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EE462L, Fall 2011 Motor Drives and Other Applications

EE462L, Fall 2011 Motor Drives and Other Applications. Three-Phase Induction Motors. Reliable Rugged Long lived Low maintenance Efficient. (Source: EPRI Adjustable Speed Drives Application Guide).

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EE462L, Fall 2011 Motor Drives and Other Applications

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  1. EE462L, Fall 2011Motor Drives and Other Applications

  2. Three-Phase Induction Motors • Reliable • Rugged • Long lived • Low maintenance • Efficient (Source: EPRI Adjustable Speed Drives Application Guide)

  3. Slip frequency (about 5% of no load speed), so induction motors are almost constant speed devices At no load, the motor spins at grid frequency, divided by the number of pole pairs. Usually this is 3600 / 2 = 1800RPM

  4. High slip corresponds to low efficiency

  5. It’s much more efficient to reduce operating speed by lowering the frequency of the supply voltage. But how?

  6. Adjustable-Speed Motor Drives (ASDs) (Source: EPRI Adjustable Speed Drives Application Guide)

  7. Some Prices for Small 3-Phase, 460V Induction Motors and ASDs $50 - $75 per kW $150 - $200 per kW For Comparison, Conventional Generation: $500 - $1,000 per kW Solar: $4,000 - $6,000 per kW (but the fuel is free forever!)

  8. Pump Application: Adjustable Flow rate Bad news – inefficient! Equivalent to reducing the output voltage of a DBR with a series resistor Payback in energy savings is about 1 year • Fixed versus adjustable speed drive Chapter 14 Induction Motor Drives Source: Ned Mohan’s power electronics book

  9. Per-Phase Representation (assuming sinusoidal steady state) Because of the shunt inductance term, we must reduce the applied voltage magnitude in proportion to applied frequency to avoid serious saturation of the iron near the air gap This is what is called “Constant Volts per Hertz Operation,” which is the standard operating mode for ASDs Chapter 14 Induction Motor Drives Source: Ned Mohan’s power electronics book

  10. Torque-Speed Characteristics • The linear part of the characteristic is utilized in adjustable speed drives Chapter 14 Induction Motor Drives Source: Ned Mohan’s power electronics book

  11. Acceleration Torque at Startup • Intersection represents the equilibrium point Chapter 14 Induction Motor Drives Source: Ned Mohan’s power electronics book

  12. Torque Speed Characteristics at various Frequencies of Applied Voltage For a constant torque load • The air gap flux is kept constant Chapter 14 Induction Motor Drives Source: Ned Mohan’s power electronics book

  13. Adjusting Speed of a Centrifugal Load • The load torque is proportional to speed squared Chapter 14 Induction Motor Drives Source: Ned Mohan’s power electronics book

  14. Frequency at Startup An important property of ASDs is the ability to “soft start” a motor by reducing the applied frequency to a few Hz Zero speed • The torque is limited to limit current draw Chapter 14 Induction Motor Drives Source: Ned Mohan’s power electronics book

  15. PWM-VSI System A three-phase inverter A three-phase DBR • Diode rectifier for unidirectional power flow Chapter 14 Induction Motor Drives Source: Ned Mohan’s power electronics book

  16. Three-Phase Inverter (called a six-pack) • Three inverter legs; capacitor mid-point is fictitious Chapter 8 Switch-Mode DC- Sinusoidal AC Inverters Source: Ned Mohan’s power electronics book

  17. Three-Phase PWM Waveforms Chapter 8 Switch-Mode DC- Sinusoidal AC Inverters Source: Ned Mohan’s power electronics book

  18. Three-Phase Inverter Harmonics Chapter 8 Switch-Mode DC- Sinusoidal AC Inverters Source: Ned Mohan’s power electronics book

  19. Three-Phase Inverter Output • Linear and over-modulation ranges Chapter 8 Switch-Mode DC- Sinusoidal AC Inverters Source: Ned Mohan’s power electronics book

  20. Improving Energy Efficiency of Heat Pumps How does inserting an ASD save energy in single-phase applications? But a three-phase motor is 95% efficient, compared to 80% efficiency for a single-phase motor Some losses • Used in one out of three new homes in the U.S. Chapter 16 Residential and Industrial Applications Source: Ned Mohan’s power electronics book

  21. Loss Associated with ON/OFF Cycling The big efficiency gain is here • with conventional air conditioners, the first few minutes after start-up are very inefficient as the mechanical system reaches steady-state • with ASDs, the air conditioner speed is lowered with demand, so that there are fewer start-ups each day • The system efficiency is improved by ~30 percent Chapter 16 Residential and Industrial Applications Source: Ned Mohan’s power electronics book

  22. Electronic Ballast for Fluorescent Lamps • Lamps operated at ~40 kHz save energy Chapter 16 Residential and Industrial Applications Source: Ned Mohan’s power electronics book

  23. Induction Cooking • Pan is heated directly by circulating currents – increases efficiency Chapter 16 Residential and Industrial Applications Source: Ned Mohan’s power electronics book

  24. Industrial Induction Heating Chapter 16 Residential and Industrial Applications Source: Ned Mohan’s power electronics book

  25. HVDC Transmission • There are many such systems all over the world Chapter 17 Electric Utility Applications Source: Ned Mohan’s power electronics book

  26. HVDC Poles • Each pole consists of 12-pulse converters Chapter 17 Electric Utility Applications Source: Ned Mohan’s power electronics book

  27. HVDC Transmission: 12-Pulse Waveforms Chapter 17 Electric Utility Applications Source: Ned Mohan’s power electronics book

  28. Reducing the Input Current Distortion Like DBR current (high distortion) Chapter 18 Utility Interface Source: Ned Mohan’s power electronics book

  29. Power-Factor-Correction (PFC) Circuit The boost converter is operated to make the DBR current look sinusoidal on the AC side To be sold in Europe, this is a necessary feature in high-current single-phase power electronic loads It also permits more power to be drawn from conventional wall outlets because the harmonic currents are minimal Chapter 18 Utility Interface Source: Ned Mohan’s power electronics book

  30. Power-Factor-Correction (PFC) Circuit The boost converter is instructed to “close” when the current is below the sinewave envelope, and “open” with the current is above the sinewave envelope open close • Operation during each half-cycle Chapter 18 Utility Interface Source: Ned Mohan’s power electronics book

  31. Power Electronics Has Made Wind Farms Possible • The choices used to be • Use an efficient induction generator, which has very poor power factor, or • Use a synchronous generator, but constantly fight to synchronize the turbine speed with the grid. • Now, • Either use a DC bus and inverter to decouple the generator and grid AC busses, or • Use a doubly-fed induction motor, operate the wind turbine at the max power speed, and use power electronics to “trick” the wind generator into producing grid-frequency output. This is what you see in West Texas. Chapter 18 Utility Interface

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