Grounded Transformer

[speer007]

I had a Air Handler that has reznor gas duct heaters for heat. Everything is 220 volts and 24 volts. I measured to ground at the gas valve at the reznor unit i had 115 volts on one terminal and 115 volts on the other terminal on a 24 volt gas valve. I went back to the AHU and checked across the control transformer i measured 24 volts, i checked the same terminals on the transformer to ground and had 115 volts, A journeyman told me to ground the transformer and see if that fixed it, it did. Can someone explain why this happens, and what this is, so i can understand it.

[rtruett1]

I am interested in what you read between the two wires, try taking a measurement and see what the reading is. You have a potential there as the 24 volt transformer is supplied from a 110 volt source, and if one side of the 24 volts is not grounded you have what is called a floating neutral. Also if using a Fluke meter make sure the reading you are getting to ground isn't Millivolts as this is a common mistake when using that paticular piece of equipment.

[speer007]

The tranformer is supplied with 230 volts and 24 volts leaves. From the hot to the neutral i read 24 volts, but with one lead to ground and the other to hot i read 115 volts. I made sure it wasnt millivolts, before i called the journeyman. Can u explain the floating neutral?

[jc47]

If neither of the leads on the secondary side of the transformer are grounded, the voltage to ground on either lead can be anything, since there is nothing 'forcing' the voltage to be any particular value with respect to ground. The only thing that is guaranteed is that there will be 24VAC across the two secondary wires. Some transformers, autotransformers for example, have the grounded side already tied together inside the transformer.

[jogas]

If the secondary of a transformer is not grounded, it has no relationship to ground. The magnetic coupling of the transformer prevents this.
I know this is way out there, but did you happen to check resistance between the transformer primary and secondary windings (with nothing attached to the transformer? There shouldn't be any. But it does happen, that's why transformer secondaries are grounded in the first place. It's for safety reasons.
Jogas

[speer007]

Ya the resistance was like .5 ohms

[speer007]

Isnt your neutral line considered your ground? That pops another quick question why do you need a neutral on 115 and 24 volts but on something like 208 you can just use two lines of 115 and no neutral. I know the neutral has a zero voltage potential but what is it actually doing in the circuit, i know without it, it wont work.

[rtruett1]

All the neutral does is carry the unbalanced load, it comes from the center tap of the transformer being it a wye or delta configuration. If the neutral is ungrounded it will vary in voltage which the term floating neutral is derived. You mention the 208 volts being 2 115 volt wires, this is just a different transformer hook up known as a wye configuration. These are both wires with potential on them just like you have in your house which is more times than not 240 volts. You actually get the 120 volts by going to the center tap on the transformer which should definetly be grounded.

[TitleLess]

but 240 res comes from one leg of a threee phase circuit right ?

One leg, not two legs right? so it is at 180* not 120* as you would get from pulling two wires off a threee phase circuit.

[Edited by titleless on 12-24-2005 at 04:58 PM]

[wonder_boy]

Hey Speer007, A transformer works by mutual inductance.Basically a transformer consists of two coils. A primary and secondary,which are seperate from each other. When you apply voltage to the primary coil the magnetic field generated, expands outward and cuts across the wires of the secondary coil. This induces current.Since there is less windings in the secondary coil,the voltage is less.And since A/C current alternates (on and off 60 times a second)this is possible. There is no such thing as a D/C transformer. Anyways there should be infinite resistance between either lead of your primary coil to either lead of your secondary coil

[jc47]

"but 240 res comes from one leg of a threee phase circuit right ?

One leg, not two legs right? so it is at 180* not 120* as you would get from pulling two wires off a threee phase circuit. "



Yes, this is correct. That is why you get 240 (120*2) volts instead of 208 (120*3^0.5) volts.

[jogas]

Originally posted by speer007
Ya the resistance was like .5 ohms

I think you did not understand me. There should be infinite resistance between the primary and secondary windings. NOT across the primary and secondary windings.
I mean, with no wires attached to the transformer, what is the resistance when one of the meter's lead is on the primary terminal and the other lead is on the secondary terminal.
Jogas

[youngkj]

If secondary of the transformer is not grounded, measured voltage to ground can vary randomly. If the secondary voltage is consistant, chances are the measurement you are getting is controled by the load side circuit. Before you ground the secondary you will need to know if the load needs to be isolated. It is also possible that the winding have shorted to the primary side. Disconnect all leads and measure resistance from secondary to primary windings... should be megaohma range. Also each winding should be open to ground. If not the transformer is failing.