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Here is the scenario; A brand new Trane AHU was installed by a contractor. It has a 3hp 460vac 3ph motor. The service factor is 1.25 with a temp rise of 40c. The motor will run for either hours or days then it blows a fuse in the disconnect. Here lately it has been blowing fuses within hours.

I double checked everything I could think of, wire size is correct for the motor size and length of run (12awg), the fuses are Bussman 600v dual element time delay fuses rated at 15 amps, the overloads were initially set at 125% but bumped up to 140% because they were tripping out also. The only discrepancy I found was the branch breaker was 20A and it should be 15A. I captured inrush at start up, it was 35.2A and running current was 5.0A and the voltage was 482vac ph-ph and 278vac ph - gnd while running and measured at the motor. The panel is a Square D I-line panel.

Trane pulled the motor last week and tested it, they said it was fine. I'm still waiting to find out what tests they did. No one wants to be forth coming with information.

For the last two days I've had my Fluke 1735 hooked capturing data and there is no anomaly there. My thoughts are it is a mechanical issue with either bearings on the motor or bearings on the squirrel cage blower. But of course the GC and Mechanical contractor are saying it's electrical regardless of the data I show them.

The only thing I haven't done is meggar the motor myself which I will do next week. I'll do my own DAR on the motor. I believe all Trane did was measure winding resistance.

Looking for any thoughts, suggestions or just dumb comments at this point. If anyone wants to look at the Fluke data just pm me and I'll email it in either a PDF format or the Fluke native format.
 

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Discussion Starter · #3 ·
Different phase every time. It has me scratching my head.
 

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If the fuse always varies, it means it's likely not a fault. If it were, it would have an impedance in the circuit based on it's position in the supplying phase or motor winding, and that impedance wouldn't change, meaning it would likely take out the same phase (or in the case of L-L fault, phases) repeatedly.

Given the above and the fact that the motor runs at all, sometimes for days, I am really leaning towards a mechanical problem with the driven load.

You said you got 5A running. Was that balanced on all phases? What's the FLA on the compressor?
 

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Fuse opens or Link melts? Thermal scan done?... Come to think of it ,I had the same problem with a Trane ahu. Random tripping of a 3 h.p. unit. Did everything you did then changed breaker to a 20 from a 7 amp. Had a single phase on the site and ended up putting in a new motor ...
 

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Discussion Starter · #6 ·
If the fuse always varies, it means it's likely not a fault. If it were, it would have an impedance in the circuit based on it's position in the supplying phase or motor winding, and that impedance wouldn't change, meaning it would likely take out the same phase (or in the case of L-L fault, phases) repeatedly.

Given the above and the fact that the motor runs at all, sometimes for days, I am really leaning towards a mechanical problem with the driven load.

You said you got 5A running. Was that balanced on all phases? What's the FLA on the compressor?
It is an industrial AHU fed by a 200 ton package chiller. I got 5.0A running on all phases. No compressor, no UV lamps just a blower motor and chill water coils.

Fuse opens or Link melts? Thermal scan done?
No thermal scan done. The fuse blows and sometimes the OLs open but not every time.

The AHU in question is a Trane and the package chiller is a Trane controlled by JCI controls.
 

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Discussion Starter · #7 ·
Inrush

Phase to Phase A-B

Phase A to Gnd

I didn't post all of the pictures but this should show the electrical characteristics.
 

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How long does it for the blower to reach running speed? I had a similar problem because the motor was undersized for the size of the fan. On a second occasion, it happened after the belts were changed and tightened too much.
 

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Here is the scenario; A brand new Trane AHU was installed by a contractor. It has a 3hp 460vac 3ph motor. The service factor is 1.25 with a temp rise of 40c. The motor will run for either hours or days then it blows a fuse in the disconnect. Here lately it has been blowing fuses within hours.

I double checked everything I could think of, wire size is correct for the motor size and length of run (12awg), the fuses are Bussman 600v dual element time delay fuses rated at 15 amps, the overloads were initially set at 125% but bumped up to 140% because they were tripping out also. The only discrepancy I found was the branch breaker was 20A and it should be 15A. I captured inrush at start up, it was 35.2A and running current was 5.0A and the voltage was 482vac ph-ph and 278vac ph - gnd while running and measured at the motor. The panel is a Square D I-line panel.

Trane pulled the motor last week and tested it, they said it was fine. I'm still waiting to find out what tests they did. No one wants to be forth coming with information.

