Variable frequency drive, Soft Start

[Electrical guy]

Need to figure this one out, this morning I came across a 3PH 480V 40Hp air handling unit. The question is why would there be a Soft Start and a VFD hooked up to this unit? The unit runs at 50-60% capacity under normal conditions. The only thing I can see is that the soft start would kick in first then VFD would take over if the VFD was undersized. :-?

Thanks,

 

[winnie]

Connected in series? In parallel? With separate disconnects?

A soft start between VFD and motor would probably let the magic smoke out.

A VFD with a 'soft start' on the front end might make sense, if the two were somehow coordinated so that the soft start provided capacitor precharge.

The two connected with suitable bypass switches so that only one is used at a time might simply be used for redundancy.

-Jon

 

[Besoeker]

Need to figure this one out, this morning I came across a 3PH 480V 40Hp air handling unit. The question is why would there be a Soft Start and a VFD hooked up to this unit? The unit runs at 50-60% capacity under normal conditions. The only thing I can see is that the soft start would kick in first then VFD would take over if the VFD was undersized. :-?

Thanks,

Maybe the soft-start is there as a back up if the VFD fails.

 

[Jraef]

Maybe the soft-start is there as a back up if the VFD fails.

Yes, it's actually really common to use a soft starter as a service bypass in case the VFD fails. It's an issue when the line supply would have trouble starting the motor across-the-line if it had to be operated when the VFD is being serviced. This is especially important on things like lift station pumps where they must have an emergency backup generator to power the pumps if the utility service is out.

 

[daleuger]

Need to figure this one out, this morning I came across a 3PH 480V 40Hp air handling unit. The question is why would there be a Soft Start and a VFD hooked up to this unit? The unit runs at 50-60% capacity under normal conditions. The only thing I can see is that the soft start would kick in first then VFD would take over if the VFD was undersized. :-?

Thanks,

I have a somewhat similar problem right now same kind of units only they're short cycling. HVAC had a guy that did all the control wiring I just put the real voltage to it. He's not been to the job in a couple of days but should be tomorrow. I have had some higher priorities in the meantime anyway. My problem or his? Should be interesting to see how it goes.

 

[weressl]

Yes, it's actually really common to use a soft starter as a service bypass in case the VFD fails. It's an issue when the line supply would have trouble starting the motor across-the-line if it had to be operated when the VFD is being serviced. This is especially important on things like lift station pumps where they must have an emergency backup generator to power the pumps if the utility service is out.

...and the winner is:......

I would hate to see either the ASD or SSRV backfeed on the secondary side, so isolating contactors or a selector switch should be in place.