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Load on Transformer

05/14/2014 8:03 AM

We have 2 nos 375kW pumps having duty point at 290kW. Is it possible to run the pumps from a 1000kVA transformer. The starting of the pump is through soft starters

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#1

Re: Load on transformer

05/14/2014 8:39 AM

With soft start units a 100KVA transformer should be well on top of the job.

The person you should really ask is the designer. Unless it happens to be you.

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#2
In reply to #1

Re: Load on transformer

05/14/2014 8:53 AM

100 or 1000?

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#8
In reply to #1

Re: Load on transformer

05/14/2014 10:01 PM

Let see if these simple calculation can help you to determine your 1000Kva Transformer to support both of your pumps of 375KW. Assume the data's as follows:

a) Transformer capacity 1000KVA

b) Voltage at 415V

c) Motor Capacity 375KW

d) Starter - Soft Starter

Your total transformer output capacity will be = 1000Kva/415V*1.732 = 1,391 Ampere.(total 100% output capacity)

Pump 1 full load running current = 375KW/415V*1.732*0.85 (assuming PF) = 613 Amp

Assuming your pump running at 290KW which is about 77.34% of the total full load, therefore the pump 1 running current will be = 474 Ampere.

Pump 1 starting current with soft-starter is 2.5 time of the full load current = 613 x 2.5 = 1,532 Amps

From the calculation above it shown that your 1000Kva transformer cannot even start a single pump with a soft-starter. You either need a bigger capacity of the transformer or replace your soft starter to frequency inverter.

Even with the 10% extra KVA capacity that most of the transformer have, the total output capacity calculation of the transformer is at about 1,530 Ampere which is able to allow you start only 1 pump but not 2 pump. You will need all the luck you can get to ensure you don't damage the transformer.

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#12
In reply to #8

Re: Load on transformer

05/15/2014 10:01 AM

Shri Raj Nath,

Please do not get scared by this calculations.

I do not see any problem of starting and running the motors on 1 no. 1000kVA Transformer.

Up to first motor you are using well within capacity.

When you start the second motor with one running, you are exceeding current rating of transformer.

Your starting current as calculated by this Member is 1532A at 2.5 times rated current. First of all this current is momentarily and is not going to harm the transformer in any way as Transformer short time rating is much high (may be available in IS and IEC codes. For example, Short time rating of motor and generator is 150% for 30 Secs and 300% for 10 Secs. There shall be similar momentarily short time rating of Transformers).

Next effect can be due to voltage dip due to starting current. Considering 6% Pct impedance of Transformer under discussion, with Total current of (running + starting) = 474 +1532 = 2006A.

At 6% Zx, approximate momentary (for few seconds) dip in voltage shall be 6 x 2006 /1391 = 8.65% which is nominal. Motors can easily start at dip up to 20% to 22%.

Hence do not worry and go ahead.

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#13
In reply to #12

Re: Load on transformer

05/15/2014 10:51 AM

I agree that the OP should try and run these two motors off of this transformer.

I recommend that the two motors are not started simultaneously.

I do agree that Simon Wan has provided a scenario with the numbers and sparse information provided that exceeds a specification of this transformer. However, there are several plausible assumptions in those calculations. [Not all AC motor soft starter circuits draw 2.5 times the running current.]

Since this is a "temporary" installation [I have many 20 year old temporary designs still working] I recommend taking start-up and running current readings to see if this is a marginal or acceptable size transformer for these motors or to document why a new transformer must be installed.

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#11
In reply to #1

Re: Load on transformer

05/15/2014 6:14 AM

Like the last sentence! LOL

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#3

Re: Load on Transformer

05/14/2014 10:52 AM

Yes you may go ahead

I started 2 no. 160 KW motors at 400 KVA transformer using soft-Starters

But One thing you must do that keep capacitor bank on during motor running

also if possible keep p.f. above 0.96

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#4

Re: Load on Transformer

05/14/2014 10:59 AM

What are your doubts? Do you have additional load on the transformer when both pumps are started? If so what type and how much?

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#10
In reply to #4

Re: Load on Transformer

05/15/2014 4:17 AM

Dear Ram

there is no additional loads and the pumps are the only loads. My concern is about the high inrush current during the starting of the pump and its effect on the transformer. The pump may start at least 10 time an hour depending on the sewage flow(by the way the pumps are for sewage application). please note that no capacitor bank is installed as its a temporary arrangement for some time

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#5

Re: Load on Transformer

05/14/2014 12:06 PM

Can two 375kW pump motors be powered and run properly by a 1MW transformer?

Absolutely yes it can be done.

Can two 375kW pump motors fail to run properly when powered by a 1MW transformer and possibly cause catastrophic damage to either the electric motors, the transformer, the wiring, the building and even people standing too close?

Absolutely yes, this can be a disaster waiting to happen.

Which scenario will you assemble? I cannot tell from the information you've given us. Since you had to ask a public forum a simple power distribution problem I expect the latter scenario. Then again, you might get lucky.

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#6

Re: Load on Transformer

05/14/2014 1:09 PM

Even though duty point at 290 KWs, both pumps rating in KVA are more than 1000 KVA, therefore, running the two pumps from 1000 KVA transformer is not recommended.

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#7
In reply to #6

Re: Load on Transformer

05/14/2014 7:26 PM

OP said "We have 2 nos 375kW pumps having duty point at 290kW "

How so? 2 x 375 kw = 750 kw. Choose your PF.

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#9

Re: Load on Transformer

05/15/2014 2:06 AM

that transformer will be overloaded

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#14

Re: Load on Transformer

05/15/2014 11:36 AM

Couple of things to consider. Until the bypass contactor kicks in there will be additional losses in the soft starter. (look for the heat sink)

You give very little information about the transformer. Is is ONAN or would it be possible to install ONAF and effectively increase the rating by 20 or 30%.

I'm not totally convinced you will have the capacity to successfully start the second pump. Its a close call and I think you will saturate the transformer.

It might be possible if you can take the mechanical load of pump 1 by throttling back on the valves so it is cosuming much less power. This could release the transformer capacity you need to start pump 2. Once you have spinning loads you can run them.

I wouldn't like to buy this transformer off you when you have finished with it though.

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#15

Re: Load on Transformer

05/16/2014 12:18 AM

If the number of starts per hour were less (say 3-5), the transformer should be able to handle them. Ten starts per hour is getting up there; a VFD on one or both pumps, or a throttling means, could keep the pump(s) running longer between starts.

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#16

Re: Load on Transformer

05/20/2014 11:49 AM

1000 kVA transformer with efficiency of min. 99% is assumed. We cannot consider duy point as it may shift. Total load is 750 kW. You can definiely run witth soft starter.

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Users who posted comments:

Candlez (1); Codemaster (1); DmanEng (1); navdeep_goyal (1); powersolutionsFBD (1); PWSlack (1); rajnathr (1); RAMConsult (1); redfred (2); silverfox (1); Simon Wan (1); TonyS (1); Tornado (1); VIKRAKTS (1); wareagle (1)

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