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What happens when the voltage limit is ignored

  1. May 18, 2010 #1
    i'm looking at these little ac motors that generate about 6 hp for 30 min for maximum load. it says it uses 120v standard er something and rotates at 3000rpm, what happens if i put wayyyy more volts in the motor? examples, first a small number like 400v, then the real amount i wanna do 6,000v from a transformer.
  2. jcsd
  3. May 18, 2010 #2
    120v 6hp ac motor? Do you have a link? Thinking along the lines of a more conventional motor I would say that at 400v it would get really hot and make some smoke (if a breaker doesn't trip), along with the smell of burning enamel. At 6000v I would guess that you would see a flash of light and a pop. The brightness of the flash and the loudness of the pop will depend on how much current the transformer can supply. Hopefully a breaker will trip somewhere. The highest we ever tested magnet wire insulation was 3000 volts. It would very likely break down at 6000.

    You probably will not get much power increase from ac motors by increasing the volts. The field cores will magnetically saturate and the only thing that will increase is the heat.
  4. May 18, 2010 #3
    Sorry I don't have a link. They're called "farm motors" at my local tractor supply for like $200. So how do u make an electric motor rotate real fast. I'm shootin around 10,000rpm. Is it possible?
  5. May 18, 2010 #4
    Generally AC motor speed is synchronous, meaning it spins at a speed determined by the 60 hertz line frequency. To double the speed you'd double the frequency. To double the frequency you need to connect an electronic device called a cycloconverter between the motor and the 60 hertz power line. This is not a cheap solution.

    Don't overvoltage your motor. Problems could range from excess heating, shortened life, and bad smells to fire. Motors have information associated with them, called "ratings," that define a range of operation that is generally proven to be safe. They are usually somewhat conservative, but you are pushing your luck whenever you exceed them. You need a lot of knowledge about how motors are designed and tested to assess what the real limit is. If you caused damage or injury, the motor company could state you wilfully exceeded the ratings, and hence any responsibility they might have is negated.
  6. May 18, 2010 #5
    The short answer is that you'd break it.

    This is the same as asking to take a car engine that has a max revs at 5000rpm and asking to spin it to 20000rpm. The components would just shatter.

    If you pump enough volts through something, the electricity would jump through the insulation and you'd get the same effect as a short (causing a spark which would burn it), if you get enough voltage you can set the insulation on fire. In the event that the wire doesn't fail instantly (which is pretty much impossible) you'd get the motor to spin faster, exceeding design limits of the materials (usually through friction) causing it to heat up and set on fire.

    There is a reason larger powered electric motors use larger comonents and larger wires (and much more insulation).
  7. May 18, 2010 #6
    ah, ic. sorry i've been a mechanical type person for ever with automotives, i thought more juice ment more power, like more vaccuum in a petrol motor means more hp, well hm. what about dc motors, i heard those can handle torque very well and can spin pretty fast, is that true? got any sites that show the specs on these?
  8. May 19, 2010 #7
    I'd start with Wikipedia.

    There are a bunch of different types of electrical machines. As motors, torque is generally increased by feeding it more current, and that current is forced by higher voltage. But there are safety ratings that should be adhered to. If you violate them badly enough, you get a catastrophic and potentially very ugly failure.

    With DC motors more torque gets you more speed, limited by the mechanics of the load. With AC synchronous motors, more current enables the motor to maintain synch speed with higher load. It's still a higher torque and mechanical power output.

    Another big difference is that the torque curves wrt RPM are very different for different motor types. DC motors have max torque at zero rpm - it's why Prius' are relatively quick off the line, and changes the requirement for a transmission in an EV.
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