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How far can a motor be overloaded?

  1. Jun 12, 2010 #1
    According to the caculation, we need a 40kW 3 phase AC asychorne motor. Could we use 37kW AC motor instead of 40kW? If yes, what kind of precautions have to be taken?

  2. jcsd
  3. Jun 12, 2010 #2


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    I don't know what kind of calculations you did, but are you sure you didn't understimate the power requirement?
    If you ignored things like friction and people holding it with their fingers then you might prefer something higher than 40 kW instead?
  4. Jun 13, 2010 #3


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    Staff: Mentor

    Most motors have a service factor of 1.15, meaning they can be run for shot durations at 115% of nameplate rating. But as CompuChip suggested, you're not giving yourself any safety factor and you really should ask yourself just how confident you are in your calcs and whether the peace of mind and assurance of lifespan of having a properly sized motor is worth the extra 5% cost.

    Put another way, the cost difference between the two motors is equivalent to the cost of the first four days of running the motor (assuming it runs 24 hrs a day). So is it really worth it to skimp?
  5. Jun 14, 2010 #4
    Service factor (SF) is an indication of how much overload a motor can withstand when operating normally within the correct voltage tolerances. For example, the standard SF for open drip-proof (ODP) motors is 1.15. This means that a 10-hp motor with a 1.15 SF could provide 11.5 hp when required for short-term use. Some fractional horsepower motors have higher service factors, such as 1.25, 1.35, and even 1.50. In general, it's not a good practice to size motors to operate continuously above rated load in the service factor area. Motors may not provide adequate starting and pull-out torques, and incorrect starter/overload sizing is possible.
    Traditionally, totally enclosed fan cooled (TEFC) motors had an SF of 1.0, but most manufacturers now offer TEFC motors with service factors of 1.15, the same as on ODP motors. Most hazardous location motors are made with an SF of 1.0, but some specialized units are available for Class I applications with a service factor of 1.15.

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