GROUNDED B PHASE

[timebuilder]

Yes i'm still here but that 3 phase motor is toast.

If the voltages between the phases are correct, you should be able to change direction and have no toasting of the motor.

Maybe there is a more detailed explanation of the sequence of events???

[wolfdog]

On "corner grounded delta", B phase is grounded. Si if you want to revese rotation you can swap A and C phase with no damage to the motor.

[wolfdog]

I called the power company out . and to be completely honest I still don`t completely understand it. Nuetral in a single phase makes perfect sense to me , ie negative on the sine wave yet still carrying current, this just blows my mind, there has been other really long and informative discussions here in the past, I have to put my hand up and say I am still not 100% on it stan

Perhaps some of your difficulty comes from the misconception of the neutral.
It is not the negative of the sine wave. It is derived from a center tap of the winding on the transformer. A 240 volt winding tapped mid way provides 120 volts from tthe center tap to either end....240 volts to L1 and L2.

Now if you ground that point in the system it will be at zero potential to ground and 120 volts to the ends of the winding or L1 and L2. If you don't ground the neutral it will drift and not be a stable voltage.

Same principle with corner grounded delta. If you ground B phase, it will be at zero potential to ground or earth. But it will still be at 240 or 480 volts potential to the other phase conductors.

[J/D]

On "corner grounded delta", B phase is grounded. Si if you want to revese rotation you can swap A and C phase with no damage to the motor.

Why only A and C swap any two phases to change direction does not matter.

[stanbyyourword]

Wolfdog thank you for reply ,right you are, just did a little refresher. A person could spend a lifetime studying electrical engineering and power distribution , whats even more amazing is how a technician grows technically in other areas, even electrically, some things get foggy with time over a 20 year period, good example. regards stan

[timebuilder]

On "corner grounded delta", B phase is grounded. Si if you want to revese rotation you can swap A and C phase with no damage to the motor.

Unless the motor is grounded where it shouldn't be, a grounded B phase should behave just like the A and C phases.

That's why I am asking.

[VTP99]

If the voltages between the phases are correct, you should be able to change direction and have no toasting of the motor.

Maybe there is a more detailed explanation of the sequence of events???

wolfdogs post # 22 after yours is the correct answer. Switch A phase with C phase to reverse but leave B alone.

timebuilder,
The "sequence of events" was i replaced a 3 Phase RTU blower motor, same as old one, and added a motor starter. I needed to reverse the direction so i just picked any two phases and reversed. The starter tripped out. I took my amp probe and found the amps on each phase were way out of wack. Like 5-2.5-0 on each of the 3 phases. I then reversed things back and used the other phase. Guess what all 3 phases even out and no tripping overload.
I don't understand it completely myself and maybe wolfdog can explan it in more detail.

[timebuilder]

wolfdogs post # 22 after yours is the correct answer. Switch A phase with C phase to reverse but leave B alone.

timebuilder,
The "sequence of events" was i replaced a 3 Phase RTU blower motor, same as old one, and added a motor starter. I needed to reverse the direction so i just picked any two phases and reversed. The starter tripped out. I took my amp probe and found the amps on each phase were way out of wack. Like 5-2.5-0 on each of the 3 phases. I then reversed things back and used the other phase. Guess what all 3 phases even out and no tripping overload.
I don't understand it completely myself and maybe wolfdog can explan it in more detail.

I don't see a lot of grounded B. High B, yes. There's a bunch of that in Philly. It used to be that way for most three phase services.

I guess my point is that it should make no difference to the motor if the B phase is grounded, and the practice of switching any two phases to reverse the rotation should work just fine.

That said, there may be some issue involving the motor starter, and how it is wired, since a control circuit is often derived from two of the phase legs for a single phase control for the starter.

You wouldn't have a schematic to post, would you?

[bobboan]

"I don't see a lot of grounded B. High B, yes. There's a bunch of that in Philly. It used to be that way for most three phase services.

I guess my point is that it should make no difference to the motor if the B phase is grounded, and the practice of switching any two phases to reverse the rotation should work just fine.

