Random RPC starting failures

[JMG]

I'm having random non starts on a new build 15hp RPC. Fitch style but with a timer for cap disconnect. It now has 1000 uf worth of starting caps with bleeder resistors, no run caps yet. Starts in 1 sec. (When working properly.) Tried about 800 and 1200 uf but no difference in failures. Occasionally on start the idler motor shaft just barely wiggles and never really rotates. This may happen maybe 1 in 4 starts. After a non start it may or may not start after that, everything from 1 to 3 non starts in row have happened. When it does start everything is perfect. Both start and run contactors have 220 on the outputs and both main motor leads draw just under 200 amps and the 3rd about 65 amps when this happens, only slightly higher than when it does start normaly. All these readings are max hold on a Fluke ammeter. Fed with a 60 amp breaker, never tripped it yet. Leeson motor is used but megs good and all windings read the same with an ohm meter, runs quiet with good bearings. I'm leaning towards a bad motor but not much to go wrong there considering the way it checked out. Any ideas?
Thanks
John

 

[rons]

This happened to me with a Baldor motor. When I swapped the motor the stalls went away.

If you are curious then:
Make little orientation marks on the shaft.
When it starts good then note the mark position.
When it starts bad note the mark position.
Now after a bad start orientate the shaft to the good position. See what happens.
And then do the reverse from good to bad.

The clean "out":
You can always add a Pony motor to start the 15 Hp motor.
If no change then this is what I would do.

 

[BT Fabrication]

id say dead caps , i think you need 30-50 mfd per hp or something like that at 50 you need 750ish

had one die on me and did this.

 

[johansen]

id say dead caps , i think you need 30-50 mfd per hp or something like that at 50 you need 750ish

had one die on me and did this.

He appears to indicate LRA i65 amps on the capacitor leg. This is approximately correct for 1000uf.


If the motor's rotor is bad, you may hear a pronounced beat frequency of 60hz and the rotor frequency which might be 59hz. So once every 10 seconds you will see the line amps increase and decrease, as the motor runs.

 

[JST]

Yes, that might be noticeable.

One cause of what you see is cracked rotor bars. If the motor stops at random positions, then at some, the cracked bar will not conduct when it should, and the rotor magnetic field will not be correct to turn the rotor.

In other positions, the cracked bar(s) are in "neutral" zones and the motor starts OK.

I think more than just one or two might need to be bad, depending on how many the rotor has.

 

[JMG]

Did some more testing. It will not start in certain rotor positions. So I'm assuming it's a cracked rotor or whatever. We had a 5hp motor also available and that works first time every time. So we now have a 5hp RPC which is sufficient for the time being. No one here knows the history of either motor so who knows what happened to them and why they were just laying around here.
Thank for the help!
John

 

[jim rozen]

Wondering how a rotor bar would fail in service.

 

[JST]

Heating, or "just because".

Use of DC braking is good for heating up the rotor. Aluminum expands more than steel, so it can "work" back and forth, so over time it could break.

Or just expanded and got "caught" somehow and did not contract back, so it was stressed and broke. Lots of possible reasons.

The aluminum conductors are just cast in place, as a rule, and the laminations have sharp corners, so lots of ways to have stress risers.

 

[jim rozen]

Those are welded to the aluminum end plates, yes?

 

[JST]

Usually cast all as one part, conductors and end plates, all one piece often including fan impeller blades.

Old copper conductor types were brazed to the end caps.

 

[Froneck]

I had a similar problem. I was using a Chines contactor, thought the amperage rating was more than double the needed amperage the contacts would fail after time. After replacing the contactor twice I switched to a Made in USA unit and problem went away and hasn't happened again. Another suggestion is I don't use bleed resistors. Their main function is to discharge the cap when not in use to avoid shock hazard. Since I'm the only one in my shop I know they will not discharge. If your concerned connect bleed resistors thru closed contacts when Cap. is not being used.

 

[mmurray70]

A friend of mine had the exact same problem with a phase converter. It was built by a small company that seems to use whatever motors are available surplus at the time. His failed to start maybe 1 in 10 times. Also seemed to be related to rotor position.

He called the company who made it and they sent him an extra start capacitor to add in parallel and its been fine ever since. Might be worth trying even more capacitors then you think you need.

 

[JST]

The motor probably did have a problem, but the capacitor added a bit of current that was just enough to get it going.

Ideally, the rotors are symmetrical in all planes, but it is always possible to have a peculiar one. Or, of course, an actual fault.