Grounding the secondary of a step-up 3 phase transformer

[rcwilson]

I could lie and say that I never had an alligator clip fall off and do minor damage and blow fuses. (That's why I am no longer hands on.)

A quick bolt and tape job is good.

 

[ichudov]

While I test a fair amount of stuff, I never, ever, use alligator clips.

If the machine in question has a long enough cable, I just put it into the screw terminals of a fused disconnect.

if not, I use an extension cable, and use wire nuts and insulation tape. I connect two cables together with electrical tape, and then put wire nuts on the pairs that I want to connect.

 

[Joethemechanic]

I could lie and say that I never had an alligator clip fall off and do minor damage and blow fuses. (That's why I am no longer hands on.)

A quick bolt and tape job is good.

I'll tell you the best alligator clips are the ones I make out of vise grips with a 3/8" bolt welded to the side. They are hard to insulate properly though.


I always think that some day I will finish up that engineering degree and never have to touch anything hands on again. But then again, I was VP of a small engineering, fabrication, and construction company for a few years, although I took a lot of heat for being in work clothes and being out on a jobsite or in the shop. :slaphead:

 

[templdl]

The bolts, are they part of the current carrying path and are the steel?

 

[Joethemechanic]

Yes they carry current, yes they are steel, as are the vice grips. It's kinda hard to weld anything to a pair of steel vice grips, unless it is also steel. Although I would imagine you could braze a copper bolt to the side of a pair of steel vice grips, but I can't imagine why you would want to.

Why do you ask?


BTW, they make excellent welding ground clamps, and a really first class pair of jumper cables.

 

[Joethemechanic]

BTW, the head of a 3/8 bolt is about .27 square inches in cross sectional area. The current travels for about 1/4" through the .27 square inch portion of the bolt head.

All figures above are ignoring the added cross sectional area created by the weld fillet.

If you are going to try to tell me this is going to overheat and melt off due to I^2R heating, people in china are going to hear me scream.

BTW, the weld fillet probably takes my 9/16 regular hexagon out to be a 3/4" circle with a cross sectional area of around .44 square inches

Now, what is the resistance in ohms of a piece of carbon steel, with a cross sectional area of .44 inches and a length of 1/4 inch?

 

[ActionDave]

How do you test motors on the bench?

For single phase motors it is a pair of alligator clips wired in series with a 100W light bulb on a switch.

With the switch down the bulb is in and lights if there is a complete circuit. With the switch up the bulb is bypassed and 120V full current is given to the motor.

Three phase motors are either burnt or not, so a good looking over is first. The three phase power on our bench is hooked up to a transformer that will step up the voltage up from 120V to 240V. An unloaded motor will start on lower voltage so 240V is the most we ever need.

 

[Joethemechanic]

For single phase motors it is a pair of alligator clips wired in series with a 100W light bulb on a switch.

With the switch down the bulb is in and lights if there is a complete circuit. With the switch up the bulb is bypassed and 120V full current is given to the motor.

Three phase motors are either burnt or not, so a good looking over is first. The three phase power on our bench is hooked up to a transformer that will step up the voltage up from 120V to 240V. An unloaded motor will start on lower voltage so 240V is the most we ever need.

Yeah I do a lot of those test runs wired for 480, but tested with 240. Good if you are just checking for noise and leaks. Sometimes what I'm test running might have a pump or something where I have to test head pressure or flow so I got to have the 480. I transform up to get the 480 too.

Really you never know what you are going to have to test, so you need to be able to think on your feet.

 

[Joethemechanic]

Now, what is the resistance in ohms of a piece of carbon steel, with a cross sectional area of .44 inches and a length of 1/4 inch?

That is about 560 kcmil, steel conductor approximately one quarter of an inch long,,,,.

 

[templdl]

Because of the steel's magnetic properties the current flowing threw the bolt as well as the steel around it will get hot because of induction. These are some examples of where the magnetic field around a current carrying conductor can cause trouble:
This is why you never run a single phase conductor through a steel conduit, which you never run a single phase conductor through a hole by itself in a steel enclosure. All conductors must be run with one another to cancel out the magnetic flux or a nonmetallic raceway or a nonmetallic ?gland plate? must be used in an enclosure,
The magnetic field which surrounds the conductor will induce current to flow if the steel the surrounds it, that current flow will cause the steel to get hot. And with bolted connections it is preferable to use a nonmagnetic fastener such as stainless steel.

If you don?t seem to have an issue with heating then it works for you then it does not pose to be a problem for you. But using steel as a current carrying path is normally not a good electrical practice. I always like to think that it is important to know what kind of trouble you can get into before you do, that is to understand the ramifications. I things work, fine, if they don?t then you may have an understanding why they don?t.

