1. ECE 8830 - Electric Drives Topic 1: Introduction to Electric Drives
Spring 2004

2. Introduction
“Nearly 65% of the total electric energy produced in the USA is consumed by electric motors.”
- R. Krishnan, “Electric Motor Drives”

3. Some Applications of Electric Drives
Electric Propulsion
Pumps, fans, compressors
Plant automation
Flexible manufacturing systems
Spindles and servos
Appliances and power tools
Cement kilns
Paper and pulp mills; textile mills
Automotive applications
Conveyors, elevators, escalators, lifts

4. Energy/Cost Savings
System efficiency can be increased from 15% to 27% by introducing variable-speed drive operation in place of constant-speed operation.
US energy bill would be reduced by an estimated $90 billion!
For a large pump variable-speed drive, payback period ~ 3-5 years whereas operating life is ~ 20 years.

5. Power Devices Power Diode Power BJT SCR/Thyristor
Gate Turn-Off Thyristor (GTO)
Power MOSFET
Insulated Gate Bipolar Transistor (IGBT)
MOS Controlled Thyristor (MCT)

6. Categories of Switches
There are three categories of switches:
Diodes (rectifiers) - on/off determined by the power circuit.
Thyristors (SCRs, Triacs) - latched on by a control signal but turned off by the power circuit.
Controllable Switches (BJTs, MOSFETs, GTOs, IGBTs, MCTs) - turned on and off by control signals.

7. Power Diodes Circuit Symbol: Current-Voltage Characteristics: A + iDvD - K iD iD I
vrated vD vD vF
reverse
blocking
reverse
blocking
Ideal
Real

8. Diode Switching Characteristics
Reverse Forward
Forward Reverse iD IF t iD IF
trr t
Qrr
 -IF

9. Thyristors Circuit Symbol: Current-Voltage Characteristics: A iA + vAKON
reverse
breakdown
voltage
OFF ON if gate
voltage applied
ON-state
OFF ON if gate
voltage applied
OFF
vAK
vAK
reverse
blocking
reverse
blocking
forward
blocking
forward
breakdown
voltage
Ideal
Real

10. Thyristor Switching Characteristicsvs iA + + t vs
vAK - - iG iG t
trr = reverse recovery time
tq = circuit-commutated
recovery time
(the time that the thyristor must
have reverse voltage applied
before entering the forward
blocking state)
Note: trr  tq iA
trr t
“fires”
vAK tq t

11. Controllable Switches
These devices do not depend on power
reversal to go off - they may be triggered off.
In many applications, the switch current
flows through a series inductance.
Idealized Circuit
The current source
approximates the
current that would
actually flow due to
inductive current storage. I0 vd + - + vT - iT
control
switch
Controllable
switch

12. Controllable Switches (cont’d)
Switching Waveforms
Switch
control
signal
off on off t
vT, iT VS VS IS
tD(on)
tD (off)
Von t
tri
tfv
trv
tfi
tC(on)
tC(off)
tc = cross over ON and OFF times

13. Power Device Losses
Conduction energy loss, Esc=ISVON[ton+tD(off)-tC(on)-tD(on)]
Sum of turn-on and turn-off energy loss, Est 0.5VSIS[tc(on)+tc(off)]
Total power loss,
where fs is switching frequency

14. Transistor Switches BJTs, Monolithic Darlingtons (MDs) and MOSFETs
MOSFETs are easier to parallel than BJTs because of their positive temperature coefficient of on-state resistance (although paralleling MOSFETs is an art more than a science).

15. Gate Turn-Off Thyristors (GTOs)
GTOs can be turned off
by applying a negative
gate current.
Circuit Symbol:
Current-Voltage Characteristics: A iA +
vAK G - K
ON OFF if negative
gate voltage applied iA iA ON
reverse
breakdown
voltage
ON-state
OFF ON if positive
gate voltage applied
OFF-state
OFF
vAK
vAK
reverse
blocking
reverse
blocking
forward
blocking
forward
breakdown
voltage
Ideal
Real

16. Switching Waveforms for GTOsvS t iG t
large in magnitude ~ 1/3 iA iA t

17. GTOs (cont’d)
GTOs are sensitive to dv/dt. Therefore, snubber circuits are used to minimize dv/dt and di/dt.
GTOs are available to handle 1000’s of V,A up to 10kHz.

18. Insulated Gate Bipolar Transistors (IGBTs)
Circuit Symbol:
Characteristics:
High impedance gate (similar to MOSFETs)
Von ~ 2V in a 1000V device !
Voltage ratings up to 2 kV, 100’s of A, ~ 1sec. switching time. iD D +
vDS G - + S
vGS -

19. MOS Controlled Thyristors (MCTs)
Circuit Symbols:
Characteristics:
Current-voltage characteristics similar to GTOs
Two main advantages over GTOs:
1) Smaller turn-off current
2) Faster switching speeds (~ sec)
Voltage ratings up to 1500V;
current ratings ~ few hundred Amps
P-MCT
N-MCT A A G G K K

20. Motor Drive Components
A modern variable-speed drive has four components:
(i) Electric machines - ac or dc
(ii) Power converter - rectifiers,choppers,
inverters, and cycloconverters
(iii) Controllers -matching the motor and
power converter to meet the load
requirements
(iv) Load

21. Motor Drive Schematic
Ref: R. Krishnan, “Electric Drives: Modeling, Analysis and Control”

22. Subdisciplines of Electrical Engg.
Semiconductor Devices
Magnetic Materials
Power Electronics
Control Systems
Electromagnetics
Sensors
Analog and Digital Electronics
Signal Processing

23. Electric Machines
“An engineer designing a high-performance drive system must have intimate knowledge about machine performance.”
- Bimal K. Bose, “Modern Power Electronics and AC Drives”

24. Electric Machines (cont’d)
DC Machines - shunt, series, compound, separately excited dc motors and switched reluctance machines
AC Machines - Induction, wound rotor synchronous, permanent magnet synchronous, synchronous reluctance, and switched reluctance machines.
Special Machines - switched reluctance machines

25. Electric Machines (cont’d)
All of the above machines are commercially available in fractional kW to MW ranges except permanent-magnet, synchronous, synchronous reluctance, and switched reluctance which are available up to 150 kW level.

26. Selection Criteria for Electric Machines
Cost
Thermal Capacity
Efficiency
Torque-speed profile
Acceleration
Power density, volume of motor
Ripple, cogging torques
Peak torque capability

27. Power Converters
Controlled Rectifiers; fed from single-phase or three-phase ac mains supply and provide dc output for motor drive.
Inverters; convert dc output of battery or rectified ac source to provide variable ac voltages and currents at desired frequency and phase.
Cycloconverters; Directly convert fixed frequency ac voltage/current to variable voltage/current of variable frequency for driving ac machines.

28. Controllers
Controllers embody the control laws governing the load and motor characteristics and their interaction.
Controller
Torque/speed/
position commands
Vc, fc, start,
shut-out,
signals, etc.
Torque/speed/
position feedback
Thermal and
other feedback

29. Load
The motor drives a load that has a characteristic torque vs. speed requirement.
In general, load torque is a function of speed and can be written as:
Tl  mx
x=1 for frictional systems (e.g. feed drives)
x=2 for fans and pumps