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Reduced Voltage Starter vs. Solid State

The three types of soft starters most commonly used are: solid state, part winding and reduced voltage starter with an autotransformer. Learn about the comparision between a reduced voltage starter and a solid state starter.

Reduced Voltage Starter vs. Solid State

Reduced Voltage Starter

Reduced voltage starter systems very often will use an autotransformer to adjust the voltage during starting. The autotransformer will have 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. The taps are typically set to either 50%, 65%, or 80% of full-voltage, allowing for a starting current of 25%, 42% or 64%.

For example, a centrifugal compressor will be set at a 65% tap, allowing for 42% of full inrush current. If the autotransformer was set at the 50% tap, there would not be enough current going to the motor to allow it to run, while the 80% tap would allow for more current than was necessary.

Once the motor gets up to full voltage, the autotransformer will then be switched out of circuit, and the motor will be able to run across the line at full voltage.

A reduced voltage starter can come with solid-state motor protection relays, vacuum contactors and a chopper circuit.

Autotransformer starters are also sometimes known as Korndorfer starters.

Solid State Starter

A solid state soft starter contains integrated motor protection, with electrical components rather than mechanical components. Other types of starts, including those with autotransformers, need to have external protection from such problems as overload and locked rotor.

A solid state soft starter uses six thyristors that come out of the three lines from the starter control power to the motor. The thyristors can rapidly switch large currents at high voltages. Only a portion of power is supplied to the motor during start.

Solid state soft starter are able to be programmed to ramp up to a certain voltage in a set amount of time, giving them increased flexibility over other starts.They can use a number of connection types for for interacting with other devices, including ethernet, ModBus RS232.485, analog and discrete triggered input terminals.


A reduced voltage starter uses an autotransformer to lower the voltage and current supplied to the motor during startup. A solid state soft starter contains integrated motor protection, with electrical components rather than mechanical components.

Flexibility and Pricing

Since the motor protection is included in solid state starters, they are typically less costly overall than their counterparts.


Solid state soft starts also come with more flexibility in the amount of inrush current, as they can be programmed to get up to a certain current in a set amount of time. Since autotransformers use taps, they are limited in the the amount of current that can be supplied to the motor.

Autotransformer starters have a more limited duty cycle; if a motor needs more than 15 seconds of reduced voltage, the autotransformer starter will trip. A solid state starter can extend ramping time up to two minutes.

A reduced voltage starter also offers less flexibility in terms of starting voltage than solid state starters, cannot accommodate changing loads and does not provide a soft stop, which will slow down the stop time of the motor, instead of having it simply shut off.

The biggest difference between the two may be how they bring the motor up to current. Sold state starters have a smooth, consistent, ramp up. While an autotransformer starter can supply more current to the motor than other types of soft starters, while also concurrently keeping the voltage low, it does not have consistent acceleration due to the difference in torque from the start period to the transition period.

Instead of a consistent ramp up, the autotransformer gives the motor two consecutive torque bumps, creating mechanical stress which can cause damage to the motor.

Motor Protections and Maintenance

While solid state starts come with built in motor protection, and autotransformers do not, autotransformers will come with a surge capacitors to protect the motor from voltage spikes, something the solid state starter will not have.

Solid state soft starters provide longer performance with less maintenance since they do not have contacts that can burn out or armatures that can break.

Autotransformer starters, on the other hand, have 9 sets of contacts that can burn out.

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