No, not the stator, the winding, the rotor, or the frame. We don’t mean the shaft or bearings. Certainly all those components are essential to the motor’s operation. They make it work.

But keeping those items in good order, to maintain the motor’s proper function, is up to the user, the maintenance technician, and the repair shop. Our concern with the motor is more with these issues:

    1. Is it the right motor? How does it compare to others?
  • What do we know about this motor’s capabilities?
  • If we have a question about what this motor can do, how do we get the answer?


Such issues can be resolved only when we are familiar with, and possess complete data from, the motor’s single most important component — its nameplate.

No two manufacturers use exactly the same nameplate format. But NEMA standards require certain information on all a-c motor nameplates, no matter where or by whom they’re made; the basic requirements have hardly changed in decades. (Similar standards apply to generators, d-c motors, etc.) The major items any nameplate must display are:

    1. Manufacturer type and frame designation.
  • Horsepower output and time rating (e.g., “continuous”).
  • Phase, frequency, and full-load rpm
  • Voltage and full-load amperes.
  • Code letter indicating locked-rotor kVA (usually a letter from D through K).
  • Design letter (usually B, C, or D; unrelated to the code letter).
  • Service factor, if other than 1.0.
  • Maximum ambient temperature and insulation system designation.
  • Nominal full-load efficiency when applicable (required for 1-125 hp, single speed, low voltage, etc.)


Other data may include bearing identification, style and/or serial numbers, and reference to accessories such as thermal protectors.

What’s the importance of all this? Without knowing the serial number, for example, the manufacturer probably can’t identify the specific design and answer questions about it. Without the frame size designation, no dimensional questions can be answered. The rpm is a must when comparing two motors to drive the same load. Temperature rating defines suitability to different surroundings. The code letter permits prediction of motor starting condition. There’s a reason for everything shown.

Whenever you’re involved in an energy survey, in a possible motor replacement, or in a user’s concern about application suitability, read the nameplate. Make a clear record of everything it says before starting to ask questions. Many of those questions may answer themselves — if you have all the nameplate data.

Richard L. Nailen, P. E