On a 2-pole ac induction motor operating a cooling fan for a dc motor I have noted a sudden drop in running speed from 3596 to about 3300 rpm.

There are no abnormal vibrations at 2 x lf or rotor bar passing frequencies and running speed is only about .1 ips. No rise in temperature as checked with an IR gun.

What could cause that much slip?

Thanks,

Danny

I would check the current on each lag with a clamp on ammeter. If one phase has opened or is otherwise damaged it will be detectable. Such a malfunction will cause a serious drop in speed often accompanied by overheating. I'm not certain of the phase to phase tolerance on amperage but seem to remember that it is on the order of 5 to 10%.

John

Some thoughts in rough ranking of probability imo.

1 - mechanical overload beyond breakdown torque. If it were an axial flow fan, this might correspond to completely blocking off the outlet.
2 - Severe unbalance of voltage or complete loss of a phase.
3 - intermittent severe rotor malfunction - for example endring intermittently opening (doesn't just interrupt one bar but, all current past that point of the endring).
4 - Speed measurement error?
5 - weird effect of harmonic voltages on the power system (unlikely but possible).
6 - intermittent turn-to-turn short circuit in the stator. I'm a little skeptical of this scenario since I always thought turn to turn inevitably lead to rapid failure including ground fault, but someone in the motor forum insists it can happen.

There is an excellent article on dignosing motor related faults at http://www.manufacturing.net/ple/article/CA6332256.html

Thanks for the replies.

John,

I would like to check the current and do current analysis but this is about 1/2 hp or maybe less and will probably just be changed out and thrown away.

Thanks for the link. I always print that kind of thing out and store it in a file using the paper management software Paper Tiger that Walt Strong told me about. I'll refer to it next time.

Pete,

1. Quite likely a clogged filter. These folks are not very good about changing them and I didn't notice anything while I was collecting data.

2. Probably a single phase motor.
3. Could be, but if it is I'll never know.
4. Possible. I had a problem with a cable connection during this route but I've never know a connector problem to effect frequency, just amplitude.
5. and 6. Possible, but I'll never know.

Too bad some of my clients aren't more indulgent of my technical curiousity. In the long run, they would be better off if they were because I could be a better analyst.

Thanks again,

Danny

btw, John. Did you have any thoughts about my post about the shaft mounted drives on the common shaft?

This message has been edited. Last edited by: Danny Harvey,