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AC Motors

  1. Feb 27, 2005 #1
    I have no idea how AC Motors work. Can anyone point me in the way of a basic book or website which explains how they operate :confused: I need to know about them for my new job :blushing:
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  3. Feb 27, 2005 #2
    You may know that when a generator has a load put on it that the torque required to turn the generator is increased. The wire cutting the magnetic field inside the generator would rather just ‘hang out’ in between the lines of force instead of cut through them so more effort is required to move the wire through the lines. The same thing happens in an AC induction motor. The rotor is the wire (actually a group of them) that would prefer to ‘hang out’ in between the lines of force instead of cut through the lines. You may ask why it turns. The reason is that the magnetic field around the rotor appears to rotate so the in order for the wires in the rotor to not cut the lines it has to turn with the field. In 3 phase systems this is easily accomplished just by wiring various windings in the stator to the 3 phases. In single phase systems a capacitor is used to create a phase shift in the current in one of the windings. Most single phase motors under a certain horsepower only use the capacitor on startup. Once up to synchronous speed they are able to run only on the main windings and the startup windings are switched out with a centrifugal switch.

    It has been a number of years, but at one time there were motors built that were repulsion start/induction run. The motor started on brushes and behaved as any motor with brushes. But as it came up to speed a centrifugal switch shorted out the brushes and it run as an induction motor. These motors had the advantage of well regulated speed and the ability to break loose the hardest of loads on startup.
  4. Feb 27, 2005 #3


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    Well, the standard AC induction motor just sets up a rotating magnetic field. There's a squirrel cage made out of conducting wires inside the motor that forms the rotating part.

    It's a little hard to explain why this makes a motor. If you look at the magnetic flux, though, you'll see that if the rotor (the squirrel cage) isn't rotating at the same rate as the magnetic field, that there will be a current induced in the rotor. This induced current interacts with the magnetic field to generate torque. Hence the name "induction motor".

    It's worth noting that no torque is generated upon startup - an induction motor requires some special startup circuitry.

    Didn't find any good websites, "howstuff works" is usually pretty good, but I didn't see any listing for them when looking up "induction motor".
  5. Feb 27, 2005 #4


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  6. Feb 27, 2005 #5
    Then would you care to explain what this startup circuitry is on a 3 phase induction motor? Because I've never seen it. I've also never seen special startup circuitry on a shaded pole single phase motor.
  7. Feb 27, 2005 #6


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    The shaded pole is the startup circuit.
    Somewhat inefficient since it operates even after rotation starts.
  8. Feb 28, 2005 #7


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    Induction motors usually have some startup circuitry, but it looks like I was probably wrong when I said the torque was zero at startup without it (I'm not even postive I was wrong - it's been too long since I had my one and only motor course).

    I did find a reasonable URL on induction motors, though


    Some of the torque vs slip graphs in the URL above are what makes me think I was wrong. It looks like the problem on startup is that the motor looks a lot like a transformer with a short-circuited winding on it - this leads to very large currents under startup unless the startup circuitry prevents it. If the URL is right, motors are sometimes designed to just take these high currents for a short time period as far as startup goes, though at a minimum there have to be fuses that will blow or circuit breakers that will trip if it goes on too long (say the shaft is jammed solid so the motor can never start).
  9. Feb 28, 2005 #8


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    I'm pretty sure the starters supplied with the HVAC equipment I spec have capacitors in them to "soften" the startup amperage spike. And while the startup torque is pretty low (I don't think its zero), its not a problem for things like fans because the load is also low at low rpm.
  10. Feb 28, 2005 #9
    The starting capacitor is used to create a phase shift in the current in the starting windings. Larger horsepower motors (typically larger than 1 HP) will also have a running capacitor. Induction motors starting torque can vary. One of the things that determines the starting torque of an induction motor is how far the copper bars are buried in the iron of the rotor. Farther into the rotor means less startup torque. I can't recall exactly what they are, but there are advantages to having the bars buried deep. It is entirely possible that as the bars get deeper (to a point) the efficiency goes up. Very often we need a high starting current in order to 'break loose' the load such as an air compressor.

    There is also a motor called a split-phase motor. They are usually less than a half horse. It is a standard induction motor with NO starting capacitor. The starting winding is wound with a higher resistance wire. This is enough to cause a phase shift needed for starting. The blower fan inside your furnace is typically a split-phase motor. Some new furnaces are coming with solid state controls for the blower and NOT a set of contacts. Not 100% sure, but they are probably soft started by simply inserting a capacitor in series with the whole motor to reduce the voltage. Once the motor is partially up to speed they may switch it out. One reason I can see for doing this is to reduce the stress on the solid state switch. There is simply no reason for letting the blower motor draw a large amount of current on startup. It's not a hard to start load.

    Incidentally, I still stand by my earlier claim that 3-phase motors do not require any special startup circuits or devices. It can be advantageous to use a soft start controller in large industrial blowers that use 3 phase motors since it is not necessary to have a large startup current on a fan. Soft startup is becoming more common but I suspect the main reason is to protect the startup switch (solid state or otherwise) and not the motor.
  11. Mar 11, 2005 #10
    Thanks for the help it has given me more of an insight into their operation :smile: I have managed to get a good book on the subject which should take my knowledge the rest of the way :wink:
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