For the last two days I've had my Fluke 1735 hooked capturing data and there is no anomaly there. My thoughts are it is a mechanical issue with either bearings on the motor or bearings on the squirrel cage blower. But of course the GC and Mechanical contractor are saying it's electrical regardless of the data I show them.

The only thing I haven't done is meggar the motor myself which I will do next week. I'll do my own DAR on the motor. I believe all Trane did was measure winding resistance.

Looking for any thoughts, suggestions or just dumb comments at this point. If anyone wants to look at the Fluke data just pm me and I'll email it in either a PDF format or the Fluke native format.
Overloads tripping at a setting of 125% is a huge clue as to the real cause. The fuses blowing is likely the result of having turned up the OL relay setting, the OL should be tripping FIRST, the fuses are really only supposed to deal with short circuits. In this case, the OL relay WAS tripping first, you elected to move the symptom further up stream.

Most likely it is in the motor, and you are probably right, the Trane guy just dropped his VOM on it, which means next to nothing. You need to do a proper megger reading on it to either find the real issue or eliminate that from the probable cause list.

If it isn't the motor, it might be the load. When you say you put your Fluke 1735 on it for a couple of days, were you just looking for obvious anomalies? Or did it blow fuses during that time period? If the fuses never blew while the Fluke was on there, then the only thing you have eliminated is odd ball utility supply anomalies. That's good, but it's not complete.

One possibility (further down the line) is that there is something in the way the system is operating that is causing this. For example, most likely the reason they picked a motor with a 1.25SF is because the system air balance is such that the fan blows too much air when the doors open, but they only expect that to be very temporary. Someone may be proping a door open for hours at a time, the static duct pressure drops and the fan overloads trying to make it up.

A similar more far fetch version is if there is a duct somewhere that is supposed to be closed, and some joker is manually opening it, the fan may be overloading as a result. I saw that happen once; a janitor was using an unused supply room as his nap spot, but there was no AC in there. So he cut a hole in a duct somewhere near by, ran his own duct to his nap room, and hid the louver for it. When he went in to take a nap, he opened the vent and sometimes the supply fan tripped on overload. The HVAC company came out several times to check the system, finding everything in order. But of course that vent was not in the design, so it wasn't until they happened to see the extra flow one day by being there at nap time that they hunted it down (thinking it was a leak in a duct).

Another possibility is if the fan is controlled by a contactor, the contactor may be chattering sometimes either because of a low control voltage condition that happens occasionally, or because whatever is telling that contactor to turn on, is fluttering on and off itself.
 

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I'm working on a 220 ton chiller package from trane right now with 14 blowers. No motors with a sf higher than 1.15 though and also no fuses. All of the fans have their own drive. Do you have trane guys on site? If it's a new commission and your electrical checks out I'd say it's their burden
 

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What does the supply voltage drop to under start up ?
You measured the current?
What about the voltage ?
Maybe it dips and more current flows.
 

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Greg said:
Here is the scenario; A brand new Trane AHU was installed by a contractor. It has a 3hp 460vac 3ph motor. The service factor is 1.25 with a temp rise of 40c. The motor will run for either hours or days then it blows a fuse in the disconnect. Here lately it has been blowing fuses within hours. I double checked everything I could think of, wire size is correct for the motor size and length of run (12awg), the fuses are Bussman 600v dual element time delay fuses rated at 15 amps, the overloads were initially set at 125% but bumped up to 140% because they were tripping out also. The only discrepancy I found was the branch breaker was 20A and it should be 15A. I captured inrush at start up, it was 35.2A and running current was 5.0A and the voltage was 482vac ph-ph and 278vac ph - gnd while running and measured at the motor. The panel is a Square D I-line panel. Trane pulled the motor last week and tested it, they said it was fine. I'm still waiting to find out what tests they did. No one wants to be forth coming with information. For the last two days I've had my Fluke 1735 hooked capturing data and there is no anomaly there. My thoughts are it is a mechanical issue with either bearings on the motor or bearings on the squirrel cage blower. But of course the GC and Mechanical contractor are saying it's electrical regardless of the data I show them. The only thing I haven't done is meggar the motor myself which I will do next week. I'll do my own DAR on the motor. I believe all Trane did was measure winding resistance. Looking for any thoughts, suggestions or just dumb comments at this point. If anyone wants to look at the Fluke data just pm me and I'll email it in either a PDF format or the Fluke native format.
Breaker size is usually bigger for inrush current not smaller with a fused disconnect protecting the branch circuit. The only thing that should be tripping is your OL on your starter. If it's still tripping I would start megging out the leads on the motor, if motor is good it's probably a mechanical issue. I am not a motor guru but that's where I would start lol
 

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Check for obstructions in the duct. Incorrectly installed fire dampers, modulating dampers that were miswired or entirely forgotten...
 

