two speed motor question

[tankfarms]

Recently, I've had a two-speed motor tripping motor feeder breaker (instantly) on high speed only.
First off, shown below are the motor specs
low speed: 480V, 100HP, FLA = 161amps, SF=1.0
high speed: 480V, 200HP, FLA = 225amps, SF=1.0
Motor starter is using a three contactor configuration: one for low speed, and one for high speed, and the third contactor is used to bring leads T1, T2, and T3 together during high-speed operation.

Normal opeartion use: blower
Operation procedure: start motor at low-speed first; after motor speed is established (and process conditions satisfied), hit "stop" button and then hit fast-speed start button to bring the motor to hi-speed.

Now, back to the question and how we troubleshoot: We disconnected the motor at the service head, and then energized the feeder and tried both low and hi speed, breaker did not trip. So we eliminated the potential problem of a feeder fault; then we de-coupled the motor from load and connected the leads back to the motor and solo run the motor-----it turned out that low speed is running fine, but as soong as we switch to hi-speed, it trips, and this is the same symptom with the motor having load on.

Interesting thing that we observed was that, during solo motor run, the motor was already pulling 175amps on all three phases,while its FLA for low-speed is 161.

We suspect that the motor is bad and possibly has a internal winding fault, however insulation resistance test didn't show any obvious results--we megged all six leads of the motor to ground, but each lead is reading above 6 Mohms.

So my question is, what could possiblly cause the trip? What kinda of internal motor internal fault could cause the motor to operate on low-speed only but not on hi-speed?

 

[templdl]

I this an existing installation where it had been working just fine and then started to trip the breaker recently?

 

[Cow]

We suspect that the motor is bad and possibly has a internal winding fault, however insulation resistance test didn't show any obvious results--we megged all six leads of the motor to ground, but each lead is reading above 6 Mohms.

I don't know what voltage you tested at, but if I tested a motor at 1000V and I got 6 Mohms, I'd be telling the customer to be looking for a replacement IMMEDIATELY. I use 50Mohms as a rule of thumb for a cutoff.

 

[sgunsel]

How much current when the motor is at high speed? Speeding up a fan motor requires a LOT more current. If this was operating OK before, something in the system has changed (motor, fan, drive, ???).

 

[Jraef]

I this an existing installation where it had been working just fine and then started to trip the breaker recently?

I agree this is the place to start. If it was working and now has a problem, I would suspect the motor. If it is new and has never worked, I would suspect the connections. For example you don't say if it is a 2S2W or 2S1W motor. 3 contactors is a 2S1W starter and if you have a 2S2W motor, there is a big problem right there!

 

[tankfarms]

I agree this is the place to start. If it was working and now has a problem, I would suspect the motor. If it is new and has never worked, I would suspect the connections. For example you don't say if it is a 2S2W or 2S1W motor. 3 contactors is a 2S1W starter and if you have a 2S2W motor, there is a big problem right there!

This is the same application which is in service for years. Just to bring some clarification that this is a 2S1W motor with 3 contactors in the stater (like said, one contactor is used to bring the leads 1,2,3 together).

We have a spare motor in the shop and we just finished solo run,
No-load current low-speed: 140 amps
No-load current hi-speed: 65 amps

This spare again is a 2S1W motor, and its specs are
low-speed, 900rpm, FLA181 amp, 100HP
hi-speed, 1800rpm, FLA 230 amps, 200HP

Still not sure why the no-load low-speed current is always higher than the no-load hi-speed current. How can I quantitatively verify that these amps are correct? Any formulas/equations out there?

 

[tankfarms]

How much current when the motor is at high speed? Speeding up a fan motor requires a LOT more current. If this was operating OK before, something in the system has changed (motor, fan, drive, ???).

Like said, main feeder breaker trips as soon as the motor is switched onto hi-speed, for both with load and without load on. Process condition/fan, blower stay the same, also, we've energzied the cable and hi-speed/lo-speed switching has no problem---this told us that if there was a fault, it was not on the feeder.

 

[Jraef]

By "no load" do you mean the motor is uncoupled? If so, the current readings are essentially meaningless, although I wouldn't expect them to appear THAT high. Uncoupled, the power factor is horrible, as in 0.20pf, so the current that you are measuring has next to nothing to do with real motor power.

With that motor showing the low speed HP as 1/2 of the high speed HP, that is a 2S1W Constant HP connection.

