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#61805 - 02/02/06 09:39 PM single/three phase confusion  
Merlin  Offline
Member
Joined: Jan 2004
Posts: 170
NW Indiana
I am a little confused on a situation that I ran across the other day. The garage where I work has a "240 volt three phase" service. It measures L-G 120,208,120 and L-L 240. Is this considered a 208Y120?.

The issue is that I have a 115/230 vac single phase chop saw that is "custom" wired. It has a 2 conductor cord on it, plugged into this 3 phase 4 wire service. They have one conductor to the ground pin and the other to the 208 pin. I don't have a wiring diagram for this motor so I'm not sure exactly how this is. If I remember correctly, it has 4 wires in the motor. The green wire in the motor is connected to the ground pin on the cord. One wire is connected to the 208 pin. And two others are tied together.

Can anyone help me with this confusion? How is this working? I know its not right and I want to fix it correctly.

Thanks, Merlin


Work Gear for Electricians and the Trades

#61806 - 02/02/06 09:47 PM Re: single/three phase confusion  
electricman2  Offline
Member
Joined: Nov 2003
Posts: 269
Thomasville, NC USA
The service you describe is a 3 phase, 240Vdelta "high leg". So named because the "B" phase is 208V to ground or neutral. I am unsure about the motor wiring, but if it is 115/230 single phase it can be operated connected to any two phases.


John

#61807 - 02/02/06 09:56 PM Re: single/three phase confusion  
Dnkldorf  Offline
Member
Joined: Dec 2004
Posts: 1,064
nowhere usa
Another issue when dealing with this type service is the use of full voltage breakers.

You can not use the standard 240/120 breakers when the high leg is used to power single phase 240v loads, or the 3phase loads.

Dnk..


#61808 - 02/02/06 09:59 PM Re: single/three phase confusion  
RobbieD  Offline
Member
Joined: Feb 2003
Posts: 231
Canada
Check out the diagram for the
CENTER-TAP GROUNDED DELTA
http://www.bmillerengineering.com/elecsys.htm


#61809 - 02/02/06 10:38 PM Re: single/three phase confusion  
frenchelectrican  Offline
Member
Joined: Feb 2003
Posts: 939
Wi/ Paris France { France for ...
Merlin:

the motor connection is sometime very tricky there and depend on which voltage you got it on it can be either 120 or 240 volts you have to check the motor name plate for correct connections

a quick warning do not hook 120 volt motor[s] on 208 circuit on delta system it will blow it up.

retest the plug for the motor where it will be plug in and make sure you stay off the wild leg unless the motor is for 3 phase then it will be ok


Merci , Marc


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)


#61810 - 02/02/06 11:27 PM Re: single/three phase confusion  
Radar  Offline
Member
Joined: Apr 2004
Posts: 349
Los Angeles, CA
In a high leg delta system, the B phase bus bar in a panel is supposed to be identified somehow, like orange or red in color, to indicate that something is different, and not to install a 1-Pole circuit breaker on phase B.

I'll try posting a graphic from the link Robbie gives.

[Linked Image]

Radar

Edited for spelling (duh)

[This message has been edited by Radar (edited 02-02-2006).]


There are 10 types of people. Those who know binary, and those who don't.

#61811 - 02/02/06 11:58 PM Re: single/three phase confusion  
Merlin  Offline
Member
Joined: Jan 2004
Posts: 170
NW Indiana
Thanks for the great responses. This is what I love about this site, the timely and knowledgeable responses.

Can anyone explain how this is currently working from the "wild" and ground legs only?

Thanks again, Merlin


#61812 - 02/03/06 12:29 AM Re: single/three phase confusion  
Radar  Offline
Member
Joined: Apr 2004
Posts: 349
Los Angeles, CA
In the sketch above, you see the lower winding, the A to C one, which is center tapped with the center point grounded. The A to C winding is 240V overall, so from the center point to either A or C is 120V, exactly half.

However, you also see there is no direct connection from the center point up to B, and the voltage from the center point up to B will be around 208V, an unfortunate coincidence. And, as luck would have it, 208 is probably close enough to operate the 230V chop saw.

The right way to do this is to run 230V from any 2 points A-B, B-C, or C-A, or to convert the saw to 120V and run it from either 120V leg (A or C) to neutral.

Radar


There are 10 types of people. Those who know binary, and those who don't.

#61813 - 02/03/06 12:31 AM Re: single/three phase confusion  
gfretwell  Online Content


Member
Joined: Jul 2004
Posts: 9,058
Estero,Fl,usa
It sounds like they have the motor tapped for 230-240 (taps with "2 wires connected together" usually means 2 windings in series).
It is seeing 208 and running somewhat OK.

We never liked to see line to neutral loads on an "open vee" service. It tends to make the line to line around the phases shift a bit.


Greg Fretwell

#61814 - 02/03/06 08:04 AM Re: single/three phase confusion  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Quote
The green wire in the motor is connected to the ground pin on the cord. One wire is connected to the 208 pin. And two others are tied together.


What type of plug and receptacle are in use? Is this actually connected to a grounding (U-shaped) prong on the plug?

It would work that way, since the ground is bonded to the neutral (the mid-point of the A-C winding in the diagram above), but actual GROUNDING pins should not be used for current-carrying conductors.

It does sound as though it might be a motor designed for 120 or 240V operation (by altering the windings to parallel/series) which is being undserrun a little on 208V.


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