Controlling Motor Starting

Posted on Feb 5, 2014

Motor starters generally fall into two categories: full-voltage or reduced-voltage. The choice of which type of starter to use depends on various factors, such as current-carrying capacity of plant wiring, ability of plant power supplies to absorb power transients, size of the motor, and other special control requirements. The first type to be covered is the full-voltage starter.

Controlling Motor Starting
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Full-voltage starters, sometimes referred to as ``across-the-line`` starters, are the most widely used type of starter. A starter of this type, shown in ```Figure 1```, directly connects the motor leads and line leads together through either a manual starter or the main line contacts of a magnetic starter and does not provide any means of reducing the applied voltage or limiting the starting current. The full-voltage starter is used on almost all three-phase, squirrel-cage, and single-phase machines. The use of this type of starter for squirrel-cage motors is limited by the strain imposed on the installed wiring systems and by the starting current and torque when the motor is first energized. Whenever the starting of a motor at full voltage would cause serious and unacceptable voltage transients or excessive torque when unneeded, reduced-voltage starters are used to reduce the starting transients. There are, however, other reasons for using this type of control. The effect on the equipment must be taken into account in the selection of motor starters. When a large motor is started across the line, it puts a tremendous strain or shock on such things as gears, fan blades, pulleys, and couplings. When the load is heavy and hard to bring up to speed, reduced-voltage starting may be necessary. Belt drives on heavy loads are apt to have excessive slippage unless the torque is applied slowly and evenly until full speed is...

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