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.
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.
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 […]
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 […]
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 […]
Mr. Harvey Ursaki is the Director of Programming and Operations for Electrical Design Management Software Ltd. He has over 37 years experience in the electrical industry.
Experienced in thermal and hydro generation, radial distribution substations, multi-breaker, ring bus transmission terminals. Along with many years in the oil and gas industry, he has a well-rounded knowledge of the electrical consulting industry.
Prior to forming EDM, Mr. Ursaki was Director of CLA Utility Services Ltd. an electrical consulting service, specializing in developing electrical engineering standards, serving clients in Canada, USA and in the Caribbean.
He also served as a Supervisor of Transmission Engineering for a privately- owned utility in southern British Columbia, Canada.
He now brings his years of experience to EDM company, developing an electronic standards toolbox for Engineers, Technologists and Electricians worldwide.