Advanced methods to determine a short in a single phase motor winding?

[fastline]

We are converting a machine to 3 phase I guess but my brain wants to know what happened to this 7.5HP 1P motor! Was running for years, then decided to have fits and will now continually blow the overload.

The windings tested fine with a megger, the aux winding measures approx 1.0 ohms, and main winding at 0 ohms. Not unheard of. the motor will start and seems to get close to sync speed without load, but the amps are very high and it growls like it is struggling. I even made a switch to uncouple the start caps thinking the start switch might be hung up. Nope.....

So... no load on motor, cap start/cap run, pulling 220-240A at 240V.

Took motor apart expecting something to be evident. NOTHING! Clean, no burns, no smoke, looks perfect. I just cannot understand it! Usually when a motor goes, you know it. This is very odd! I want to test the main winding for an internal short but how?

 

[dknguyen]

Could you test the inductance? If it actually tests 0 ohms though...that's really low (I think? I guess it depends on the size of the motor though).

 

[shortbus=]

So... no load on motor, cap start/cap run, pulling 220-240A at 240V.

That surely is a misprint. A single phase motor of that size should only draw around 22Amps full load.
From this chart - https://www.aic-controls.com/motor-fla

 

[fastline]

That is not a misprint the the exact reason I made the thread. She is pulling basically LRA, which that amperage would be normal at inrush, but after about 2sec, we disconnect power because it won't come out of it. If we leave it, it will just pop the overload obviously.

Its like we cannot get an inductive field going BUT the motor certainly spools right up! It baffles me to no end! I hate that my brain cannot "let it go"..

 

[mbarazeen]

The standard test to determine motor winding short is impulse test (bench test at a testing facility), but measuring inductance value may reveal some information.
Are the bearings ok? it may rotate freely for hand test but air gap may become small when energized if the bearing is lose. check the play.
Some time crack on rotor can cause such effects but extremely rare case.

 

[fastline]

Bearings are good. No indications there. Rotor has not made contact with stator and spins free and easy.

 

[shortbus=]

Maybe this PDF will help. Never seen it but it could possably be what is shown in section 2.5, broken rotor bar.

 

[unclejed613]

there's an "old school" method of testing the armature for shorted turns, known as a "growler" which is basically one half of a motor stator fed with AC. when the pole pieces of an armature are held in contact with the growler, you get some vibration, but if the armature has a shorted turn you get a lot of vibration.

 

[shortbus=]

there's an "old school" method of testing the armature for shorted turns, known as a "growler" which is basically one half of a motor stator fed with AC. when the pole pieces of an armature are held in contact with the growler, you get some vibration, but if the armature has a shorted turn you get a lot of vibration.

I know that works with 'wound' rotors/armatures, but does it work with a squirrel cage rotor in an induction motor? I don't know.

 

[unclejed613]

I know that works with 'wound' rotors/armatures, but does it work with a squirrel cage rotor in an induction motor? I don't know.

from what i can see, a squirrel cage rotor IS a shorted turn...

 

[MaxHeadRoom78]

The Growler test is intended for DC commutator motors.
If the centrifugal switch is OK then it sounds like a shorted turn, very hard to detect other than evidence of burnt stator winding etc.
The rotor does consist of shorted turns, the current in them is only reduced by coming in to near sync, IOW a few cycles within synchronization, the closer to it, less current.
Max.

 

[JonSea]

In the naval shipyard, we used to check 3 phase motors for loose/broken rotor bars using a "simulated single phase growler test." It's actually fairly simple to perform.

The stator is powered with a single phase, and vibration at 50/60 Hz (or maybe twice that) is monitored with an accelerometer as the rotor is slowly rotated. As I recall, the vibration level will drop as a broken rotor bar passes by the energized stator section. Make several rotations, and if a drop in level is noted at the same point in several rotations, there's a loose/broken rotor bar.

Using this technique, identifying which rotor bar is the problem would be tough, but it's a way to identify a problem.

I think your problem is with a single phase motor, but this technique may be a useful tool for some.

 

[dknguyen]

In the naval shipyard, we used to check 3 phase motors for loose/broken rotor bars using a "simulated single phase growler test." It's actually fairly simple to perform.

The stator is powered with a single phase, and vibration at 50/60 Hz (or maybe twice that) is monitored with an accelerometer as the rotor is slowly rotated. As I recall, the vibration level will drop as a broken rotor bar passes by the energized stator section. Make several rotations, and if a drop in level is noted at the same point in several rotations, there's a loose/broken rotor bar.

Using this technique, identifying which rotor bar is the problem would be tough, but it's a way to identify a problem.

I think your problem is with a single phase motor, but this technique may be a useful tool for some.

Is there any other kind of shipyard other than a naval one? One day...one day...

 

[JimB]

Is there any other kind of shipyard other than a naval one?

Yes, I think so.

A naval shipyard could be either:
A shipyard owned by the navy for their own use
or
(Maybe) a privately owned shipyard which only does work for the navy.

Another kind of shipyard could specialise in building only ships for commercial operations,
ie fishing, containers, oil/gas carriers, ferries etc.
No navies involved, hence not naval.

JimB

 

[dknguyen]

Yes, I think so.

A naval shipyard could be either:
A shipyard owned by the navy for their own use
or
(Maybe) a privately owned shipyard which only does work for the navy.

Another kind of shipyard could specialise in building only ships for commercial operations,
ie fishing, containers, oil/gas carriers, ferries etc.
No navies involved, hence not naval.

JimB

Huh...right. I guess I associate "naval" to just mean ocean but I guess it really means the navy.

 

[MaxHeadRoom78]

In the naval shipyard, we used to check 3 phase motors for loose/broken rotor bars using a "simulated single phase growler test." It's actually fairly simple to perform.
.

That must have been an older motor?
Years ago, bars were made of copper and connected at each end of the bar with copper soldered or brazed on, in the case of an overheated motor, it often used to fling the solder off and the bars became open circuit, this essentially turned the motor into a choke and although the rotor was not turning, the current was much lower than the overloads or fuses, IOW Very Low.
Nowadays it is Very rare for rotor bars to go open as the aluminum squirrel cage is cast into the rotor.
Max..

 

[shortbus=]

But broken bars in the rotor would make it magnetic and not give the needed BEMF to make it turn up to speed, wouldn't it? That would then cause a high amp draw.

But I still can't wrap my head around a circuit that single phase circuit capable of 220 -240Amps.

 

[MaxHeadRoom78]

No, the bars in a rotor essentially represent a shorted secondary turn(s). In with respect to the primary (stator).
The bars or 'secondaries' causes the high initial current and until the bars reach near synchronous speed with the rotating field, current is high .
If you remove the bars, (secondary coils) the rotor becomes a simple lump of steel. making an inductor , result is the motor does not turn and the current is minimal.
The motor is now a choke.
Max.

 

[unclejed613]

Huh...right. I guess I associate "naval" to just mean ocean but I guess it really means the navy.

you are close... "nautical" would actually be the word you're looking for.

 

[JonSea]

A naval shipyard in this case is a government-owned facility working on.... wait for it.... navy ships.

Let me revise that to read "a large industrial facility capable of rebuilding all manner of electric motors from the smallest to the largest, from 1940s technology onward."

As I recall, we did have some rotor bar problems with small electric motors with cast aluminum rotors (say fractional to maybe 10 horsepower). In a vibration signature, this shows up at rotor-bar-rate - the number of rotor bars × the rotational speed.

The simulated single phase growler test is probably not used much if at all in the industrial world but it may prove useful for someone.