120v Motor Reversed...

[4-20mA]

Hello all, love the forum, been reading for a little while now. I'm Matt and from Charleston SC. I do I&E maintenance for the last 9 years but also have over a decade of commercial electrical starting in 98'.

So, yesterday, the neighboring shop next to our plant had a 120v fan that tripped the breaker. The guys tried to reset the breaker and it failed. So they unplugged the fan then reset the breaker. When they plugged it back in and turned the fan on it was blowing out the back of the shroud. By time I got there it was very hot to the touch and I told them to unplug it. It does have a start cap on it. It's only a few years old, but I suggested they replace it anyway.

What occurred for this fan to reverse? Winding burn out and touch somewhere It's not supposed to? I've seen some things but this one is new to me.


Sent from my SM-G998U using Tapatalk

 

[ptonsparky]

Single phase motors need to be 'kicked' in the direction of run when they start. Start windings are placed differently in the motor to accomplish it. Failure of the start capacitor or winding would allow it to run backwards. All it takes is for the blade to be turning at start.

Motor guys will have a better explanation

 

[GoldDigger]

1. The main reason for the fan motor to run hot when running backwards would be that it depended on normal airflow for proper cooling. For most types of fans running backwards will not draw more power than running forward.
2. For many motor applications it is normal for the motor to be unpleasantly hot to touch. Depending on the motor type, "normal" temperature rise can go above 212F.
3. There is a chance that there was damage to the start winding which caused it to draw excess current and still not produce a proper magnetic field to start the motor from a stopped condition. Many motors will have a centrifugal speed switch that disconnects the start winding. If the motor is rotating in the reverse direction fast enough it can disengage the start winding and the motor will happily continue in the reverse direction under power.
4. There is a good chance that the motor initially failed in such a way that it actually drew too much current through the breaker. If left to run in the reverse or forward direction long enough it might have tripped the breaker again. Overload which are just over the breaker trip point can take many minutes to trip the breaker.

 

[4-20mA]

Thanks for the thoughts and answers. Looking forward to learning from and possibly even contributing to the forum. Trying to find where I can edit my sig line now on pc.

 

[paulengr]

Thanks for the thoughts and answers. Looking forward to learning from and possibly even contributing to the forum. Trying to find where I can edit my sig line now on pc.

If you just see a fat box on one side it’s not a starting cap.

There are 3 different single phase motors. Previous explanation in post #2 is spit on…need some way to “kick start” it.

Simplest is “shaded pole”. It’s a magnetic trick but lousy efficiency. Google it. Only used on little low efficiency fan motors.

Second is permanent split capacitor but there are three variations. You have either a start, run, or both capacitors permanently wired in with a high impedance start winding. I rarely see anything but one capacitor for running.

Third is for larger motors, capacitor start-run with a relay. These have s big starting capacitor and a good size run capacitor. Much higher torque and efficiency. The start capacitor and coils are switched off at about 85% speed. This is done with an inertial switch, a timer, or a potential relay.

Capacitors run $10-20 each. Potential relays are about $20. It takes less time to just replace all 3 than troubleshoot and chances are one is bad.

Local small motor shops usually stock capacitors.

 

[kwired]

My guess is since it was running backward there was already air flow pushing it that direction at time it was energized, it is very likely a permanent split capacitor motor, and the capacitor is bad. That kind of makes most sense to me to let it run backward as well as run at a high temperature. Capacitor start- capacitor run motors will start just fine with a bad run capacitor but will draw high current (because the run capacitor isn't assisting) and you would basically get same thing out of a PSC motor if an outside force was there to assist getting it started.

 

[4-20mA]

Great info y'all! I'll stop in at their shop Monday to take a closer look and see what I can see. Will be back with some pics and all then.

Sent from my SM-G998U using Tapatalk

 

[4-20mA]

Look what I found. It's a loud one with plenty of noise and vibration, vibrated right through that suckered! Repaired it for em and taped it all up pretty good. Should get another few years out of it.

Sent from my SM-G998U using Tapatalk

 

[kwired]

Look what I found. It's a loud one with plenty of noise and vibration, vibrated right through that suckered! Repaired it for em and taped it all up pretty good. Should get another few years out of it.

Sent from my SM-G998U using Tapatalk

Most likely a PSC motor, the lead was probably the aux winding and on the grounded conductor otherwise it would have caused short circuit/ground fault protection to open when it first faulted. Once it developed to an open circuit on the aux winding, that motor would still be able to run though would probably draw more than normal current as long as it were already spinning when energized, and would run either direction in that condition.