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Reduced Voltage Motor Starter vs. Part Winding

There are three types of soft starters that are most commonly used: solid state, part winding and reduced voltage motor starter with an autotransformer. A reduced voltage motor starter uses an autotransformer to lower the voltage and current supplied to the motor. A part winding starter involves a motor with a split winding, where only part of the coil is initially powered up.

Reduced Voltage Motor Starter vs. Part Winding

Reduced Voltage Motor Starter

Reduced voltage motor starter systems have an autotransformer that will adjust the voltage during starting and will have preset voltage allowances, also known as taps. Taps allow for greater 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, which will also reduce the current.

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

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

Autotransformer starters are also called Korndorfer starters.

Part Winding Starter

For a part winding start, the winding in the motor must be split, so only a portion of it is connected when the motor is started. Dual voltage motors typically have split windings, but they can only be started on the lower voltage. For example, a 230/460 volt motor will have the 230 winding powered initially.

Only those loads that have a low starting torque requirement, including low inertia fans and blowers, can be started using part-winding starters.

Usually one half of the winding will be powered initially, but sometimes it can be two thirds. The rest of the winding is added in with a relay or a timer when the motor has achieved a certain speed.

The part winding motor will initially draw full voltage, but only 65% of full current, and will rise to full current when the rest of the winding is added into the system. 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 given speed, then the motor will lock in at that speed until the second part of the winding is connected. The motor will then develop full torque at that speed. The lower that speed is, the larger the surge of current the motor will experience when the second part of the winding is introduced into the system.

Pumping is the most common application for part winding starting.



While part windings are the least expensive starter, they requires a special motor, which can be hard to find a replacement for and may be more expensive.


The taps of the autotransformer limits the amount of initial current that can be supplied to the motor. Part winding starts have even less flexibility: they will start at between 60% and 70% of inrush current.

A reduced voltage motor starter has a limited duty cycle; if a motor needs more than 15 seconds of reduced voltage, the autotransformer starter will trip. A part winding will typically take two or three seconds to fully ramp up. Neither the autotransformer or the part winding starts can have their ramp up programmed.

Like the autotransformer, a part winding start cannot provide soft stop, an essential feature in pumping applications. The deceleration feature is only available with softstarters.

Instead of a consistent ramp up, the autotransformer and the part winding have an uneven ramp up. A reduced voltage motor starter gives the motor two consecutive torque bumps. The inconsistent ramping can do damage to the motor.

Motor Protections and Maintenance

Neither a part winding or a reduced voltage motor starter will come with built in motor protection, but the autotransformer will have surge capacitors to protect the motor from voltage spikes. In some cases, the higher initial torque of a part winding starter may cause surges and equipment damage on start up.

Call us at 877.474.8209 to speak with an application engineer about your project.