Making good motor decisions

Correct motor sizing is critical for maximum efficiency, as is making the right rewind-versus-replace determination.


The motor nameplate is the first step of a motor survey because it supplies valuable information, such as speed rating and full-load current, to help in determining the correct motor size.Correctly sizing an ac motor is important; overloaded motors can overheat and under-loaded motors waste energy. Because a motor’s energy usage accounts for more than 95% of its lifetime cost, achieving maximum energy efficiency is crucial.

But this doesn’t guarantee that the latest highest efficiency motor is the best solution for every application. While premium efficiency motors are important, it’s equally important to size the motor correctly. Otherwise, optimal energy efficiency won’t ever be realized. In addition, there are times when older efficiency motors can be rewound and actually improve their efficiency.

Sizing and output speed

The two most important factors when sizing any type of motor are torque and output speed. Finding the required output speed is relatively easy and can be determined by the design specifications.  Determining the correct torque is typically more problematic.

Many motors in use today are oversized as this is often a substitute for more precise up-front engineering. For example, if an application really requires slightly more than 5 hp at infrequent intervals, a 7.5 hp motor is often installed. In this situation, the 7.5 hp motor will definitely work, but it will be running well below full-load torque (further down the efficiency curve) and wasting a lot of energy.

In applications that only require the motor to operate above full load for short periods of time, a better solution may be to pick the right-size motor with a higher service factor. For example, if a motor has a 1.15 service factor, it can handle an additional 15% load occasionally without damaging the motor.

Conducting a motor survey

The best way to correctly size a replacement motor is by conducting a motor survey, which begins by reviewing and cataloging the nameplate information on the current motor to check rated speed, efficiency, full-load current, etc. 

Next, monitor the current the motor is drawing by using a clamp-on meter. In most systems, there are many unknown factors, such as friction and mechanical transmission efficiencies, which affect motor loading. Therefore, getting an actual measurement of the current going into the motor helps determine the true required motor size needed.

Determining the load requirement accurately is important because motors operate most efficiently near full load. Best efficiency is achieved above 70% of full-load torque. Below 60%, efficiencies start to drop off dramatically. There are several websites that provide information on how to determine motor load size, such as the U.S. Dept. of Energy. 

Rewind or replace?

If a motor fails before a motor survey can be performed, examining the age and type of the motor helps to determine if the motor should be repaired or replaced. If the failed motor isn’t an EPAct motor, the repair-versus-replace decision is easy, since the motor should be replaced in most cases.

If an EPAct motor fails, then rewinding should be considered. It used to be that rewinding a motor often meant losing efficiency, but that’s no longer the case. In most instances the original motor efficiency can be maintained. In some cases, the rewound motor can actually achieve increased efficiency. An efficiency discussion with your local motor repair shop can help determine the options. The considerations on whether to repair or replace also include the type of motor involved, how often the motor is running, and its efficiency. 

If the failed motor is a special or custom motor, additional factors determine whether to repair or replace (longer lead times for custom motors, higher costs, etc.). For many custom motors, rewinding is a more attractive proposition. For standard motors, replacement is often the better way to go.

If the motor is running constantly, the return on investment (ROI) for a new, premium-efficiency motor will happen faster. If the motor is run sporadically, then the cost calculations for replacing versus rewinding require more careful analysis. Once again, there are several websites, such as the Department of Energy site listed above, that can help with these calculations.  The Dept. of Energy also provides a free software package, MotorMaster+, which assists in creating a motor survey and helps with motor repair/replace decisions. 

When trying to cut costs by increasing energy efficiency, selecting the right-sized motor is as important as the energy efficiency of the new motor. Conducting a thorough motor survey is the best method for determining the right-size motor, as well as for making the correct replace-versus-rewind decision.

Joe Kimbrell is product manager of drives, motors, and motion for AutomationDirect.

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