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Summary:
"Motor Starting From 'Isolated' Power Sources" With
trilingual summary
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.
From "Motor
Starting From 'Isolated' Power Sources"
...by
Richard L. Nailen, EA Engineering Editor - published in
Electrical Apparatus
February 2005
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Barks Publications, Inc. All Rights Reserved.
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