vfd on ups

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anthony001

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I have a 7.5 hp 3phase 208v motor that is needed to be fed from a ups. the motor is for a fan and needs a 90min run time. it also has a vfd. with the vfd doesn't it reduce the startup consumption from the flra and couldn't the ups then be sized to the setting on the vfd instead of the size of the motor? need some clarification on this one. thanks
 
Yes, it has been done, but you need to factor in the distortion power factor (not displacement) for running the VFD when sizing the UPS. I used a .8pf and it worked fine for me. So take the kW of that motor (7.5 / .746 = 10kW) divided by .8pf = 12.5 kVA on your UPS loading, along with whatever other loads you need to power for that length of time. I was powering a 10HP HVAC unit on a remote radar site and we had to keep it running for 2 hours to allow service people to get there with a portable generator. Worked great.

But here's a hint; if all the rest of your loads are 1 phase at that site, compare the price of using a 3 phase UPS and a 7.5HP VFD, to using a larger 1 phase UPS and a 15HP VFD. By larger I mean take your 12.5kVA and multiply by 1.732 = 21kVA, plus your other loads. The VFD will provide the 3 phase for the motor from that 1 phase supply and a 1 phase UPS is significantly less expensive than a 3 phase UPS. I found that doing it with the larger 1 phase UPS and larger VFD was still significantly less expensive. I think the reason was that 1 phase UPSs are priced for more "commercial" projects and therefore more competitively, whereas 3 phase UPSs are usually more industrial and the suppliers think they can get a better price on them. Can't hurt to check it out. Report back what you find if you don't mind, I'd like to know if it still works out that way.
 
Jraef said:
So take the kW of that motor (7.5 / .746 = 10kW) divided by .8pf = 12.5 kVA ...
Click to expand...

The formula HP/746 is for determing the OUTPUT kW of the motor, you also new to include the effciency. The input power of the motor is HP/(746 x PF x Eff).

Also, the UPS sees the input power requirement of the VFD not that of the motor so check with your VFD manufacturer.
 
Good points, although rarely is the distortion pf really at .8 for a fully loaded VFD, so I figured I had those issues covered in the fudge, and then UPSs only come in certain sizes which means you always end up with a smidgen more than you need.

But it was sloppy engineering I'll admit.
 
thanks this is what i thought. this will be th only load on the ups for a 90 min run time do you belive a 20 kva should be more than enough to handle startup.
 
VFD's can run three phase motors from single phase inputs but have to be derated by half. The input rectifier section is only using one side to energize the DC bus. So you have to upsize the VFD so the DC bus can handle the loads.

There are several posts here in the archives about doing this.

Hope this helps.
 
would there be a concern about the bypass on the vfd then unavoidably giving the full load to the ups and if it was in start up and the ups was sized to the vfd and not the flra of the motor than the ups woul go into bypass as well? and if so what can be done to eliminate this or what product is know for this situation?
 
I am not clear on your exact question, but using a bypass contactor will create the same situation as having the motor on an across the line contactor. The inrush of starting the motor may overload the UPS. There are interlocks available on the major VFD bypass assemblies will will prohibit bypass operation if a contact is open.

Hope this helps.
 
I think you are expressing concern about the fact that some UPS designs have a "bypass" feature that connects your load directly to the line source if necessary. That kind of feature will not be present on what is called a "double conversion" or "true on-line" UPS. Those types ALWAYS rectify the incoming AC to DC, then recreate the AC output from the DC bus, so there is no "bypass" feature.

But as to your concern, a VFD will be capable of accelerating most common variable torque loads, such as your fan, with no more than FLA of the motor; in other words no inrush. That is why I used it on my AC unit; in fact I never needed to vary the speed at all.
 
thank you far all the great input. I know the last post was unclear. what I meant to say was that my concern is, the vfd going into bypass mode. with having sized the ups at 20 kva should the system have to be sized to the motor flra because of the bypass on the vfd or is there a way for the vfd to not go into bypass so I can keep this 20 kva ups?
 
Bypass operation on VFDs is an option, typically used to allow equipment to keep running in the event of a failure or during routine maintenance. VFDs don't automatically go into Bypass mode (when supplied) unless someone commands them to. Simply put some logic in the Bypass mode selector that disables that capability if running on the UPS. I would do it by just having the entire Bypass contactor coil circuitry fed off of utility power only, tapped ahead of the UPS, so that if the utility power fails, there is no power to operate the Bypass contactor coil(s).

It's either that or buy a UPS with surge capacity to handle 600% of your motor FLA!
 
thank you for the clarification. I was being told that due to the fact that the vfd has a bypass it would allow the motor to give out its flra to the ups inevitably causing the ups to fail on start up. they are also requesting a 30 kva ups to run this motor. it seems very extreme to me with having the vfd in place.
 
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