Reduced voltage starter systems are soft starts that often use autotransformers to adjust the voltage during starting. Once the motor gets up to full speed, the autotransformer will then be switched out, and the motor will be able to run at full voltage.
The autotransformer in the reduced voltage start has preset voltage allowances also known as taps. Taps allow for control over the power output to the motor by determining the initial voltage and inrush current. Taps are typically set to either 50%, 65%, or 80% of full-voltage, which will also reduce the current.
For example, a centrifugal compressor will be set at a 65% tap, allowing for 42% of inrush current. If it were to be set at the 50% tap, there would not be enough current, and the 80% tap would allow for more than was necessary.
A reduced voltage start can come with solid-state motor protection relays, vacuum contactors and a chopper circuit.
It will also come with a surge capacitors to protect the compressor motor from voltage spikes. Surge capacitors are not used with solid state starters.
A big advantage to the autotransformer starter is it can supply more current to the motor than other types of soft starters, while keeping the voltage low at the same time. The transformer is used to step up the current.
A reduced voltage start does not have consistent acceleration because of the difference in torque from the start period to the transition period. A reduced voltage start also offers less flexibility in terms of starting voltage than solid state starters.
It cannot accommodate changing loads and does not provide a soft stop.
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