Voltage imbalance trips breaker

[MD84]

I was on a service call for a breaker tripping on a packing machine. The machine would work properly until the vacuum motor started and then the circuit breaker would trip.

This circuit originates in a 480v panel from a 3 pole 20A CB. It runs over to a 480V:240V delta-delta transformer. The transformer feeds a 3 pole 40A CB located on the machine.

When the vacuum motor comes on the 20A 480v CB trips. Before the vacuum comes on a delta connected resistive heating element runs for a few moments.

I checked voltage at the machine and it was not balanced. I had 240, 240, and 230. I checked line to ground and I had 140, 120, 140. I suspected the secondary was floating. I confirmed no SBJ. I bonded the center tap and checked voltage again. Line to ground was 114, 114, 212. I knew something was open or had a bad connection. I checked the primary windings and they checked out ok.

Next I checked the 480. I had 490, 480, 465. Line to ground were all around 275. I checked at the breaker and had the same. I checked the main lugs and had perfect balanced 480. I isolated to the B phase breaker connection.

Long story short the B phase breaker connection to the bus was not tight. There was pitting which likely formed over time and resulted in enough voltage drop to cause a problem.

I fixed the circuit breaker connection and all my voltages were correct and balanced. Secondary line to ground was 120 and 208 as expected. The machine ran properly at this point.

I knew that increased current was possible in this situation for a motor. But I do not completely understand why. I know that for the resistive loads the current will be less for a bad connection and lower voltage on one phase. My standard troubleshooting techniques and general understanding got me through this one fairly quickly. It leaves my asking why.

Why did a high resistance connection cause the breaker to trip when a motor was started?

 

[Jraef]

For the motor to work, the first thing that has to happen at the instant the contactor closes is that the windings have to magnetize the core. At that instant the only thing slowing the rise in current is the wire resistance itself, it's essentially a short circuit. So current on any given phase can be as much as 20x the FLC for the first cycle. If one phase isn't contributing to that process in the same way, the other two have to make up for it. Normally even though it's that high, the time frame is too short for anything to react to it and we don't see it; what we see is the 8-10X the FLC that we call the "Inrush Current" that lasts for a few cycles, but is dampened by the mutual induction taking place after the core is magnetized. But if that one phase isn't delivering its fair share at that first initial moment, it either makes the whole initial magnetizing process take longer and/or the increased current on the other 2 phases is so MUCH higher that it takes too long to decay again with the mutual induction and is seen by the trip sensors.

 

[MD84]

Thank you. I can understand that.

What would happen if the motor was already moving and one leg had a voltage drop? Would the other legs compensate?

 

[Saturn_Europa]

Thank you. I can understand that.

What would happen if the motor was already moving and one leg had a voltage drop? Would the other legs compensate?

Depends on how low the voltage went. Slight imbalance the motor will probably keep running. Serious voltage drop the motor will go out on overload.

 

[Jraef]

It also depends on the load on the motor at the time. If already running and a phase is lost, the motor can keep spinning because it already was spinning. But the unpowered winding will cause negative sequence currents to flow in the rotor cage, which then create negative torque and the rotor begins to "fight itself" resulting in the motor doing less work per unit of current, so the overall current pulled by the other two phases increases by more than just the missing amount. It also means that added current is turning into heat in the motor instead of useful work, so unless the OL relay trips, the motor can be damaged. If the motor is fully loaded the increased current on the other two phases will cause the OL to trip. However if the motor load is low so current through the other legs is still below the trip threshold of the OL relay, it may never trip and the damage happens without warning. That's why important motors should be protected against phase current imbalance (which also covers phase current loss).

IEC bi-metal OL relays claim to offer this protection, and they do to a certain extent, but the motor still has to be fairly close to fully loaded for it to work. That's why I have switched to using modern Solid State OL relays that have true phase current imbalance protection.

 

[GoldDigger]

The current through a motor winding depends on the inductance, winding resistance and counter EMF caused by rotation speed.
Since the same rotation speed and therefore counter EMF applies to all three windings any "stiff" difference in phase (line to line) voltage will cause a disproportionately large current difference. Even for an unloaded motor.
Single phasing is not a "stiff" voltage difference and so is not necessarily as hard on the motor. Depends on the load details.