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ECE 8830 - Electric Drives

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**1. ** ECE 8830 - Electric Drives

**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:

**8. ** Diode Switching Characteristics Reverse Forward
Forward Reverse

**9. ** Thyristors Circuit Symbol:
Current-Voltage Characteristics:

**10. ** Thyristor Switching Characteristics

**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

**12. ** Controllable Switches (cont’d) Switching Waveforms

**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.

**16. **Switching Waveforms for GTOs

**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.

**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

**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

**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

**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