Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives
Introduction Nearly 65% of the total electric energy produced in the USA is consumed by electric motors. - R. Krishnan, Electric Motor Drives
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
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.
Power Devices Power Diode Power BJT SCR/Thyristor Gate Turn-Off Thyristor (GTO) Power MOSFET Insulated Gate Bipolar Transistor (IGBT) MOS Controlled Thyristor (MCT)
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.
Power Diodes Circuit Symbol: Current-Voltage Characteristics: iDiDiDiD + vDvDvDvD - A K iDiDiDiD vDvDvDvD I vFvFvFvF v rated reverseblocking iDiDiDiD vDvDvDvD Real Ideal reverseblocking
Diode Switching Characteristics Reverse Forward Forward Reverse 0 IFIFIFIF iDiDiDiD 0 IFIFIFIF iDiDiDiD t t -I F t rr Q rr
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
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
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
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
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
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).
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
Switching Waveforms for GTOst t iG iG iG iG t iA iA iA iA vSvSvSvS large in magnitude ~ 1/3 i A
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.
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
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
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
Motor Drive Schematic Ref: R. Krishnan, Electric Drives: Modeling, Analysis and Control
Subdisciplines of Electrical Engg. Semiconductor Devices Magnetic Materials Power Electronics Control Systems Electromagnetics Sensors Analog and Digital Electronics Signal Processing
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
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
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.
Selection Criteria for Electric Machines Cost Thermal Capacity Efficiency Torque-speed profile Acceleration Power density, volume of motor Ripple, cogging torques Peak torque capability
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.
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.
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