ECE1750, Spring 2017 Week 12 – Motor Drives and Other ...akwasins/power electronics...
Transcript of ECE1750, Spring 2017 Week 12 – Motor Drives and Other ...akwasins/power electronics...
ECE1750, Spring 2017
Week 12 – Motor Drives and Other ApplicationsApplications
1
Three-Phase Induction Motors
• Reliable
Rugged• Rugged
• Long lived
• Low maintenance
• Efficient
(Source: EPRI Adjustable Speed Drives Application Guide)
The rotor also has “windings”
St t i di l ki f
• Current convention: Positive
Stator winding looking from the front (3-phase system)
Current convention: Positive current….
• Goes “in” in terminal A
G “ t” f t i l• Goes “out” from terminal a
(Source: EPRI Adjustable Speed Drives Application Guide)
• A 3-phase system of circulating currents in the stator creates a rotating (varying) magnetic field.• The varying rotating magnetic field induces a 3-phase system of currents in the rotor.• The 3-phase system of rotor currents creates a rotating magneticThe 3 phase system of rotor currents creates a rotating magnetic field.• The interaction of both rotating magnetic fields makes the rotor to turn (it is like one magnet pushing into another).
Per-Phase Representation(assuming sinusoidal steady state)( g y )
Ohmic losses = mechanical power output
The shunt inductance term represents the flux in the machine pair gap between rotor and stator.
The flux is proportional to Vs / f
(This characteristic will lead to an important conclusion in a few slides…. Stay tuned)
Source: Ned Mohan’s power electronics book
Torque-Speed Characteristics
s rli s r
s
slip s
speed of the stator rotatingmagnetic field
s magnetic field
speed of the rotorr
.slf s f
• The linear part of the characteristic is utilized in padjustable speed drives
Source: Ned Mohan’s power electronics book
Acceleration Torque at Startup
racc em load
dJ T T T
dt
• Intersection represents the equilibrium pointp q p
Source: Ned Mohan’s power electronics book
0acc em loadT T T
Slip frequency (about 5% of no load speed), so induction motors are almost constant speed devices
B
At no load, the motor spins at grid frequency, divided by the number of pole pairs (in each phase).
Usually this is 3600 / 2 = 1800RPM
High slip corresponds toHigh slip corresponds to low efficiency
Speed Adjustment by Changing the Input Voltage
Source: Ned Mohan’s power electronics book
Speed Adjustment by Changing the Line Frequency
It’s much more effective to reduce operating speed by lowering the frequency of the supply voltage. q y pp y g
But how?
Per-Phase Representation(assuming sinusoidal steady state)( g y )
Ohmic losses = mechanical power output
The shunt inductance term represents the flux in the machine pair gap between rotor and stator.
The flux is proportional to Vs / f
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 p pthe standard operating mode for ASDs
Source: Ned Mohan’s power electronics book
PWM-VSI System
A three phase DBR
A three-phase inverter
A three-phase DBR
• Diode rectifier for unidirectional power flowSource: Ned Mohan’s power electronics book
Three-Phase Inverter(called a six-pack)
• Three inverter legs; capacitor mid-point is fictitious g p p
Source: Ned Mohan’s power electronics book
Three-Phase PWM
WaveformsNOTE: ModulationNOTE: Modulation index is different from that on single-phase inverters In three-inverters. In three-phase inverters:
V ,3 2
2
ph rmsph
dc
Vm V
Source: Ned Mohan’s power electronics book
Three-Phase Inverter Harmonics
Compare with slide 22 in “Inverter Basics presentation”
Source: Ned Mohan’s power electronics book
Three-Phase Inverter Output
Li d d l ti• Linear and over-modulation rangesSource: Ned Mohan’s power electronics book
Adjustable-Speed Motor Drives (ASDs)
(Source: EPRI Adjustable Speed Drives Application Guide)
Some Prices for Small 3-Phase, 460V Induction Motors and ASDs
Power Motor ASD
10kW $750 $2,0000 $ 50 $ ,000
100kW $5,000 $15,000
$50 - $75 per kW $150 - $200 per kW
For Comparison, Conventional Generation: $500 - $1,000 per kW
Solar: $5,000 - $8,000 per kW (but the fuel is free forever!)So a $5,000 $8,000 pe (but t e ue s ee o e e )
Pump Application: Adjustable Flow ratej
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 drivej p
Source: Ned Mohan’s power electronics book
Torque Speed Characteristics at various Frequencies of Applied Voltage
For a constant torque load
• The air gap flux is kept constantg p p
Source: Ned Mohan’s power electronics book
Adjusting Speed of a Centrifugal Loadj g g
• The load torque is proportional to speed squared
Source: Ned Mohan’s power electronics book
Operating regionsg g
Pull-out torque line
V/f = constant
Source: Ned Mohan’s power electronics book
Frequency at StartupAt startup f=fsl so this fictitious resistance is a short circuit and the (Rotor portion of
current is high( pthe equivalent
circuit)
So the current can be reduced by reducing the input frequency f,
which, in turn, equals fsl at start
An important property of ASDs is the ability to “soft start” a
, , q slup (because the ratio with respect
to fsl tends to increase)
• The torque is limited to limit current draw (e.g. max startup current 150% of the rated current)
motor by reducing the applied frequency to a few Hz
max startup current 150% of the rated current)
Source: Ned Mohan’s power electronics book ,
startstart sl rated
rated
Tf f
T
Braking(Regenerative braking)
Initial operation point
(Regenerative braking)
(Motor)
Fi l ti i t E bFinal operation point(Generator)
Energy can be dumped in
resistances or used to recharge batteries g(e.g. hybrid cars)
Source: Ned Mohan’s power electronics book
Improving Energy Efficiency of Heat Pumpsg gy yHow does inserting an ASD save energy in single-phase applications?
9 %
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
Loss Associated with ON/OFF Cyclingy gThe 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 percenty y p y p
Source: Ned Mohan’s power electronics book
Electronic Ballast for Fluorescent Lamps
• Fluorescent lamps operated at ~40 kHz save energyp p gy
Source: Ned Mohan’s power electronics book
Induction Cookingg
• Pan is heated directly by circulating currents – increases efficiency (and can make the panefficiency (and can make the pan levitate)
Source: Ned Mohan’s power electronics book
http://web.mit.edu/cjoye/www/photo/0610MrMagnet/
Industrial Induction Heating
• Used in steel industry
Source: Ned Mohan’s power electronics book
• Used in steel industry
HVDC Transmission
• There are many such systems all over the world
•Issue in power electronics: switching frequency goes
Source: Ned Mohan’s power electronics book
p g q y gdown as voltage goes up (and filters become bulkier)
HVDC Poles
• Each pole consists of 12-pulse convertersp p
Source: Ned Mohan’s power electronics book
HVDC Transmission: 12-Pulse Waveforms
Source: Ned Mohan’s power electronics book
Reducing the Input Current DistortionReducing the Input Current Distortion
Lik DBR t (hi h di t ti )Like DBR current (high distortion)
Source: Ned Mohan’s power electronics book
Power-Factor-Correction (PFC) Circuit( )
The boost converter is operated toThe 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
Power-Factor-Correction (PFC) Circuit( )
The boost converter is instructed to
“ l ” h th t i b l th“close” when the current is below the sinewave envelope, and
“open” with the current is above the sinewave envelope
• Operation during each half-cycle
p
p g y
Source: Ned Mohan’s power electronics book
Typical telecom rectifier schematic
Isolationbarrier
High-frequencytransformer
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• 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• 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 Texaswhat you see in West Texas.