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Old 09-15-2019, 04:44 PM   #1
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Default 25 hp Wye Delta motor problem

We have a customer with a 25hp motor connected to a plant wide vacuum blower motor, The complaint is C phase is blowing a 35 amp fuse when trying to start.
The motor has been in service for about 3 years.
Its a Wye-Delta motor that is set up to run configured as a delta.

The maintenance guy had a megger he insisted on using, I couldn't figure out how to use his, he didnt have a clue and mine was in my truck.
I used a meter to check out the windings. All readings were consistent with each set, none were shorted to ground.
The motor will turn by hand, I couldn't feel any kind of load or a rough bearing
My opinion was that it needed to be removed and sent to a motor shop.

Question,
Is it common practice to set up a wye/ delta motor up to run on a single 3 pole contactor wired as Delta?
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Old 09-15-2019, 05:05 PM   #2
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I would expect a 25 HP motor to blow a 35 amp fuse if it starts across-the-lines. I would use 50 amp dual-element time-delay fuses, especially on a fan that takes a couple of seconds to come up to speed.

It's very common to start a Y-∆ motor across-the-lines, if so, it needs to be connected ∆. The configuration of the power source has no effect on the configuration of the motor.

If it's a 12 lead motor (most 25s are), then the lead connections would be L1 = T1 & T12. L2 = T2 & T10. L3 = T3 & T11. T4 & T7 splice together as do T5 & T9 and T6 & T9.

In your case, it's almost certainly connected correctly, the fuses are too small.

Generally speaking, for a 460 Volt 3Ø motor, the breaker and fuse size is double the HP. This works even at several hundred HP.
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Old 09-15-2019, 05:31 PM   #3
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,,,,,,,,,,,,
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Old 09-15-2019, 05:36 PM   #4
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So many tests have not been done it impossible to tell. Basic things like checking to see if its dropped a phase or has a short anywhere from the breaker (fuses) to motor winding including the stater.

Micromind is correct about both fuse type and size. In this industry sometimes you don't get the full story which means you have to troubleshoot as a un-know system rather than taking for granted what you are told.

If c phase is the only fuse blowing that to me says

1/ c has a short to ground any where from fuse to motor winding
2/ a or b has a open winding
3/ motor/blower had a problem in the past that blew c phase and plant maintenance replaced with wrong type of fuse. (common one)
4/ possible winding shorting (hard to prove unless you have the meter)
5/ some type of device used to reduce the blower load on start up is no longer working
6/ what looks new or out of place

Its always interesting to have only one fuse blow. Blowing C means you now have a single phased motor which should quickly overload A or B and blow one of them unless the control transformer is hooked to C which then adds another possible problem.
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Old 09-15-2019, 06:08 PM   #5
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I going to assume this is a 460 system correct? If so then as @micromind says at least double the FLA, my old CH slipstick says to use a 60 amp TD fuse. @gpop gives good advice also, look for a broken damper that closes on startup, or the controls which activate it. If C phase is the consistent fuse blown, pop open the contact cover on the starter and inspect them [If NEMA]. A consistent blow points to a voltage drop and spent contacts can and will blow a fuse or trip a breaker, especially on startup. A data logger or an old [or new] Simpson 260 can see a voltage drop problem on startup that a digital [Fluke] can't follow.
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Old 09-15-2019, 06:12 PM   #6
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Quote:
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,,,,,,,,,,,,
The same fuse, C phase but, I believe they installed a different type. I did disconnect the motor from the circuit and energized it to test out the branch wire.
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Old 09-15-2019, 07:56 PM   #7
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The same fuse, C phase but, I believe they installed a different type. I did disconnect the motor from the circuit and energized it to test out the branch wire.
This is basically a megger test using line voltage. It'll prove whether or not the writing is good all the way to the motor itself.
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Old 09-15-2019, 09:04 PM   #8
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This is basically a megger test using line voltage. It'll prove whether or not the writing is good all the way to the motor itself.
He didn’t have clips for the leads so, I was out.
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Old 09-15-2019, 09:04 PM   #9
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Quote:
Originally Posted by 460 Delta View Post
I going to assume this is a 460 system correct? If so then as @micromind says at least double the FLA, my old CH slipstick says to use a 60 amp TD fuse. @gpop gives good advice also, look for a broken damper that closes on startup, or the controls which activate it. If C phase is the consistent fuse blown, pop open the contact cover on the starter and inspect them [If NEMA]. A consistent blow points to a voltage drop and spent contacts can and will blow a fuse or trip a breaker, especially on startup. A data logger or an old [or new] Simpson 260 can see a voltage drop problem on startup that a digital [Fluke] can't follow.
It’s 480 volt
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Old 09-15-2019, 09:30 PM   #10
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25hp is about 32 amp name plate with a 6-7 instantaneous so maybe 190amps on starting.
As they claim it has worked for years that means the fuses must be time delayed and the load must be easy to spin up.

What gets me is a motor that's either spinning up, at speed or during the inrush blows a fuse then doesn't instantly go unbalanced (single phase) and take out a second fuse.

Phase protection relay after the fuses, control transformer is using c leg or one phase is already open would account for only C blowing.
If the motor has lost a phase then its already single phased and on starting will only need to blow one fuse to cut power and fastest fuse wins.

Knowing what the motor does during starting would be real useful as that can tell you alot about whats happening.
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Old 09-15-2019, 11:45 PM   #11
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Looking at the melt curves for Buss FRS (600 volt, time delay), a 35 amp model will blow in about 5 seconds at 180 amps. A 50 amp one will blow in about 45 seconds.

