Post on 06-Apr-2018
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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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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 perHertz Operation, which isthe 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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14-14Chapter 14 InductionMotor 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 reducingthe applied frequency to a few Hz
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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8-16Ch
apter8
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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8-17Ch
apter8
Switch-
Mode DC-
Sinusoidal AC Inverters
Three-
Ph
asePWM
Waveforms
Source: Ned Mohans powerelectronics book
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8-18Ch
apter8
Switch-
Mode DC-
Sinusoidal AC Inverters
Three-Phase Inverter Harmonics
Source: Ned Mohans powerelectronics book
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8-19Ch
apter8
Switch-
Mode DC-
Sinusoidal AC Inverters
Three-Phase Inverter Output
Linear and over-modulation ranges
Source: Ned Mohans powerelectronics book
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16-20Ch
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
apter 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 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
ans powerelectronics book
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16-22Ch
apter 16 Residential andIndustrial Applications
Electronic Ballast for Fluorescent Lamps
Lamps operated at ~40 kHz save energy
Source: Ned Moh
ans powerelectronics book
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16-23Ch
apter 16 Residential andIndustrial Applications
Induction Cooking
Pan is heated directly by circulating currents
increases efficiency
Source: Ned Moh
ans powerelectronics book
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16-24Ch
apter 16 Residential andIndustrial Applications
Industrial Induction Heating
Source: Ned Moh
ans powerelectronics book
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17-25Ch
apter 17
ElectricUtility Applications
HVDC Transmission
There are many such systems all over the world
Source: Ned Moh
ans powerelectronics book
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17-26Ch
apter 17
ElectricUtility Applications
HVDC Poles
Each pole consists of 12-pulse converters
Source: Ned Moh
ans powerelectronics book
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17-27Ch
apter 17
ElectricUtility Applications
HVDC Transmission: 12-Pulse Waveforms
Source: Ned Moh
ans powerelectronics book
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18-28Ch
apter 18
Utility Interface
Reducing the Input Current Distortion
Like DBR current (high distortion)
Source: Ned Moh
ans powerelectronics book
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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
ans powerelectronics book
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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
ans powerelectronics book
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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.