Self Reversing Motor?

Status
Not open for further replies.

big john

Senior Member
Location
Portland, ME
I have a 3 phase motor that is undersized for the load it drives. Occasionally, the motor will bog down almost to the point of stopping, and then, spontaneously it will start operating in reverse.

Is this "normal"? Has anyone ever heard of this? I didn't know it was possible.

-John
 
We had a 3 phase conveyor motor change direction roughly every fifth or sixth time you threw the power to it. I can't explain how it did it but it burned up a short while later.:-?
 
I've seen self reversing controls on different pieces of equipment in the past. Like industrial equip that is prone to clogging up from process flow. They use a torque switch to initiate the reversing. So when the load is too great, it will reverse and try to clear the clogged process....
 
Cow, anything unusual about the type of motor or drive?

This could be a control issue, but I'd be surprised. There is a reversing contactor, but it's a very simple setup, there are no automatic controls. So, something would have to be causing the contactor to change position independently of the selector switch that controls it... which would be just as strange.

-John
 
Is the contactor actually changing state? Or is the motor changing direction regardless of contactor state? What kind of FW/RV contactor is it, mechanically tied or electrically, IEC, NEMA, brand, size? All useful info.

Is the motor winding up a really big spring? ;)


Doug S.
 
There is a reversing contactor, but it's a very simple setup, there are no automatic controls.
Is the controller itself reversing (which I guess would be one contactor dropping out and the other engaging)?


Added: Sorry to repeat Doug's question.
 
Someone showed me this phenomenon at the end of the day today and my reaction was "Holy smokes, I've never seen that before!" and then "Holy smokes, it's after 4:00!" I'll have some more information for you guys tomorrow.

-John
 
It is not uncommon for 2 stroke chainsaw motors to start in reverse if you dont press the compression release. When you are pulling your broken knuckles out of the cooling fins you will never forget to press that button. When the local landscaper tells you he will not fix the 36"50cc Stihl because I will cut my #@$%^&&*( leg off I had to fix it myself. And I didnt believe the reverse starts untill I was peeling my knuckles out of the engine.
 
If this was a single phase motor it would be easy to understand, but a 3 phase? chain saw two stroke yes, but a 3 phase motor? something has to change two of the legs to reverse the rotation, or it is single phasing, and that is why it is bogging down in the first place. a 3-phase motor without a phase or with a phase with a loose or bad connection can run in reverse as now it has no direction, and the fact you said it was bogging down till it almost stopped, so this make the most sense?

If it had proper over loads and fusing it should have tripped these as the remaining two windings will pull much more current
 
I have a 3 phase motor that is undersized for the load it drives. Occasionally, the motor will bog down almost to the point of stopping, and then, spontaneously it will start operating in reverse.

Is this "normal"? Has anyone ever heard of this? I didn't know it was possible.

-John
Well yes it is possible if a 3 phase motor is running single phase . Meaning lost one phase of the three phase ! ITS CALLED SINGLE PHASEN

Heres why if your running with only phase A / B and C is lost your motor will start up and run in the direction of least resistance to torque .

A single phase motor with out a start capacitor can do the same thing .


So your missing one leg or that motor has a winding burnt out or a bad tap inside broken & burnt out and the field is weak so your motor what ever it is doing strange to you is do to its magnetic field can start it the least direction of magnetic force applied . It will go in that direction of least pressure or force you can spin a 3 phase motor shaft by hand in any direction and with only 2 of the phases it will go in that direction .
 
Last edited:
"undersized for the load it drives. Reverse running"

"undersized for the load it drives. Reverse running"

I have see this. T leads 5 and 6 at motor had been reversed to 8 and 9 causing 1 phase coil of two having been reversed. The motor would run "no load" or "light load" but occasionaly run reverse, never very long. Check for 4-5, 5-8 and 6-9. :smile:
Quick Check:
Most motors run half of FLA with no load. Mixed Leads run 3/4 FLA.

My ID was "replacer matching 'mismatched' T-leads" on replacement when it burned up and not diagram.
 
I have see this. T leads 5 and 6 at motor had been reversed to 8 and 9 causing 1 phase coil of two having been reversed. The motor would run "no load" or "light load" but occasionaly run reverse, never very long. Check for 4-5, 5-8 and 6-9. :smile:
Quick Check:
Most motors run half of FLA with no load. Mixed Leads run 3/4 FLA.

