Motor question....I don't get it?

Status
Not open for further replies.

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
I've been troubleshooting a pump for the last couple of days that intermittently quits. I did get the pump working again but don't understand some of my meter readings.

Its a 480v submersible, I'm guessing around 60HP just by the amp readings. The problem is B phase runs about 9-10 amps lower than A/C, despite shifting motor leads one phase over. Here's my readings, I verified with 3 different meters and used the meter that was the closest average of all three to do all the testing with.

Pump Off:

a-b 495
b-c 492
a-c 501

a-gr 247
b-gr 247
c-gr 429

Pump On(measured at T leads):

a-b 486
b-c 482
a-c 491

a-gr 242
b-gr 242
c-gr 419

Motor runs about 83 amps on A/C and about 73 amps on B phase.

Did a voltage drop test across the starter/breaker combined and only read about .2 ohms on all phases.

Megged it, and had .8 megaohms at 1000volts on all 3 motor leads. Which seems exceptionally low, but isn't the first time a submersible has ran just fine with this low of a reading.

The problem ended up being the programmable O/L relay was set for a 7% current unbalance and was dropping the motor out. I turned it up to 20% to have some single phase protection still but don't understand why the pump is so unbalanced? The problem isn't following the motor leads, so I assume it's an incoming power issue. But the voltage is so close, I wouldn't think it'd make a 10 amp difference?

Any ideas?
 
Last edited:

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Look at the transformers on the pole. The copper straps that connect the case to the center secondary bushing should be removed on an ungrounded delta. The copper strap could be connected to one transformer at one side, or a phase wire could be crimped to neutral for a corner grounded delta.
With the readings you provided, it sounds (reads) like there is a copper strap connected to one of the center secondary bushings on the transformer, giving you a center grounded delta.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
I would say it's an ungrounded delta with those voltages, you probably have a high resistance fault somewhere causing the higher voltage to ground on "C". Could possibly be an ungrounded WYE, though not likely unless for some reason the ground was broken or disconnected. This is a POCO transformer?
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
I most likely will not be back unless the pump fails again, so I can't answer your question about the transformers. I did not think one leg being higher to ground would have any affect since I figured it was the L-L that mattered?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I can not provide much useful input other than to say I have found such readings commonplace on submersible pumps. I'm told that because of their dimensions, the winding characteristics are different from "standard" polyphase motors.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The ratio of voltage imbalance to current imbalance in a motor can be higher than you might think, it's not linear. So without doing the math, your readings and results don't surprise me at face value. Add to that any location issues and that can easily explain the trips.

What i mean by location issues is the fact that some types of three phase loads (for example submersible pumps in wells or lift stations) that are in amongst a lot of residential users, will experience greater voltage imbalance swings throughout a day than a typical industrial installation might. The issue is, residences are all single phase and although the PoCo can attempt to balance loads by connection, they cannot control who is home consuming vs who is away not consuming. So their voltage balance can be easily thrown off. Then you set it up and everything works fine, but later the same day or a day or a week or a month later there is the right mix of residential users that makes the voltage imbalance swing more and bingo, your relay trips. By the time you get there, the resi mix changed and you no longer see the issue. It's almost impossible to catch without a recording meter.

That's the problem sometimes with electronic motor protection relays on small motors in areas like this. The factory defaults on imbalance are often low, because they expect these things to be used in more controlled environments. Your response of raising the trip threshold is pretty much what you have to do. If you look at most motor specs, they will not give you a CURRENT imbalance protection threshold, they will tell you the VOLTAGE balance must be within 5% (sometimes less). If I remember correctly, a 2% voltage imbalance can result in as much as a 10% current imbalance, which makes your 7% setting pretty difficult to maintain. But I'm going strictly on memory with that number, I'd have to look it up again and I'm too lazy this morning. But it is like I said, more dramatic than one would think.
 
Last edited:

mull982

Senior Member
I would say it's an ungrounded delta with those voltages, you probably have a high resistance fault somewhere causing the higher voltage to ground on "C". Could possibly be an ungrounded WYE, though not likely unless for some reason the ground was broken or disconnected. This is a POCO transformer?