 

[Joethemechanic]

Because of the steel's magnetic properties the current flowing threw the bolt as well as the steel around it will get hot because of induction.

Who said I was working with AC current?

And just about every electrical device made has steel as some part of the current path.

For that matter I can induce a current in any conductor and produce heat. Have you ever taken apart a "mechanical" speedometer. It is really an electrical device. There are permanent magnets in there, inside a little aluminum drum. The relative motion sets up a little induced current in the aluminum drum. The induced currents create a small magnetic field in the little drum and the thing acts like a little induction motor with a ton of slip. Increases the relative motion, the induced current in the little drum increases, the field increases, the torque increases, and it overcomes the spring and the needle moves.

We have magnetically induced current in things all the time. Somebody puts an electric motor together in the field and bangs the laminations a little bit and they short together, increase in eddy-currents and heat.

Why is it you think my vice grips are an engineering no-no, what are alligator clips made out of?

And I can tell you, after 40 some years of breaking my fingers in the field, I can tell you how they fail. The guys either don't clamp them tight, or they clamp them on something dirty The resistance from the loose/dirty connection heats them up to where the jaw loses it's strength. and they loosen up more, eventually the jaw loosens up enough for an arc to form and then,,,,,,,,,,,,,

But I can tell you this, they last about a million times longer than anything you can buy off the shelf, and that is just from empirical knowledge


Here is a little thing to try, Take an old squirrel cage motor, yank the rotor out of it, make up a jig to hold a piece of conductive material of your choice in the center of the armature, energize the armature, and watch whatever you are holding in the center get hot.

Don't burn your hands

That is the kind of stuff I did as a child, my mom said I had no interest in toys. Just books and things my grandfather and his friends gave me to take apart to see how it worked.

And you know how those old boys were, they let me smash fingers, bust knuckles, get minor shocks, and burns. Thank God they did.


These are clamps for emergency power hook ups, and for welder's ground clamps. NASA is not involved

 

[weressl]

These are clamps for emergency power hook ups, and for welder's ground clamps. NASA is not involved

..and your emergency is?:lol:

 

[Joethemechanic]

..and your emergency is?:lol:

Usually something to do with me getting to dinner.


But seriously, most often to do with some pump or generator. I was pretty heavily into waste-water, storm-water a few years ago.

 

[Open Neutral]

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.

All well and good, but I would also be concerned with arc flash type disasters on 480V. Do you have a say 0.375" polycarbonate shield between you & the UOT? How far back are you, 3 ft or 15? There is NO substitute for setback distance....Are you further than the suddenly loose lead can fly??

I second the "lamp in series" approach as I have used such for current limits as well. Better stock up cuz those new CF's will not do the job. Not so sure re: 3ph but consider a bank in the transformer primary.

 

[ActionDave]

...I second the "lamp in series" approach as I have used such for current limits as well. Better stock up cuz those new CF's will not do the job...

Traffic bulbs may be excluded from the ban. That is what we use since they hold up well.

If not I have an operating hypothesis that since Medical Marijuana makes pot easier to get in a lot of states the smugglers will switch to sneaking in incandescent lamps from Mexico.

 

[Joethemechanic]

Yes I always wear proper PPE when testing electric motors on the bench,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,





But I'm also a big time lier








Although 35 or so years of getting paid to work on machinery, and another 10 or so as an unpaid apprentice, and I am still here.

The best safety equipment is between your ears, all the arc flash suits in the world can not compete with knowing what you are doing

BTW my favorite piece of safety equipment is distance. And that includes getting away from people who do dumb and careless things.

 

[iwire]

Usually something to do with me getting to dinner.


But seriously, most often to do with some pump or generator. I was pretty heavily into waste-water, storm-water a few years ago.

But seriously this thread is not about you, it is about the OP setting up a safe and compliant test bench.

 

[Joethemechanic]

The OP and I seem to see things pretty much eye to eye. It seems that we are in a very similar business and have discussed the matter "off forum"

To tell you the truth, he seems like a pretty smart guy, and I would say his electrical engineering skills rival most on this forum.

 

[Besoeker]

Here's a picture of the transformer data plate.

http://yabe.chudov.com/tmp/Transformer-Data-Plate.jpg

In post No. 1, you say your supply voltage.
The transformer name plate shows 208V on the low voltage side.
If you supply that with 240V you will overvolt it by about 15%.
The transformer might not like that.
Or a floating star point if, as suggested above, your 240V supply is delta.

Message to self:
Read the whole thread....:roll:

 

[templdl]

Joethemechanic,
Interesting, now you are saying it's not AC. Does that transformer you bought have AC in and DC out now?

The plot thickens.