Motor Instantly Tripping Breaker

[jerjwillelec]

I wish I had more opportunities to troubleshoot motors because my inexperience causes me to second guess my procedures and finally my diagnosis. Here's what I've got on the two 460 volt 2 HP motors that are tripping the breaker on startup.

Motor A has 498 vac across all three phases prior to startup. My resistance readings are (1 to 2) >22k ohm, (2 to 3) >22 k ohm, (3 to 1) 8.64 ohm. I megged all three leads individually to the case and none of them would take a charge.

Motor B has 498 vac across all three phases prior to startup. My resistance readings are (1 to 2) 6.85 ohm, (2 to 3) 8.62 ohm, (3 to 1) 7.22 ohm, (1 to 4) .84 ohm, (2 to 5) 2.23 ohm, (3 to 6) 2.24 ohm. I megged all three leads individually to the case each of them read 445 M ohm at 1000 vdc.

I do appreciate any elementary explanations of this. Thank you!

 

[kwired]

Would you please expand on what you mean by this? This is a shaft mounted fan. Guess I'm wondering where bearing failure comes into play. Thank you!

On such equipment as I mentioned daily temperature swings cause condensation which if it gets into the bearings shortens the life of the bearing. If the bad bearing adds enough load to the input current, overload protection may respond, but often this kind of load doesn't run at 100% of motors rated current so any heat produced in the bearing still migrates to motor winding with overload devices that are current sensing only having no idea of the actual winding temp. And there are cases like mentioned where the rotor will rub on the stator. Most of those cases end up in winding failure also as there is too much heat even if current was not too high.

 

[GoldDigger]

I wish I had more opportunities to troubleshoot motors because my inexperience causes me to second guess my procedures and finally my diagnosis. Here's what I've got on the two 460 volt 2 HP motors that are tripping the breaker on startup.

Motor A has 498 vac across all three phases prior to startup. My resistance readings are (1 to 2) >22k ohm, (2 to 3) >22 k ohm, (3 to 1) 8.64 ohm. I megged all three leads individually to the case and none of them would take a charge.

Motor B has 498 vac across all three phases prior to startup. My resistance readings are (1 to 2) 6.85 ohm, (2 to 3) 8.62 ohm, (3 to 1) 7.22 ohm, (1 to 4) .84 ohm, (2 to 5) 2.23 ohm, (3 to 6) 2.24 ohm. I megged all three leads individually to the case each of them read 445 M ohm at 1000 vdc.


I do appreciate any elementary explanations of this. Thank you!

Motor A appears to have an open lead, allowing current to flow in only one winding. This single phasing will prevent the motor from starting, since there will be no torque.

Motor B, on the other hand, appears to have an internal turn to turn short in one winding, which will cause a high current in that winding.

PS the open circuit motor A may be the result of a failed internal overload protector. The very low resistance.to the case is harder to explain, since it is inconsistent with the high line to line meter readings. Unless your Megger will not read as low as 22k.

 

[meternerd]

Do you have ANY motors that run correctly? If so, take the readings (Winding Ohms and Megohms to ground) at the load side of the starter. If they are different than the motors that trip, the motors are bad. Once you have that information, take the same readings at the motor pecker head. Compare good to bad again. Same deal. Minor differences are OK, but large differences aren't. Winding resistances are typically only a few ohms and almost the same on each phase. Have you tried spinning the fans by hand? Any noise or binding could be a problem. Are the breakers sized correctly" Starting current can be 5 to 7 times full load running current.

Instantaneous trips usually indicate a short. Mechanical damage or a single phase condition will usually cause a bit of a delay before tripping.

 

[petersonra]

Do you have ANY motors that run correctly? If so, take the readings (Winding Ohms and Megohms to ground) at the load side of the starter. If they are different than the motors that trip, the motors are bad. Once you have that information, take the same readings at the motor pecker head. Compare good to bad again. Same deal. Minor differences are OK, but large differences aren't. Winding resistances are typically only a few ohms and almost the same on each phase. Have you tried spinning the fans by hand? Any noise or binding could be a problem. Are the breakers sized correctly" Starting current can be 5 to 7 times full load running current.

Instantaneous trips usually indicate a short. Mechanical damage or a single phase condition will usually cause a bit of a delay before tripping.

megging from the starter end includes the downstream wiring so it does not prove the motor is anything. it will tell you that the downstream stuff is either Ok or something is bad, but it could be the wiring or the motor.

spinning the fans by hand is a good thing to try as well. if there is mechanical binding of some sort it should be fairly obvious.

one thing that might be at issue as well is that if the fans don't have brakes on them, wind might be blowing them backwards and that will create a very high starting current. I would make sure that the fans are not moving when you try to start them after you check for mechanical binding of some sort.

if the fans have brakes on them you might want to make sure they are being released when the starter picks up.