That said, there may be some issue involving the motor starter, and how it is wired, since a control circuit is often derived from two of the phase legs for a single phase control for the starter.

You wouldn't have a schematic to post, would you?"


Timebuilder,
You're right on this. The motor only responds to 3 phases that are of equal potential 120* apart. It does not matter that one of them is grounded because the potential is the same between any two of them.

There is either a short in the motor winding or the wiring that ceases to cause a fault when connected to the grounded phase or something in the control wiring.

[wolfdog]

Unless the motor is grounded where it shouldn't be, a grounded B phase should behave just like the A and C phases.

That's why I am asking.

It makes no difference. the motor sees 3 phase and the line voltage is where the grounded phase is.

The significance of my statement is that the "B" phase is grounded and should maintain that standard on all line side disconnects, starters, and contactors. As ling as you are just swapping the load side, standards are maintained and the next poor shmuck may not get fried.

[timebuilder]

It makes no difference. the motor sees 3 phase and the line voltage is where the grounded phase is.

The significance of my statement is that the "B" phase is grounded and should maintain that standard on all line side disconnects, starters, and contactors. As ling as you are just swapping the load side, standards are maintained and the next poor shmuck may not get fried.

I'm not sure what you are trying to say by "maintain that standard."

The B phase is only grounded at the transformer. Are you saying that the B phase is always in second position? I would agree with that.

What I don't get in the thread is the assertion that a motor was fried because motor leads were swapped where one lead was for the B phase. Electricity has taught me nothing that says the B phase lead cannot be swapped with the A or C phase for motor reversal.

So, some piece of this situation is still missing.

[VTP99]

What I don't get in the thread is the assertion that a motor was fried because motor leads were swapped where one lead was for the B phase. Electricity has taught me nothing that says the B phase lead cannot be swapped with the A or C phase for motor reversal.

So, some piece of this situation is still missing.

I here what your saying timebuilder. Before this i was with the same thinking. I will try to get the motor & overload #'s for us. How does one explain the amps leveling out after the switch ? Same motor start.

[jammzzville]

Is it possible that there is a ground fault somewhere else in the building causing some sort of ground loop? This may not be what caused the motor to burn but may be the cause of your limited choices of phasing or it may be both.

We had an issue where strange things were happening and it turned out it was being caused by a motor with a shorted winding to ground in a completely different part of the building. It was off of the same substation which had an ungrounded delta secondary, which is an important fact.

It didn't exhibit the problem when the motor was off, when the motor would start the shorted winding to ground would actually corner ground the system and we would see this in another area of the building as an intermittent thing causing some unusual things to happen. Zero current to ground on that phase means no tripping of the overcurrent device and the motor continued to cycle on and off.

My point is, strange and unusual things can happen in systems like these. I would investigate the power system of this building and its loads (if possible) and see if anything raises any eyebrows.

[timebuilder]

I here what your saying timebuilder. Before this i was with the same thinking. I will try to get the motor & overload #'s for us. How does one explain the amps leveling out after the switch ? Same motor start.

In order to explain what happened after the switch, we would have to know what was happening BEFORE the switch. We just don't have enough info.

If there is no problem with the starter, I have to think there was a defect in the first motor. A coincidence can lead to incorrect conclusions.

[VTP99]

In order to explain what happened after the switch, we would have to know what was happening BEFORE the switch. We just don't have enough info.

If there is no problem with the starter, I have to think there was a defect in the first motor. A coincidence can lead to incorrect conclusions.

