3 phase motors, missing phase

[GeorgeB]

I should be ashamed to ask this, but am very puzzled. I was helping a customer with hydraulic problems; numerous internal leaks warming a reservoir with about 4 kW of waste energy ... and they were having problems with the motor sometimes not starting. They blamed the pump since their electricals had been checked thoroughly. I blamed the 5.5 HP of extra-ordinary load at startup ... which was well under the 20 HP motor.

I'm told today that the problem was that they were single phasing.

I thought that a running motor would continue running at significant over-current on the 2 powered phases until something happened ... motor failure, overload trip, or fuse trip ... but that full power could not be delivered to the load. I still think that is true.

The confusion ... sometimes (at light load, for what it's worth), this 20 HP motor would start. After it started, we were pulling over 50% of rated power in our tests. No one looked at current.

This is a standard 460-3-60 motor started across the line. How can it start?

Thanks!

 

[GoldDigger]

Unless there is some motion of the motor shaft before voltage is applied, a complete single phasing will not allow the motor to start.
But if there are other already running motors or if the single phasing is actually just a severely reduced voltage on one phase conductor, then the motor could start under low load and keep running at reduced maximum power.

 

[ActionDave]

How can it start?

You basically answered your own question..

they were having problems with the motor sometimes not starting.

sometimes (at light load, for what it's worth), this 20 HP motor would start.

 

[Jraef]

You basically answered your own question..

Well, actually the load would not determine whether it would start or not, but it does play a part in why nothing trips. But if a phase is completely missing, even an uncoupled motor will not spin unless something else STARTS it spinning first, even a little bit. But when you only have 2 phases hot, there is no relative phase rotation so the motor just sits there vibrating back and forth.

The "how can it start" issue is what GD said about OTHER loads running at the same time (on the SAME circuit by the way), or the loss not being complete on the missing phase. Even a SMALL amount of voltage on the missing phase will give a relative rotation for the other two and make it spin. Other loads on the same circuit already running when the phase was lost on that circuit will keep running, and acting like a "rotary phase converter" to create a voltage on the missing leg so that a load that is energized under those conditions might start and rotate. But ALL of those loads will lose about 50% of their torque, so if the load on them is more than that, the motors will stall. The bigger danger is if they DON'T stall and keep running under single phase, because that makes the motor heat up disproportionally to the current flowing to it, so it can burn up without tripping anything until its too late.

This is why it is so important to have phase monitor relays on anything fed by fuses. I now prefer to just use solid state overload relays that provide true phase loss protection based on motor current rather than voltage. Voltage based phase loss relays can often be fooled by that RPC effect too.

 

[ActionDave]

Well, actually the load would not determine whether it would start or not, but it does play a part in why nothing trips. But if a phase is completely missing, even an uncoupled motor will not spin unless something else STARTS it spinning first, even a little bit. But when you only have 2 phases hot, there is no relative phase rotation so the motor just sits there vibrating back and forth.

The "how can it start" issue is what GD said about OTHER loads running at the same time (on the SAME circuit by the way), or the loss not being complete on the missing phase. Even a SMALL amount of voltage on the missing phase will give a relative rotation for the other two and make it spin. Other loads on the same circuit already running when the phase was lost on that circuit will keep running, and acting like a "rotary phase converter" to create a voltage on the missing leg so that a load that is energized under those conditions might start and rotate. But ALL of those loads will lose about 50% of their torque, so if the load on them is more than that, the motors will stall.

This is why it is so important to have phase monitor relays on anything fed by fuses. I now prefer to just use solid state overload relays that provide true phase loss protection based on motor current rather than voltage. Voltage based phase loss relays can often be fooled by that RPC effect too.

That's what I meant. The fact that the motor was lightly loaded meant it would not take much of some kind of kick to get the rotation ball rolling and why nothing tripped or burned up and why the the problem was intermittent, is it not?

 

[GeorgeB]

Thanks guys; what all of you say is what I understood. They called me today and said it wasn't really a lost phase, rather as is suggested above, a bad connection. There was a bad connection in the motor peck ... oops, language ... junction box. The electrician had checked voltage on the T leads at the starter, but not at the motor. I'd guess there would have been a significant current imbalance, but he didn't check.

Thanks again!

George

 

[ActionDave]

Thanks guys; what all of you say is what I understood. They called me today and said it wasn't really a lost phase, rather as is suggested above, a bad connection. There was a bad connection in the motor peck ... oops, language ... junction box. The electrician had checked voltage on the T leads at the starter, but not at the motor. I'd guess there would have been a significant current imbalance, but he didn't check.

Thanks again!

George

Note to the electrician,

Always check amps, always check amps, always check amps, always check amps......

 

[Jraef]

Thanks guys; what all of you say is what I understood. They called me today and said it wasn't really a lost phase, rather as is suggested above, a bad connection. There was a bad connection in the motor peck ... oops, language ... junction box. The electrician had checked voltage on the T leads at the starter, but not at the motor. I'd guess there would have been a significant current imbalance, but he didn't check.

Thanks again!

George

I may have mentioned it before, but I have actually been "written up" by HR for using the term "peckerhead" in a presentation about motors and VFDs I did for training our salespeople. I said it was an accepted term for our industry, they wanted me to show its origins and prove it was not a "sexually charged term". I did research it and could not find anything refuting that, meaning, it actually might be what it sounds like!

