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

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<ul><li> Slide 1 </li> <li> Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives </li> <li> Slide 2 </li> <li> Introduction Nearly 65% of the total electric energy produced in the USA is consumed by electric motors. - R. Krishnan, Electric Motor Drives </li> <li> Slide 3 </li> <li> 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 </li> <li> Slide 4 </li> <li> 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. </li> <li> Slide 5 </li> <li> Power Devices Power Diode Power BJT SCR/Thyristor Gate Turn-Off Thyristor (GTO) Power MOSFET Insulated Gate Bipolar Transistor (IGBT) MOS Controlled Thyristor (MCT) </li> <li> Slide 6 </li> <li> 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. </li> <li> Slide 7 </li> <li> Power Diodes Circuit Symbol: Current-Voltage Characteristics: iDiDiDiD + vDvDvDvD - A K iDiDiDiD vDvDvDvD I vFvFvFvF v rated reverseblocking iDiDiDiD vDvDvDvD Real Ideal reverseblocking </li> <li> Slide 8 </li> <li> Diode Switching Characteristics Reverse Forward Forward Reverse 0 IFIFIFIF iDiDiDiD 0 IFIFIFIF iDiDiDiD t t -I F t rr Q rr </li> <li> Slide 9 </li> <li> 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 </li> <li> Slide 10 </li> <li> 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 </li> <li> Slide 11 </li> <li> 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 </li> <li> Slide 12 </li> <li> 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 </li> <li> Slide 13 </li> <li> 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 </li> <li> Slide 14 </li> <li> 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). </li> <li> Slide 15 </li> <li> 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 </li> <li> Slide 16 </li> <li> Switching Waveforms for GTOst t iG iG iG iG t iA iA iA iA vSvSvSvS large in magnitude ~ 1/3 i A </li> <li> Slide 17 </li> <li> 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. </li> <li> Slide 18 </li> <li> 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 </li> <li> Slide 19 </li> <li> 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 </li> <li> Slide 20 </li> <li> 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 </li> <li> Slide 21 </li> <li> Motor Drive Schematic Ref: R. Krishnan, Electric Drives: Modeling, Analysis and Control </li> <li> Slide 22 </li> <li> Subdisciplines of Electrical Engg. Semiconductor Devices Magnetic Materials Power Electronics Control Systems Electromagnetics Sensors Analog and Digital Electronics Signal Processing </li> <li> Slide 23 </li> <li> 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 </li> <li> Slide 24 </li> <li> 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 </li> <li> Slide 25 </li> <li> 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. </li> <li> Slide 26 </li> <li> Selection Criteria for Electric Machines Cost Thermal Capacity Efficiency Torque-speed profile Acceleration Power density, volume of motor Ripple, cogging torques Peak torque capability </li> <li> Slide 27 </li> <li> 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. </li> <li> Slide 28 </li> <li> 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. </li> <li> Slide 29 </li> <li> 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 </li> </ul>

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