Motor Drives and Other Applications

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    EE462L, Fall 2011

    Motor Drives and Other Applications

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    At no load, the motor spins at grid frequency, divided by thenumber of pole pairs. Usually this is 3600 / 2 = 1800RPM

    Slip frequency (about 5% of no load speed), so inductionmotors are almost constant speed devices

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    High slip corresponds tolow efficiency

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    Its much more efficient to reduce

    operating speed by lowering thefrequency of the supply voltage.

    But how?

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    Adjustable-Speed Motor Drives (ASDs)

    (Source: EPRI Adjustable Speed Drives Application Guide)

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    Some Prices for Small 3-Phase, 460V Induction Motorsand ASDs

    $50 - $75 per kW $150 - $200 per kW

    Power Motor ASD

    10kW $750 $2,000

    100kW $5,000 $15,000

    For Comparison, Conventional Generation: $500 - $1,000 per kW

    Solar: $4,000 - $6,000 per kW (but the fuel is free forever!)

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    14-8Chapter 14 InductionMotor Drives

    Pump Application: Adjustable Flow rate

    Fixed versus adjustable speed drive

    Bad newsinefficient!

    Equivalent to reducing theoutput voltage of a DBR with aseries resistor

    Payback in energysavings is about 1 year

    Source: Ned Mohans powerelectronics book

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    14-9Chapter 14 InductionMotor Drives

    Per-Phase Representation(assuming sinusoidal steady state)

    Because of the shunt inductance term, we must reduce theapplied voltage magnitude in proportion to applied frequencyto 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 Mohans powerelectronics book

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    14-10Chapter 14 InductionMotor Drives

    Torque-Speed Characteristics

    The linear part of the characteristic is utilized in

    adjustable speed drives

    Source: Ned Mohans powerelectronics book

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    14-11Chapter 14 InductionMotor Drives

    Acceleration Torque at Startup

    Intersection represents the equilibrium point

    Source: Ned Mohans powerelectronics book

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    14-12Chapter 14 InductionMotor Drives

    Torque Speed Characteristics at various

    Frequencies of Applied Voltage

    The air gap flux is kept constant

    For a constanttorque load

    Source: Ned Mohans powerelectronics book

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    14-13Chapter 14 InductionMotor Drives

    Adjusting Speed of a Centrifugal Load

    The load torque is proportional to speed squared

    Source: Ned Mohans powerelectronics book

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    14-15Chapter 14 InductionMotor Drives

    PWM-VSI System

    Diode rectifier for unidirectional power flow

    A three-phase DBR

    A three-phaseinverter

    Source: Ned Mohans powerelectronics book

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    8-16Chapter 8 Switch-Mode DC-Sinusoidal AC Inverters

    Three-Phase Inverter

    Three inverter legs; capacitor mid-point is fictitious

    (called a six-pack)

    Source: Ned Mohans powerelectronics book

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    8-17Chapter 8 Switch-Mode DC-Sinusoidal AC Inverters

    Three-

    Phase

    PWMWaveforms

    Source: Ned Mohans powerelectronics book

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    8-18Chapter 8 Switch-Mode DC-Sinusoidal AC Inverters

    Three-Phase Inverter Harmonics

    Source: Ned Mohans powerelectronics book

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    8-19Chapter 8 Switch-Mode DC-Sinusoidal AC Inverters

    Three-Phase Inverter Output

    Linear and over-modulation ranges

    Source: Ned Mohans powerelectronics book

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    16-20Chapter 16 Residential andIndustrial 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 Mohans powerelectronics book

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    16-21Chapter 16 Residential andIndustrial 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 minutesafter start-up are very inefficient as the mechanicalsystem reaches steady-state

    with ASDs, the air conditioner speed is lowered withdemand, so that there are fewer start-ups each day

    Source: Ned Mohans powerelectronics book

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    16-22Chapter 16 Residential andIndustrial Applications

    Electronic Ballast for Fluorescent Lamps

    Lamps operated at ~40 kHz save energy

    Source: Ned Mohans powerelectronics book

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    16-23Chapter 16 Residential andIndustrial Applications

    Induction Cooking

    Pan is heated directly by circulating currents

    increases efficiency

    Source: Ned Mohans powerelectronics book

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    16-24Chapter 16 Residential andIndustrial Applications

    Industrial Induction Heating

    Source: Ned Mohans powerelectronics book

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    17-25Chapter 17 ElectricUtility Applications

    HVDC Transmission

    There are many such systems all over the world

    Source: Ned Mohans powerelectronics book

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    17-26Chapter 17 ElectricUtility Applications

    HVDC Poles

    Each pole consists of 12-pulse converters

    Source: Ned Mohans powerelectronics book

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    17-27Chapter 17 ElectricUtility Applications

    HVDC Transmission: 12-Pulse Waveforms

    Source: Ned Mohans powerelectronics book

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    18-28Chapter 18 Utility Interface

    Reducing the Input Current Distortion

    Like DBR current (high distortion)

    Source: Ned Mohans powerelectronics book

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    18-29Chapter 18 Utility Interface

    Power-Factor-Correction (PFC) Circuit

    The boost converter is operated to

    make the DBR current looksinusoidal on the AC side

    To be sold in Europe, this is anecessary feature in high-currentsingle-phase power electronic loads

    It also permits more power to bedrawn from conventional walloutlets because the harmoniccurrents are minimal

    Source: Ned Mohans powerelectronics book

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    18-30Chapter 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 envelopeclose

    open

    Source: Ned Mohans powerelectronics book

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    18-31Chapter 18 Utility Interface

    Power Electronics Has Made Wind FarmsPossible

    The choices used to be

    Use an efficient induction generator, which has very poorpower 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 andgrid AC busses, or

    Use a doubly-fed induction motor, operate the wind turbine atthe max power speed, and use power electronics to trick the

    wind generator into producing grid-frequency output. This iswhat you see in West Texas.