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Duty cycle and motor example

  1. Aug 14, 2011 #1

    I found these two definitions of duty cycle:

    1: The duty cycle of a machine refers to how long it can keep operating before it needs a rest, or what percentage of the time it's designed to be in use. For instance, a machine gun might only be able to fire continuously for 100 rounds before it needs to be allowed to cool.

    For some machines, you could wear out the mechanism well before it came to the end of the warranty period if you left it printing continuously.

    For example, a printer that is capable of printing 30 copies per minute would probably wear out in weeks if it was always printing. Therefore the manufacturer quotes a duty cycle such as "10,000 copies per month".

    2: In engineering the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration.

    A motor runs for one out of 100 seconds, or 1/100 of the time, and therefore its duty cycle is 1/100, or 1 percent.

    What is 'exact' definition of duty cycle in your opinion? Please keep it simple so I can understand it.

    I didn't understand at all that motor example. Isn't motor running all the time? Is the example speaking of DC motor or AC?

    Thank you for the help.

  2. jcsd
  3. Aug 14, 2011 #2
    The motor example states that the motor only runs for one second when watched for one continuous minute. So the duty cycle is 1%. A good example of this scenario would be with an automobile starter motor. It runs for a second and then doesn't run for a while. However, to convey a real "duty cycle" value I think it should be watched through several cycles. Otherwise one might watch a car starter motor for only the one second it was running and conclude its duty cycle is "normally" 100%--yet we know a typical starter motor would last long at that duty cycle!

    "Duty cycle" can apply to devices with any repetition rate, from something that runs a portion of the year to an electronic device that cycles billions of times a second. Heat dissipation (and resulting damage) is typically the issue but there may be other reasons one needs to limit the "on time" of a device. For example, efficiency may go down while a devices is "on", so you might not want to it run too long at a stretch.
  4. Aug 16, 2011 #3
    Many thanks, fleem.

    But that's what troubles me. To me, it's counterintuitive to even imagine that motor runs for 1 second. If the motor is rated 50W, then in 60 seconds wouldn't it consume 3000W rather than only 50W? If you agree with me that it will take in 3000W then what's the reason for saying that it only runs for 1 second? Please help me. Thanks

  5. Aug 16, 2011 #4
    'Watts' is a measure of power (energy per time), not energy. Perhaps you want to talk about watt-seconds?
  6. Aug 16, 2011 #5
    Sorry. I wanted to say:

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