The wrong motor for the application; the wrong choice involving an operating problem; the wrong interpretation of applicable standards — all these can result from using the wrong words when describing motor characteristics in motor specifications, purchase orders, or trouble reports.
Sometimes another, more subtle, effect results from the use of inappropriate terminology. The hearer concludes that the speaker really doesn’t know much — about the world of electric motors — and that can lead to more trouble.
Here, then, are just a few of the commonly encountered terms used in describing motor usage, along with some of the alternatives that sometimes lead people astray:
Starting Torque. That seems simple enough — but there’s no standard definition for it. Some people consider this to mean the torque a motor develops at the instant it’s energized. To others, it means the average torque a motor develops while accelerating to full speed. And it’s sometimes been used to denote the average difference between motor torque and load torque during acceleration. The industry standard term is “locked-rotor torque,” meaning only the first of the three interpretations just mentioned. That value by itself is seldom useful in judging whether the motor will safely accelerate a given load.
Breakdown Torque. This is the maximum torque a motor can develop at any point on its speed-torque curve. If a connected load demands more torque than that, the motor will stall. Hence, this value is sometimes called either “stall torque” or “pullout torque.” But breakdown is the industry standard term.
Accelerating Torque. To some, this means the total torque exerted by the motor as it comes up to speed. Others interpret accelerating torque as the difference between motor and load torques during acceleration (that’s also one of the interpretations of “starting torque,” as we have already seen). Whenever you see this term used, ask which meaning is intended, because published standards do not govern.
Starting Current. This one can really be fuzzy. Again, it’s a nonstandard term lacking any accepted meaning. The standard term here is “locked-rotor current.” There’s an added complexity compared to the torque situation, because when a motor is first energized it draws a short-time transient current that is much higher than the locked-rotor value. The latter is a “steady-state” condition — a value that would be measured as a continuous current if the motor sat indefinitely with its rotor locked against rotation, and rated voltage applied to the stator winding. That initial transient, lasting usually no more than 1/50 of a second, can cause tripping of high-speed circuit breakers especially troublesome with “high efficiency” motors. The value can vary with the position of the rotor when the circuit is closed, and with how nearly together the three contacts close in a polyphase starter. The term “inrush,” often used interchangeably to denote both the initial transient current and locked-rotor value, properly applies to the former.
Richard L. Nailen, P. E