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10-10-2012 10:40 AM

Variable frequency drive VS soft starter


My company purchased ABB variable frequency drive to run a 30HP motor, I set the acceleration time 15 second and it is working very smoothly; and a soft starter with no adjustable frequency, can I use it for a high torque application just like the VFD? It looks like that VFD is by far the best VS soft starter.
10-10-2012 07:43 PM
Top #2
10-10-2012 07:43 PM
Yes, a soft starter can be used to start a high inertia load provided that the motor is capable of starting that load, but a soft starter does not directly control the acceleration of the motor, like the variable frequency drive.
A solid state soft starter is a reduced voltage starter and does not alter the frequency of the voltage applied to the motor. During start, the acceleration field is at line frequency and the torque is a function of the full voltage starting torque curve of the motor and the current reduction caused by the soft starter. The torque is reduced by the current reduction squared.

For a high torque start, the start current must be high, I have started many motors with a starting current as high as 450 - 500% of rated current.

The minimum start current is determined by the minimum start torque of the load and the motor current and torque/speed curves.

The best way to start a high inertia load, is to set the starter up for a current limit only. There is nothing gained from a current ramp on a high inertia load.

The acceleration time is determined by the start torque (controlled by the start current) and the load inertia.

Try a start with the current limit set at 400% and see how long it takes to start. You can shorten the start time by increasing the start current and lengthen the start time by reducing the start current.

During start, there is high slip power loss in the rotor. The total rotor loss is basically equal to the full speed kinetic energy of the load. It is important that the rotor ha sufficient thermal mass to start the load inertia.

With the soft starter, set parameter 1 to equal the rated current of the motor, parameter 2 to 400%, parameter 3 to 400% and parameter 4 to 1. If the acceleration is too slow, increase 2 to 450% and try again.

We have started many high inertia rock crushers and chippers using soft starters with start times as long as 45 seconds.
10-10-2012 10:15 PM
Top #3
10-10-2012 10:15 PM
I agree totally with that, high quality soft starters are designed to start high torque applications which require no speed manipulation. For this application a soft starter which has a current limit capability is the correct product to use – although we would need to investigate the settings closely to ensure that the motor accelerates efficiently.

Although a variable frequency drive is a very good product they are more expensive than soft starters and have greater loses at full speed. With fixed speed applications there is no need to have a product which has extra features you might never use.

Finally it is not always necessarily to bypass a soft starter at top of ramp, if your application is running at less than 50% load then using an energy saving motor controller may give you the added benefit of reducing your running costs and allow you to achieve the technical outcome you require.

If you would like any more support with this please feel free to message me.

Best regards

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Top #4
11-19-2012 12:21 AM
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