jim sutton
Senior Member
Is it required to ground one leg of the secondary of a 480v pri / 120v sec single phase transformer?
Here's what I ran in to: A control panel for a conveyor system has line conditioning transformers installed to power PLC's and other devices on the line. We determined that one of these (5KVA) was bad due to the secondary reading about 35 volts. We could not get an exact replacement quick enough so the plant engineer got a 3KVA to replace it temporarily. I did not ground the secondary, only the case. The problem was that the devices on one conveyor did not power up even though we had 120v leaving the control panel. We had two 120v circuits on this conveyor line, one from the transformer that I replaced and one from another transformer. I spent a lot of time looking for a broken wire since correct voltage was leaving the panel but not reaching the devices. Later that night at home, I was thinking it through and came up with this: If those two circuits "neutrals" were crossed, then one would go to the existing transformer and one to the new one. If the existing transformer had one leg bonded and the new one did not, then that would explain why I did not have power. If both were bonded to the control panel, even if they were crossed, the circuits would be completed through the case of the cabinet. This would explain how it worked before. I called back this morning and they said they had fixed the problem by replacing the transformer.
By the way, the transormer I replaced did not appear to have one leg externally bonded. Are these sometimes internally bonded?
Any of you guys have any comments or suggestions?
I don't normally do industrial troubleshooting and can sometimes be baffled!
Thanks for your help.
Here's what I ran in to: A control panel for a conveyor system has line conditioning transformers installed to power PLC's and other devices on the line. We determined that one of these (5KVA) was bad due to the secondary reading about 35 volts. We could not get an exact replacement quick enough so the plant engineer got a 3KVA to replace it temporarily. I did not ground the secondary, only the case. The problem was that the devices on one conveyor did not power up even though we had 120v leaving the control panel. We had two 120v circuits on this conveyor line, one from the transformer that I replaced and one from another transformer. I spent a lot of time looking for a broken wire since correct voltage was leaving the panel but not reaching the devices. Later that night at home, I was thinking it through and came up with this: If those two circuits "neutrals" were crossed, then one would go to the existing transformer and one to the new one. If the existing transformer had one leg bonded and the new one did not, then that would explain why I did not have power. If both were bonded to the control panel, even if they were crossed, the circuits would be completed through the case of the cabinet. This would explain how it worked before. I called back this morning and they said they had fixed the problem by replacing the transformer.
By the way, the transormer I replaced did not appear to have one leg externally bonded. Are these sometimes internally bonded?
Any of you guys have any comments or suggestions?
I don't normally do industrial troubleshooting and can sometimes be baffled!
Thanks for your help.
