March 2010 Electrical Apparatus

March 2010 Electrical Apparatus

This is a summary of the Electrical Apparatus March 2010 featured technical article,  by Richard L. Nailen, P.E.    

The speed-torque curve is the essential indicator of an a-c motor’s ability to accelerate its load. It’s a plot of available shaft torque at all values of RPM from standstill to the maximum running speed allowed by the connected load. Except for those two end points, the curve displays only transient conditions, because the motor will not remain in stable operation at any intermediate speed unless forced to by load counter-torque.

Motor voltage governs motor torque. High accelerating current causes a voltage drop in the supply system, resulting in torque reduction that can be severe enough to prevent acceleration, or to extend acceleration time enough to cause tripping of overcurrent devices.

Two misconceptions hamper accurate prediction of those conditions. One is that accelerating torque varies as voltage squared. Because of magnetic saturation, the exponent is typically nearer 2.3 than 2.0. Secondly, the assumption that current remains constant throughout acceleration is unduly pessimistic. Current actually decreases significantly by the time the motor reaches about 75% of full speed.

Field tests to verify a speed-torque curve are seldom possible because of instrumentation and power supply limitations. Although NEMA and other standards stipulate minimum torque values for locked-rotor, minimum, and breakdown (stalling) conditions, other points on the curve are neither defined nor subject to specific tolerances.

Users often calculate expected motor terminal voltage during starting based on the voltage drop in the supply circuit that would result from the flow of locked-rotor current to the motor. However, that assumption is always misleading because the motor current cannot be that high if voltage is reduced. Meaningful motor current and voltage values can be derived only by solving the entire series circuit, including the motor impedance, back to a source where voltage can be considered unaffected by the start.

NEMA standards require motors to be capable of starting at a minimum of 90% of rated voltage, provided that the load has the inertia and torque characteristics specified in those standards. Many industrial motor loads do not meet those criteria, however.

To order a back issue with the full article, “Understanding the speed-torque curve,” call 312-321-9440 or visit our online webstore.

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