How far can a motor be overloaded?

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Discussion Overview

The discussion revolves around the suitability of using a 37kW AC motor instead of a 40kW motor for a specific application, focusing on the implications of motor overload, service factors, and the importance of proper sizing in relation to power requirements.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the calculations leading to the 40kW requirement, suggesting that factors like friction may have been underestimated, implying that a higher power rating might be necessary.
  • Another participant mentions that most motors have a service factor of 1.15, allowing for short-term operation at 115% of their rated capacity, but emphasizes the importance of considering safety and longevity when selecting motor size.
  • A further contribution explains that service factors indicate how much overload a motor can handle and warns against continuous operation above rated load, citing potential issues with torque and starter sizing.
  • There is a mention of different service factors for various types of motors, indicating that some motors can handle higher overloads, but also cautioning against improper sizing practices.

Areas of Agreement / Disagreement

Participants express differing views on the adequacy of the 37kW motor for the application, with some advocating for caution and proper sizing while others question the initial calculations. No consensus is reached regarding the best approach.

Contextual Notes

Participants highlight the importance of considering various factors such as service factors, starting torque, and operational conditions, but do not resolve the specific calculations or assumptions made in the initial inquiry.

Who May Find This Useful

Individuals involved in motor selection, electrical engineering, or those interested in the operational limits and safety factors of electric motors may find this discussion relevant.

kyaw min
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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?

thanks
 
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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?
 
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?
 
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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