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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” 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: iD A + vD K Current-Voltage Characteristics: iD vrated iD I reverse blocking Real vF vD reverse blocking Ideal vD Diode Switching Characteristics Reverse Forward iD IF 0 t Forward Reverse iD IF trr 0 -IF t Qrr Thyristors iA Circuit Symbol: G A + vAK K Current-Voltage Characteristics: reverse breakdown voltage iA ON OFF ON if gate voltage applied iA ON-state OFF ON if gate voltage applied OFF reverse blocking Real vAK forward breakdown voltage reverse forward blocking blocking Ideal vAK Thyristor Switching Characteristics vs R + - vs iG iA + vAK - t iG trr = reverse recovery time tq = circuit-commutated iA recovery time (the time that the thyristor must have reverse voltage applied before entering the forward “fires” vAK blocking state) Note: trr tq t trr t tq t 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 I0 vd The current source approximates the current that would actually flow due to inductive current storage. + - iT + vT - control switch Controllable switch Controllable Switches (cont’d) Switching Waveforms Switch control signal off vT, iT on VS tri VS IS tD(on) Von t off tD (off) tfv trv tfi tC(on) tC(off) tc = cross over ON and OFF times t 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, Est Esc Psw f s ( Est Esc ) ton toff where fs 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. Circuit Symbol: iA G Current-Voltage Characteristics: iA reverse ON breakdown OFF ON if positive voltage gate voltage applied reverse blocking Real ON OFF if negative gate voltage applied iA ON-state OFF-state OFF vAK forward breakdown voltage A + vAK K reverse forward blocking blocking Ideal vAK Switching Waveforms for GTOs vS t iG t large in magnitude ~ 1/3 iA iA t 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. Insulated Gate Bipolar Transistors (IGBTs) Circuit Symbol: G Characteristics: + i D D + vDS S vGS - High impedance gate (similar to MOSFETs) Von ~ 2V in a 1000V device ! Voltage ratings up to 2 kV, 100’s of A, ~ 1sec. switching time. MOS Controlled Thyristors (MCTs) Circuit Symbols: P-MCT N-MCT A A G Characteristics: K G 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 K 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 highperformance drive system must have intimate knowledge about machine performance.” - Bimal K. Bose, “Modern Power Electronics and AC Drives” 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 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. 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 singlephase 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. Torque/speed/ position commands Torque/speed/ position feedback Thermal and other feedback Controller Vc, fc, 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: Tl mx x=1 for frictional systems (e.g. feed drives) x=2 for fans and pumps