Transcript 14_EE462L_Fall2011_Motor_Drives_and_Other_Applications

EE462L, Fall 2011
Motor Drives and Other Applications
1
Three-Phase Induction Motors
• Reliable
• Rugged
• Long lived
• Low maintenance
• Efficient
(Source: EPRI Adjustable Speed Drives Application Guide)
2
Slip frequency (about 5% of no load speed), so induction
motors are almost constant speed devices
At no load, the motor spins at grid frequency, divided by the
number of pole pairs. Usually this is 3600 / 2 = 1800RPM
3
High slip corresponds to
low efficiency
4
It’s much more efficient to reduce
operating speed by lowering the
frequency of the supply voltage.
But how?
5
Adjustable-Speed Motor Drives (ASDs)
(Source: EPRI Adjustable Speed Drives Application Guide)
6
Some Prices for Small 3-Phase, 460V Induction Motors
and ASDs
Power
Motor
ASD
10kW
$750
$2,000
100kW
$5,000
$15,000
$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!)
7
Pump Application: Adjustable Flow rate
Bad news – inefficient!
Equivalent to reducing the
output voltage of a DBR with a
series resistor
Payback in energy
savings is about 1 year
• Fixed versus adjustable speed drive
Source: Ned Mohan’s power
electronics book
Chapter 14 Induction
Motor Drives
14-8
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
Chapter 14 Induction
Motor Drives
14-9
Torque-Speed Characteristics
• The linear part of the characteristic is utilized in
adjustable speed drives
Source: Ned Mohan’s power
electronics book
Chapter 14 Induction
Motor Drives
14-10
Acceleration Torque at Startup
• Intersection represents the equilibrium point
Source: Ned Mohan’s power
electronics book
Chapter 14 Induction
Motor Drives
14-11
Torque Speed Characteristics at various
Frequencies of Applied Voltage
For a constant
torque load
• The air gap flux is kept constant
Source: Ned Mohan’s power
electronics book
Chapter 14 Induction
Motor Drives
14-12
Adjusting Speed of a Centrifugal Load
• The load torque is proportional to speed squared
Source: Ned Mohan’s power
electronics book
Chapter 14 Induction
Motor Drives
14-13
Frequency at Startup
An important property of ASDs is the
ability to “soft start” a motor by reducing
the applied frequency to a few Hz
Zero speed
• The torque is limited to limit current draw
Source: Ned Mohan’s power
electronics book
Chapter 14 Induction
Motor Drives
14-14
PWM-VSI System
A three-phase
inverter
A three-phase DBR
• Diode rectifier for unidirectional power flow
Source: Ned Mohan’s power
electronics book
Chapter 14 Induction
Motor Drives
14-15
Three-Phase Inverter
(called a six-pack)
• Three inverter legs; capacitor mid-point is fictitious
Source: Ned Mohan’s power
electronics book
Chapter 8 Switch-Mode DCSinusoidal AC Inverters
8-16
ThreePhase
PWM
Waveforms
Source: Ned Mohan’s power
electronics book
Chapter 8 Switch-Mode DCSinusoidal AC Inverters
8-17
Three-Phase Inverter Harmonics
Source: Ned Mohan’s power
electronics book
Chapter 8 Switch-Mode DCSinusoidal AC Inverters
8-18
Three-Phase Inverter Output
• Linear and over-modulation ranges
Source: Ned Mohan’s power
electronics book
Chapter 8 Switch-Mode DCSinusoidal AC Inverters
8-19
Improving Energy Efficiency of Heat Pumps
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
• Used in one out of three new homes in the U.S.
Source: Ned Mohan’s power
electronics book
Chapter 16 Residential and
Industrial Applications
16-20
Loss Associated with ON/OFF Cycling
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
• The system efficiency is improved by ~30 percent
Source: Ned Mohan’s power
electronics book
Chapter 16 Residential and
Industrial Applications
16-21
Electronic Ballast for Fluorescent Lamps
• Lamps operated at ~40 kHz save energy
Source: Ned Mohan’s power
electronics book
Chapter 16 Residential and
Industrial Applications
16-22
Induction Cooking
• Pan is heated directly by circulating currents –
increases efficiency
Source: Ned Mohan’s power
electronics book
Chapter 16 Residential and
Industrial Applications
16-23
Industrial Induction Heating
Source: Ned Mohan’s power
electronics book
Chapter 16 Residential and
Industrial Applications
16-24
HVDC Transmission
• There are many such systems all over the world
Source: Ned Mohan’s power
electronics book
Chapter 17 Electric
Utility Applications
17-25
HVDC Poles
• Each pole consists of 12-pulse converters
Source: Ned Mohan’s power
electronics book
Chapter 17 Electric
Utility Applications
17-26
HVDC Transmission: 12-Pulse Waveforms
Source: Ned Mohan’s power
electronics book
Chapter 17 Electric
Utility Applications
17-27
Reducing the Input Current Distortion
Like DBR current (high distortion)
Source: Ned Mohan’s power
electronics book
Chapter 18 Utility Interface
18-28
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
Chapter 18 Utility Interface
18-29
Power-Factor-Correction (PFC) Circuit
The boost converter is instructed to
open
close
“close” when the current is below the
sinewave envelope, and
“open” with the current is above the
sinewave envelope
• Operation during each half-cycle
Source: Ned Mohan’s power
electronics book
Chapter 18 Utility Interface
18-30
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.
Chapter 18 Utility Interface
18-31