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I have a square D quick connect transformer in a control box system connect to the vacuum pump. The input supply for the primary side of the transformer is 220V. The output side is 110V. When i place the multimeter probe on the input side terminals, i get a voltage reading of 210V. When i do the same thing on the output side, i get 110V. This would suggest that the transformer is working alright. However, when i do the multimeter reading with respect to ground, the whole thing changes. I get a reading of 120V for the input terminals with respect to the ground and i get 0V for the output terminal with respect to ground. Do you think this is normal? Then, i used a electric tester with the input supply on and the output terminals disconnected and open. In this case. i had light ON in both the input terminals. In the case of output terminals, the light was ON only on one of the terminal and not in the second one. What do you think would be going wrong?

I have attached the picture of the circuit which consist of a relay, transformer and contactor (starter). It is a control box of a vacuum pump. There is a remote start/stop button outside which is used to start the pump. The problem is pump does not start after pressing the start button. However, if i manually push the contactor, it goes in and the pump starts. This indicates there is a problem in the control circuit which consist of transformer and relay. Any guides on how should i troubleshoot the problem? I do not have a wiring diagram for this circuit.

  • Your transformer is working. You likely have a control issue. That relay at the lower left corner is a key part of the latching start/stop circuit, and you will note that it is connected to wires that leave the enclosure going...somewhere (control switches, limits, safeties, etc). This looks like commercial three phase wiring and is straightforward work for a controls guy, even without a schematic. I suspect this is beyond the scope of this forum.
    – user39367
    Commented Sep 25, 2015 at 16:08

1 Answer 1


Entirely normal.

On the input side, 240VAC (at least in US residential wiring) is center-tapped to yield two 120V legs relative to ground.. These are out of phase to each other, so if you use a phase-to-phase connection you get the full 240 relative to each other but if you use either phase relative to ground it's 120VAC. That's an easy way to make both voltages available for use in the home.

On the output side: Unless there is a reference connection tying one side of the transformer to ground, it is a completely ungrounded, "floating", circuit -- effectively, what you have there is an isolation transformer. Without a ground connection, there is no circuit through ground and hence no voltage, just as you don't read voltage when one of the probes is in mid-air. If you did ground one side, of course, that side would read 0 volts and the other would read as hot relative to ground.

Now you'll ask what the voltage relative to the incoming 220V should be. Without a ground on the output side that too is 0V since there's no circuit. With the output grounded, the result is going to depend on how much phase delay the transformer has introduced -- it's a big inductor, after all.

  • Oh! Your answer cleared some confusion from my head. Moving to the output side, i used the tester with the power supply on. I had light on one terminal and did not see any light in the other terminal. What would this mean? The terminal in which i did not see the light goes to the relay circuit. So, i do want current flowing through them. Do you think there is something wrong with the terminal?
    – skonda2
    Commented Sep 24, 2015 at 21:38
  • Not familiar enough with the rest of that setup to have a valid opinion.
    – keshlam
    Commented Sep 24, 2015 at 21:55

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