14 ee462l fall2011 motor drives and other applications

8/3/2019 14 EE462L Fall2011 Motor Drives and Other Applications

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EE462L, Fall 2011

Motor Drives and Other Applications


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2(Source: EPRI Adjustable Speed Drives Application Guide)

Three-Phase Induction Motors

Reliable

Rugged

Long lived

Low maintenance

Efficient


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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 induction

motors are almost constant speed devices


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High slip corresponds tolow efficiency


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Its much more efficient to reduceoperating 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 news inefficient!

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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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 perHertz Operation, which isthe standard operating mode for ASDs



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Torque-Speed Characteristics

The linear part of the characteristic is utilized in

adjustable speed drives



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Acceleration Torque at Startup

Intersection represents the equilibrium point



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Torque Speed Characteristics at various

Frequencies of Applied Voltage

The air gap flux is kept constant

For a constanttorque load



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Adjusting Speed of a Centrifugal Load

The load torque is proportional to speed squared



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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 reducingthe applied frequency to a few Hz



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PWM-VSI System

Diode rectifier for unidirectional power flow

A three-phase DBR

A three-phaseinverter



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8-16Ch

apter8

Switch-

Mode DC-

Sinusoidal AC Inverters

Three-Phase Inverter

Three inverter legs; capacitor mid-point is fictitious

(called a six-pack)



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8-17Ch

apter8

Switch-

Mode DC-


Three-

Ph

asePWM

Waveforms



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8-18Ch

apter8

Switch-

Mode DC-


Three-Phase Inverter Harmonics



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8-19Ch

apter8

Switch-

Mode DC-


Three-Phase Inverter Output

Linear and over-modulation ranges



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16-20Ch

apter 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 Moh

ans powerelectronics book


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16-21Ch


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 mechanicalsystem reaches steady-state

with ASDs, the air conditioner speed is lowered with

demand, so that there are fewer start-ups each day

Source: Ned Moh



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16-22Ch


Electronic Ballast for Fluorescent Lamps

Lamps operated at ~40 kHz save energy

Source: Ned Moh



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16-23Ch


Induction Cooking

Pan is heated directly by circulating currents

increases efficiency

Source: Ned Moh



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16-24Ch


Industrial Induction Heating

Source: Ned Moh



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17-25Ch

apter 17

ElectricUtility Applications

HVDC Transmission

There are many such systems all over the world

Source: Ned Moh



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17-26Ch

apter 17


HVDC Poles

Each pole consists of 12-pulse converters

Source: Ned Moh



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17-27Ch

apter 17


HVDC Transmission: 12-Pulse Waveforms

Source: Ned Moh



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18-28Ch

apter 18

Utility Interface

Reducing the Input Current Distortion

Like DBR current (high distortion)

Source: Ned Moh



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18-29Ch

apter 18

Utility Interface

Power-Factor-Correction (PFC) Circuit

The boost converter is operated tomake 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 Moh



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18-30Ch

apter 18

Utility Interface

Power-Factor-Correction (PFC) Circuit

Operation during eachhalf-cycle

The boost converter is instructed toclose when the current is below thesinewave envelope, and

open with the current is above thesinewave envelopeclose

open

Source: Ned Moh



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18-31Ch

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

14 ee462l fall2011 motor drives and other applications

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