Internal Motor Fault ?

[TwoBlocked]

Usually, but not always, I can point to a bad resistance between terminals or a bad megger resistance to ground to say a motor is bad. Today was not one of those times. It was a vibrator 480V 3-ph with 3 FLA, WYE configuration. It was tripping OL, drawing 25 amps an each of the three legs. Took the covers off and it spun freely by hand. OK, measuring from the top of the contactor with the contacts pushed in it's not an open, ground, or mechanical binding, nor an obvious short. That leave an internal fault. I was expecting to see a current imbalance between the legs. That points to one of the coils in this wye motor, but they all read the same. That leaves the stator and the rotor. That also leaves me behind...

So motor gurus: what sort of fault might cause high current in a motor that passes ordinary test?

 

[retirede]

Overloaded mechanically?

 

[ptonsparky]

Do a Smell Test. Fail? Doesn't matter what the rest of the tests indicate. 25 amps on a 3 amp motor?

Don't waste time looking if it's not a mechanical overload.

 

[TwoBlocked]

Overloaded mechanically?

No, motor and load spins freely.

 

[TwoBlocked]

Do a Smell Test. Fail? Doesn't matter what the rest of the tests indicate. 25 amps on a 3 amp motor?

Don't waste time looking if it's not a mechanical overload.

I'm just curious what would cause it. The overload caught it before it could burn up.

 

[retirede]

I'm just curious what would cause it. The overload caught it before it could burn up.

Does the motor run properly when de-coupled from the load?

 

[TwoBlocked]

Does the motor run properly when de-coupled from the load?

It would have been difficult to do, and no need to. The way it works is: The shaft is double ended. At each end there is a non-centric weight. When the motor spins, the motor and what it is bolted to vibrates. The motor and the weights (load) spin freely with the power off.

 

[TwoBlocked]

While I have some of you folk's attention, got a question. These are 6 lead, wye connected motors. The coil terminations are landed on studs. Three of them are shorted with a bar. The other three are fed from the starter.

A common problem with these is over-torquing of the nut on the motor lead studs resulting in stripped threads on the studs and loose motor lead connections. Am thinking one solution is to switch the ends of the leads. Like remove the shorting bar and connect the motor leads to what was the wye end of the coils, and just bug the other ends of the coils leads together to make the new wye.

I see no reason this would not work. Comments?

 

[Jraef]

No, motor and load spins freely.

Bad axial thrust bearing. Axial thrust is the movement along the direction of the motor shaft that only happens when the rotor is magnetized by the stator. Thrust bearings keep it from jamming when it does that. If the thrust bearings go bad, the rotor jams, but ONLY when energized. De-energized, it will appear fine.

 

[TwoBlocked]

Bad axial thrust bearing. Axial thrust is the movement along the direction of the motor shaft that only happens when the rotor is magnetized by the stator. Thrust bearings keep it from jamming when it does that. If the thrust bearings go bad, the rotor jams, but ONLY when energized. De-energized, it will appear fine.

Huh! Good to know. Don't know if these have thrust bearings, though.

 

[Jraef]

Huh! Good to know. Don't know if these have thrust bearings, though.

Sure, maybe not, but axial thrust happens anyway, so it can affect regular bearings, especially in high vibration applications. Like vibrators. I’ve worked on a lot of vibrating screens in the mining and rock crushing industry, that’s a common symptom; seems to spin fine unpowered, jams when energized.

Sleeve bearings typically allow a certain amount of axial thrust (also called “end play” by the way), but roller or ball bearings, not as much. So another thing that can happen is that if there are some other components in the mechanical power transmission system (gearbox, couplings, driveshaft etc.), and THOSE devices cannot handle axial thrust, so your motor has to be the type that limits it. What I’ve seen happen is that a motor that had ball bearings goes bad, someone has a sleeve bearing motor at hand, so he grabs it and puts it on, not understanding that the allowable thrust from that design is too much for the other mechanical components. The way to tell if that’s an issue it to bench test the motor. If it spins fine under power, but jams under load, you might have the wrong motor.