# Step up transformer size



## sonnyvxdl (Dec 5, 2011)

What KVA transformer should I use in a step up application from 240 3ph to 460 3ph, with motors that total 47 FLA with a 1.0 SF? It has a 20hp, 15 hp, 2hp and a 1/2hp motor on the circuit. Since the two larger motors will be started several times per hour, the KVA may need to be upped a little. I haven't done this before, so thought I'd see iff anyone with more experience could guide me on this. Thanks


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## Peerless Design (Dec 3, 2011)

If these are common squirrel cage motors, they add up to 43.63 kVA based on horse power per the NEC. You need to add 25% of the largest motor. This brings the total to 49.24 kVA. You will need a 75 kVA transformer (the next size up from a 45 kVA).

You need to be careful sizing a transformer for just a few motors. The transformer may not be able to handle the starting load even if you add 25% of the largest motor. The momentary starting current will be equal to the locked rotor current. This should not be a problem with 75 kVA.


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## wildleg (Apr 12, 2009)

I thought most transformers were built to handle at least 150% of their rating ?


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## Peerless Design (Dec 3, 2011)

wildleg said:


> I thought most transformers were built to handle at least 150% of their rating ?


The locked rotor current of most common squirrel cage motors is about 600% of the full load current.


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## brian john (Mar 11, 2007)

Peerless Design said:


> The locked rotor current of most common squirrel cage motors is about 600% of the full load current.


There are tables that show the maximum motor starting loads a transformer can handle, no time to look now, but I am sure someone will post them.


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## sonnyvxdl (Dec 5, 2011)

Actually, only the two larger motors would start, at first start up. This is a two head sander. The other motors work screw jacks, for changing the deck height.


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## sonnyvxdl (Dec 5, 2011)

Also found out the 20hp and 15 hp have six leads. A separate starting circuit is in each motor. Each circuit is fed from a different contactor. The run contactor has the overload block. I haven't seen any like this before. So I would think starting current would not be as high as a 3 lead motor.


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## varmit (Apr 19, 2009)

This sounds like a wye-delta starter configuration. This will limit the start inrush to about half of across the line starting. I would normally size a transformer for 200% of multiple motor loads. If sizing a transformer for a single motor load, you may need to go 300%, depending on the starting characteristics. If the transformer is the power limiter, the motors will not last long.


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## micromind (Aug 11, 2007)

sonnyvxdl said:


> Also found out the 20hp and 15 hp have six leads. A separate starting circuit is in each motor. Each circuit is fed from a different contactor. The run contactor has the overload block. I haven't seen any like this before. So I would think starting current would not be as high as a 3 lead motor.


How many contactors for each motor. 

If there are 3, then I agree with Varmit; they're wye-syart, delta-run. 

This type of starter will limit starting current to about 3X running current. 

Do both motors start at the same time?


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## sonnyvxdl (Dec 5, 2011)

There are 2 contactors each, for the 2 larger motors. The starting contactor has no overload block while the run contactor does. This is a used piece of equipment with no schematic. Not sure about the starting sequence. I have contacted the company and hope to get a wiring schematic.


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## frenchelectrican (Mar 15, 2007)

There are two possiblty is one is Wye Delta starting system this normally need 3 contractors otherwise if you have only two contractors then you may have two speed motour.

The only way you can confirm this answer is look at the motour nameplate that is the only surefire way to confirm this.

Merci,
Marc


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## sonnyvxdl (Dec 5, 2011)

Inside the cover of each motor are six studs; one set is for the start, the other for the run circuit. This sander was bought used by a cabinet maker I know. He was wanting to get it running with the least cost and has only 240 3ph in his shop. I thought finding a used tranformer would be the cheapest route, without having to increase the size of the motor starters for 240V (which is possible since the motors are dual voltage); this would be quite costly for the type and and number of starters involved. Plus there is limited space in the cabinet to mount them. Also the original starters are sized for the 460V amp load.


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