The first motor was cooked and the story goes from a single phase scenario(brown out) . This is what i was told. I installed the same motor and reversed two phases to get the correct rotation. At that time i did not check the amps on each phase. I also did not install a motor starter at that time. The next day the call came again that the blower was off. The new motor was cooked the same as the first motor. I installed a second new motor and a motor starter to protect this second motor. Wiring was not changed for this second motor as rotation was correct. When i started the motor the overload tripped so i amped my phases and found them way unbalanced. Like 5 amps 2.5amps & allmost 0 amps. It was then that i realized i had used the grounded B phase to reverse rotation the last time. So i moved the B phase back and reversed A with C phase and the amps evened out and the overload did not trip. Now my guess is that the first motor did get cooked from a true single phase condition. That motor was a little over a year old and had no protection. My first motor got cooked i believe because i did not check amps and did not install a motor overload. Had i installed a motor overload at that time i think my first motor would still be good. The overload would have tripped and i would have been alerted at that time.

[timebuilder]

I'd have to say that it sounds as though the real culprit is the power quality rather than the existence of a grounded B phase, because all three phases will behave identically at the motor regardless of the B being tied to ground, or a high B where the voltage to ground exceeds that of the other two phases.

I wish I was there to look into this further.

[VTP99]

came across a k body copeland tripping it's overload. Amped out the 3 phases and got 18/7/7 reversed phases and got 7/7/7 rounded off. Once again a B phase Corner Grounded Delta system.

[bobboan]

came across a k body copeland tripping it's overload. Amped out the 3 phases and got 18/7/7 reversed phases and got 7/7/7 rounded off. Once again a B phase Corner Grounded Delta system.

Did you meg the windings? How long had the compressor been installed and running? Has anyone recently changed any wiring or controls?

I have to agree with "timebuilder" that the motor only "sees" the 3 phases at 120* apart. It doesn't recognize whether one phase is grounded or high leg to ground; it only "sees" the potential between the phases - unless a winding is shorted.

Please, I would really like more info. Can you check it out and get some answers to these questions?

[VTP99]

Did you meg the windings? How long had the compressor been installed and running? Has anyone recently changed any wiring or controls?

I have to agree with "timebuilder" that the motor only "sees" the 3 phases at 120* apart. It doesn't recognize whether one phase is grounded or high leg to ground; it only "sees" the potential between the phases - unless a winding is shorted.

Please, I would really like more info. Can you check it out and get some answers to these questions?

The wiring appeared to be in it's original position. Wires on load side of contactor were marked left to right 1/2/3. I switched 1&2 around and the amps leveled out. My only theory is that the electric co. switched two phases around. Be a reciprocating compressor rotation was not a concern.

On a side note. Yesterday i got a no cooling call on a Heil dry unit i installed last year. This is at the same shopping center where i had the blower motor problem that started this thread. Get this i check the unit and found the phase monitor flashing. Panal cover says phase problem. I reversed two phases and the light stops flashing. Runs a few seconds and shuts down. I then switch it back and same thing again. I can not get the board to reset. I call tech support and explan. He tells me it will not work with this B phase ground and has me install a Carrier CLO ( compressor lockout ) Yet it ran all last summer no problem.

[bobboan]

The wiring appeared to be in it's original position. Wires on load side of contactor were marked left to right 1/2/3. I switched 1&2 around and the amps leveled out. My only theory is that the electric co. switched two phases around. Be a reciprocating compressor rotation was not a concern.

On a side note. Yesterday i got a no cooling call on a Heil dry unit i installed last year. This is at the same shopping center where i had the blower motor problem that started this thread. Get this i check the unit and found the phase monitor flashing. Panal cover says phase problem. I reversed two phases and the light stops flashing. Runs a few seconds and shuts down. I then switch it back and same thing again. I can not get the board to reset. I call tech support and explan. He tells me it will not work with this B phase ground and has me install a Carrier CLO ( compressor lockout ) Yet it ran all last summer no problem.

Interesting. I don't think I've ever seen a corner grounded secondary Delta used at a shopping center. The obvious reason being that a grounded neutral would be necessary for 120v loads and it would not seem make sense to have a separate corner grounded system to supply relatively small to medium hvac loads. But, I haven't seen everything either.

Are you certain it was corner grounded or could there have been a fault in a phase? Could someone have changed something not realizing the type of system it was?

I doubt if the poco would have changed phasing. They are very careful about that and one of their standard tools is a phase rotation meter.