Reduced voltage starting systems have an autotransformer that allows for greater control over the voltage. Changing the voltage during starting, with preset voltage allowances, also known as taps, will also change the current. The taps are typically set to either 50%, 65%, or 80% of full-voltage, which will also reduce the current to 25%, 42% or 64% of rated current.
Motors can have either a normal or a heavy duty start, which may also affect the require soft starter size. Reduced voltage starting capacity is dependant on the type of load being driven by the motor.
Normal start has a load that has small or medium moment of inertia. A normal start will have a starting time of less than 5 seconds.
Heavy duty start has a load that has a high or very high moment of inertia. A heavy duty start will have a starting time of more than 5 seconds.
Motors with normal duty that can use reduced voltage starting include bow thrusters, compressors, elevators, centrifugal pumps, short conveyor belts and escalators.
Motors with heavy duty starts that commonly use reduced voltage starting are centrifugal fans, crushers, mixers, long conveyor belts, mills and stirrers.
Motors will be either a normal or heavy duty start.
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 starts include agriculture, manufacturing, water, waste management and mining.
Fans, or other systems with belt drives, can use reduced voltage starting to avoid belt slipping.
For fans that have a small or medium flywheel, they will have a normal start, while fans with a larger flywheel will have to have a heavy duty start.
Reduced voltage starting can help to avoid pressure surges that may occur in centrifugal pumps.
The soft starter is used to increase the voltage of the motor in order for it to accelerate the pump up to speed, without the motor experiencing any torque or current peaks. The soft starter lets a centrifugal pump start up at 400% rated motor current.
Centrifugal pumps will have a normal start.
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.
Smaller compressors, such as a piston compressor, will have torque that increases at the same rate as the speed. A screw compressors, on the other hand, has torque that will go up with the squared value of the speed.
Using reduced voltage starting eases stress on the compressor couplings and bearings.
A soft starter will allow the compressor to start with current that is 3 or 4 times the rated motor current.
Compressors that has a start time of less than 5 seconds will have a normal start; compressors with a start time higher than 5 seconds will need a heavy duty start.
To avoid jerk and stress on drive components, such as gearboxes and couplings, conveyor belt systems can use reduced voltage starting.
A motor soft starter will reduce starting current to 3 or 4 times rated motor current during start.
A conveyor belt with a short or light load will have a normal start, while belts with longer, and full, loads have to have a heavy duty soft starter.
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