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EE462L, Fall 2011 Motor Drives and Other Applications 1 Three-Phase Induction Motors • Reliable • Rugged • Long lived • Low maintenance • Efficient (Source: EPRI Adjustable Speed Drives Application Guide) 2 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 3 High slip corresponds to low efficiency 4 It’s much more efficient to reduce operating speed by lowering the frequency of the supply voltage. But how? 5 Adjustable-Speed Motor Drives (ASDs) (Source: EPRI Adjustable Speed Drives Application Guide) 6 Some Prices for Small 3-Phase, 460V Induction Motors and ASDs Power Motor ASD 10kW $750 $2,000 100kW $5,000 $15,000 $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!) 7 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 Source: Ned Mohan’s power electronics book Chapter 14 Induction Motor Drives 14-8 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 Source: Ned Mohan’s power electronics book Chapter 14 Induction Motor Drives 14-9 Torque-Speed Characteristics • The linear part of the characteristic is utilized in adjustable speed drives Source: Ned Mohan’s power electronics book Chapter 14 Induction Motor Drives 14-10 Acceleration Torque at Startup • Intersection represents the equilibrium point Source: Ned Mohan’s power electronics book Chapter 14 Induction Motor Drives 14-11 Torque Speed Characteristics at various Frequencies of Applied Voltage For a constant torque load • The air gap flux is kept constant Source: Ned Mohan’s power electronics book Chapter 14 Induction Motor Drives 14-12 Adjusting Speed of a Centrifugal Load • The load torque is proportional to speed squared Source: Ned Mohan’s power electronics book Chapter 14 Induction Motor Drives 14-13 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 Source: Ned Mohan’s power electronics book Chapter 14 Induction Motor Drives 14-14 PWM-VSI System A three-phase inverter A three-phase DBR • Diode rectifier for unidirectional power flow Source: Ned Mohan’s power electronics book Chapter 14 Induction Motor Drives 14-15 Three-Phase Inverter (called a six-pack) • Three inverter legs; capacitor mid-point is fictitious Source: Ned Mohan’s power electronics book Chapter 8 Switch-Mode DCSinusoidal AC Inverters 8-16 ThreePhase PWM Waveforms Source: Ned Mohan’s power electronics book Chapter 8 Switch-Mode DCSinusoidal AC Inverters 8-17 Three-Phase Inverter Harmonics Source: Ned Mohan’s power electronics book Chapter 8 Switch-Mode DCSinusoidal AC Inverters 8-18 Three-Phase Inverter Output • Linear and over-modulation ranges Source: Ned Mohan’s power electronics book Chapter 8 Switch-Mode DCSinusoidal AC Inverters 8-19 Improving Energy Efficiency of Heat Pumps How does inserting an ASD save energy in single-phase applications? Some losses But a three-phase motor is 95% efficient, compared to 80% efficiency for a single-phase motor • Used in one out of three new homes in the U.S. Source: Ned Mohan’s power electronics book Chapter 16 Residential and Industrial Applications 16-20 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 Source: Ned Mohan’s power electronics book Chapter 16 Residential and Industrial Applications 16-21 Electronic Ballast for Fluorescent Lamps • Lamps operated at ~40 kHz save energy Source: Ned Mohan’s power electronics book Chapter 16 Residential and Industrial Applications 16-22 Induction Cooking • Pan is heated directly by circulating currents – increases efficiency Source: Ned Mohan’s power electronics book Chapter 16 Residential and Industrial Applications 16-23 Industrial Induction Heating Source: Ned Mohan’s power electronics book Chapter 16 Residential and Industrial Applications 16-24 HVDC Transmission • There are many such systems all over the world Source: Ned Mohan’s power electronics book Chapter 17 Electric Utility Applications 17-25 HVDC Poles • Each pole consists of 12-pulse converters Source: Ned Mohan’s power electronics book Chapter 17 Electric Utility Applications 17-26 HVDC Transmission: 12-Pulse Waveforms Source: Ned Mohan’s power electronics book Chapter 17 Electric Utility Applications 17-27 Reducing the Input Current Distortion Like DBR current (high distortion) Source: Ned Mohan’s power electronics book Chapter 18 Utility Interface 18-28 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 Source: Ned Mohan’s power electronics book Chapter 18 Utility Interface 18-29 Power-Factor-Correction (PFC) Circuit The boost converter is instructed to open close “close” when the current is below the sinewave envelope, and “open” with the current is above the sinewave envelope • Operation during each half-cycle Source: Ned Mohan’s power electronics book Chapter 18 Utility Interface 18-30 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 18-31