Contrary to popular opinion, bigger isn’t always better—especially when it comes to electric motors.

Plant maintenance and engineering departments like having a little extra power available “just in case,” so they sometimes specify larger motors than applications require.

 But oversized motors cost more to operate—sometimes a lot more.

Fortunately, there’s a simple procedure for determining the actual hp required by a load, without expensive equipment or engineering.

Bear in mind that loads should be determined when the motor is operating at its maximum load. Loads that vary widely are good candidates for variable-frequency drives (VFDs), which offer the added benefit of controlling rate of production.

During design stage of motor selection, calculating the maximum amperes of the motor running at full load is obtained by using the standard motor formulas. This is referred to as Running Load Amps (RLA). This is used to determine the starter size and breaker or fuse protection of the motor.

The running load amps are sometimes referred to as Full Load Amps (FLA). However, the full load amp designation is determined when the motor is tested after manufacturing is complete. Each motor will have a unique FLA rating depending on the manufacturer, materials etc. Once determined, this value is then added to the nameplate.

The designer’s goal is to bring the design stage of the motor calculations as close as possible to a similar manufactured FLA nameplate rating.

One way this achieved is by using a manufacturer’s motor performance data sheet.

From these data sheets the designer can match the power factor and efficiency of the listed motor with his, which is critical to arriving at the correct design values for the motor.

Also, each motor has a service factor designation, ranging from 1.15 to 1.25 depending on the manufacturer’s design.

This being the case, some national codes have added a multiplier of 1.25 to the running load amps to incorporate this added load. 

This value is the motor’s Full Load Current (FLC) and used in determining the cable size for the motor.

Also, manufacturers added a Service factor which indicates how much over the nameplate rating a motor can be driven without overheating. In other words, multiplying nameplate horsepower by the service factor tells how much the motor can be overloaded without overheating.

Note: A change in NEMA standards for service factors has been brought about because of better insulation. A service factor — once standard for all open motors — is no longer standard above 200 hp motors.

Selection of Electric Motors is an essential part of having a successful design ensuring a long motor life cycle as well as cost effective operation. This article has covered a just a small but important part of motor selection.