Are you absolutely sure you have the contactors and motor terminals connected correctly? Sounds kind of like that is not the case. Here is what your contactors should be doing:

For Low Speed, L1 - T1, L2 - T2, L3 - T3, then L4, 5, 6 left open.

For High Speed, L1 - T6, L2 - T4, L3 - T5, then the 3rd contactorshorts L1, 2, and 3

 

[tankfarms]

By "no load" do you mean the motor is uncoupled? If so, the current readings are essentially meaningless, although I wouldn't expect them to appear THAT high. Uncoupled, the power factor is horrible, as in 0.20pf, so the current that you are measuring has next to nothing to do with real motor power.

With that motor showing the low speed HP as 1/2 of the high speed HP, that is a 2S1W Constant HP connection.

Are you absolutely sure you have the contactors and motor terminals connected correctly? Sounds kind of like that is not the case. Here is what your contactors should be doing:

For Low Speed, L1 - T1, L2 - T2, L3 - T3, then L4, 5, 6 left open.

For High Speed, L1 - T6, L2 - T4, L3 - T5, then the 3rd contactorshorts L1, 2, and 3

You're absolutely correct, three contactors do exactly like you said here. We disconnected the motor leads, and energized the cable and all three contactors function the way they're supposed to, that is, we were able to "run" with lo and hi speed without any issues. As soon as the motor is connected and even run with no load (yes, that's uncoupled without load), motor trips on high speed.

 

[Jraef]

Just a minor correction to my previous post, I typed "constant HP" when I meant constant torque.

If you are sure that the motor leads you are connecting to the contactors are as I showed above, and nothing has changed since it worked last, then you have only two remaining possibilities.
1) you have a bad motor feed on 4, 5, or 6. Since these are left open in low speed, a short to ground goes unnoticed. As soon as you switch to high speed you have a ground fault. Since you said (I think) that you did this test, it is already eliminated.

2) you have a short inside of the motor. Same place, 4, 5, or 6. You said that you meggered all 6 leads at 6Mohms to ground. The basic rule of thumb on megger readings is 1 Meg per 1000V of operating voltage, with a minimum of 1 Meg. So by that you should be OK. But did you short them all together and Meg them all at the same time? Or did you check each motor lead individually? In Y connected motors, shorting them all together can mask a leakage problem, and residual polarization can give you false resistance readings if you do individual tests too quickly. If individually, did you wait at least 4x the test voltage in minutes between tests to allow the circuit to discharge? If you tested at 1000V, wait 4 minutes between tests, 2 minutes if done at 500V (although you should be doing this at 1000V).

 

[tankfarms]

Just a minor correction to my previous post, I typed "constant HP" when I meant constant torque.

If you are sure that the motor leads you are connecting to the contactors are as I showed above, and nothing has changed since it worked last, then you have only two remaining possibilities.
1) you have a bad motor feed on 4, 5, or 6. Since these are left open in low speed, a short to ground goes unnoticed. As soon as you switch to high speed you have a ground fault. Since you said (I think) that you did this test, it is already eliminated.

2) you have a short inside of the motor. Same place, 4, 5, or 6. You said that you meggered all 6 leads at 6Mohms to ground. The basic rule of thumb on megger readings is 1 Meg per 1000V of operating voltage, with a minimum of 1 Meg. So by that you should be OK. But did you short them all together and Meg them all at the same time? Or did you check each motor lead individually? In Y connected motors, shorting them all together can mask a leakage problem, and residual polarization can give you false resistance readings if you do individual tests too quickly. If individually, did you wait at least 4x the test voltage in minutes between tests to allow the circuit to discharge? If you tested at 1000V, wait 4 minutes between tests, 2 minutes if done at 500V (although you should be doing this at 1000V).

Hi, thanks for your input.

First of all, we have confirmed that it was indeed a moto issue---we installed our new spare motor and motor is in service with no problem on both low and hi speeds. So as shown in your previous reply post, the second one was the issue.

Now, in the process of troubleshooting, we did check each of the 6 leads individually, and I got 6~7 Mohms on the megger reading. But, no, we did not short all the leads together as I thought that wouldn't make a difference b/c I think if there was a fault/short (granted, it was a fault to ground), checking individual lead would pick up. Now, knowing what I know now, it's definitely an internal fault in the motor and Megger in this case is limited to make the detection.

I'm now thinking of making a proposal in our plant to make some investement on better motor testing equipment (i.e. PdMA set), as we've had so many cases that Megger wasn't able to identify motor fault but Megger is pretty much the only thing our technicians have.