# 3 phase motor reversing when single phasing



## stuiec (Sep 25, 2010)

cdnrebel said:


> I am trying to understand how all 3 phase 575v motors on a machine would run in reverse when the main fuse blew to the machine?


Just one fuse blew?


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## stuiec (Sep 25, 2010)

What was done afterwards, prior to restart?


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## micromind (Aug 11, 2007)

If only one fuse blew and the other two held, than all motors would be single-phased. If most or all motors were loaded enough to stop while being single-phased, then they would not know what direction to go. They'd be sitting there waiting for any sort of 3rd phase input. 

If one of the motors was mechanically run in reverse (a hydraulic pump might do this), and it was ∆ wound, it would generate the 3rd phase, but in reverse sequence. All other motors would follow. 

A motor wound ∆ makes a far better generator than one wound Y. A lot of motors over 15 HP are ∆. 

This is just one possibility........


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## cdnrebel (Feb 25, 2013)

I think the hydraulic motor for the pump might have went first and after the operator continuing to try to start the machine blew one fuse in the main. On startup the electric motor conveyor starting running in reverse and also another electric motor driving another motor that would only generally only go in the forward rotation with no reverse motion. Replaced fuses and everything was fine?


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## AK_sparky (Aug 13, 2013)

micromind said:


> A motor wound ∆ makes a far better generator than one wound Y. A lot of motors over 15 HP are ∆.


I didn't know this. Why is delta a better generator?


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## JRaef (Mar 23, 2009)

micromind said:


> If only one fuse blew and the other two held, than all motors would be single-phased. If most or all motors were loaded enough to stop while being single-phased, then they would not know what direction to go. They'd be sitting there waiting for any sort of 3rd phase input.
> 
> If one of the motors was mechanically run in reverse (a hydraulic pump might do this), and it was ∆ wound, it would generate the 3rd phase, but in reverse sequence. All other motors would follow.
> 
> ...


I like it! 
Been there, done that, but not from a hydraulic pump. Mine happened on a rock plant with a big jaw crusher I worked on. If the Main fuse blew, the jaw would stop because of losing torque of course, but then the mechanical pressure of the rocks remaining in the jaw against the tension of the drive belts would make the 500HP jaw motor spin backward for just a few seconds, which made all of the smaller conveyor motors run backward for a few seconds and create a huge mess. Solved the problem by using a main breaker instead of fuses.


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## JRaef (Mar 23, 2009)

AK_sparky said:


> I didn't know this. Why is delta a better generator?


Probably?...
On a wye wound motor acting to generate the missing phase, it would only generate 58% of the line voltage on that phase (inverse of the sq. rt. of 3), so the torque it can create on the other motors is usually not enough to make this happen. On a delta wound, it regenerates at the full line voltage so no loss of torque in the other motors.


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## cdnrebel (Feb 25, 2013)

That makes sense. I will double check the winding of the hydraulic motor. I still don't quite understand why the smaller 3/4 hp 575v motors all went in reverse. Is it the back feed from the 20 hp hydraulic motor switching the phasing? Would I get a voltage reading across all phases?


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## telsa (May 22, 2015)

cdnrebel said:


> That makes sense. I will double check the winding of the hydraulic motor. I still don't quite understand why the smaller 3/4 hp 575v motors all went in reverse. Is it the back feed from the 20 hp hydraulic motor switching the phasing?
> 
> YES
> 
> ...


The fuse that blew -- setting the stage for single-phasing -- is up the line... from multiple motors.

They are still all connected -- on the load side of that blown fuse.

When (the big) induction motor is single phased -- it's also going to induce current into the third leg... and if it got restarted bouncing backwards in rotation -- it's going to stimulate all of the other motors to synchronize with it -- going backwards.

Regardless of the rotation, you must IMMEDIATELY shut down such single-phasing motors -- as the condition burns up the windings -- and rather quickly, too.

Lacking the third phase, only one winding out of the three is getting current. 

Soon, the smoke gets out.


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## JRaef (Mar 23, 2009)

If you want to understand more, research a "rotary phase converter", which is essentially a large 3 phase motor, called an "idler" driven by a single phase motor, called a "pony motor", then the single phase power is connected to the 3 phase idler, which recreates the missing 3rd phase for everything connected down stream of it. The rotation of the idler is what determines the phase rotation of the 3 phase power from that point down.


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## Big John (May 23, 2010)

micromind said:


> If one of the motors was mechanically run in reverse (a hydraulic pump might do this), and it was ∆ wound, it would generate the 3rd phase, but in reverse sequence. All other motors would follow....


 Seen this happen with a large high inertia gear drive: It was mechanically binding and overloading the hell out of the motors that didn't have proper protection. A fuse would pop, and as soon as the fuse popped the motors would single phase and then the mechanical force of the driven load would push them in the other direction. Well, it was a hell of a lot easier for the motors to drive the load in the direction gravity wanted it to travel, so they would take off at full speed in the wrong direction.

I beat my face against the wall for a while trying to figure out how the hell it was auto-reversing.


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## JRaef (Mar 23, 2009)

Yeah, when I was dealing with the rock plant thing, we couldn't see the jaw crusher because of guarding, all we saw was all of the conveyors suddenly reverse and start dumping gravel allover the place. I beat my head against the wall for a couple of hours on that before it dawned on me.


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## Wirenuting (Sep 12, 2010)

micromind said:


> If only one fuse blew and the other two held, than all motors would be single-phased. If most or all motors were loaded enough to stop while being single-phased, then they would not know what direction to go. They'd be sitting there waiting for any sort of 3rd phase input. If one of the motors was mechanically run in reverse (a hydraulic pump might do this), and it was ∆ wound, it would generate the 3rd phase, but in reverse sequence. All other motors would follow. A motor wound ∆ makes a far better generator than one wound Y. A lot of motors over 15 HP are ∆. This is just one possibility........


Yup, seen it with air handlers that had a return fan inside the same unit and the mixing dampers were stuck open.


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## cdnrebel (Feb 25, 2013)

Thanks guys for all the great input. I understand it a lot better and it makes sense


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## JW Splicer (Mar 15, 2014)

Dang you guys are smart!


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## Wirenuting (Sep 12, 2010)

JW Splicer said:


> Dang you guys are smart!


It's not as easy as it seems.


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