Replacing & Sizing Your AC Motor

Posted by Advanced Controls & Distribution on Aug 16, 2017 11:27:35 AM

replacing and sizing.pngRising utility costs on one side and higher initial investment costs on the other are becoming problems for businesses with every passing day. Getting higher Return on Investment (RoI) is vital for a business to sustain its operations, and engineers and managers have been working hard to keep the balance sheet in their favor.

There are three major motor efficiency standards, namely pre-EPAct, EPAct and NEMA Premium. Each standard has a comprehensive set of guidelines, written under the US Department of Energy (DOE). 

Rewinding or Replace

One of the very first decisions engineers are faced with upon motor failure is whether to rewind or replace it. Factors that play an important part in this decision include:

  • Age of motor
  • Type of motor
  • Availability of parts
  • Frequency of operation
  • Efficiency

Replacement is mostly preferred in case of pre-EPAct motors as greater savings can be made through installation of NEMA Premium motors. However, in case a NEMA Premium motor fails, the lines become blurry.

In the past, rewinding motors was often assimilated with lesser efficiency, but today that is no longer true. It is even possible to improve the motor’s efficiency through rewinding, so it can’t be ruled out.

Due to the complexity, the extent of improvement should be consulted with an expert.

The decision becomes easier for custom motors, as they take longer times to replace and cost greater as well. However, for motors that follow non-custom standards of efficiency, replacement is often better due to efficiency gains.

The frequency of operations of a motor also plays a great role in making this decision. The calculations are complex, fortunately the Department of Energy provides a free software, i.e. MotorMaster+ that helps in making the rewind/replace decision based on the amount of time the motor runs.


A common misconception is that the procurement of a NEMA Premium motor would deliver optimum efficiency, irrespective of the application. Note that when trying to boost savings, the factor of “sizing” gains paramount importance. If the new motor is oversized, the energy savings would be minimized by the initial cost. Correctly sizing a motor is necessary as overloaded motors are prone to overheating while underloaded motors can waste energy.

Two of the most important factors which affect sizing any type of load are:

  • Speed
  • Torque

Determining the speed is usually a straightforward process, and requires only the design specifications; the latter however is not so simple. Conducting a motor survey is a great way to make sure the replacement motor is correctly sized.

The process involves:

  • Reviewing the current motor’s specifications through the nameplate data
  • The use of clamp-on meter to monitor the current being drawn under normal operation
  • Considering factors such as friction and mechanical efficiencies

Conduction of a motor survey may seem like a complex task; however, it is one that is necessary to maintain optimum performance. Before skipping this vital step, keep in mind that the energy usage accounts for more than 95% of a motor’s cost!

Interested in learning more on how you can keep your motor operating properly?

Speak to an Expert

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Topics: AC Motors, Motors

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