# 3 Phase Motor, Wired for One Direction Rotation, but Turning in Reverse? Explanations?



## NjacobesEMT (Mar 18, 2021)

So, bit of background on the machine. Simple tester for industrial production. Motor in question was for the conveyor to bring product in to the test area. Pneumatic staging stops would stop the product where needed, and another motor would bring the test head down, and once test was completed, back up. Staging stops are supposed to release the product, and the conveyor, which has been continuously running the entire time, carries it out to be rejected or sent down the line. All motors are relatively small, all run on 480v 3 phase, though, there are no VFD's throughout the entire machine. The test head motor is controlled in two directions through the use of 2 contactors and an overload to achieve the reversing circuit, but the conveyor is wired for only one direction, and uses the same setup of separate overload and contactor to run it. I have included a picture of the portion of the electrical prints, which mirror the machine very accurately, which show the power supply circuit for the conveyor motor and test head (Lifter).









Now, here's the weird part. A couple days ago I got sent to this machine for the conveyor running in reverse, then going back and forth between forward and reverse if allowed to continue long enough. This was only happening AFTER the test head would raise after the test, and the conveyor functioned normally in bypass and manual operations, and up until the test head would raise in automatic. Now, being wired for a single direction and not having any VFD supply, this situation was incredibly odd, to everyone I've spoken to and myself. 

What I did find originally was the run contactor for the conveyor motor(-12CON1) and the overload(-4OL3) both had abnormally higher resistance values across the contacts for phase 3, and slightly higher resistance values than normal for phase 2. I thought this may be the source of the issue and replaced them both, along with some of the wiring in the panel, to be sure. It did not, however, fix the problem. After further troubleshooting, I found that the third leg was only being fed about 170v phase-to-ground on the line side of the overload. I followed the circuit back to contactor -3CON3 and found that it was where I was losing the power for phase 3 from 277v to 170v phase-to-ground. I replaced that contactor as well, and the problem was aleviated.

So, my question to anyone who has been brave enough to get this far is this. What on earth could have been causing this reaction? Could it have been a back feed of phase 1 into phase 3 through -3CON3 from the reversing of the lifter motor? That is my best guess, but I'm curious to know what else could've possibly caused this, if anything. Thank you for your time and consideration.


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## Wardenclyffe (Jan 11, 2019)

replaced them both, along with some of the wiring in the panel, 


put it back the way it was


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## Wirenuting (Sep 12, 2010)

Welcome to ET,
Please finish filling out your profile 
Are you an electrician?


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## gpop (May 14, 2018)

Why oh why do people struggle with the basic concept that measuring to ground is not the way to troubleshoot a problem on a 3 phase system. 

You dropped a leg but here is the fun part. The missing leg was replaced by a regenerative leg from a different motor. The 170v you read was the other phases back feeding via the motor. 

You will normally only see problems like this if one of the motors has a load that helps it find direction. Lets say a air ram pushes a box on a 3 phase conveyor that has dropped a leg. The friction in turn nudges the motor forwards so now you have direction and the motor will run (very weak but will run). Now you have a regenerated 3rd leg for the second motor to use.

Rapidly reverse the second motor and sit back and watch the fun as you are now kicking back a different regenerated leg at a different phase angle.


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## 460 Delta (May 9, 2018)

gpop said:


> Why oh why do people struggle with the basic concept that measuring to ground is not the way to troubleshoot a problem on a 3 phase system.


^^^^^ This!


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## Kevin (Feb 14, 2017)

Please take a few minutes to fill out your profile. It's required. Here's a link with instructions to assist you with this simple task.


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## NjacobesEMT (Mar 18, 2021)

Wirenuting said:


> Welcome to ET,
> Please finish filling out your profile
> Are you an electrician?


industrial electro-mechanical tech. In short, you could say it's a part of my job, sure.


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## NjacobesEMT (Mar 18, 2021)

gpop said:


> Why oh why do people struggle with the basic concept that measuring to ground is not the way to troubleshoot a problem on a 3 phase system.
> 
> You dropped a leg but here is the fun part. The missing leg was replaced by a regenerative leg from a different motor. The 170v you read was the other phases back feeding via the motor.
> 
> ...


Thank you for this explanation. I don't solely go off of phase to ground readings. Just included that to shorten the post a bit and because I thought the voltage I did get was a bit odd. I knew I had dropped a leg, but usually then it gives something comparable to 277v phase to phase, and nominal 0 to ground. Very rarely have I seen anything else, and you explanation shed light on as to why. I had thought that the back-fed phase was the reason for the functionality initially, and the reason as to why it was acting strangely after another motor had run, but I wasn't entirely sure, as I had never encountered or heard of anything comparable to this before. Thank you for your feedback and explanation. It was incredibly helpful, and I'll make sure to keep in mind that phase to ground in 3 phase systems is unreliable. Thanks again!


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## gpop (May 14, 2018)

NjacobesEMT said:


> Thank you for this explanation. I don't solely go off of phase to ground readings. Just included that to shorten the post a bit and because I thought the voltage I did get was a bit odd. I knew I had dropped a leg, but usually then it gives something comparable to 277v phase to phase, and nominal 0 to ground. Very rarely have I seen anything else, and you explanation shed light on as to why. I had thought that the back-fed phase was the reason for the functionality initially, and the reason as to why it was acting strangely after another motor had run, but I wasn't entirely sure, as I had never encountered or heard of anything comparable to this before. Thank you for your feedback and explanation. It was incredibly helpful, and I'll make sure to keep in mind that phase to ground in 3 phase systems is unreliable. Thanks again!



Always test phase to ground before you touch something as it can save your life especially if you drop 2 legs. (one leg gets hung in a disconnect or a breaker).

I would never expect to see 0 volts phase to ground due to capacity coupling and the possibility of a back feed. On a back feed i would also never expect to see 0 volts phase to phase. Lets say A phase to single phase transformer back to missing B phase. Between the meter leads you may have 100' of wire (wire to transformer, wire in the transformer and wire from the transformer) which is enough to cause a small phase shift so you will see some voltage. 

You may go your whole career and never see another problem like that. So well done on fixing it.


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## micromind (Aug 11, 2007)

gpop said:


> Why oh why do people struggle with the basic concept that measuring to ground is not the way to troubleshoot a problem on a 3 phase system.
> 
> You dropped a leg but here is the fun part. The missing leg was replaced by a regenerative leg from a different motor. The 170v you read was the other phases back feeding via the motor.
> 
> ...


This!

The quickest easiest way to troubleshoot motor starters and O/Ls is a fall-of-potential test. This is where you measure voltage across a contact of the same phase. For example, if it's a contactor, put one meter lead on the top of phase A. Put the other lead on the bottom of A. If the contacts are good, you'll real millivolts. If they're bad, you'll read more than a few volts, maybe even line-to-line voltage.

This test works only if there's load on the circuit. It's much better than a simple resistance test because it tests resistance during an actual load.

This test is also useful to troubleshoot a breaker that trips without a reason.


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