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ECE 8830 - Electric Drives

ECE 8830 - Electric Drives. Topic 1 : Introduction to Electric Drives Spring 2004. Introduction. “Nearly 65% of the total electric energy produced in the USA is consumed by electric motors.” - R. Krishnan, “Electric Motor Drives”.

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ECE 8830 - Electric Drives

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  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 A Circuit Symbol: Current-Voltage Characteristics: + iD vD - K iD iD I vrated vD vD vF reverse blocking reverse blocking Ideal Real

  8. Diode Switching Characteristics Reverse Forward Forward Reverse iD IF 0 t iD IF trr 0 t Qrr  -IF

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

  10. Thyristor Switching Characteristics vs R 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) Circuit Symbol: GTOs can be turned off by applying a negative gate current. A Current-Voltage Characteristics: 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 forward blocking reverse blocking forward breakdown voltage Ideal Real

  16. Switching Waveforms for GTOs vS 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) P-MCT N-MCT 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 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

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