480 to 600vac boost transformer for machine run-off

agarb · Sep 20, 2012

We have 480vac 3 phase in our shop but our sales engineer just sold a machine going to Canada that requires 575vac. Problem is it has a 150 HP motor on it... what is the best (inexpensive) way to create a 600vac 3 phase supply to test and run-off this machine before shipment?

I was thinking a boost transformer, so I called Sola and they recommended the following arrangement (attached & below). Are there any gotchas I need to be aware of with an open delta?

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From Sola:
Two (2) units of HS5F25AS
-We don’t have a standard buck boost transformer that would satisfy the given requirements. However, you may consider using two of these single phase general purpose transformers for your application.
-This unit is a Single Phase Transformer rated 480 x 240V Primary to 120/240V Secondary at 25 KVA when used as an isolation transformer but if we use it as an autotransformer (Buck-Boost) we connect the primary and the secondary windings in series thus, changing the transformer’s electrical capability. Once the two transformers were connected using the Open Delta diagram below, they can be used for a three phase application with 480 Vac input and 600 Vac output at 216.5 KVA (208 Amps). Please note that this configuration does not have a neutral line or any grounded terminal.

jrwb4gbm · Sep 20, 2012

I see nothing wrong with this arrangement. I have done the same thing in the past. I have found that wiring up the middle phase (2) first reduces the confusion level a little. Disregard voltage readings to Ground. Be sure to use 600vac rated switches, disconnects, fuses, and and wire.

DickDV · Sep 20, 2012

If your testing doesn't involve fully loading the motor, you can get it to turn at light load up to nameplate speed with 480VAC. If you must load the motor, then you will need the transformer boost arrangement you described. The heavier the load on the motor, the higher the kva will need to be on the transformer.

At full load plus something extra for the rest of the controls on the machine, the transformer could get up as large as 200kva or more. That's not a small transformer. Now, since there are actually two transformers dividing the load, I'd have the transformer people size it for you. I'm not sure just how that sizing would be done.

jrwb4gbm · Sep 20, 2012

@DickDV and the OP,
It's kinda hard to expiain how this setup could yield such a large ampacity but here is my take on the setup.
The amperage capacity of the parallel 120 volt windings that is in series with phase 1 or phase 3 determines the total kva of this setup. 208 total amps thru the 120 volt windings on each transformer is equal to 25 kva for phase 1 or 3. Phase 2 is straight thru so no transformer is needed. If one uses an isolation transformer where all the current must flow thru the secondary windings, then the full 200 or so kva transformer could be required, just as you stated. It appears that Sola, (a transformer company), has sized the setup correctly at 25 kva to handle the running load. I'm not sure if it is enough to handle starting under load.

OkiePC · Sep 20, 2012

Since it is open delta, phase B is straight through, and the others are offset 120 degrees from phase B, the current through each secondary would only "see" 58% of full load or staring amps. So the secondary ampacity and duty cycle of peaks, I would think would have to determine the size of transformer. I am not 100%, but I think the physical mounting of the iron is important in these special applications. Definitely have them build it for you if feasible. You may also want them to give you multiple taps to make it even more universal for future use with foreign customers.

Also, check with the mfg/literature of any VFD being fed by this set up. Some drives have internal MOVs connected to ground that may need attention with an unbalanced voltage reference to ground.

EDIT: I think I am way off, nice info here:
http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm

DwSoFt · Sep 21, 2012

I'm surprised nobody mentioned this. If all u want to do is test the unit before it ships. Then why not rent a generator. Cat rentals up here has lots of 600v generators. I'm sure u would get one there. Just a suggestion. Now if u plan on selling more 600v equipment then maybe invest in a transformer arrangement.

leitmotif · Sep 21, 2012

DwSoFt said:
I'm surprised nobody mentioned this. If all u want to do is test the unit before it ships. Then why not rent a generator. Cat rentals up here has lots of 600v generators. I'm sure u would get one there. Just a suggestion. Now if u plan on selling more 600v equipment then maybe invest in a transformer arrangement.

I was also thinking this is a good option - especially so if a one time operation. You may also be able to rent a special transformer from utility or a contractor or Cat.

Dan Bentler

agarb · Sep 21, 2012

Thanks for all the good responses...

We actually have a 3 phase variac assembly that gets us 0-600vac at up to 70 amps that we use for most of our international projects. We also have a 380VAC step-down transformer for our Asian friends. But this is the first time we've ever needed something this large for an international customer.

Sola sized the 25kva transformers for me in the original post so I'm OK with that. But they are $2200 each and I need two of them.

Regarding a rental... I called around this morning looking for a large 600vac generator. Nobody had one that does more that 480. But, CAT said they could rent me a 480vac genset *with* a transformer! So then I inquired about renting just the transformer by itself; the 300kva transformer rents for $850 a month. Seems like this is the best way to go. Thanks to DwSoFt and leitmotif for mentioning CAT.

DwSoFt · Sep 21, 2012

agarb said:
Thanks for all the good responses...

We actually have a 3 phase variac assembly that gets us 0-600vac at up to 70 amps that we use for most of our international projects. We also have a 380VAC step-down transformer for our Asian friends. But this is the first time we've ever needed something this large for an international customer.

Sola sized the 25kva transformers for me in the original post so I'm OK with that. But they are $2200 each and I need two of them.

Regarding a rental... I called around this morning looking for a large 600vac generator. Nobody had one that does more that 480. But, CAT said they could rent me a 480vac genset *with* a transformer! So then I inquired about renting just the transformer by itself; the 300kva transformer rents for $850 a month. Seems like this is the best way to go. Thanks to DwSoFt and leitmotif for mentioning CAT.

No problem at all. Glad to help. And good luck

leitmotif · Sep 21, 2012

DwSoFt said:
No problem at all. Glad to help. And good luck

Me too - have fun and good luck

Dan Bentler

480 to 600vac boost transformer for machine run-off

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