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Learn how to size a motor soft starter and about common applications they are used for.

Motor Soft Starter Sizing

How Do You Size a Motor Soft Starter?

A motor's power requirements are measured in current, voltage, kilowatts and horsepower.

The inrush current, which is the current required for startup, of the motor may also affect the required soft starter size. Motors can have either a normal or a heavy duty start. The starting capacity of the motor is dependant on the type of load being driven by the motor.

A motor with a normal start has a load that will have a small or medium moment of inertia. A normal starting motor will have a starting time of less than 5 seconds.

A motor with a heavy duty start has a load that will have a high moment of inertia. A heavy duty starting motor will have a starting time of more than 5 seconds.

The more powerful the motor, the larger the soft starter that will be required to power it.

Motor Soft Starter for Mining

Common Applications For a Motor Soft Starter

Some common applications for normal duty motors include bow thrusters, compressors, elevators, centrifugal pumps, short conveyor belts and escalators.

Motors with heavy duty starts include centrifugal fans, crushers, mixers, long conveyor belts, mills and stirrers.

Some applications will have both a normal and heavy duty start mode.

There are very few motors that cannot use a soft start, and the manufacturer will most likely note this. Some motors may be designed for full torque at 0 RPM, and these would not be able to use a soft start.

Some common industries that use soft starters include agriculture, manufacturing, water, waste management and mining.

Motor Soft Starter for Mining

Centrifugal Fans

Fans or other systems with belt drives can be started slowly to avoid belt slipping. They will start with reduced load torque and will often start with a closed damper, which leads to a short start.

Fans that have a small or medium flywheel, will have a normal start. Larger flywheels will require a heavy duty start.

Centrifugal Pumps

In pump applications, the use of a soft starter can prevent pressure surges. This may eliminate the negative effects of water hammer on your equipment.

The motor soft starter gradually increases the voltage applied to the motor so that it can accelerate the pump up to speed without experiencing any torque or current peaks.

Centrifugal pumps only have normal start capacity and a motor soft starter will allow a centrifugal pump to start at 4 times rated motor current.


Different types of compressors have different ways of starting.

For example, in piston compressors, which are generally small, torque increases at the same rate as the speed. In screw compressors, torque will increase at a rate equal to the squared value of the speed.

Using a motor soft starter reduces stress on couplings and bearings in the compressor.

A compressor started with a motor soft starter will pull an inrush current that is 3 or 4 times the full load current that the motor is rated for. Compare this to an across the line start, which will require up to 6 or 7 times the rated full load current.

Compressors with a start time that is less than 5 seconds will need a normal start; a start time longer than 5 seconds will require a heavy duty start.

Conveyor Belt

Conveyor belt systems can be smoothly started using a motor soft starter, minimizing jerking and lowering stress on drive components.

A motor soft starter leads to less stress on gearboxes and couplings and will reduce starting current to 3 or 4 times the full load current rating.

A short or light loaded conveyor belt will require a normal start. Longer and fully loaded conveyor belts need a heavy duty soft starter.

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