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Soft start selection to overcome poor power??

Guest Anonymous

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Hello gentleman- nice forum you have here!

I am currently having some diffiiculty selecting a soft starter for my application.

A 20hp480v 3 phase pump motor that will be run constant speed,24-7.

The reason I am interested in a soft start is that I need to power this pump with a feed that is smaller than desired.

I have a 60 amp feed in the building that the pump is going in. While this is big enough to support the pump at running speed(27fla ) It is NOT big enough to handle the inrush that this motor will have!

If I use a soft start I figure I can get around the inrush current and not have to bump the building feeder.

Any thoughts or suggestions????

Thanks very much.

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The installation of a soft starter on a pump, will not reduce the running current, despite what you may be told by some overzealous salesmen promoting the Nasa energy saving system. The soft starter will however, reduce the starting current to the motor.

Typically, the start current to an induction motor is 6 - 9 times the rated current of the motor when started at full voltage. Reducing the start voltage, will reduce the start current, and in the case of a pump, from my experience, the start current will be reduced to between 3 and 4 times the current rating of the motor.

In the case of submersible pumps, the start current is commonly reduced to 2.5 - 3 times the current rating of the motor.


If this reduction is sufficient for your needs, then there is your answer.

If not, the other (more expensive alternative) is to use a variable speed controller to controll the pump. This will limit the start current to less than the rated current of the motor, and it will enable you to slow the pump down if full flow is not required. This in turn will reduce the pump losses and save you some energy.

The disadvantage of using the variable speed controller, is the high level of harmonics on the supply (caused by the drive) and also the potential for RFI from the drive. Filters can be used to reduce these problems and they may or may not be an issue depending on the installaton.


Best regards,

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Only the inrush. The panel is big enough to support the running amps, but not big enough for inrush current.

On a typical soft start the incoming current is rated at 200- 500 % curreent. Does this mean it starts at 200-500 percent below the rated current of the motor?;p;

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From experience, you will need about 300% current to start a pump.


Full voltage starting will draw 600% to 900% or the motor rated durrent, so the softstarter will approximately halve the starting current.


For example, if your motor was rated at say 10 amps run current, under a Full Voltage start, I would expect it to draw 60 - 90 amps during start.

With a soft starter, it would draw around 30 amps during start, and with a VSD, it would draw less than 10 amps during start.


Best regards,

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