UPS Rating and Capacity

Here is the calculation:

You didn't mention some important information such as UPS output voltage, load power factor, Inverter efficiency, allowable discharge voltage of batteries etc. Let's assume it:

UPS output voltage = 380 V

Power factor = 0.8

Inverter efficiency = 0.9

Allowable discharge voltage of batteries = 11 V (to obtain 40 AH from the batteries, the battery is allowed to discharge up to 11 V)

Based on your load, required UPS output current = 8500/(1.732 x 380 x 0.8) = 16.14 A

The minimum UPS size required = 8.5/0.8 = 10.6 KVA.

With 10% reserve capacity, you should select 12 KVA (or higher) size UPS.

12 KVA UPS output power = 12 x 0.8 = 9.6 KW

Your nominal DC voltage is 12 x 32 = 384 V. The minimum battery voltage = 11 x 32 = 352 V (when the load is on batteries and the batteries are discharged to the minimum allowable voltage).

The average discharge DC voltage = (384 + 352) / 2 = 368 V

The required input DC power to the inverter = UPS output power / Inverter Efficiency

(9.6 x 1000) / 0.9 = 10667 Watts

The Average Battery discharge current = 10667 / 368 = 29 A.

The required support time = 0.5 Hour

The Battery AH required = 29 x 0.5 = 14.5 AH

Considering 15% design factor and 25% aging factor, the actual battery AH required

= 14.5 x 1.15 x 1.25 = 20.84 AH

You have 40 AH batteries, so your battery capacity is enough to support even for 1 hour (based on the assumptions above).

For more detail, you can see the previous thread: http://cr4.globalspec.com/thread/38821#comment405933

-MS

Hi MS, Thank you for the information.

I will contact my supplier to get the missing information. We were totally lost when they tried to explain to us on the rating and capacity.

This forum rockz !!!!! Cheers

Hi you mentioned you have a 1kW freezer. The compressor during start up will be 8 times the nominal current.

1000/380/1.732 = 1.51 Amps. x8 will be 12Amps.So you need to cater 12Amps for your freezer.

Otherwise during every compressor cycling (turning on and off) the UPS will go into Bypass for a split second.

During power failure, the first cycling will shut down the UPS due to an overload(as bypass is not available).

Your calcs are correct but the OP has not expanded upon the base information.

Ventilation fan = 3.7Kw

AHU fan = 2.8Kw

These are output wattages. These fans are around 85% efficient at best. This equals 7.6 kW.

At 0.8 PF this equals 9.6 kVA.

Freezer =1kW

This is around 70% efficient at best. This equals 1.4 kW. But likely to have horrendous PF so at 0.6 PF this equals 2.4 kVA.

Other electrical appliances=1Kw

Lets take these at face value but at 0.8 PF this equals 1.25 kVA.

We now have a total of 10 kW & 13.25 kVA on the output. You can use these figures for your running.

So 10% reserve = 11 kW

90% eff = 12.2 kW

Note that UPS manufacturers typically use 0.8 PF for defining the output, so 12.2 kW = 15.3 kVA. Chloride, however, use 0.7 so for their machine 12.2 kW = 17.4 kVA.

The above can only be used for the run-time calculations. The start-up current of the motors will be 4-6 times their running current, but of course only for a few seconds. As mentioned in a later post, when bypass is avilable the UPS may flick to bypass to cope, but during mains failure bypass doesn't exist and the UPS will trip. Since this is obviously a high risk or emergency supply, you will need a much larger unit to cope.

Assuming our fans can't start together (which is unlikely to be correct but your call) the vent fan has a running current of about 8 amps (at 380V). Locked rotor current is going to be about 30-40amps. A 20 kVA UPS might cope, but you would be better with 30.

Note that the runtime of the UPS will be substantially unchanged; the larger capacity is purely to cope with motor start.

Hi Everybody,

Thank you for the detailed explanation.

We are speaking to our contractor to get the information. We are still waiting for the reply.

We were unaware that there are so many factors to consider when buying an UPS. Through some googling we noticed that there is this term "non-linear high crest factor loads" which was discussed earlier.

We are hoping that the UPS comes with this Crest factor capability (some circuitry)which allow the inverter to be compatible with loads exhibiting a high crest factor. We are not sure if this is a good idea. Can someone kindly advise?

Cheers

Hi Surfs,

My advice is to work out what you need and add half again. There is always things you never think of like lighting. Though I realise this is not a large item it really depends if you want to light just one corner of your factory or have everything working as normal? In which case and judging by what you need and the total number of cells. It may pay to get a Generator and perhaps a smaller version of an 'UPS'?

Just a thought.

Hi, generator might be a good idea. There might be a few seconds of down time which i am not sure how it will affect the airflow thru the facility. It is a safety requirement to keep the facility under -ve conditions at all times.

Just a thought.

If nobody is going to suffocate without ventilation during 30 or so minutes once a week, maybe provide with a modest UPS just the freezer (to save perisheables) and the "other" - needing its 1kW of power (lighting, working PCs, faxes, printers etc) ?

Hi, there are some safety requirements for the facility. It is some how unique and ventilation is essential. Some thing like a BSL3 idea ...... . Contamination must be avoided.

For now, we imposed it on the vendor to provide a backup supply and UPS is one of their suggestion.

Ah as I suspected and I will reinforce my post by saying you need a minimum of 30 kVA UPS.

One way to reduce motor start current is to use a VSD. However the UPS unfortunatley does not cope well with the harmonic currents generated and you end up back at square one, with a bigger UPS than you thought.

Now, a UPS converts the incoming AC to DC, then DC back to AC. The VSD works by converting the AC to DC, then the DC back to AC.....sound familiar?

What is needed is a UPS with built-in VSD. The VSD can be supplied direct from the DC bus. All problems get simultaneously solved. The batteries cope very well with short-cycle deep loads. Now why didn't I think of that?

Oh, I just did.