# What size motor can I start with a 45 kVA transformer

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We'll never forget you, Tom! :)

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No matter -how- hard we try, huh?

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Hey Need some help. I have a 50 kva Transformer. Hpw do calculate how many of the given below machines I can run? Its a 4 head imoulder Powers engines

1) Lower head 7,5 kW - 8000 RPM 2) Right head 7,5 kW - 8000 RPM Power Lock system 3) Left head 7,5 kW - 8000 RPM Power Lock system 4) Overhead 7.5 kW - 8000 RPM 5) Lower head 5,5 kW - 8000 RPM

-Installed power of 52 kW Feed motor with 4 kW Motor height adjustment 0,55 kW

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What is the power factor ? that derates the transformer ability by drawing more than rated. Maybe the rated has the real value when running, not the ideal...

Are all going to be running at the same time ? Or is it one or two at a time ?

Mart> >> I understand that there is a bit of fudge factors involved, but I want

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Not true, really; the back-emf is being completely neglected here, and under light load, only some REAL power (a phase shift with respect to the 'idle' reactionary power) is drawn. The current at idle is very small, and out of phase. At light load, it's STILL very small, but closer to in phase. Light load doesn't increase the ohmic losses at all.

It takes substantial load, and nearly in-phase current and voltage, before the motor starts to stall (and draws high current and melts the wiring). At startup, the motor is aways 'near stall', which is why such motors don't turn on with a simple switch, but have a sequenced start process.

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This is an old thread, don't know where it popped up from. Well, the guy wanted to run a 55 HP motor on a 25 KVA transformer. Not clear what voltage we're talking about, but most likely 480 for a motor that large. Roughly, the rated line current for a typical 480 V 55 Hp motor would be about 90 A. At idle, such a motor would draw at least 50 A, although the power factor would be close to zero. So, the real power draw would be small, but the line current would still be pretty large, and close to 40 KVA, WAY above the rating of the transformer. Yes, for a quick test, the transformer would handle it, but you'd not get away with actually running the machine that way. Possibly, with a big bank of power factor correcting caps, it would be OK.

Jon

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