480/277V Battery Charger, 3 Phase Wiring Question

[MD84]

I was called in to replace a control power transformer on a 36Vdc forklift charger. The charger was originally setup for 208V. When the charger was reconfigured to 480V the CPT taps were not adjusted for the higher voltage. This smoked the CPT. I replaced the CPT.

After checking everything over I switched on the breaker. I quickly found that I had no voltage. The wires were not connected in the JB. I connected the wires and switched it on again. This time I had some voltage.

I had good voltage each phase to ground but did not have voltage a phase to c phase. The charger did work in this state. I am assuming that somehow the wires were mixed up in a JB upstream or in the panel and the two wires with no voltage happen to be the same phase.

My questions are:

1. How will this three-phase full wave rectifier react to having 0 volts across two poles?

2. What other scenarios could cause the 0 volts.

Thanks,
MD84

 

[ActionDave]

I had good voltage each phase to ground but did not have voltage a phase to c phase. The charger did work in this state. I am assuming that somehow the wires were mixed up in a JB upstream or in the panel and the two wires with no voltage happen to be the same phase.

My questions are:

1. How will this three-phase full wave rectifier react to having 0 volts across two poles?

I guess not to badly because you said it's charging, you won't be charging the batteries as fast.

2. What other scenarios could cause the 0 volts.

Thanks,
MD84

Open connection somewhere causing a lost phase.

 

[GoldDigger]

If the charger has current limiting other than just the internal resistances, then the charger will be trying to deliver the same average current (and power) through only two of the three internal transformer windings and at most four of the six diodes in the rectifier bridge. That runs the risk of smoking either transformer or rectifier in the long term. (Like an 8 hour battery charge.)
You may also have twice the design current in the B conductor back to the panel, which is not a good idea either.

 

[MD84]

Open connection somewhere causing a lost phase.

There was still 277V to ground on all three phases. The only absence of potential was between phase a and c. How would I see potential to ground if there is an open?

Thank you for the response.

 

[GoldDigger]

There was still 277V to ground on all three phases. The only absence of potential was between phase a and c. How would I see potential to ground if there is an open?

Thank you for the response.

With a 480Y/277 supply there will always be 277 to neutral from all three phase conductors. If the neutral is grounded there will be 277 to ground. If you measure one of the conductors twice you will still get the same number.
So if the three wires at the charger are connected with a(at charger) connected to A (service), b connected to B and c connected to C you will have normal operation.
If you connect a-A, b-B and c-A you will get exactly the voltage readings that you report.
Your measurement of 277 to ground from each wire does not tell you the phases of each wire.
Now if one of the phase wires is open instead, you may still get a voltage on the open lead because of either capacitance or conduction through the charger windings. But it would not be the nice numbers that you got.

 

[MD84]

If the charger has current limiting other than just the internal resistances, then the charger will be trying to deliver the same average current (and power) through only two of the three internal transformer windings and at most four of the six diodes in the rectifier bridge. That runs the risk of smoking either transformer or rectifier in the long term. (Like an 8 hour battery charge.)
You may also have twice the design current in the B conductor back to the panel, which is not a good idea either.

I do not believe there was any active current limiting. It was a very basic design. I am not sure what the charging characteristics are. This was my concern and I will track down the issue before returning the unit to service.

I know a motor can increase current when single phasing. This issue seemed different and I was curious if I would see additional current on 2 of 3 transformer windings.

I appreciate the reply.

Unfortunately I did not have any further time to check current draw or troubleshoot further on Friday but I will check into it further on Monday.

 

[ActionDave]

There was still 277V to ground on all three phases. The only absence of potential was between phase a and c. How would I see potential to ground if there is an open?

Thank you for the response.

Backfeed through the equipment.

 

[MD84]

Now if one of the phase wires is open instead, you may still get a voltage on the open lead because of either capacitance or conduction through the charger windings. But it would not be the nice numbers that you got.

Thanks. This is what I was thinking. I think it must be as you have stated a-A b-B c-A. Otherwise I don't see how I could get those voltages. They were closer to 280 but we're all very similar.

I was just wondering if there are any other situations where those readings could be found. I could not think of any.

 

[MD84]

Backfeed through the equipment.

Thank you. I will try isolating the equipment as part of my troubleshooting. This sub panel has this charger and two other three phase chargers.

I can check to see if I have good balanced 3 Phase at the panel and/or on the other two circuits.

 

[MD84]

There was an open connection. I guess it was just a coincidence the backfeed voltage appeared so balanced. When I checked it today it was 50V. Everything is looking good now that I have made up the connection.

Thanks for the help.