Confusion with supply voltages.

rejoe.koshy · Jun 6, 2018

Hey Guys,
I am facing a peculiar problem , while commissioning a new CNC machine wherein the machine is running fine but I am not getting proper voltage readings on my multimeter to declare the system to be healthy.

Standard input voltage- 220 V (single phase) at the load end
Line to neutral - 220 V (Which is fine)
Line to ground - 110 V .....confused!
Neutral to ground - 109 V ....confused!

The point is if I get neutral ground voltage as 109 V , I should not be getting line to neutral as 220 V .

I am totally confused regarding this & I have also checked my source end , where all readings were within limits

Is the load playing a role in it?Will it cause any harm to electronics of the CNC machine?

Pls help...at wits end

Dirt · Jun 6, 2018

Center tapped transformer where center is grounded? Your neutral in that case isn't really neutral.

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BryanG · Jun 6, 2018

I would guess that the supply is from a transformer with the centre tap grounded, that would explain your voltage readings. Not sure why they would do it that way, you would need to check with the machine manufacturer. With any system that is like your 110/0/110 you have to be vary careful with switching only one leg. what I mean is a 220V contactor may not drop out if it is receiving 110V, so an electrical fault can lead to a dangerous condition where a contactor doesn't drop out.

Jeff23spl · Jun 6, 2018

You seem to experiment north American standards. For safety, we keep domestic voltage lower from the ground by using a center tap on our TFO and most of the control stuff works on 110-120v. This way it is very hard to take more than 120v touching a live wire. If any live wire touch ground, it makes the protection to trip/blow...
Larger domestic load use both legs to get 240v.

But as long as all your 220v load use solid connection to each phase of the transformer it would work and usually protections have both phase connected toghether to make sure if you short 1x120v, it trip the other one with it.

rootboy · Jun 6, 2018

It looks to me that it is wired correctly (for North America). On the secondary side of the transformer, what you would consider your Neutral is just the other side of your output phase. You can't think of it as your neutral at this point since it is not connected to ground.

The center tap has been connected to ground, which is the standard for North America. And since it's the "Grounded Conductor", it becomes the neutral. Simply meaning that it is "neutral" compared with the ground.

For more info on the subject: https://www.ecmag.com/section/codes-standards/grounding-separately-derived-systems-part-1-basics

If all of your hardware is rated at 220 volts, then connecting the center tap to ground makes no sense since you have no need for 110 volts.

Assuming that you aren't commissioning this piece of equipment in North America, then removing the center tap from ground and tying one end of the secondary to ground might be the appropriate thing to do.

If you do have hardware that is powered at 110 volts, then it is absolutely the wrong thing to do. Leave it as it is.

Consult your local codes and electricians.

Some helpful followup info on your part would be:

1) What is the make and model of your transformer.
2) Do you have any hardware that runs on 110 volts in the panel.
3) Where is the equipment intended to be used.

rejoe.koshy · Jun 7, 2018

rootboy said:
It looks to me that it is wired correctly (for North America). On the secondary side of the transformer, what you would consider your Neutral is just the other side of your output phase. You can't think of it as your neutral at this point since it is not connected to ground.

The center tap has been connected to ground, which is the standard for North America. And since it's the "Grounded Conductor", it becomes the neutral. Simply meaning that it is "neutral" compared with the ground.

For more info on the subject: https://www.ecmag.com/section/codes-standards/grounding-separately-derived-systems-part-1-basics

If all of your hardware is rated at 220 volts, then connecting the center tap to ground makes no sense since you have no need for 110 volts.

Assuming that you aren't commissioning this piece of equipment in North America, then removing the center tap from ground and tying one end of the secondary to ground might be the appropriate thing to do.

If you do have hardware that is powered at 110 volts, then it is absolutely the wrong thing to do. Leave it as it is.

Consult your local codes and electricians.

Some helpful followup info on your part would be:

1) What is the make and model of your transformer.
2) Do you have any hardware that runs on 110 volts in the panel.
3) Where is the equipment intended to be used.

Thank you so much for bringing clarity to this problem.I checked the drawing but could not find any centre tap, although I have to check the same physically...

It is a control transformer with 440 input & two 220 V AC output & one 24 v ac output.

keithkyll · Jun 7, 2018

You are reading a floating winding. The capacitors inside the input filter of the power supply is causing the voltage to read 110 at each side, but this is low current. You're still floating.
One side of the 220 VAC output should be connected to ground. This establishes your neutral. Connect Blue and Green/yellow to this terminal. Fuse/breaker followed by Brown wire at other terminal.
Also connect one side of the 24 VAC winding to ground.
This is also a safety issue. The ground protects from overvoltage if there was ever a primary/secondary short.

rootboy · Jun 14, 2018

rejoe.koshy said:
Thank you so much for bringing clarity to this problem.I checked the drawing but could not find any centre tap, although I have to check the same physically...

It is a control transformer with 440 input & two 220 V AC output & one 24 v ac output.

No problem!

Can you post a picture of your transformer please?

Confusion with supply voltages.

rejoe.koshy

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Dirt

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BryanG

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Jeff23spl

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rootboy

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rejoe.koshy

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keithkyll

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rootboy

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