My ID was "replacer matching 'mismatched' T-leads" on replacement when it burned up and not diagram.

I totaly miss read the original post about the motor bogging down, YES.., your making alot of sense with your comments. I've seen alot of motor issues due to lack of attention to proper winding connections......I've also seen simular issues where one line phase had a loose connection on poco side, thus causing the motor to run in the right direction, but having one high current reading on the next phase in rotation.
 
I have see this. T leads 5 and 6 at motor had been reversed to 8 and 9 causing 1 phase coil of two having been reversed. The motor would run "no load" or "light load" but occasionaly run reverse, never very long. Check for 4-5, 5-8 and 6-9. :smile:
Quick Check:
Most motors run half of FLA with no load. Mixed Leads run 3/4 FLA.

My ID was "replacer matching 'mismatched' T-leads" on replacement when it burned up and not diagram.


Talk about digging up old memories...

I've seen that happen myself, a long time ago.
Talk about a suprise.:confused:
If I remember correctly, there was also damage done to some driven equipment when the motor suddenly reversed.

Your solution was the correct one, if my memory serves.:)

steve
 
Here's a possible scenario that fits ohmheads explanation:

The load is actually expressing a negative torque on the motor, which you are seeing only as as an increased load at first. But as it continues, something in your system, i.e. a lose connection or bad starter contact, is heating up and becoming an extreme resistor, causing the voltage in that phase to all but go away. The motor is then effectively becoming a single phase motor and the negative torque expressed by the load, after actually stopping the motor, actually reverses it's direction because as a 3 phase-become-single-phase motor, there is no longer a relative rotation for it to fight against.

The problem I have with the mis-wired connection theory is, how was it generating any torque to run at all?
 
I would say that jraef and hurk27 have it right. It was single phasing. Could have been loose connection, broke wire,etc. in motor but I bet the problem is external. So if you just put a new motor on and left it, I would troubleshoot a little further. Or make sure you have plenty of motors in stock.
 
If I hadn't seen it I wouldn't have believed it. Here's the skinny:

The motor wouldn't always bog down, and it wouldn't always reverse. Sometimes it would run perfectly. I originally discounted single phasing because I was thinking it wouldn't rotate at all, it would just sit there and grind.

Turns out there was a loose connection on one of the phases. There was a lot of vibration, the terminal would loosen up intermittently, and then the motor would go single phase. Once it single phased, it couldn't move the load forward and it would bog down. The chain-drive it was hooked to would yank it in reverse, and because there was much less force needed to drive the load in reverse, it would just start sailing away in that direction.

I just happened to be running the motor with the pecker head off and saw arcing at one of the Stakons (which was a phase being lost), and right as that happened, the motor did it's little magic trick.

Didn't know single phasing would do that. You guys were all over it! Thanks!

-John
 
If I hadn't seen it I wouldn't have believed it. Here's the skinny:

The motor wouldn't always bog down, and it wouldn't always reverse. Sometimes it would run perfectly. I originally discounted single phasing because I was thinking it wouldn't rotate at all, it would just sit there and grind.

Turns out there was a loose connection on one of the phases. There was a lot of vibration, the terminal would loosen up intermittently, and then the motor would go single phase. Once it single phased, it couldn't move the load forward and it would bog down. The chain-drive it was hooked to would yank it in reverse, and because there was much less force needed to drive the load in reverse, it would just start sailing away in that direction.

I just happened to be running the motor with the pecker head off and saw arcing at one of the Stakons (which was a phase being lost), and right as that happened, the motor did it's little magic trick.

Didn't know single phasing would do that. You guys were all over it! Thanks!

-John
Good to see you fund the problem. The bad news however is that your motor protection should (could) have picked up on the single phasing condition. Most likely you have an older NEMA starter with bi-metal or melting alloy overloads. It might be time to upgrade to newer solid state OLRs that would have protected that motor better.
 
I've only seen this one time,,,,,,,but I saw a three phase motor with a bad connection in the peckerhead slow down,,,,,,,,then start running backwards. But it never switched back and forth like this one is. There could be problems in the control wiring if it's a reversible starter
 
Status
Not open for further replies.
Top