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

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  • Slide 1
  • Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives
  • Slide 2
  • Introduction Nearly 65% of the total electric energy produced in the USA is consumed by electric motors. - R. Krishnan, Electric Motor Drives
  • Slide 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
  • Slide 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.
  • Slide 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)
  • Slide 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.
  • Slide 7
  • Power Diodes Circuit Symbol: Current-Voltage Characteristics: iDiDiDiD + vDvDvDvD - A K iDiDiDiD vDvDvDvD I vFvFvFvF v rated reverseblocking iDiDiDiD vDvDvDvD Real Ideal reverseblocking
  • Slide 8
  • Diode Switching Characteristics Reverse Forward Forward Reverse 0 IFIFIFIF iDiDiDiD 0 IFIFIFIF iDiDiDiD t t -I F t rr Q rr
  • Slide 9
  • Thyristors Circuit Symbol: Current-Voltage Characteristics: iAiAiAiA + v AK - A K iAiAiAiA reversebreakdownvoltage reverseblocking iAiAiAiA v AK v AK Real Real Ideal Ideal reverseblocking G ON forwardbreakdownvoltage OFF ON if gate voltage applied OFF forwardblocking ON-state ON-state OFF ON if gate voltage applied
  • Slide 10
  • Thyristor Switching Characteristics iAiAiAiA + - v AK R + - vsvsvsvs vsvsvsvs t t iG iG iG iG t t rr fires iA iA iA iA iG iG iG iG v AK t tqtqtqtq t rr = reverse recovery time t q = circuit-commutated recovery time recovery time (the time that the thyristor must have reverse voltage applied before entering the forward blocking state) Note: t rr t q
  • Slide 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 I0I0I0I0 + - vdvdvdvd iT iT iT iT + ++vTvT - -++vTvT - - - controlswitch The current source approximates the current that would actually flow due to inductive current storage. Controllable switch switch
  • Slide 12
  • Controllable Switches (contd) Switching Waveforms off on off t off on off t Switchcontrolsignal v T, i T t ISISISIS t D (on) t D (off) VSVSVSVS VSVSVSVS V on t ri t fv t C (on) t rv t fi t C (off) t C (off) t c = cross over ON and OFF times
  • Slide 13
  • Power Device Losses Conduction energy loss, E sc =I S V ON [t on +t D (off)-t C (on)-t D (on)] Sum of turn-on and turn-off energy loss, E st 0.5V S I S [t c (on)+t c (off)] Total power loss, where f s is switching frequency
  • Slide 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).
  • Slide 15
  • Gate Turn-Off Thyristors (GTOs) GTOs can be turned off by applying a negative gate current. Current-Voltage Characteristics: iAiAiAiA + v AK - K iAiAiAiA reversebreakdownvoltage reverseblocking iAiAiAiA v AK v AK Real Real Ideal Ideal reverseblocking G ON forwardbreakdownvoltage OFF ON if positive gate voltage applied OFF forwardblocking ON-state ON-state Circuit Symbol: A ON OFF if negative gate voltage applied OFF-state
  • Slide 16
  • Switching Waveforms for GTOst t iG iG iG iG t iA iA iA iA vSvSvSvS large in magnitude ~ 1/3 i A
  • Slide 17
  • GTOs (contd) GTOs are sensitive to dv/dt. Therefore, snubber circuits are used to minimize dv/dt and di/dt. GTOs are available to handle 1000s of V,A up to 10kHz.
  • Slide 18
  • Insulated Gate Bipolar Transistors (IGBTs) Circuit Symbol: Characteristics: High impedance gate (similar to MOSFETs) V on ~ 2V in a 1000V device ! Voltage ratings up to 2 kV, 100s of A, ~ 1sec. switching time. D S G + v GS - + - v DS iDiDiDiD
  • Slide 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 A A G G K K P-MCT N-MCT
  • Slide 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
  • Slide 21
  • Motor Drive Schematic Ref: R. Krishnan, Electric Drives: Modeling, Analysis and Control
  • Slide 22
  • Subdisciplines of Electrical Engg. Semiconductor Devices Magnetic Materials Power Electronics Control Systems Electromagnetics Sensors Analog and Digital Electronics Signal Processing
  • Slide 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
  • Slide 24
  • Electric Machines (contd) 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
  • Slide 25
  • Electric Machines (contd) 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.
  • Slide 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
  • Slide 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.
  • Slide 28
  • Controllers Controllers embody the control laws governing the load and motor characteristics and their interaction. Controller Torque/speed/ position commands Torque/speed/ position feedback Thermal and other feedback V c, f c, start, shut-out, signals, etc.
  • Slide 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: T l m x x=1 for frictional systems (e.g. feed drives) x=2 for fans and pumps