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Learn about the three common design types of softstarts:

Part-winding, reduced voltage, and solid state (also known as digital).

Softstart Types


In a part winding softstart, the motor will contain a winding that is split, so only a portion it is connected when the motor is started, usually one half but sometimes two thirds. The rest of the winding is added in with a relay or a timer when the motor has achieved a certain voltage. The softstart will contain two contactors.

The part winding motor will initially draw 65% of locked rotor current. When the rest of the winding is added into the system, the current will rise to full locked rotor current. This means that one of the contactors will be carrying 65% of the current during start-up. While the motor is running, each contactor will carrying half the voltage.

If load torque exceeds the torque provided by the part winding at a certain speed, then the motor will remain at that speed until the second part of the winding is connected. The motor will then develop full torque at the same speed. The slower that speed is, the bigger surge of current the motor will experience when the second part of the winding is introduced.

A part winding system does not provide internal motor protection, so it must be added separately.

Part Winding Softstart Motor

Reduced Voltage

Reduced voltage softstart systems often use autotransformers, which can be used 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 has taps, or preset voltage allowances, that limit the voltage to either 50%, 65%, or 80% of full-voltage.

Autotransformer starters can come with solid-state motor protection relays, vacuum contactors and a chopper circuit.

Autotransformer starters are relatively cheap and simple to operate, but they do not have consistent acceleration due to the difference in torque from the start period to the transition period. They also offer less flexibility in terms of starting voltage than solid state starters.

Reduced Voltage Softstart

Solid State (Digital)

Solid state softstarts have integrated motor protection, which other starts do not have, so they are the most commonly used type of start. They can include digital programming, as well as other control features.

Each line on the a solid state softstart uses two thyristors, switches that has can quickly switch large currents at high voltages, that are connected to the motor. The six thyristors that come out of the three lines from the starter control power to the motor, though only a portion of power is supplied to the motor during start.

A softstart can also be programmed to ramp up to a certain voltage in a certain amount of time. This gives them a greater amount of flexibility over autotransformer starts, which have taps that only allow for certain voltages.

In every system, a softstart is used to limit the inrush current, which improves the power supply stability, as well protecting against voltage drops.

Solid State Softstart

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