Wouldn't you expect the G-Gnd voltage to be zero in the case of a high impedance fault on an ungrounded delta system?
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
Great articles guys!

Going along with what Jraef said, it does look like a small voltage imbalance really does have the potential to make a large current imbalance!

One article mentioned:
"Also, measure the current in each supply phase because the current unbalance is often about
6 to 10 times greater than the voltage unbalance."

That seems to be right on target. Mystery solved.:)

Now I won't have to wonder anymore when I come across this in the future....

Thanks again.
 
Great articles guys!

Going along with what Jraef said, it does look like a small voltage imbalance really does have the potential to make a large current imbalance!

Thanks again.

Correct. The other comment concerning the winding resistance tolerance of these type motors being wider than normal motors is also true. So you can have the 'perfect storm' when both of these factors play together. I seem to recall that we had this topic before, and I am making the same suggestion as I did then: rotate your connections, eg.move Phase A where Phase B is now, and so on, until you get the least current imbalance.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Now I won't have to wonder anymore when I come across this in the future....
That's how I learned it. I had done a job for a big vineyard in Napa, 18 submersible well pumps between 25 and 50HP, all with soft starters. Everything worked great, but the owner insisted that I use MotorSaver 777 solid state OL relays instead of what was built-in to the soft starters because the pump supplier would extend the warranty (he was also a MotorSaver dealer and I had to buy them from him). No problem, but a month after I commissioned them the 777s started tripping on Current Imbalance. I had left them at the factory default of I think 10%, but it turns out the pump dealer went in and set them all for 5%, because that's what the submersible motor mfr recommended. But when I read the pump manual, it said 5% VOLTAGE imbalance, not current, and they had the residential mix issue going on. Put a BMI recorder on it over a week and sure enough, the voltage imbalance would swing +-2-3%, always at about 6:00AM and again at 7:00 PM, basically when people left for work and again when they came home. That 2-3% voltage imbalance resulted in over 20% current imbalance, so I reset all of the Current Imbalance settings to 30%, never had another problem.
 

mull982

Senior Member
That's how I learned it. I had done a job for a big vineyard in Napa, 18 submersible well pumps between 25 and 50HP, all with soft starters. Everything worked great, but the owner insisted that I use MotorSaver 777 solid state OL relays instead of what was built-in to the soft starters because the pump supplier would extend the warranty (he was also a MotorSaver dealer and I had to buy them from him). No problem, but a month after I commissioned them the 777s started tripping on Current Imbalance. I had left them at the factory default of I think 10%, but it turns out the pump dealer went in and set them all for 5%, because that's what the submersible motor mfr recommended. But when I read the pump manual, it said 5% VOLTAGE imbalance, not current, and they had the residential mix issue going on. Put a BMI recorder on it over a week and sure enough, the voltage imbalance would swing +-2-3%, always at about 6:00AM and again at 7:00 PM, basically when people left for work and again when they came home. That 2-3% voltage imbalance resulted in over 20% current imbalance, so I reset all of the Current Imbalance settings to 30%, never had another problem.

Great discussion.

With a motor that has a reduced voltage on one of the phases would you expect to see more current on that phase compared to the others? Does it matter weather the source of this voltage reduction is located weather it is at the motor itself or somewhere upstream in the system?

I've also heard that impedance caused from loose connections can also cause voltage unbalance. If the impedance is large enough is it possibe that the current on this phase of the motor with the impedance in the system can be lower than the other two phases?
 

mbeatty

Senior Member
Location
Illinois
Great discussion.

With a motor that has a reduced voltage on one of the phases would you expect to see more current on that phase compared to the others? Does it matter weather the source of this voltage reduction is located weather it is at the motor itself or somewhere upstream in the system?

I've also heard that impedance caused from loose connections can also cause voltage unbalance. If the impedance is large enough is it possibe that the current on this phase of the motor with the impedance in the system can be lower than the other two phases?


I have seen this many times (high resistance connection due to loose or corroded connections), especially in marine environments. The phases with good connections had significantly higher current flow.
 
Status
Not open for further replies.
Top