w Torq

ue

Speed

Va Constant torque

Constant power

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

Principle of Operation

Q1   Q3  

Q2   Q4  

Vdc  

DC  voltage  

ate IkT ⋅=

Ia

Ra

La

4 quadrant chopper topology:

uController

driver

P1-4

T1-2 T3-4

Va

+

-

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

Principle of Operation

•  What is 4 quadrant operation?

ate IkT ⋅=

Ia

Ra

La Va +

-

Va

Ia

rpm

Te

I

IV III

II

Forward Motoring

Reverse Generating

Forward Generating (braking)

Reverse Motoring

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

Principle of Operation

•  Chopper in 1st quadrant – step-down -> buck converter:

Q1   Q3  

Q2   Q4  

Vdc  

DC  voltage  

ate IkT ⋅=

Ia

Ra

La Va

+

-

•  Chopper is DC-DC converter used to obtain variable DC voltage from a constant DC voltage source.

•  Widely used for trolley cars, battery operated vehicles, conventional traction motor control,

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

Principle of Operation

Q1  

Q4  Vdc  

ate IkT ⋅=

Ia Ra

La Va +

-

•  ton switch is ON •  toff switch is OFF

dca

onondca

VdVTtdif

TtVV

⋅=

=⋅= ,

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

1st Quadrant Operation P1

-3

T1-

3 Va

, Ia

time

3.3V

12V

Vdc

Q1   Q3  

Q2   Q4  

Vdc  

ate IkT ⋅=

Ia Ra

La Va +

-

Q1   Q3  

Q2   Q4  

Vdc  

ate IkT ⋅=

Ia Ra

La Va +

-

ton

toff

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

1st Quadrant Operation

Va, I

a

time

Vdc

on

tLRt

LR

dca tteIe

REVti <<⋅+−

−=

−−0,)1()(

)(

min

)(

offon

tLRt

LR

a ttteREeIti <<+−−⋅=

−−,)1()(

)()(

max

Imin

Imax

T 2T 3T

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

1st Quadrant Operation

•  Other option for switching pattern: Q1   Q3  

Q2   Q4  

Vdc  

ate IkT ⋅=

Ia Ra

La Va +

-

Q1   Q3  

Q2   Q4  

Vdc  

ate IkT ⋅=

Ia Ra

La Va +

-

ton

toff

D2  

D3  

on

tLRt

LR

dc tteIeREVti <<⋅+−

−=

−−0,)1()(

)(

min

)(

time

offon

tLR

dctLR

ttteREVeIti <<+−

+−⋅=

−−,)1()(

)()(

max

Ia

Vdc

-Vdc

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

1st Quadrant Operation

•  Drawbacks of the other option for switching pattern: –  Switching losses double – 2 power devices are turned off,

not only one –  Higher DC voltage deteriorates the machine –  Higher rate of change of current - vibrations –  Switching harmonic current is higher

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

Example:

Chopper is operating with switching frequency of 20kHz, powered by Vdc=460V. If load voltage is 350V, what would be conduction time of switching device for each switching period?

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

CCM vs. DCM

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

2nd Quadrant Operation

•  Negative current/positive voltage •  Negative torque/positive speed •  Forward generating

Va

Ia

rpm

Te

II

Forward Generating (Braking)

ton

Q1   Q3  

Q2   Q4  

Vdc  

ate IkT ⋅=

Ia Ra

La Va +

-

D1  

Q1   Q3  

Q2   Q4  

Vdc  

ate IkT ⋅=

Ia Ra

La Va +

-

toff

D1  

D4  

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

2nd Quadrant Operation

aadc RIVE ⋅+=

•  ton building current: aa RIE ⋅−=0•  toff charging DC source:

Ia

time

Va

dcadc VIdP ⋅⋅−=

Power is negative:

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

3rd Quadrant Operation

•  Negative current/negative voltage •  Negative torque/negative speed •  reverse motoring

Va

Ia

rpm

Te III Reverse

Motoring

Q1   Q3  

Q2   Q4  

Vdc  

ate IkT ⋅=

Ia Ra

La Va +

-

ton

toff

Q1   Q3  

Q2   Q4  

Vdc  

ate IkT ⋅=

Ia Ra

La Va +

-

D4  

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

3rd Quadrant Operation

aa RIEVa ⋅+=•  ton building current:

aa RIE ⋅−=0•  toff charging DC source:

Ia

time

Va

dcadc

aadc

VdIPVIP

⋅⋅=

−⋅−= )()(Power is positive:

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

4th Quadrant Operation

•  Positive current/negative voltage •  Positive torque/negative speed •  reverse generative

Q1   Q3  

Q2   Q4  

Vdc  

ate IkT ⋅=

Ia Ra

La Va +

-

ton

Va

Ia

rpm

Te IV Reverse

Generating

Q1   Q3  

Q2   Q4  

Vdc  

ate IkT ⋅=

Ia Ra

La Va +

-

toff

D4  

D2  

D3  

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

4th Quadrant Operation

aa RIE ⋅−=0•  ton building current:

aaa RIEV ⋅−=•  toff charging DC source:

Ia

time

Va

dcadc VIdP ⋅⋅−=

Power is negative:

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

Four Quadrant Operation

•  Robotics/hybrid-electric vehicles (control start and stop) •  Machine is operating at certain speed that need to be brought to zero:

a)  Cut-off power supply and rotor will eventually stop b)  Make machine work as generator, and decrease (control) the speed, save

energy – recuperative braking •  In this case drive operates in I and II quadrant •  Some machine tools require rotation in both directions - I and III quadrant

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

Chopper types:

•  Class A – First quadrant chopper •  Class B – Second quadrant chopper •  Class C – combination of first and second quadrant

Q1  

Q2  

Vdc  

DC  voltage  

ate IkT ⋅=

Ia Ra

La

•  Class D – combination of first and fourth quadrant •  Class E – fourth quadrant operation

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

Example:

•  A DC motor is driven by the chopper with a voltage source of 24V, and frequency of 1kHz. Determine variation in duty cycle required to have a speed variation of 0 to 1pu at constant load of 2pu?

•  1hp, Vrated=10V, 2500rpm, 78.5 efficiency, Ra=0.01Ohm, La=2mH, kb=0.03819V/rad/s, ∆V=1V

ECEN 5737 CU Boulder Electrical , Computer, and Energy Engineering Dept.

Example: