Written by Harvey Ursaki, February 14, 2020
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
Selection of Electric Motors is an essential part of having a successful design ensuring a
Calculating the Voltage Drop in An Electrical Circuit is Critical to the Success of Any Electrical Design!
by Harvey Ursaki, February 26, 2020
Voltage drop is defined as the amount of voltage loss that occurs through all or part of a circuit due to impedance. Understanding voltage drop is the key to a successful circuit design. A common analogy used to explain voltage, current and voltage drop is a garden hose. Voltage is like the water pressure supplied […]
Bigger Isn’t Always Better When It Comes to Motor Design
by Harvey Ursaki, February 14, 2020
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 […]
DC Motors are Still Relevant in Today’s Modern Industries
by Harvey Ursaki, January 28, 2020
While some may claim that direct-current (DC) motors are no longer relevant, that is definitely not the case. DC motors and DC converters/drives are alive and well in industry, driven by many applications in which they are the best option. Alternating-current (AC) motors have certainly decreased DC motor sales, and they do have advantages in […]