 

[kwired]

megging from the starter end includes the downstream wiring so it does not prove the motor is anything. it will tell you that the downstream stuff is either Ok or something is bad, but it could be the wiring or the motor.

spinning the fans by hand is a good thing to try as well. if there is mechanical binding of some sort it should be fairly obvious.

one thing that might be at issue as well is that if the fans don't have brakes on them, wind might be blowing them backwards and that will create a very high starting current. I would make sure that the fans are not moving when you try to start them after you check for mechanical binding of some sort.

if the fans have brakes on them you might want to make sure they are being released when the starter picks up.

If wind is turning it backward, yes you will have even higher starting current, but in my experience most of these types of fans don't have motors with a real high torque production, and unless the wind is really strong that fan will not have that much speed to overcome. The fan he is describing if what I think it is, is usually an axial type fan mounted directly on motor shaft, usually is a 3600 RPM motor (less slip so about 3450 in most cases). High speed motors have less torque than lower speed motors and are designed with higher starting current in mind because of the lower torque they anticipate longer starting time. I have yet to encounter from my recollection, a fan of this type that failed to start because it was rotating backward before energizing. If you set overcurrent protection too tight, I suppose it can happen though.

 

[ActionDave]

I wish I had more opportunities to troubleshoot motors because my inexperience causes me to second guess my procedures and finally my diagnosis. .....

I do appreciate any elementary explanations of this. Thank you!

In short you either got a bad motor or bad conductors feeding the motor.

A megger is a good tool for tracking down an intermittent problem or heading of a problem coming on sometime later.

It the motor is not working you either have a bad motor or bad conductors powering the motor. Unhook the motor, if something still trips it's the wires to the motor. If nothing trips it is the motor.

If it is the motor it is either bad windings or bad bearings, or if you are lucky, something obvious that is stopping the motor from spinning. Bad windings trip breakers and smell awful; very similar to the smell of bad conductors that feed the motor. Bad bearings trip overloads and make a lot of noise, unless they are really bad and the motor is dragging iron and can't start.

Motors small enough to be carried by one person- replace the motor. Motors that need two or more people to load in the truck are worth taking to a motor shop.

 

[GoldDigger]

It the motor is not working you either have a bad motor or bad conductors powering the motor. Unhook the motor, if something still trips it's the wires to the motor. If nothing trips it is the motor.

This is only true if the trip is truly at a short circuit level of current.
If one of the conductors is open, the resulting single phasing of the motor will cause very high currents in the two remaining phase wires (assuming for the moment a delta wired motor) which could trip the OCPD promptly although not instantly, and would not cause the motor to turn, just hum, during the time it stays on.

 

[kwired]

Motors small enough to be carried by one person- replace the motor. Motors that need two or more people to load in the truck are worth taking to a motor shop.

Somewhat of a general rule- my version goes more like - if it is a NEMA standard frame, is 1800 or 3600 RPM (synchronous speed), and is 10HP or less, you generally replace it, rewind costs just as much if not more. If it has unique properties such as non standard or typical shaft characteristics, is multispeed, or is not a NEMA general purpose type of motor rewinding may or may not cost more, but you also need to consider how long it may take to get a replacement vs rewinding if you need to get equipment running ASAP.

Once had a motor shop rewind what I though would be an easy motor to replace only because they said they could have it rewound faster than I could get the replacement, of course if willing to spend the time looking or possibly pay extra shipping I probably could have found a replacement faster.

Often if you have say a C-face frame needing replacement and one isn't readily available you can get same motor with rigid base and change the ends if in a pinch. I have also had motors with special shaft but other specs being same and just replaced the rotor in the new motor with the rotor from the old motor. This not only saved time needed to get the replacement, but the general purpose motor cost much less than the one with OEM specs on the shaft.

Of course this kind of thing could void warranties, but when it is cold outside and customer has no circulating pump on their boiler, loss of warranty on a $300 motor is not much of a loss.

 

[jerjwillelec]

Thank you all! You have all been very helpful! I took both motors in to be looked at and will let you know what the problem(s) were. I'm also going to be looking into a class I can take to learn more about motors.

 

[cowboyjwc]

I don't know since we don't have any grain silos here, but I do know grain dust is highly combustable, but is it also conductive? Just wondering if grain dust was getting sucked into the motors if that would cause a short. Or maybe gum up the bearings so that it was having trouble turning.

 

[ActionDave]

This is only true if the trip is truly at a short circuit level of current.
If one of the conductors is open, the resulting single phasing of the motor will cause very high currents in the two remaining phase wires (assuming for the moment a delta wired motor) which could trip the OCPD promptly although not instantly, and would not cause the motor to turn, just hum, during the time it stays on.

Good clarification. Thanks.

 

[ActionDave]

..... If it has unique properties such as non standard or typical shaft characteristics, is multispeed, or is not a NEMA general purpose type of motor rewinding may or may not cost more, but you also need to consider how long it may take to get a replacement vs rewinding if you need to get equipment running ASAP.

And is the only reason our shop would ever consider rewinding a small motor.

