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.
http://www1.eere.energy.gov/manufacturing/tech_deployment/pdfs/10097517.pdf
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. Read More
