Grounding the secondary of a step-up 3 phase transformer

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GearMan

Member
Location
WI
Thanks. For my education, practically speaking, if I am operating the primary windings at 240 volts input, as opposed to 208 that the primary is rated for, just how much trouble can happen, realistically?
I had to view your profile for that question.:happysad:
 
T

T.M.Haja Sahib

Guest
The OP may consult with the manufacturer of the subject transformer for making it suitable for receiving 208v supply.If additional tapping or rewinding is necessary,let them do it through authorized service personnel and produce certificates for compliance with standards.
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
You guys want to spend how much money on a junk transformer that this poor guy just uses for 5 minutes at a time (and probably at less than 50%capacity)????


Really, get it rewound????????//


Am I the only one who understands he uses this transformer for testing on a bench
 

ichudov

Member
Location
United States
You guys want to spend how much money on a junk transformer that this poor guy just uses for 5 minutes at a time (and probably at less than 50%capacity)????


Really, get it rewound????????//


Am I the only one who understands he uses this transformer for testing on a bench
I am using it sitting on a pallet, I move it to test location with a pallet jack.

Second, I want to say that I appreciate everyone's advice, it was an invaluable discussion and I am learning a lot about electrical safety and code issues.

To explain myself a little, I buy and sell surplus equipment. The current setup (transformer, input cable, and disconnect box) cost me $67 or so. I bought the transformer for $50 plus buyer's premium at a liquidation auction.

The intended use it to plug in 460v machines or motors that I sell, run them, maybe shoot a little video of a machine running, and not too much beyond. this. It is not intended for any kind of permanent installation. The machines would not be plumbed into metal air or water lines, etc. Most of the time, they would be sitting on a pallet.

i
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
Can you imagine how much money it would cost to get it rewound? I'll bet I could find 2 brand new transformers and buy them for less than a authorized service center would charge just to rewind this one old junk thing.

They used to say that,

"An engineer can do for a dollar, what any idiot can do for 10 bucks"


This guy asked for engineering help, not help reading a catolog
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
I am using it sitting on a pallet, I move it to test location with a pallet jack.

The intended use it to plug in 460v machines or motors that I sell, run them, maybe shoot a little video of a machine running, and not too much beyond. this. It is not intended for any kind of permanent installation. The machines would not be plumbed into metal air or water lines, etc. Most of the time, they would be sitting on a pallet.

i
As I said before, for single machine installations I would run it ungrounded by adding a simple ground indication scheme. Because you are connecting to some existing wiring, you could not be sure that your 'being tested' equipment does not already have a L-G connection in it.
 

ichudov

Member
Location
United States
As I said before, for single machine installations I would run it ungrounded by adding a simple ground indication scheme. Because you are connecting to some existing wiring, you could not be sure that your 'being tested' equipment does not already have a L-G connection in it.
Jim, thanks. Would you have an example of a particular ground indicating device, so that I would know what to look for?

Thanks
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
Jim, thanks. Would you have an example of a particular ground indicating device, so that I would know what to look for?

Thanks
It seems most indicators are 'home-built', but I know there are commercially available ones.
There are many schemes, some involving 240V light bulbs connected in series and others that use 480V transformer style pilot lights. A common scheme is to wire 480V lights into a wye configuration, then ground the center point of the light. With no ground faults the lights will each see only 277V, so they will glow dimly. If there is a ground the voltage across that light goes to 0V and the reaming light will see 480V each resulting in one dark lamp and two bright ones. This provides an easy visual check - if all three lights are not 'equal intensity' you need to do some troubleshooting.
 
Today, I set myself up with a three phase, 45 kVA, step up transformer, converting my 240v to 480v. My building is 240v three phase and I need 460 volts to test various 460v equipment.

This transformer really is a step down transformer, that I hooked up backwards. The "high side" is delta connected, and I do not see any way to change that.

Picture is here:

https://picasaweb.google.com/100231737176390367041/DropBox?authkey=Gv1sRgCIDl5a7JuZq35gE#5693510756662101026

The transformer seems to be working fine, however, I have a grounding question. Given that this is 460v delta, what, if anything, should I connect to ground? Would it be proper to ground one of the legs of the 460v three phase secondary, to ground?

Thanks

i
I would suggest to read the relevant sections of the Code to see if your installation meets the exceptions under which an ungrounded system is allowed. If the application does not meet the provisions of exceptions, ground one leg, again according to the requirements for system/supply grounding.
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
I would love to know if the NEC even applies to situations where we are testing on the bench. I have a magnetic motor starter with a remote momentary contact switch on a cord. There is a piece of SO cord coming out of it with 3 large alligator clips. Each clip is a phase. I also have a piece of welding cable connected to ground on the bench. It has a pair of needle nosed vice grips with a bolt welded to the side and connected to this ground cable.