Often if you have say a C-face frame needing replacement and one isn't readily available you can get same motor with rigid base and change the ends if in a pinch. I have also had motors with special shaft but other specs being same and just replaced the rotor in the new motor with the rotor from the old motor. This not only saved time needed to get the replacement, but the general purpose motor cost much less than the one with OEM specs on the shaft.

Which is the one reason we throw old motors in the bone pile instead of the dumpster. The one time you get to use an old part is like a gamblers high when the slot machine hits 7 7 7.

Of course this kind of thing could void warranties, but when it is cold outside and customer has no circulating pump on their boiler, loss of warranty on a $300 motor is not much of a loss.

True dat.

 

[kwired]

I don't know since we don't have any grain silos here, but I do know grain dust is highly combustable, but is it also conductive? Just wondering if grain dust was getting sucked into the motors if that would cause a short. Or maybe gum up the bearings so that it was having trouble turning.

OP mentioned his fans in question happened to be roof fans. I really don't know the intent of fans located on roof other than to assist removing heat from the roof area of the bin. They certainly are never large enough fans to pull any significant amount of air through the stored grain, and you maybe only see them on about 10-20% of the bins at commercial elevators, I can't ever recall seeing one on a bin on a farm.

As far as dust goes, the roof fan will experience more dust than the main "crop drying" fans do, but really will only see dust when bin is being filled, as that is when dust will be in the air. The main "crop drying" fans are not really subjected to much dust as they are usually pulling fresh air and pushing it into the bin, the bin has a raised floor with slots in the floor so the air will push through the floor and through the grain and exit via vents or fans mounted on the roof. Depending on size of bin, and amount of desired air flow these fans may range from a couple of horsepower to 40 or 50 horsepower or even larger.

Most of the dust from corn is what they call chaff, it is kind of reddish material that is essentially not from the kernel but is part of the cob the kernels come from. There is finer white powder that develops that is essentially corn starch for the most part, that simply is removed from the kernels by abrasion when handling. Augers are hard on the kernels. If you ever work around places that are handling "seed" intended for planting they want as little seed damage as possible and never use augers, always belt conveyors.

I don't think the dust is generally all that conductive, but it would depend some on moisture levels I would think also, but higher moisture grain probably creates less dust too, but higher moisture also requires more air through the grain or it will spoil faster.

 

[cowboyjwc]

OP mentioned his fans in question happened to be roof fans. I really don't know the intent of fans located on roof other than to assist removing heat from the roof area of the bin. They certainly are never large enough fans to pull any significant amount of air through the stored grain, and you maybe only see them on about 10-20% of the bins at commercial elevators, I can't ever recall seeing one on a bin on a farm.

As far as dust goes, the roof fan will experience more dust than the main "crop drying" fans do, but really will only see dust when bin is being filled, as that is when dust will be in the air. The main "crop drying" fans are not really subjected to much dust as they are usually pulling fresh air and pushing it into the bin, the bin has a raised floor with slots in the floor so the air will push through the floor and through the grain and exit via vents or fans mounted on the roof. Depending on size of bin, and amount of desired air flow these fans may range from a couple of horsepower to 40 or 50 horsepower or even larger.

Most of the dust from corn is what they call chaff, it is kind of reddish material that is essentially not from the kernel but is part of the cob the kernels come from. There is finer white powder that develops that is essentially corn starch for the most part, that simply is removed from the kernels by abrasion when handling. Augers are hard on the kernels. If you ever work around places that are handling "seed" intended for planting they want as little seed damage as possible and never use augers, always belt conveyors.

I don't think the dust is generally all that conductive, but it would depend some on moisture levels I would think also, but higher moisture grain probably creates less dust too, but higher moisture also requires more air through the grain or it will spoil faster.

Gee you sound like you're from Nebraska.

 

[kwired]

Gee you sound like you're from Nebraska.

Certain times of year grain bins and associated equipment is all I see at work on daily basis.

When it comes to service calls on this equipment, guess when the heaviest amount of calls come in? And guess how many of those problems could have been found sooner if they just ran the equipment for a little bit before they actually needed it?

 

[ptonsparky]

Certain times of year grain bins and associated equipment is all I see at work on daily basis.

When it comes to service calls on this equipment, guess when the heaviest amount of calls come in? And guess how many of those problems could have been found sooner if they just ran the equipment for a little bit before they actually needed it?

Are you trying to tell me that you get calls this time of year where they actually have power to the bin site? Before they have a grain cart waiting? And you live in.what state?

 

[kwired]

Are you trying to tell me that you get calls this time of year where they actually have power to the bin site? Before they have a grain cart waiting? And you live in.what state?

To be truthful, I have not really had much of that kind of problem. I have many times been installing wiring while bin is being filled or even after it is full. They almost always have power before the bin goes up because the bin erecting crew usually wants power. That said, I will not give much praise about some of what I have seen them connect for their needs when erecting the bin.