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AC induction motor for EVs?

Discussion in 'Electronic Basics' started by BobG, Mar 25, 2007.

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  1. BobG

    BobG Guest

    Evidently there is some engineering reason that AC propulsion makes
    their EV motor and controller use 300+ VAC. I'm a little foggy on the
    theory of operation of induction motors... To make the rotor
    magnetic, you have to induce current into the copper loops in the
    rotor by putting AC thru the coils in the stator surrounding the
    rotor. This sounds like a synhcro or a resolver used in aircraft
    instruments, where the coils get sin and cos of varying amplitude but
    constant 400hz, but lots heftier? What freq does the AC propulsion
    motors and controllers use? 400hz? 15KHz? So the stator coils are
    energized in sequence to spin the rotor? I guess it would be a real
    bummer if they 'cogged' like synchronous motors or stepper motors, so
    I don't think my mental model of AC induction motors is exactly right
    yet. Is it possible to build a permanent magnet motor with equivalent
    power to this induction motor (200HP?), or is there a limit to the
    power of PM motors? That is, is the AC induction motor more effcient
    than a PM motor of the same power? All those permanent magnets have to
    be good for something dont they?
     
  2. AC induction motors are (1) cheap, (2) rugged, and (3) easy to control.
    Large ones are almost always 3 phase, and may have integral multiples of 2
    poles for an assortment of rated speeds, up to 3600 (nominal) at 60 Hz. A
    240 VAC motor requires peak voltage of 1.4 times that, or 340 VDC. A three
    phase controller synthesizes the voltage waveforms by using PWM (Pulse
    Width Modulation) on a 3 output H bridge consisting of 6 IGBTs or MOSFETs.
    The "carrier frequency" of the PWM is usually on the order of 15 kHz.

    Motors are usually built for optimal performance at 50 or 60 Hz, but VF
    drives can produce up to 400 Hz or more to overdrive the speed. Mostly,
    however, they are operated at lower frequencies to achieve speed and torque
    control. Induction motors operate by creating a rotating magnetic field in
    the stator, which induces a field in the rotor and causes it to follow the
    field, at a speed a little bit slower than the applied voltage. This is
    called slip, and the rate at which the applied field rotates is call
    synchronous speed. A 2 pole motor has a synchronous speed of 3600 RPM, but
    will run at something like 3450 RPM.

    The motor can also be thought of as a transformer, and like any
    electromagnetic device, the steel laminations saturate at a voltage which
    depends on number of turns and frequency. The ratio of voltage to frequency
    is a constant, and AC drives are often called VF or V/F drives. The torque
    is proportional to current, and is limited by winding resistance (heating)
    and magnetic saturation. So an induction motor has essentially constant
    torque from start-up to full speed. Its horsepower is determined by Torque
    * RPM / 5252.

    So torque is a function of current, and speed a function of voltage, and
    the product is power. Their is phase shift, so Watts = Volts * Amps *
    Cos(Angle). At no load, the motor may draw considerable current at rated
    voltage, but consumes little power. With more load, the phase shift gets
    closer to zero, so power goes up with little current change. If you run the
    PWM controller from a battery, however, you will see the current increase
    proportional to loading.

    Motors can be wound to run at lower voltage, and also at higher frequency,
    to get much more horsepower from a given frame size. There are motors
    designed for 400 Hz, which are about 1/6 the size and weight of equivalent
    60 Hz motors. They are usually high RPM (about 12,000), and use thinner
    laminations of higher grade steel to reduce losses that are the limiting
    factor at higher frequency.

    I have rewound several single phase motors to run on three phase at about 8
    VAC at 60 Hz, and I have then used a VF controller to operate it at 180 Hz,
    at three times the RPM. I made it a 12 pole motor, which has a nominal
    speed of 600 RPM at 60 Hz, and it ran nicely at 1800 RPM. It was a 3/4 HP
    motor that now might be able to produce over 2 HP (using 24 VAC). Rewinding
    a motor is an interesting learning experience.

    Permanent magnet motors may have a slight advantage in efficiency, and
    series wound DC motors have much higher starting torque, so they are often
    used in EVs, but the best PM motors need expensive rare earth magnets,
    which can be damaged by heat and vibration, while commutator type motors
    have brushes which can burn or wear out, requiring maintenance, and they
    are hard to control.

    That's a lot to read, and there is much more to motors and controllers, but
    I hope this helps to explain things. Feel free to ask more questions.

    Paul
     
  3. Eric R Snow

    Eric R Snow Guest

    Greetings Bob,
    I'm no motor expert by far, but can answer some of your questions.
    AC induction motors do indeed work by inducing current in the rotor
    from the stator. Single phase motors commonly have a starting winding
    in electrical quadrature that is only energized long enough to get the
    motor up to about 85% rated speed. There are other schemes used in
    single phase motors that involve capacitors, shaded poles, etc. But
    three phase AC motors don't need a starting winding and the technology
    is mature. These motors are well understood, cheap to build, robust,
    and about the only thing that wears out are the bearings. AC induction
    motors can be speed controlled by varying the frequency. This is
    easily done these days with inverters. Inverter technology has
    advanced greatly in recent years. Inverter power supplies are used in
    computers and TVs and washing machines and robots and etc. Welders are
    built around inverters now. Doing this makes it possible to use
    smaller inducters which use less copper in the windings. So using an
    AC 3 phase motor with a variable frequency drive with a nominal 300
    volt output can be a relatively inexpensive way to power a car.
    ERS
     
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