Electrical Apparatus magazine, February 2005 Motor Starting From 'Isolated' Power Sources

An important concern in starting any a-c motor, especially in the larger sizes, is voltage drop at the motor terminals caused by high starting current flowing through system impedances. In the so-called "captive transformer" scheme, the motor is directly connected to the transformer secondary, so that closing the primary circuit will simultaneously energize the transformer and starts the motor.

That has the advantage of reducing the voltage drop because of lower conductor impedance between transformer and motor, and also isolating the voltage drop from other parts of the electrical system.

Calculating the drop requires determining actual current and voltage relationships based on the impedances of both motor and transformer in series, rather than by assuming that the current will remain at its full-voltage value during acceleration. Equally important, the captive transformer must be capable of withstanding mechanical and thermal stresses caused by that current. Also, some machines (such as reciprocating compressors) impose cyclic torque peaks on the motor, which will appear as current peaks in the transformer windings.

The thermal basis for transformer sizing is calculated on a root-mean-square basis, involving the time and kVA loading for both starting and running conditions. Mechanical stress is accounted for based on empirical data concerning fatigue resulting from repetitive overloads. A transformer rating is chosen based on whichever condition is more severe.

When the power source is a generator, with the motor as its only significant load, selecting the proper generator size and performance is much more complex. Unlike the transformer, the generator is capable of adjusting its output to restore the voltage decrease during motor starting. The voltage regulation system is responsive to the high current. At the same time, the generator prime mover deals with the momentary power overload through its speed and load governing system. Proper generator selection for such an application calls for careful examination of the characteristics of all these components. Arbitrary choice of the generator kVA rating as some multiple of motor horsepower is not a wise design practice.

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Featured Technical Article
February 2005

On this page is a summary of the Electrical Apparatus February 2005 featured technical article, by Richard L. Nailen, P.E. , "Motor Starting From 'Isolated' Power Sources" Starten von Motoren aus 'isolierten' Energiequellen ... Démarrage des moteurs avec des sources d'alimentation 'isolées' ... Arranque de Motor por Fuentes de Energía 'Aisladas'

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