Supporting Motors on UPS

[EEIre]

Hi,

I have an application where we have of small number of "essential" motors (~10 motors, total load 15kVA) from a total load of 1200kVA.

As the load is so small I would prefer not to use a diesel back up generator. I was thinking of specifying a solid state UPS to support the load.

I was wondering does anyone have experience of this?

If so how did you deal with starting the motors? Size the UPS for the DOL starting current or install soft starters?

Any feedback would be most appreciated

 

[jraef]

Not really a problem, it is done all the time, but inrush current can be problematic. Soft starters will work on the output of a UPS but require some serious filtering, i.e. pure sine wave output. I have done it wil HVAC motors on the output of a Toshiba UPS and have had no problems, but that UPS was a pure sine output and very expensive (it was feeding other equipment that needed that as well). Also consider putting on small VFDs on each motor, even though you may not need variable speed. VFDs couldn't care less where they get power and essentially eliminate inrush current altogether.

Both of these solutions assume you have 3 phase motors. 1 phase motors present a whole new list of issues because of capacitors in some designs.

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[rbulsara]

I am not sure UPS is substitute for a generator. What is the purpose?

How long do you expect them to run on the UPS? Generators can proivde extended run time, from hours to days as long as you can replenish fuel. While a UPS will have very limited run time. If you exceed say 30 mins, you are talking about quite a set of batteries.

Also UPS are one shot deal, once batteries are discharged it takes hours to recharge. Plus the maintenance of batteries is a recurring cost. In fact you will end up relacing batteries every 3-5 years.