Energy efficiency tip of the day (Motors repair/replace practices)

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Energy efficiency tip of the day (Motors repair/replace practices)

When the initial costs of repair versus replacement are compared, repair is usually less expensive. However, instead of making a decision based solely on the initial cost, users should do a life-cycle cost analysis. This considers two important factors:

  1. Hours of operation and electricity costs.
  2. Purchase and repair costs.

Many motor repairs involve rewinds, replacing burned out wiring and insulation with new windings. In some industries more than half the motors have been rewound. Careful rewinding can maintain motor efficiency at previous levels, but flawed rewinds result in efficiency losses.

Motors less than 30 kW or 40 HP in size and more than 15 years old (especially previously rewound motors) have efficiencies significantly lower than currently available energy-efficient models. It is usually best to replace them. It is almost always best to replace non-specialty motors under 10 kW or 15 HP.

If the rewind cost exceeds 50% to 65% of the new energy- efficient price, buy the new motor as the increased reliability and efficiency should quickly recover the price premium. This is especially true for high-use motors. You can quickly calculate the operating costs of the rewound motor versus the new one to see how long the new motor would take to pay for itself.

The impact of rewinding on motor efficiency and power factor can be easily assessed if the no-load losses of a motor are known before and after rewinding. Information on no-load losses and no-load speed can be found in documentation of motors obtained at the time of purchase. An indicator of the success of rewinding is the comparison of no load current and stator resistance per phase of a rewound motor with the original no-load current and stator resistance at the same voltage.

In summary any good motors purchasing policy should have the following objectives:

  1. Consistency in procurement.
  2. Choosing the most appropriate, cost effective motor.
  3. Streamlining the approval process
  4. Supporting decisions based on life-cycle costs.

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Magdy Aly

Energy manager, Energy efficiency consultant Passionate to help others to save Energy and Environment.

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