These ratings assume the start was made when the fuses were at room temperature, not within a few minutes of running or quite a few minutes after a successful start.
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Old 09-16-2019, 04:59 AM   #12
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I think the first order of business is to see if there is voltage drop on start up, I would test at the motor JB to test the whole system. If there is no significant drop, and if you can fuse it stout enough to get a run, then a careful check of current with a tong meter is next. A shorted turn in the motor could be the cause then, or an open or loose connection in the make up connections in the JB would be a culprit.
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Old 09-16-2019, 07:58 AM   #13
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Quote:
Originally Posted by 460 Delta View Post
I think the first order of business is to see if there is voltage drop on start up, I would test at the motor JB to test the whole system. If there is no significant drop, and if you can fuse it stout enough to get a run, then a careful check of current with a tong meter is next. A shorted turn in the motor could be the cause then, or an open or loose connection in the make up connections in the JB would be a culprit.
I did have a chance to open and re-splice each of the connections in the peckerhead.
I didn't spend as much time on it as I should have. I was using someone else's tools, it was getting late and now that I think about it, the fuse that kept tripping looked like a different brand or different color than the others, two green, one red. The fuse holder was in the IEC rotary handle disconnect and the maintenance guy was afraid to handle the fuses for some reason.
I think he might have had a problem inside of the cabinet, got bit or something.
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Old 09-16-2019, 05:42 PM   #14
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Until you do the suggested tests, we can only guess.


My guess is that they have an unbalanced system with a number of single phase loads (through transformers or otherwise) tapped off of C phase disproportionately. So when this motor starts, there is more of a voltage drop on C phase than on the other two phases, causing higher current on that phase, causing that to be the first fuse to blow. Couple that with mismatched fuses and you keep repeating the same actions, expecting different results (the definition of insanity).
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Old 09-16-2019, 11:22 PM   #15
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I collected a couple of days worth of data once every second.
It does show somewhat of a sag on C phase.

https://www.dropbox.com/s/wudm9msb4r...20MDP.csv?dl=0
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Old 09-17-2019, 06:57 AM   #16
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Everything has "always worked before" until it doesn't.

Yes, the fuses are too small, so some dynamic has changed to cause the fuse to begin blowing.

If "C" phase is drooping more than the other phases, there could be a problem with connections or fuse holders/breakers any where in the system, even on the utility side. Often as mechanical equipment ages and begins to wear and deteriorate, the load on the motor will increase a little. If the if the over current device is at the lower limits, you have a fuse tester. This being a fan, you never know what has happened to change the air flow. If a fan of this size starts at full air flow, the starting current will not drop off as quickly since as soon as the fan is moving at about half speed, it is moving a lot of air.

It could be as simple as someone having thrown away the inlet filter. As with most problems, there is probably some combination of issues.
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Old 09-17-2019, 07:32 AM   #17
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Quote:
Originally Posted by Southeast Power View Post
I collected a couple of days worth of data once every second.
It does show somewhat of a sag on C phase.

https://www.dropbox.com/s/wudm9msb4r...20MDP.csv?dl=0
I would like to see the starts that blow the fuse marked on that graph.
Attached Thumbnails
25 hp Wye Delta motor problem-chart.png  

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Old 09-17-2019, 07:27 PM   #18
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Quote:
Originally Posted by varmit View Post
Everything has "always worked before" until it doesn't.

Yes, the fuses are too small, so some dynamic has changed to cause the fuse to begin blowing.

If "C" phase is drooping more than the other phases, there could be a problem with connections or fuse holders/breakers any where in the system, even on the utility side. Often as mechanical equipment ages and begins to wear and deteriorate, the load on the motor will increase a little. If the if the over current device is at the lower limits, you have a fuse tester. This being a fan, you never know what has happened to change the air flow. If a fan of this size starts at full air flow, the starting current will not drop off as quickly since as soon as the fan is moving at about half speed, it is moving a lot of air.

It could be as simple as someone having thrown away the inlet filter. As with most problems, there is probably some combination of issues.
I once had to troubleshoot a 350HP blower for a walnut huller. He had been blowing out contactors, so he changed to a soft starter, but that just resulted in it tripping before the motor could start. Turned out he had removed the damper, so the blower was pulling too much current. In HIS mind, he removed it because having the damper closed was making the motor work HARDER, when in fact it is exactly the OPPOSITE. In a fan / blower, load = flow. So no flow, no load; full flow, full load. (Same is true for centrifugal pumps too by the way).
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Old 09-17-2019, 09:17 PM   #19
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I see people do that to pumps all of the time. Now I suspect the intake piping has changed. I’ll get that fuse problem sorted out and go from there.
I’m going to pull all three of them when I go back and post a pic.
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Old 09-20-2019, 05:34 PM   #20
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Quote:
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I would expect a 25 HP motor to blow a 35 amp fuse if it starts across-the-lines. I would use 50 amp dual-element time-delay fuses, especially on a fan that takes a couple of seconds to come up to speed.

It's very common to start a Y-∆ motor across-the-lines, if so, it needs to be connected ∆. The configuration of the power source has no effect on the configuration of the motor.

If it's a 12 lead motor (most 25s are), then the lead connections would be L1 = T1 & T12. L2 = T2 & T10. L3 = T3 & T11. T4 & T7 splice together as do T5 & T9 and T6 & T9.

In your case, it's almost certainly connected correctly, the fuses are too small.

Generally speaking, for a 460 Volt 3Ø motor, the breaker and fuse size is double the HP. This works even at several hundred HP.
My SD cheat card say s 70 amp breaker. Since we do not know the voltage or if it is nine wire motor. If nine wire delta it connects different than a wye on 240 Volt
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