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I would be looking real close at the motor starter. Is their time delays on the dampers?? Loose wire on the coil? Cycling on/off? A large load nearby dragging the voltage down??
 

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Isn't a 35A starting current a bit on the high side for a 5A running load?
 

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I think there are 2 separate issues here.

1) 5.0 amps is really high for a 3HP motor operating on 480. The vast majority of them run from 3.6 to 4.6 This would explain why the O/Ls tripped at 125% but held at 140%.

2) It's simply not possible for a properly installed 15 amp fuse to blow while carrying 5 amps. Two things come to mind here; either the motor has an internal fault (a megger could, but not necessarily would, confirm this) or it's single phasing during starting.

Does the motor run continuously? If so, what is the source of the starter coil power? If it's anything electronic, and if it's possible to do so, I would connect the coil directly to a positively known source of continuous power and see what happens in a few days.

What type of starter is involved? My money is on some type of cheap IEC or definite-purpose type. These are notorious for failing, even right out of the box.

I would find it very easy to believe that the motor is being randomly shut off and when it restarts it is somehow (likely a bad starter) single phases.

Looking at the melt curve for a FRS-R15 fuse, at 35 amps of locked-rotor current and 140% O/Ls, it would be a tight race to see which one opens a single phase starting condition first. This would explain why fuses blow on different phases.
 

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Discussion Starter · #18 ·
I think there are 2 separate issues here.

1) 5.0 amps is really high for a 3HP motor operating on 480. The vast majority of them run from 3.6 to 4.6 This would explain why the O/Ls tripped at 125% but held at 140%.

2) It's simply not possible for a properly installed 15 amp fuse to blow while carrying 5 amps. Two things come to mind here; either the motor has an internal fault (a megger could, but not necessarily would, confirm this) or it's single phasing during starting.

Does the motor run continuously? If so, what is the source of the starter coil power? If it's anything electronic, and if it's possible to do so, I would connect the coil directly to a positively known source of continuous power and see what happens in a few days.

What type of starter is involved? My money is on some type of cheap IEC or definite-purpose type. These are notorious for failing, even right out of the box.

I would find it very easy to believe that the motor is being randomly shut off and when it restarts it is somehow (likely a bad starter) single phases.

Looking at the melt curve for a FRS-R15 fuse, at 35 amps of locked-rotor current and 140% O/Ls, it would be a tight race to see which one opens a single phase starting condition first. This would explain why fuses blow on different phases.
1) I agree but at rated voltage of 460vac the name plate amperage is 4.8A.
2) Monday I'm doing a DAR on the motor to satisfy my own couristy.

The motor is a continuously run motor. It is controlled by an across the line starter. Sq D is the manufacturer. 120vac HOA starter with a JCI current switch/relay controlling it in auto.

Tomorrow I'm moving the Fluke 1735 to the motor breaker to try and catch the instance the fuse blows. I still think I have a mechanical issue and if the bearings in the motor are bad or the bearings in the blower are bad, I should see a linear increase in current until the fuse blows.

I know I'm stubborn but how many times have we, as electricians, been blamed for stuff and once the investigation is complete it winds up being something else.
 

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Discussion Starter · #20 ·
Breaker size is usually bigger for inrush current not smaller with a fused disconnect protecting the branch circuit. The only thing that should be tripping is your OL on your starter. If it's still tripping I would start megging out the leads on the motor, if motor is good it's probably a mechanical issue. I am not a motor guru but that's where I would start lol
Not splitting hairs but you should look at the code for branch vs feeder, as it pertains to a motor circuit. In either case you are not allowed to step up 2 sizes. Branch can go up one size (when calculations result in a non-standard size) while a feeder cannot, a feeder must go down.

Breaker size is also determined by the type of breaker, instantaneous=800% and inverse time =250%, while dual element time delay fuses=175%. The motor amperage comes from the NEC tables not name plate data, you use name plate data for the sizing of the OLs.
 
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