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A solid state soft starter contains electrical components rather than mechanical components, while part winding starters only initially power up part of a winding. Learn more details below.

Part Winding Starting vs. Solid State Soft Starters

Part Winding

Part winding starting can only be used in a motor where the winding is split. A portion of the winding is connected when the motor is started, usually half, but it can also be two thirds. Split windings can be found in dual voltage motors, but the lower voltage winding will be connected first. For example, a 230/460 volt motor will have the 230 winding powered initially. The rest of the winding is added in, either with a relay or a timer, when the motor has achieved a certain speed.

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

The part winding starting motor will initially draw full voltage, but only 65% of full current. Once the rest of the winding is added into the system, it will reach full current. One of the contactors will be forced to carry 65% of the current during start-up. While the motor is running, each contactor will carrying 50% of the current.

If the load torque goes beyond the torque that is provided by the part winding starting at any speed, then the motor will be locked in at that speed until the second part of the winding is added in. Full torque will develop at that speed. A lower full torque speed will mean a larger surge of current when the second part of the winding is added.

Pumping is the most common application for part winding starting.

Part Winding Starting Motor

Solid State

A solid state soft start is made with electrical components rather than mechanical components. They also come with integrated motor protection, which other types of starters, including part winding starters, need to have externally added.

A solid state soft start uses six thyristors, two of which come out of every one of the three lines from the starter control power to the motor. The thyristors are able to rapidly switch large currents at high voltages. They only supply a portion of power to the motor during start.

Solid state soft starters contain programming that will allow for the motor to be ramped up to a certain voltage in a set amount of time. This gives them increased flexibility over other starts.Solid state starters 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 solid state soft starter will use a silicon controlled rectifier (SCR), a high speed switching device that will only switch on for a short amount of time, typically less than 60 seconds.

When the SCR is conducting, it is also known as gated. Moving the "gate" point further back will increase the voltage that will build up before the SCR turns on. The speed at which the gate point is backed up is known as the ramp time.

Ramp time should not be too long, as the increased current may exceed the safety limits of the motor, or may be cut off by a current limit setting.

The SCR will switch off once the motor is at full voltage.

SCRs are often used in devices where the high power and high voltage are necessary, but must be controlled. Applications include lamp dimming, regulators and motor control.

Part Winding Starting vs. Solid State Programming



While part winding starting systems are the least expensive starter, they also require motor that can be hard to find a replacement for and may be more expensive. Since solid state starts include motor protection, they are typically less expensive than their counterparts. which will require external protection.


A part winding starting has limited flexibility in the amount of initial current that can be supplied to the motor; they will start at between 60% and 70% of inrush current. Solid state soft starts come with much more flexibility in the amount of inrush current, as they can be programmed to ramp up to a certain current over a period of time.

A part winding starting system, which cannot be programmed, will typically take two or three seconds to fully ramp up, while a solid state starter can extend ramping time up to two minutes.

A part winding starting does not have a soft stop since deceleration is only available with solid state starters.

Part winding starting provides an uneven ramp up. Solid state starters, on the other hand, have a smooth, consistent, ramp up.

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