November 2013 Electrical Apparatus

November 2013 Electrical Apparatus

This is a summary of the Electrical Apparatus November 2013 featured technical article, by Richard L. Nailen, P.E.   

Don’t be too sure of those torque values

Starting capability of an a-c motor depends upon three conditions: motor accelerating torque from standstill to full speed; torque required by the driven machine over that same speed range; and the inertia of motor and load combined. Failure of the motor to accelerate safely usually results from discrepancies in the speed-torque curves involved.

Two sources of error affect the motor characteristic. One is inaccurate prediction. Many large motors are of unique design to suit sometimes conflicting performance requirements, without the benefit of multiple tests on similar machines.

The second source of error is test inaccuracy. Standards are not entirely clear on torque measurement, particularly during an acceleration when motor temperatures and magnetic saturation are changing rapidly with speed. More important: motor specifiers and users frequently make unrealistic assumptions about motor terminal voltage during acceleration. Voltage drop in the supply circuit may be miscalculated or overlooked entirely. When it is recognized, a common practice is to consider motor torque to vary in proportion to voltage squared, whereas the actual exponent is typically in the range of 2.2 to 2.5 rather than 2.0.

Power supply limitations limit the voltage available for torque tests on most motors above a few hundred horsepower. For accurate results, tests must then be repeated for several values of reduced voltage. Plotting the results on logarithmic coordinates yields a straight line that can be extrapolated to full voltage.

For acceleration, a dynamometer is usable for recording torque versus RPM. However, dynamometers are seldom available above 1,500 kW. To obtain speed-torque readings without a coupled load, instrumentation can be provided to simultaneously measure increments of speed change and corresponding increments of time. Given the inertia involved, the torque is readily calculated. But specific accuracy limits are not given in most standards.

The inertia of the motor is either calculated or derived from a retardation test as described in IEC 60034-2. Again, accuracy is not stipulated. For the driven machine, the motor designer has no choice but to accept given load inertia and torque requirements. The latter have sometimes been found to be considerably in error.

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