Hi,
I have this transformer I pulled from a cnc machine that I'm retrofitting to a new control, and I need the 115v this transfomer can supply.
So I hooked it up with one lead on 20 and one lead on 210, and then I get 115vac between 0 and 110 on the other side. This is all good.
The transformer will supply other voltages too from the other windings, in total there are three. I only need the 150VA 115V..

When I measure voltage between the chassis of the transformer and an earth plug in my wall I get about 180V. This measurement is with my Fluke 17B+. If I measure with my a cheaper meter it reads 0. I tried to test if there is any current going here, so I connected a small 12V lamp and fluke meter in series between earth-wall and chassis and got 0ua, so no current. I can also measure a voltage between transformer chassis and one of the outputs even though the chassis is not connected to earth.

The schematic from the machine that I pulled the transformer from shows that they connected the chassis and all the 0v from the transformer to earth. I don't want to do this, I only want the transformer to be ground, and run the 0v and 115v directly to where I need it.


Why do I measure 180v between chassis and earth?
I'm thinking this is normal for the transformer and that its not dangerous because theres no current?
Why is there and earth wire going out of the transformer coil?


Dan

 

The Fluke meter will have very high input impedance, so coupling of AC through stray capacitance can give alarming high readings. You've done the right thing - looked for current. Since there is no current, it is safe.

The ground connection from the dotted line between the core and the primary winding is an electrostatic shield. Typically in a main-frequency transformer it is one turn of copper foil with the ends slightly overlapped but insulated so that it is not a complete electrical turn. It's purpose is to provide a path for any high-frequency noise present on the primary from coupling to the secondaries or vice versa. It must be connected, normally to earth for this sort of circuit, to function.

The "0" side of the secondaries should connect to earth ground at some point if there is any chance that static charge can build up in things connected to the secondaries. With moving machinery, this is distinctly possible. Without a means to bleed off the accumulated charge, sooner or later it will cause a spark to something, possibly resulting in damage. It may be necessary to connect one side of the secondaries to earth ground to comply with local safety regulations. Usually all exposed metal parts have to be "bonded" to ground.

 

Thanks for reply, that was very informative!
Safe as I thought then.

There will not be anything connected to the secondaries, as I only need the 115v.
I will ground the electrostatic shield and the chassis of the transformer by connecting to earth
Is this good enough?

How does the secondaries build up a static charge if theyre not used for anything?

Dan

 

Hi,
I have this transformer I pulled from a cnc machine that I'm retrofitting to a new control, and I need the 115v this transfomer can supply.
So I hooked it up with one lead on 20 and one lead on 210, and then I get 115vac between 0 and 110 on the other side. This is all good.
The transformer will supply other voltages too from the other windings, in total there are three. I only need the 150VA 115V..

When I measure voltage between the chassis of the transformer and an earth plug in my wall I get about 180V. This measurement is with my Fluke 17B+. If I measure with my a cheaper meter it reads 0. I tried to test if there is any current going here, so I connected a small 12V lamp and fluke meter in series between earth-wall and chassis and got 0ua, so no current. I can also measure a voltage between transformer chassis and one of the outputs even though the chassis is not connected to earth.

The schematic from the machine that I pulled the transformer from shows that they connected the chassis and all the 0v from the transformer to earth. I don't want to do this, I only want the transformer to be ground, and run the 0v and 115v directly to where I need it.


Why do I measure 180v between chassis and earth?
I'm thinking this is normal for the transformer and that its not dangerous because theres no current?
Why is there and earth wire going out of the transformer coil?
View attachment 145675 View attachment 145676 View attachment 145678

Dan

Dan,
FIRST, with nothing connected to the transformer, check the resistances between the commons on the input side and the output side. If that resistance is not very high, there is either a connection or a problem. IN the photo I think that I see a wire going from a winding to the transformer frame. If that is for the electrostatic shield, then fine, but if it is from anything else, "not fine". The resistance between the primary and the secondary should be at least 100K ohms for an isolation transformer that size. Likewise, any ground terminal should be very high resistance to both primary and secondary.
Next, connect the transformer primary to match your applied voltage with the transformer frame grounded, as well as with any "ground" terminal grounded. Under these conditions check your secondary voltage, and then the voltage between, as you did originally. It should be different this time, much closer to what you wanted.

 

You can leave unused windings completely unconnected.

I'm sorry, I misunderstood what you are doing - just using a transformer that used to be in a machine for some other purpose. There is no need to ground the secondary. If you don't ground the secondary then anything powered by it is fully isolated from your AC power line, which can be very useful on a lab bench. It removes the risk of shock from the case where you are grounded somehow and touch either of the secondary terminal. I have several transformers I use that way.