Motor Nameplates: Reading Between the Lines

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MOTOR NAMEPLATES: Reading Between the Lines Reading a motor nameplate can pose a real challenge. With all of those acronyms and industry terms, it seems to have a language all its own. However, that nameplate contains information that can really come in handy when you're making repairs, sizing a variable frequency drive or upgrading to a new unit. So, spend a few minutes learning a new vocabulary. It'll help you save money, improve motor performance, and it may even make you sound smarter at your next dinner party. Click or tap any blue indicator below to read more:
The electrical input required by the motor. Units are generally rated at less than grid voltage (460 instead of 480 volts) because a voltage drop is expected between the grid and the motor terminal. Multiple voltages may be specified, but motors will operate at less than optimal when using nonstandard minimum voltages.
Full-load speed (RPM)
The full-load speed is the motor's actual speed at rated horsepower output. It is also called the motor's slip speed. For example, for an 1,800 rpm synchronous speed, an induction motor might have a full-load speed of 1,748 rpm.
Locked rotor (LR) amps
The current drawn from the power line when the motor is energized (full voltage) but not rotating (zero speed). This occurs at start-up and whenever the motor is jammed. If this causes tripping or brownouts, a reduced voltage starter may be required. Compare this to the amps rating to determine the potential surge severity.
The current drawn from the power line when the motor is operating normally; at rated voltage, speed and horsepower output. Conditions like unbalanced phases or loose terminal connections can cause amperage to be much higher than nameplate.
Service factor (SF)
Some motors operate well at greater than rated output for a limited time, and this metric indicates the maximum percentage increase. For example, an SF of 1.15 is equivalent to an allowed continuous 15% increase. While the motor will operate at this increased output, it can degrade life expectancy.
Insulation class
The motor's insulation type expressed as a letter designation such as A, B or F. The farther from A, the higher the allowable temperature rise above ambient. Class F insulation extends the motor life expectancy at a given temperature or allows the motor to be operated at a higher temperature.
Full-load efficiency (F.L. EFF)
The electrical efficiency of the motor (output divided by input) at full load.

So now you're ready to talk like an industry pro, although we were only kidding about discussing motors at a dinner party. That might send your listeners into lockout mode really quickly.