Motor "soft" starter

[kdeangelis]

Hi everyone: I am sizing a genset to back up a 20 hp (15 kW) 3 phase motor. I understand that the typical way to do this is to multiply the motor rating by 3.5 to find the power needed to start the motor. This results in my having to purchase a 60 kW genset.

I also understand that there are things called "soft starters" that can start the motor with a much lower power requirement than the above method would suggest. Can someone out there briefly explain what these are, where I would get them, and how to select an appropriate one?

Is this device also known as a multispeed magnetic starter? I am looking at page 13-63 in Square D's Digest #172.

Thanks much!
Kevin

 

[lengould]

No it's not likely the multispeed magnetic. It's an electronics package that (used to) wire in between the motor starter output and the motor, and electronically imposes an adjustable upper limit on the inrush current at starting. Provided your motor application is not a high torque start requirement, they can significantly reduce the startup requirements.

Also very common for electric motors to be significantly oversized for the load driven. A softstarter will usually provide a large help to starting a big motor on a week service. I've applied several of them on e.g. 100hp motors on small remote services for blowers and pumps.

If not a pump, see if you can find or calculate the starting torque of the load. Failing that, put a wrench on the driven shaft and measure breakaway torque of starting the load, then calculate to % motor run torque.

 

[rbulsara]

1. Use the free tech support of generator manufactures, they will be very happy to help you size the generator. They also have small free gen sizing programs.

2. For generator sizing you need to look out for two items, kW (real Power) and ability to provide starting inrush within acceptable voltage drop (reactive power).

For 20kW motor you need 20kW (plus efficiecnly losses of course) of power. But the generator kvA should atleast meet the kvA (roughly HP) of the motor. Select a standard size meeting both requirements. Always leave some margin of saftey or room for adding more loads in the future.

For the voltage dip during starting you need to know the generator X", X' and motor impedance and see for how much voltage drop you get. While on generator you can live with much greater voltage drop (up to 30%)during starting.

3. A soft starter or a reduced voltage starter always help as they limit the starting current, provided your starting torque requirements are met.

Search web sites of Siemens, GE, Square D and similar manufacturers for information on soft starters.

 

[kdeangelis]

Thank you! I appreciate your time and information.

I just got a copy of Cummins' Power Suite software, and the local Cummins rep has promised his assistance.

Once again - thanks!
Kevin

 

[rbulsara]

Also refer to FAQ237-766 that I have posted.

 

[kdeangelis]

Hey, thanks! I just printed your FAQ237-766. It'll help me determine inputs to the Cummins software.