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Question about a Neutral in a 3 Phase Delta Circuit.

micro

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Jan 26, 2005
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I have a three phase compressor that's wired Delta. Four wires, three phases and ground (no neutral). The motor contactor energizes with switched120v. Can this be tapped off of one leg with ground instead of neutral? If it requires a true neutral does that have to be a tap on a wye transformer?
 
I agree with Jim, adding a control transformer is a good solution. Your delta supply may have a wild leg because of it being a center tapped delta system. If the controls got accidentally connected to that wild leg they would burn up in a hurry from over voltage.

A different solution would be to change the operating coil in your starter to a 230V coil, then you could connect it to wild leg without a problem, and not need a control transformer.
 
Probably you want to obtain a small control transformer - 240 volt primary (goes across two hot legs) and a 120 volt secondary. Fuse the primary and secondary appropriately and bond (ground) one end of the 120 volt winding.
And switch one of the hot legs through the pressure switch. Yes.
 
I have a three phase compressor that's wired Delta. Four wires, three phases and ground (no neutral). The motor contactor energizes with switched120v. Can this be tapped off of one leg with ground instead of neutral? If it requires a true neutral does that have to be a tap on a wye transformer?
Don't use ground. Use the transformer, or swap to 230V coil. That will always work safely, no "gotcha's", no matter what sort of 230V you hav available.

Delta will be "supplied from" a grounded service. The load may be delta or wye, and still can get power from a wye service, wild leg, or corner ground. (Wye loads do not usually connect the neutral unless supplied from "floating" Delta sources).

What 3 phase service will you have? An RPC makes wild leg 3 phase, farm service often is wild leg. If you have 208, that is always wye. And some have corner-grounded 240V service.
 
Then there's ever-popular 3 phase service where one leg is center-tapped to provide 120 volt service, AKA wild leg 240.
That's what I have in my shop. High leg delta from a solid state phase converter. It's causing me all kinds of problems. :)
 
Don't use ground. Use the transformer, or swap to 230V coil. That will always work safely, no "gotcha's", no matter what sort of 230V you hav available.

Delta will be "supplied from" a grounded service. The load may be delta or wye, and still can get power from a wye service, wild leg, or corner ground. (Wye loads do not usually connect the neutral unless supplied from "floating" Delta sources).

What 3 phase service will you have? An RPC makes wild leg 3 phase, farm service often is wild leg. If you have 208, that is always wye. And some have corner-grounded 240V service.
Thanks. I do have wild leg service. I think i understand purpose, and the differences between Delta, Star and corner tapped. I'm still trying to wrap my head around the practical application, and how to work with each of these. Specifically, when it comes to CNC equipment where the control circuitry needs a single 120V, and likewise a neutral. I think the transformer for one of my machines has a center tap neutral (star wye?) and so doesn't need either one from the panel. right now I'm just trying to get the compressor and manual machines on line and the I can look at the cnc's
 
Attach a photo of your motor starter and its part number and we may then be able to help finding the correct operating coil for 230V. Thats the best foolproof way to go.
 
If you do change coils, be sure the other parts are rated for 230VAC. Probably they are, but if it was originally 120V controls, they would not have to be rated that high.

Three is a 150VAC class, and a 300 VAC class, for wire insulation, etc.
 
This is my contactor. A friend of mine offered me one in the same size, different manufacturer, with a 240 volt coil. However, he thinks that two of the contactors are frozen to each other. How easy are these to repair?
 

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Your starter is currently configured for a single phase motor, by having only two heater elements installed. You need 3 heaters installed to properly protect a 3Φ motor. Your starter will accept a third unit, if you remove the factory installed jumper strap. The 240V operating coil that you need for that starter is a Sq D # 311041-400-51 (240V 60Hz) selection table attached.

You also need to check the sizes of your existing heater elements and compare their rating to the full load current rating (FLC) of the motor that you have. You may need a set of three to match the motor your configuring it to, since it was set up previously for a single phase motor.

Heater element selection tables are attached for your reference. The ones you have are highlighted in the table.

the starter you have shown in your picture is a quality one, if you need advice on a different type for repair or advice, you need to show us what you have, only then can we tell you if it is repairable or reconfigurable. Some types are not worth of repairing or re configuringSqD motor starter.jpg.
 

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  • schneider-electric-digest-175 Thermal Table 13.pdf
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Your starter is currently configured for a single phase motor, by having only two heater elements installed. You need 3 heaters installed to properly protect a 3Φ motor. Your starter will accept a third unit, if you remove the factory installed jumper strap. The 240V operating coil that you need for that starter is a Sq D # 311041-400-51 (240V 60Hz) selection table attached.

You also need to check the sizes of your existing heater elements and compare their rating to the full load current rating (FLC) of the motor that you have. You may need a set of three to match the motor your configuring it to, since it was set up previously for a single phase motor.

Heater element selection tables are attached for your reference. The ones you have are highlighted in the table.

the starter you have shown in your picture is a quality one, if you need advice on a different type for repair or advice, you need to show us what you have, only then can we tell you if it is repairable or reconfigurable. Some types are not worth of repairing or re configuring.
Thank you. Looking a the chart for the heaters, I realize the ones in this starter are probably too small. If I read this correctly, the B19.5 heaters are rated to 11A. This is a 5hp motor which believe can draw up to 17A at full load, so B36 heaters.
 








 
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