Product Manual

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Autotransformer Starters
• An autotranformer is a single winding transformer on a laminiated
core with taps at various points on the winding. The taps are usually
expressed as a percentage of the total number of turns and thus
percentage of applied voltage output.
• Three autotransformers are connected in a wye configuration or
two in an open delta configuration, with taps selected to provide
adequate starting current.
• The motor is first energized at a reduced voltage by closing
contacts A.
• After a short time, the autotransformers are switched out of the
circuit by opening contacts A and closing contacts B, thus applying
full voltage to the motor.
• The autotransformers need not have high capacity as they are only
used for a very short period of time.
Three-Phase
Motor
Figure 4.4
Autotransformer Starter