Soft-start question

Is there a brand of soft-start that will allow you to limit the final output speed? I contacted Rockwell Automation and this is not possible with thier units, and up until now that is all I have experience with. I'm trying to avoid using a VFD (75 H.P.), because I really do not need to vary the speed continuously, just set it and leave it.

For any speed other than base (60Hz) you need a drive or a gearbox.

Another option may be to use a motor with a lower rpm..example: 900 or 1200 instead of 1800. It may be possible to have the existing motor rewound and relabeled for the slower speed.

I am not aware of a soft starter that will do a max speed set, in many cases once the soft start is done its bypassed and the motor is DOL.

Even though a soft start shares some components with a VFD, its purpose and design is totally different. A soft start is a solid state version of and replacement for the older types of reduced voltage starters. This includes autotransformer starters, part winding starters, etc. They are all intended to limit inrush current to a motor during starting, primarily to reduce voltage drops in the distribution system. This reduces dimming lights and so on. The common abbreveation of a soft start is RVSS, which stands for Reduced Voltage Solid-state Starter.

A soft start does not change the frequency of the motor current, but instead limits the current to the motor by switching it on and off rapidly.

As Ron said, with most soft starts when you get to normal operation a bypass contactor pulls in to connect the motor across the line and reduce heating and damage to the SCRs.

A soft-start is generally a triac device that phase angle fires the triacs. It cannot change the base frequency of the line. A VFD is required for that. However, there are some basic VFDs on the market that will probably do what you need for not a lot of money, especially with just a 7.5 hp motor. Most of Rockwells VFDs are top end units that are feature loaded.

I have used several Hitachi VFDs from AutomationDirect for applications where just one preset speed is needed - and they are very reasonably priced - I've had good results with them, and the price is good.

While it is possible in theory to use a soft starter in this way, is is only possible to do it for some types of loads. Where I have seen it done (OK, only saw the report from a successfully completed university project) was for an electrical drive for a remote controlled submarine. Because of the unique loading characteristics of the water, reducing the voltage on an ordinary SCIM would reduce its speed. This can also work with fans and some types of pumps. Many other types of loads would simply stall or overheat the motor.
If you only want a single speed, Rons suggestion of coosing the correct pole count, plus possible gearing or changing the belt pulleys could be the best way.

Ken Moore said:

...I'm trying to avoid using a VFD (75 H.P.)...

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Alaric said:

...especially with just a 7.5 hp motor...

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Decimal point location will definitely affect the price...



-Eric

After I posted this question yesterday, I did a little more research, and now feel a little foolish. I just assumed that soft-starts varied the freq. but in fact they limit the current.

The cost of the VFD is a factor, but main concern is space, just don't have a good spot to put a large drive.

The application is a roots type blower for a stripper column, at the moment we only need 1000 cfm, but we plan on expanding the system and will need 1500 cfm, some time in the future. I contacted the blower mfg, and we are going to purchase the 1500 cfm unit, and the vendor will change the pulley ratio to derate the unit. Whenever we need full capacity, the original pulley arrangement can be used.

thanks to all,

Ken

Eric Nelson said:

Decimal point location will definitely affect the price...



-Eric

Click to expand...

OOOooooooooooooops!
That would definitely make a difference.