Step Up Transformer for 3 PH. Motor Apps.

Looking for some rules of thumb for sizing transformers when stepping up voltage say from 230 to 460 to run a motor. Especially looking for efficiency information.

Thanks,

maybe this could help, although it doesn't tell you how to calculate efficiency.

http://ecmweb.com/mag/electric_guidelines_transformer_application/index.html

regards
Sherine T.

Looking for some rules of thumb for sizing transformers when stepping up voltage say from 230 to 460 to run a motor. Especially looking for efficiency information.

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The voltage doesn`t matter you can figure 1 kva per motor hp. Since 1 hp is equal .746 kw and if you take this at say a power factor of 75% which is probably about average for most plants with no PF correction that comes up to .99 kva. If your PF is better than this you will just end up with a little spare power. Efficiency information what do you mean by this?

Also confused

I also do not understand the question completely. The efficiency of a motor is not affected at all by the level of voltage applied. What does matter is the frequency at which the signal is alternating. Since the frequency will be the same no matter what the voltage potential is, it is a non factor.

That being said, sizing a step up transformer would then go back to the NEC and the equipment you will be supplying.

Perhaps I am missing something? If you would like help figuring out the size and ratings of things you need, this would be another question.

I'm assuming that you mean the efficiency of the transformer.

Transformers tend to be quite efficient and losses are due to copper and iron losses. Transformers will usually have an efficiency exceeding 90% and most transformer manufacturers will be able to quote an exact figure.

Cheers,

Lee

Personally I do not think transformers have an efficiency loss in current or voltage, they are designed to work at their optimal rating. Their are numerous other considerations that have to be dealt with though, this is an FAQ list for some of those issues:
http://www.sola-hevi-duty.com/support/index.htm#transformer

Transformers are designed to power motor loads. Although output voltage may momentarily drop when subjected to the motor's inrush current, the transformer will act somewhat like a soft start device. If your application calls for a motor to be powered from one transformer, the running load amperage of the motor should not exceed 2/3 of the transformer's nameplate amperage rating (66%). The reason is as voltage decreases due to motor inrush conditions, motor torque and horsepower also drop proportionally. If voltage were to drop to 80%, torque and horsepower would drop to 64% (80% squared). If torque were to drop to within 50% nameplate rating, the motor could overheat due to excess current draw. This condition could exist without tripping the overcurrent device and could result in failure of the motor or transformer.

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Another issue is heat, transformers will always generate heat and ambient temperature can affect their ratings.

Another one is the K-factor, this concerns harmonics when electronics, like VFDs etc, are involved.

This thread actually is related to my other thread about plotting startup current. We have a customer compaining aobut the need for larger step up transformers on their systems than they have required in the past (when someone else was making their power units).

They are an OEM and have all their machines wired for 460V 3 PH. When someone without 460 3 ph buys their machines they need a step up transformer. They say they are putting in larger transformers than they had to in the past. Note the motors are typically 20hp or 25hp.

What we have promised them is that we would investigate the situation by plotting the startup current for the motor we currently use. This motor is very low cost (due to the low price they demanded from us for the entire system) and I think when we put a better motor in the system that we will not see the same problem and my tests should help prove that. They are also experiencing some noise when starting up regular 480 installations.

Anyway, I was asking about transformer efficiency and wondering whether or not running a transformer in step-up vs. step-down would be a problem.

Thanks

rsdoran said:

Personally I do not think transformers have an efficiency loss in current or voltage
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Another issue is heat, transformers will always generate heat and ambient temperature can affect their ratings.

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Any time you have heat you have inefficiency. There is less power out of a transformer than there is into it. Although the losses are small they aren't negligible. The impedance of a typical transformer is 2% to 5%, but it can vary greatly.

The lower priced motors usually have more inrush. The KVA code lets calculate the inrush current. Check the motors KVA code and see how much is needed for starting.

KVA code "G" is 5.0 to 5.6 X rated KVA for inrush current. "L" code is 9 to 10 X rated KVA inrush.

The 1 KVA per HP is a good rule of thumb too.

This thread actually is related to my other thread about plotting startup current. We have a customer compaining aobut the need for larger step up transformers on their systems than they have required in the past

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Norman just because you have the right size transfomer doesn`t mean you will not see excessive voltage drop. Who ever hooks up the machine you build needs to wire it correctly. To make it easier for them you might send a drawing with wire sizes for certain distances.

Note the motors are typically 20hp or 25hp.

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Now if you built dry kilns where there when 10 of these starting at one time that might different, but motors of this size should not be a problem to start unless they are wired with to small of wire. You might also be sure to furnish transfomers with taps so they can set their voltage to 480 volts. If you ask for a 460 volt 3 ph service here in southeast ok. the power company sets their taps to where you will measure 509 volts phase to phase with nothing running in the plant this is API`s co policy.

This is getting wierd.

Normally a xfrmr will not be far from the supply so the wire will be what is needed for the rating of it. The wiring to devices after the xfrmr may need to be sized according to distance...that depends.

In general there is no difference between step-up or step-down, as long as the xfrmr is rated to match the load, within specifications.

The 1HP per Kva is not a hard set rule, you can fudge it a little but IMO, it is not something you want to do.

In this situation you stated the machines are 20-25HP; therefore the vendor may have thought it more practical to use one specific xfrmr, in this case it could have been up to 40KVa depending on the situation. Yes that may cost more BUT it may also eliminate issues with overcurrent AND having to stock multiple replacements.

There is always a reason, some good, some dumb, behind why someone does something. I would investigate the reasoning of the other vendor.