ECE1750, Spring 2018 Motor Drives and Other Applications 1
Three-Phase Induction Motors Reliable Rugged Long lived Low maintenance Efficient (Source: EPRI Adjustable Speed Drives Application Guide)
The rotor also has windings Stator t winding looking from the front (3-phase system) Current convention: Positive current. Goes in in terminal A 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 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) Ohmic losses = mechanical power output The shunt inductance term represents the flux in the machine air 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)
Torque-Speed Characteristics slip s s r s s speed of the stator rotating magnetic field r speed of the rotor fsl s. f The linear part of the characteristic is utilized in adjustable speed drives
Acceleration Torque at Startup d dt r J Tacc Tem Tload Intersection represents the equilibrium point T 0 T T acc em load
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 to High slip corresponds to low efficiency
Speed Adjustment by Changing the Input Voltage
Speed Adjustment by Changing the Line Frequency It s much more effective to reduce operating speed by lowering the frequency of the supply voltage. But how?
Per-Phase Representation (assuming sinusoidal steady state) Ohmic losses = mechanical power output The shunt inductance term represents the flux in the machine air 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 the standard operating mode for ASDs
PWM-VSI System A three-phase DBR A three-phase inverter Diode rectifier for unidirectional power flow
Three-Phase Inverter (called a six-pack) Three inverter legs; capacitor mid-point is fictitious
Three- Phase PWM Waveforms NOTE: Modulation index is different from that on single-phase inverters. In threephase inverters: m Vph, rms 3 ph 2 Vdc 2
Three-Phase Inverter Harmonics Compare with slide 22 in Inverter Basics presentation
Three-Phase Inverter Output Linear and over-modulation ranges
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 $2,000 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!) e
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
Torque Speed Characteristics at various Frequencies of Applied Voltage For a constant torque load The air gap flux is kept constant
Adjusting Speed of a Centrifugal Load The load torque is proportional to speed squared
Operating regions Pull-out torque line V/f = constant
Frequency at Startup (Rotor portion of the equivalent circuit) At startup f=f sl so this fictitious resistance is a short circuit and the current is high So the current can be reduced by reducing the input frequency f, which, in turn, equals f sl at start up (because the ratio with respect to f sl tends to increase) An important property of ASDs is the ability to soft start a motor by reducing the applied frequency to a few Hz The torque is limited to limit current draw (e.g. max startup current 150% of the rated current) f start T T start rated f sl, rated
Braking (Regenerative braking) Initial operation point (Motor) Final operation point Energy can be (Generator) dumped in resistances or used to recharge batteries (e.g. hybrid cars)
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.
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
Electronic Ballast for Fluorescent Lamps Fluorescent lamps operated at ~40 khz save energy
Induction Cooking Pan is heated directly by circulating currents increases efficiency (and can make the pan levitate) http://web.mit.edu/cjoye/www/photo/0610mr Magnet/
Industrial Induction Heating Used in steel industry
HVDC Transmission There are many such systems all over the world Issue in power electronics: switching frequency goes down as voltage goes up (and filters become bulkier)
HVDC Poles Each pole consists of 12-pulse converters
HVDC Transmission: 12-Pulse Waveforms
Reducing the Input Current Distortion Like DBR current (high h distortion) ti
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
Power-Factor-Correction (PFC) Circuit The boost converter is instructed to close when the current is below the sinewave envelope, and open with the current is above the sinewave envelope Operation during each half-cycle
Typical telecom rectifier schematic Isolation barrier High-frequency transformer
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.