Motor Drives and Other Applications
Transcript of 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.