I clamp the ground cable to the frame of whatever I am working on, and clip my phase wires to whatever I am testing. and use my remote to energize and de-energize for testing purposes.


Is there even NEC for my test bench? I do not see how code could even be written for such a testing and repair scenario. Every situation is different, and connections may be made for less than a minute before they are changed
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
I don't see where the nec would apply to equipment that is not installed or is cord and plug connected. The may be some osha rules on safety but I don't see the nec applying there.
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
I don't see where the nec would apply to equipment that is not installed or is cord and plug connected. The may be some osha rules on safety but I don't see the nec applying there.

Sure, there are OSHA rules, Like PPE, but I think the NEC's recommendations to the AHJ end somewhere around where my SO cord leaves the motor starter enclosure.


Well anyway, that is what this guy is doing, This transformer is a piece of his testing equipment. He is a repair shop for electrical equipment and machinery. None of this is intended to be, or will be permanent.
 
I would love to know if the NEC even applies to situations where we are testing on the bench. I have a magnetic motor starter with a remote momentary contact switch on a cord. There is a piece of SO cord coming out of it with 3 large alligator clips. Each clip is a phase. I also have a piece of welding cable connected to ground on the bench. It has a pair of needle nosed vice grips with a bolt welded to the side and connected to this ground cable.

I clamp the ground cable to the frame of whatever I am working on, and clip my phase wires to whatever I am testing. and use my remote to energize and de-energize for testing purposes.


Is there even NEC for my test bench? I do not see how code could even be written for such a testing and repair scenario. Every situation is different, and connections may be made for less than a minute before they are changed
You are not testing the transformer so I would say that you are well advised to follow the NEC up to the utilization point. But hey, it's not me who are going to fry, so go ahead knock yourself out, and I don't mean that figuratively:lol:
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
You are not testing the transformer so I would say that you are well advised to follow the NEC up to the utilization point. But hey, it's not me who are going to fry, so go ahead knock yourself out, and I don't mean that figuratively:lol:

How do you define "utilization point"?

And if what I do is wrong, can someone tell me a code compliant way to test electric motors on the bench?


How do you test motors on the bench?
 
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How do you define "utilization point"?

And if what I do is wrong, can someone tell me a code compliant way to test electric motors on the bench?


How do you test motors on the bench?
The motor, the cable would be exempt from NEC but the installation, equipment and method where it connects to the power would be subject to NEC. In other words if you use a plug to connect to the transformer, the plug would fall under NEC rule but you can use a non-listed cable or even individual conductors without a raceway if you choose to do so those would not be under NEC. However, if you are subject to OSHA, then OSHA would ask that also to be NEC compliant OR equivalent. If OSHA cites you, the burden is on you to prove that the method is equivalent.
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
The motor, the cable would be exempt from NEC but the installation, equipment and method where it connects to the power would be subject to NEC. In other words if you use a plug to connect to the transformer, the plug would fall under NEC rule but you can use a non-listed cable or even individual conductors without a raceway if you choose to do so those would not be under NEC. However, if you are subject to OSHA, then OSHA would ask that also to be NEC compliant OR equivalent. If OSHA cites you, the burden is on you to prove that the method is equivalent.

So what do you use for bench testing electrical equipment? Or what have you used in the past?

Lets say you rebuild a gear-head motor, and need to check for noise or leaks with a run against a brake dynamometer before you take it out in the field.
 

rcwilson

Senior Member
Location
Redmond, WA
So what do you use for bench testing electrical equipment? Or what have you used in the past?

Lets say you rebuild a gear-head motor, and need to check for noise or leaks with a run against a brake dynamometer before you take it out in the field.
I used a motor contactor fed from a fused switch properly sized to provide overcurrent and short circuit protection for the cable. It was fed from a grounded power source and a ground lead was connected to the ground bus in the shop. Suitable boots and insulating blankets protected the alligator clips from shorting out.

We still had the possiblity of frying the motor because it was not always possible to get the motor starter overloads to match the motor rating. The contacotr and fused switch were in securely mounted enclosures that were suitably grounded. Doors were always closed and latched before testing. (One test blew up the contactor when the pushbutton was pushed and released several times during motor inrush).

If I was an OSHA inspector I would question bare alligator clips at 480V.
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
If I was an OSHA inspector I would question bare alligator clips at 480V.

You don't know how many times I end up just bolting the motor leads to the ring terminals on the end of my SO wire and wrapping them with 88 because the alligator clips are clumsy, or they are damaged somehow lol.

I think I just have them there so people don't come in and say "I'd put alligator clips on that lead"
 
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