How do you Determine Whether to Repair or Scrap an Electric Motor?

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Comments (3)

• Other factors to consider are energy savings when replacing lower efficiency motor with NEMA Premium, and expected remaining life of the system in which the motor in question resides. Threshold for decision based on HP may be much higer - up to 150 HP in some applications

  1) Posted 8:49 am, 29 July 2010 by Jack Nicholas

• The efficiency of the motor to be repaired also must be considered. Depending upon the expected life of the motor and the annual run hours, it may be better to replace the motor with a premium efficiency motor no matter how inexpensive the repair bill may be for a less efficient motor.

  2) Posted 8:54 am, 29 July 2010 by Bob McCann

• It is all about ROI and reliability. If the repaired motor cannot meet the MTBF of the equivalent new one, then you 9and your operations folks) don't want it at any price. That cost may include upgrading the wire to Inverter duty and adding bearing isolators, etc. The only exception to this is if it is a rare type and you need a TEMPORARY motor to hold you over until a new one arrives. (And that is generally the resul of lacking PdM and/ or bad spare part decisiona making.) If your repair shop can't deliver like new MTBF (on an accepatble condition core motor), or absent that ability (such as damaged beyond economic repair), recommend to decline to repair a particular motor, find a new shop.
  Next comes efficiency, the repair vs replace decision to make once the MTBF hurdle is reached is about ROI. Will the difference in cost vs difference in efficiency pay out over the expected MTBF of the motor. Simple calculations that have to be updated periodically. Forget the old rule of thumb: 3 rebuilds and you are out. Verify the core loss condition before you even start to rebuild.
  
  The motor size issue has to take into account the fixed costs of handling and administration. If we are talking standard mill motors, then 25 to 50 hp is a noraml cut range. iHowever in the discrete manufacturing business it is not uncommon to routinely repair fractional hp servos, but again it is an economic decision.
  
  The best motor repair is the one you never have to make. So always practice precision installation methods, have a well engineered and delivered lubrication program, appropriate PMs, and a WELL DESIGNED PdM PROGRAM. Good PdM alone (with appropriate decision making) can reduce the cost of motor repairs by 50% even if the basic MTBF isn't changed. why replace a bearing, when you could have fixed the misalignment? Why replace a shaft or rotor, when a simple bearing change in the early failure stages would have dealt with it?
  
  I hope this adds to Dan's excellent article.
  - Sam McNair, LCE

  3) Posted 10:54 am, 29 July 2010 by Sam McNair

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