UPS Selection for 90kW Motor

Hello Guest, You would require UPS of 180 kw with softstarter and you can alter the starting current limit in the softstarter to max of 250%. If you are going for VSD then go for 120 kw UPS. What is your desired backup time.? Your UPS size with reference to batteries (number and capacity )will largely depend on that. Good luck !!

You may need a two stage UPS (but it depends on the necessary backup time). The first stage can be a flywheel UPD, the second one a Diesel or fuel cell.

Is there any scope for installing a huge flywheel on the pump to keep it running while the electronics changes over to UPS supply? If so, and the UPS is not required to start the pump, then it can be smaller than if it is required to start it.

Installing a huge flywheel on the pump by PWSlack..

Very good suggestion. Thanks ...But it is not possible..

Dear Mr. PWSlack,

I am of the view that providing a huge fly wheel, will result in a high Inertia and resulting in very high Gd^2 Value and starting will pose a problem. The rate of speed increase will be slow, and hence the motor will undergo a severe strain in terms of starting-current to come down to full-load current level.

Hence it may not be advisable to provide a huge fly wheel.

Thanks,

DHAYANANDHAN.S

For that size pump, have you considered using an emergency generator instead? Battery-powered UPS system for 90kW which will give you significant run time would cost much more than an emergency generator and transfer switch.

Your existing soft starter could be fed from the transfer switch, so you wouldn't need to buy another one.

I had the same thought... That would take one heckofa UPS to provide that much power. I won't even speculate on the design of such a critter... other than that the batteries to store that much energy might be found in batteries from a diesel-electric submarine (which the US Navy has not used in many years).

Oh by the way Bill...

Where there is a William, there is a way...

from another Bill

Emergency generator would be a good choice for that big motor. But the question is time between power failure and supply restoration by generator! The AMF panel controlled generator may take 60 to 90 sec. to deliver the power. What is critical with this pump? Flow or pressure? Or both? u have to think aternate measures to cater for these critical parameters during this period.

Emergency generator suggestions by D. Laxman Das & Bill.

Thanks for your comments..Yes we had thoght on this but the application is mission critical & we cannot stop the pump once the process is started. So we have to go for UPS.

We are thinking of going ahead with VFD (AC Drive) so as to ramp up speed in 30-50 seconds after staring of this motor. But the issue now is what should be the rating of the UPS?

Thanks again..Cheers

If you are happy with batteries feeding in over 30-50 sec, then either a direct drive diesel pump on standby or a standby generator to provide electric power to the existing AC pump could provide the changeover equally quickly. If you had a storage pressure-vessel on a non-return valve, then that could provide a tide-over of liquid - albeit at a lower pressure and volume - until the main/standby pump restarted.
If you are going to run the system on lead-acid batteries for any time where will they go? Can the structure take the loads? what about maintenance and battery checking? Might you be better using batteries to keep the pump spinning only while a generator spools up to take over the duty?

http://www.apc.com/

I use their desktop product. They have big enough unit that can keep CNC running.

Pineapple

So there are two issues you have but do not address in your query:

1) Does it only need to KEEP running if it is already running, or does it need to be able to START running if it happened to be off and the power fails?.

2) How long do you need for this pump to run on the UPS and is anything else on it (which only goes to the battery size actually)?

In general though, soft starters and VFDs tend to not like having electronic power supplies ahead of them. The output of a UPS is already a PWM signal to create a PSEUDO-sine wave output. The diode front-end of another VFD is going to take power in "gulps" at the peaks of the sine waves. But the UPS output, unless heavily filtered, is not a true sine wave, so the rectifier in the VFD may end up creating massive harmonic currents into the UPS output, which can severely limit the output capacity of the UPS as it attempts to supply both the real current and the distortion current in the harmonics. In the case of a soft starter, it will time the SCR firing based on the zero-cross of the incoming sine wave, and again, your UPS will have a PSEUDO-sine wave output, which often confuses the gate firing timing of soft starters.

A better choice all around may be to just determine the run time you need to accomplish and connect a battery bank to the DC bus of a VFD. No need to buy the AC-DC-AC portion of the UPS, just to have the VFD go AC-DC-AC again. Total waste of energy in all the losses in that scenario. Not all VFDs are easily capable of this, but most of the better ones are. The trick is to determine the DC bus voltage level you need and put your batteries in series to get there, then determine the Amp=Hours you need to run the pump for and put these series combinations in parallel to get that. A separate battery changer is often a good idea too, because you may want a blocking doide to prevent any regen energy coming off the motors to over charge your batteries and burn out cells. Electric vehicle systems of course take advantage of that regen, but they have a system in place in the DC bus circuit to control the regen energy rate to avoid that. Off-the-shelf VFDs will not have this.