 

[ptonsparky]

I may have mentioned it before, but I have actually been "written up" by HR for using the term "peckerhead" in a presentation about motors and VFDs I did for training our salespeople. I said it was an accepted term for our industry, they wanted me to show its origins and prove it was not a "sexually charged term". I did research it and could not find anything refuting that, meaning, it actually might be what it sounds like!

All in context. Rigid Nipple. Never thought about it until the counter help was female and I needed a 2" by close. No other term that I know of. Order and go on with life.

 

[Tony S]

Depending on where the rotor cage stops a single phased motor may well start under light load. It’s a 50/50 chance which way it rotates. Modern thermal O/L’s should pick it up within seconds.

Most pumps don’t like being run backwards as it can cause considerable damage.

 

[GeorgeB]

Depending on where the rotor cage stops a single phased motor may well start under light load. It’s a 50/50 chance which way it rotates. Modern thermal O/L’s should pick it up within seconds.

Most pumps don’t like being run backwards as it can cause considerable damage.

I don't think an induction motor will move at all, but may be wrong. And, in this case, the pumps are pressure compensated piston equipment which definitely don't want to run backwards. That said, I've never seen one damaged buy the brief, typically less than 1 second, reversed direction during phase rotation confirmation.

If it were a permanent magnet rotor ... or wound rotor ... I'd agree with a jump, and maybe enough of one for it to run. I don't deal with those, so have never run into it.

 

[GoldDigger]

I don't think an induction motor will move at all, but may be wrong. And, in this case, the pumps are pressure compensated piston equipment which definitely don't want to run backwards. That said, I've never seen one damaged buy the brief, typically less than 1 second, reversed direction during phase rotation confirmation.

If it were a permanent magnet rotor ... or wound rotor ... I'd agree with a jump, and maybe enough of one for it to run. I don't deal with those, so have never run into it.

A shaded pole motor would be able to start rotating in a preferred direction too, but AFAIK those are only used for single phase motors.

 

[Tony S]

I don't think an induction motor will move at all, but may be wrong. And, in this case, the pumps are pressure compensated piston equipment which definitely don't want to run backwards. That said, I've never seen one damaged buy the brief, typically less than 1 second, reversed direction during phase rotation confirmation.

If it were a permanent magnet rotor ... or wound rotor ... I'd agree with a jump, and maybe enough of one for it to run. I don't deal with those, so have never run into it.

That says it all.

It’s unusual but it does happen under light load.

 

[Jraef]

... That said, I've never seen one damaged buy the brief, typically less than 1 second, reversed direction during phase rotation confirmation.

I know this isn't the same, but sometimes the damage can happen very quick. I did a startup of a soft starter on a 4000V 2,000ft deep well submersible pump. When I got there, they had already lowered the pump into the well, which I had told them not to do until I got there. I asked if they had at least bumped the pump to check rotation above ground, they had not, they were too afraid of it. So I explained the risks of doing a bump rotation check on a submersible and they accepted the risk because they didn't want to pull the pump again. Sure enough, the bump check broke the locking device on the King Nut that held the bottom bowl on and they lost them all at the bottom. So they had to pull the pump anyway, then when trying to retrieve the bowls with a grappler at 2,000 ft, they jammed them in the well. So they had to drill a new hole next to it and replace the bowls. They didn't lower the pump before I got there the second time.

 

[Fulthrotl]

I may have mentioned it before, but I have actually been "written up" by HR for using the term "peckerhead" in a presentation about motors and VFDs I did for training our salespeople. I said it was an accepted term for our industry, they wanted me to show its origins and prove it was not a "sexually charged term". I did research it and could not find anything refuting that, meaning, it actually might be what it sounds like!

just explain that if HR wants to remove all instances of peckerheads from their politically correct life,
you'll start with the ones that are on the chiller motors for THEIR air conditioning.

tell them to get a fan. we operate a "no peckerhead" facility here.

thank god you aren't in a refinery.
CL I DIV I explosion proof flex has, in the 35 years i've been an
electrician, always been referred to as "horsecock".
we'd have to turn that refinery off as well. no gasoline.

they will need to get a bicycle to go with the fan..... :weeping:

 

[Tony S]

OK, someone satisfy my curiosity.

Just how did a motor terminal box get the name “pecker head”.

 

[iwire]

OK, someone satisfy my curiosity.

Just how did a motor terminal box get the name “pecker head”.

I have some ideas but I don't think this is the place to share them.:lol:

 

[qcroanoke]

I may have mentioned it before, but I have actually been "written up" by HR for using the term "peckerhead" in a presentation about motors and VFDs I did for training our salespeople. I said it was an accepted term for our industry, they wanted me to show its origins and prove it was not a "sexually charged term". I did research it and could not find anything refuting that, meaning, it actually might be what it sounds like!

Then some of our guys would get fired for what they call a 200 amp generator inlet.......
And another thing. Corn hole is not a name for a bean bag toss game where I come from.......

 

[just the cowboy]

Happens that quick

Happens that quick

I've never seen one damaged buy the brief, typically less than 1 second, reversed direction during phase rotation confirmation.

A centrifugalpump will spin the locknut and impeller right off and into the housing, quick as "snap there go the fins". Saw it done more than once and bought a rotation meter just for them.

 

[Tony S]

I have some ideas but I don't think this is the place to share them.:lol:

Not even if I promise not to laugh ;-)

There is the PM system if you’re going to make lewd suggestions.

There’s a lot of terminology used in our industry specific to a country or even local areas. Some UK references would get me banned in the UK never mind here.