Motor Rotation Through a Transformer

I have a 208 3 phase mcc that is fed by a 480x208 stepdown transformer and several lighting and general use panels are fed from the main that powers the primary side of this transformer

I think the phases on the primary side may have been reversed by contractors we had working on it but i have no phase rotation/ indication meter of any kind so i cant tell.

If the phases were reversed on the primary side of the transformer will it change the motor rotation on the secondary side of the tranformer.

I can't just tryit because things will break if some of these motors run in reverse and it will be a big job to de couple them all. There is one 480 motor running backwards that is fed from the same main as the transformers primary side that is how i know that they got it backwards. Will this change rotation on my secondary side motors?

Grab a brand new motor. Wire it correctly observing phase markings. When viewed from the back the motor will turn clockwise if your phase rotation is correct.

... and can you not just start up one motor on the machine to check rotation?

Yes. Reversing the phasing of the primary side of the transformer will reverse the phasing of the secondary of the transformer.

phase angle

Yes, if you change the primary phase connections, you will also change the whole secondary side. A phase rotation meter is cheap < $500 maybe time to buy one. lol...

Bruce99 said:

Yes, if you change the primary phase connections, you will also change the whole secondary side. A phase rotation meter is cheap < $500 maybe time to buy one. lol...

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Oh wow, I thought this was like the flux capacitor. I had never seen one until I did a quick Google after reading this.

Why not just check the rotation of the motor? Even if you had a phase rotation meter it seems like it would be a smart safety check

Actually, they could have reversed the phase rotation on the transformer primary and also reversed it on the transformer secondary resulting in the motor running forward.

I'd take a small three phase 480V motor, connect it to the primary feed to determine rotation, then disconnect and reconnect on the transformer 208V side. You don't even need to change the motor connections: simply leave them on the 480V configuration and see what direction the motor spins. The voltage will be way low but high enough to make the motor spin with no load.

The Plc Kid said:

I have a 208 3 phase mcc that is fed by a 480x208 stepdown transformer and several lighting and general use panels are fed from the main that powers the primary side of this transformer

I think the phases on the primary side may have been reversed by contractors we had working on it but i have no phase rotation/ indication meter of any kind so i cant tell.

If the phases were reversed on the primary side of the transformer will it change the motor rotation on the secondary side of the tranformer.

I can't just tryit because things will break if some of these motors run in reverse and it will be a big job to de couple them all. There is one 480 motor running backwards that is fed from the same main as the transformers primary side that is how i know that they got it backwards. Will this change rotation on my secondary side motors?

Click to expand...

Oh boy this is gonna be fun. Observed this problem at Boeing - and they tried real hard to keep phasing the same because some equipment moved from plant to plant. they still had difficulties.

The first thing I would do Kid is get into the Grainger catalog and find a phase rotation / motor tester. I think mine is Ideal and cost 100 to 150. You can check proper rotation and phasing of panels and motor rotation also. Right handy rig.

Assuming your 480 is all Brown Orange Yellow then in each and every input on panel the left terminal would be B center O and right Y. If this is not how the contractor did it I would immediately stop payment and get him back to fix his out of phase. Phasing for the breakers from top to bottom on either side should be A top B next C next then repeat.

All panels should have identical phasing and thus proper rotation otherwise you are in for pulling hair out everytime you connect motors.

Yes if he got phase rotation incorrect on the transformer hi side he can fix it on secondary. If the secondary loads have correct rotation and teh primary side loads do not then this is what contractor did.

You can also do as Dick says and use a small 3 phase motor label wires ABC with the proper rotation in a panel you know is right. Connect motor wires ABC to phase terminals ie breaker ABC in panel if it runs opposite panel phasing is fouled up.

Dan Bentler

I know this doesn't help for this time, but one of these (or similar) built into the cabinet can eliminate this worry. Run the power for the control circuitry through this, or use it as a permissive in a PLC control scheme. When a panel is moved, you can power it up and if the machine is connected wrong, it doesn't move. We use something similar (not quite as fancy) on most of our equipment, though mostly for the undervoltage protection.

Brian