1PH motor fail to start, locked rotor amps, possibly rotor touching stator

[fastline12]

Got stuck putting bearings in a table saw for someone. The motor is the oddest thing I have ever seen. The Al case splits all the way down axially and the stator is removed from the cases. Technically, I do not believe it is supposed to be serviced. The stator and bearings were bedded in epoxy.

Any, I was with the saw when it failed. It was running doing light plastic cuts solid for 1hr. It just got a vibration very quickly and created some wood smoke from being hot. The bearings really were not that bad and certainly did not cause the vibration.

I found a bad run cap. changed that and the bearings. Motor will not start. Growls, shaft turns a bit, pulls about 5-6x FLA. We obviously don't leave it in this state for even a full second.

I don't know that I have ever had a motor's rotor contact the stator but in this design, I believe we are experiencing some of that BUT I am really unsure if that would really cause this problem? This should not cause a loss of magnetic flux should it? If contact with the stator would not cause a locked rotor state, any other ideas? I have an ESR meter as well and I show the start cap is still good, potential relay is normally close right now, bleed resistor is working, etc.

I don't have a specific diagram for this motor but it seems odd. It has a non contact inductive coil on the tail I assume to operate the potential relay. Never seen that. Usually a switch or relay operates off of start winding.

 

[jraef]

Most likely a bad starting switch. It's a centrifugally operated switch that switches the starting capacitor out when it gets to speed. If the switch is stuck open, the starting cap is not there, so it doesn't start.

"Will work for (the memory of) salami"

 

[fastline12]

No, as I stated, there is no centrifugal switch in this motor. It uses a potential relay and appears to have a little discrete induction coil on the back of the motor that must power the coil in the relay to kick out the start cap.

 

[jraef]

Oh sorry, missed that somehow. Might be a bad connection between the potential relay and the caps then. The symptom you are describing fits the problem of no start circuit. If it was the rotor hitting the stator, I would imagine there would not be any movement, at least not after the first attemp. Can you turn the shaft when it is de-energized now or is it locked up solid? Because if you were hitting the stator, the only way it would lock and then release is it there was a severe amount of slop in the bearings etc that would allow the rotor to skew only when it gets hit with the magnetic forces. That kind of sloppiness would, I think, be self evident.

"Will work for (the memory of) salami"

 

[bartb102]

I've seen electronic start switches in cable and chain hoists. If that fails, the motor will not start. They look like a capacitor, but aren't.

 

[bartb102]

In one brand they call it an electronic reversing switch as it switches which lead the cap is in. Not needed on a table saw.

 

[fastline12]

I have verified the potential relay is fine. BUT, for some reason it is N.O. intsead of N.C.. I opened it to verify it is ok and even bypassed it so the start could would be in circuit for sure. Still no start.

I was looking at diagrams for standard dual voltage single phase motors but they have 6 wires, this one has 5. I cannot seem to figure out which point they have tied to make the 5 wires. I figured T4/T5 but the whole thing just tests strange. Just when I think I have my main winding wires, something else conducts. Neither side of the start cap connect directly to power, they go to motor leads.

 

[fastline12]

Every test we have thrown at this motor shows the stator to be in good health. However, I am now focusing attention on the rotor. The motor picked up a very noticeable vibration just before it was shut down. The rotor will try to turn a bit when trying to start. I am curious what the signs are of a bad rotor and if that is common? We know the motor got rather warm but had minimal load on it. We believe the motor was designed only as intermittent duty and did not like running for a full hour.

Is there anything we can inspect on the rotor aside from growler testing?

 

[fastline12]

We determined that the stator is toast. Apparently when the run cap shorted out, it welded itself into the main windings. The start winding is open but one leg conducts to other main windings that it should not. I feel confident the vibration experienced during failure was the chaotic induction currents while the start winding was failing.