3-Ph Pump Control Box

[Khonwan]

When temporary switch (rear view is shown and marked as “%”) is open, “A” is ph-2, “B” is ph-3; when closed, both “A” and “B” are ph-2 (based on breaker wiring from left to right as phase lines 1-3; testing by voltage meter between A/B to lines 1/2/3). Why and/or how does “B” change from ph-3 to ph-2? Voltage of “A” to ground and “B” to ground is ca.230V whether switch “%” open or closed. The circuit correctly opens/closes the circuit to operate the pump. The contactor is a 380-415V model (not 220-240V model).

[electau]

3 phase. A, B, C. 380volts between A to B, B to C, and A to C.

A, B, or C phase to earth E or neutral N is 220V. 380V is 220V x 1,732.

Contactor coil voltage 380V if a neutral is present a 220V can be used.

Edited February 2, 2013 by electau

[Khonwan]

“3 phase. A, B, C. 380volts between A to B, B to C, and A to C.

A, B, or C phase to earth E or neutral N is 220V. 380V is 220V x 1,732.”

That’s a nice 101 but I’m way past that and can’t see anything in my post that might cause you to believe otherwise.

Not sure this contactor can be wired for single phase input – the company makes the same 3-ph contactor but with a 1-ph input so I don’t see why they would do so if this model can be used for both. Anyway, why would I want to wire it for single phase? It works perfectly already, as I said.

I was hoping for an explanation as to why one side of the switch appears to change phase when when the switch is open then closed as per my OP.

[electau]

The contactor can be wired for a single phase 220 volt motor. The phase (220V) is wired through each leg of the contactor and each leg of the thermal overload relay. the neutral is not switched. The thermal overload relay is matched to the motor current. The contactor coil is replaced by a 220VAC one. The control circuit voltage is 220V.

A 3 phase contactor may be fitted with a 220VAC coil when a 220V control circuit is used, a neutral is required.

380/415VAC control circuits are generally used where the control circuit is contained within the same enclosure, eg the standard motor starter with start/stop-reset buttons.

When the contactor closed, the voltage between phases will be 380V and to earth (or neutral) 220V on both sides of the contactor.

[Khonwan]

Sorry, Electau – I re-read your post yesterday and realised you meant it could be used on 220V by replacing the coil. I previously had the 220V coil model but, when this failed after a time and required replacement, I inadvertently purchased the 380V coil model.

Since my 380V coil set up is working, is there any reason to change to a 220V coil? The only reason I can think of is in the scenario of me extending the length of this switch cable (which I’m contemplating – 400m to my water tank at my house) since it would be safer to have a long run that, when the circuit is open, has one live conductor and one neutral conductor (potential short circuit fault of 220V) rather than two live conductors of differing phases (potential short circuit fault of 380V). This by itself is an excellent reason but is there any other reason? My reason for considering changing the coil wouldn’t be of importance were I not considering extending the cable length, but would there be another reason?

Has it been wrongly wired by me? Rather than changing the coil, can I rewire it so that both conductors in the switch cable are of the same phase whilst open? No, forget that – that would make it effectively a normally closed circuit. Perhaps it is correctly wired then, just seems odd to me that I’m getting the readings I described in my OP.

[bankruatsteve]

Schrodinger's paradox? Maybe have to go with probability. Or not.

[electau]

It is assumed that your existing motor for the pump is 3 phase. You propose to use 220V for the control circuit for the external switches to control the operating coil of the contactor. This would be preferred to a 380V control circuit. The external control circuit will require a neutral conductor at the motor starter. All you have to do is replace the 380V coil with a 220V one. The external control circuit will require protection min 10A MCB. The MCB may provide this protection for the cable.

(The single phase connection which will require rewiring of the motor contactor and overload relay to suit 220V connection as the motor current must go through all legs of the relay to prevent tripping on imbalance. It would appear that you are not going to use a single phase 220V motor.)

You will have to allow for voltage drop if the motor is located along way from the main switchboard.

One very good reason for not using a 380V control circuit is that your control devices external to the motor starter may only be rated to 250VAC max.

The control voltage is single phase 220V L and N. the motor voltage is 380V, 3 phases. The control circuit operates the contactor coil only.

Note. Use a 380V contactor coil with a control circuit voltage of 380. use a 220V contactor coil with a control circuit voltage of 220.

[electau]

The 220V control circuit runs to the line side of the MCB to the external control devices then returns to the overload relay control circuit contacts (N/C) then to the contactor coil terminal and then from the other side of the contactor coil to the neutral, N.

A 220V coil will burn out if connected to 380V. A 380V coil connected to 220V will not operate the contactor.