1 EE462L, Fall 2011 Motor Drives and Other Applications

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

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

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!)

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

14-9 Chapter 14 Induction Motor Drives 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

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

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

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

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

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

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

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

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

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

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

16-20 Chapter 16 Residential and Industrial Applications Improving Energy Efficiency of Heat Pumps Used in one out of three new homes in the U.S. 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 Source: Ned Mohan’s power electronics book

16-21 Chapter 16 Residential and Industrial Applications Loss Associated with ON/OFF Cycling The system efficiency is improved by ~30 percent 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 Source: Ned Mohan’s power electronics book

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

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

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

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

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

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

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

18-29 Chapter 18 Utility Interface 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

18-30 Chapter 18 Utility Interface Power-Factor-Correction (PFC) Circuit Operation during each half-cycle 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 close open Source: Ned Mohan’s power electronics book

18-31 Chapter 18 Utility Interface 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.