3 ph motor torque under 1ph condition.....

[Mule]

Greetings....I have a customer that had some significant mechanical damage to ice machine hut, specificaly a motor/gearbox ice rake chain assembly.
During a poco outage and single phase condition, the customer is claiming that the motor and a 1:100 !! gear reducer recoiled enough mechanical energy to back spin and actual run backwards on single phase power when it was ran manualy by the owner while trying to troubleshoot , thus overiding overload protection. While I see this as a possibility with the extreem leverage of a 1:100 reducer, I still question if enough torque would be present to do this much damage. It twisted a 1in 316 S.S. shaft like it was butter and the customer had to pay nearly $5k in parts not to mention loss of biz........now the customer is wanting me to defend him in his claim against poco. Im saying..Nah.....any comments? thanks....Mule

 

[CONTROL FREQ]

Greetings....I have a customer that had some significant mechanical damage to ice machine hut, specificaly a motor/gearbox ice rake chain assembly.
During a poco outage and single phase condition, the customer is claiming that the motor and a 1:100 !! gear reducer recoiled enough mechanical energy to back spin and actual run backwards on single phase power when it was ran manualy by the owner while trying to troubleshoot , thus overiding overload protection. While I see this as a possibility with the extreem leverage of a 1:100 reducer, I still question if enough torque would be present to do this much damage. It twisted a 1in 316 S.S. shaft like it was butter and the customer had to pay nearly $5k in parts not to mention loss of biz........now the customer is wanting me to defend him in his claim against poco. Im saying..Nah.....any comments? thanks....Mule

NO WAY MAN... I'd steer clear of that one.

 

[Jraef]

1) If the motor was ALREADY RUNNING when the phase loss occurred, it would CONTINUE running in the same direction, but more likely it would stall. That's because with one phase missing, the motor is only capable of developing about 40% of it's full rated torque. So his idea that it somehow created MORE torque under a single phase condition is ludicrous from that standpoint alone (but read on).

2) Running backwards would NOT "override motor protection" at all. The power has no idea what direction the motor is running. When the single phase happened, the motor began to slow down and the slip increased. Current increased in the other two phases by the sq. rt. of 3 (1.732X), but still proportional to the applied load. If the ice maker was seriously under loaded with regards to the motor capability, it's true that the OLR may have never tripped though, especially if you had an old NEMA style starter with replaceable heater elements. They had no ability to detect single phasing. An IEC bimetal overload relay would have biased the trip curve so as to possibly trip at a lower motor loading, but even that's dependent on how loaded the motor was. A Solid State OL relay would have most likely detected the current imbalance and tripped right away.

3) There is another possibility though: If the POCO was engaging a recloser, and it reclosed several times in rapid succession, it possibly COULD have created a scenario wherein the motor was re-energizing while still magnetically active and thereby regenerating out-of-phase with the grid. That can create very high and very dangerous torque spikes. Getting them to admit that and accept blame would be a challenge though. It's possible however that if the claim is small enough they may decide it's cheaper to pay your guy than the lawyers. Wouldn't hurt to try a small claim.

 

[Lcdrwalker]

I will agree with the previous replies. That motor would almost instantly stall and the current draw from the remaining two phases would kick the OCPDs and or the short circuit protection device.

 

[Mule]

1) If the motor was ALREADY RUNNING when the phase loss occurred, it would CONTINUE running in the same direction, but more likely it would stall. That's because with one phase missing, the motor is only capable of developing about 40% of it's full rated torque. So his idea that it somehow created MORE torque under a single phase condition is ludicrous from that standpoint alone (but read on).

2) Running backwards would NOT "override motor protection" at all. The power has no idea what direction the motor is running. When the single phase happened, the motor began to slow down and the slip increased. Current increased in the other two phases by the sq. rt. of 3 (1.732X), but still proportional to the applied load. If the ice maker was seriously under loaded with regards to the motor capability, it's true that the OLR may have never tripped though, especially if you had an old NEMA style starter with replaceable heater elements. They had no ability to detect single phasing. An IEC bimetal overload relay would have biased the trip curve so as to possibly trip at a lower motor loading, but even that's dependent on how loaded the motor was. A Solid State OL relay would have most likely detected the current imbalance and tripped right away.

3) There is another possibility though: If the POCO was engaging a recloser, and it reclosed several times in rapid succession, it possibly COULD have created a scenario wherein the motor was re-energizing while still magnetically active and thereby regenerating out-of-phase with the grid. That can create very high and very dangerous torque spikes. Getting them to admit that and accept blame would be a challenge though. It's possible however that if the claim is small enough they may decide it's cheaper to pay your guy than the lawyers. Wouldn't hurt to try a small claim.[/QUOTE

That's been my position, that if the the shaft can't handle 40% torque, how could it ever handle 100% ? Simple as that.......

 

[philly]

1) If the motor was ALREADY RUNNING when the phase loss occurred, it would CONTINUE running in the same direction, but more likely it would stall. That's because with one phase missing, the motor is only capable of developing about 40% of it's full rated torque. So his idea that it somehow created MORE torque under a single phase condition is ludicrous from that standpoint alone (but read on).

Not sure where you are coming up with 40% here. Doesnt the toruqe decrease as the square of the voltage reduction? So in a single phase condition the voltage at the motor would only be 58% of rated and .58^2 = .336 or aprox 34% torque. How did you get 40%?

 

[hurk27]

As far as the damage to the shaft goes, this is simply the machine was not design to run in reverse, and the flywheel effect of the motor spinning back in reverse can do a lot of damage through a 1:100 reduction and the fact the it had inertia from a spring load that acted as a prime mover, when it hit the point where it hit a stop the motors inertia just kept going damaging the shaft.

Now any 3 phase motor will run in reverse if a phase is lost, it no longer has direction, no different from a single phase motor that has an open start cap, the motor will run in which ever direction you spin it up, I can see if there was a spring load where this spring would spin the motor in reverse and the two good phase would keep it going, also in reverse the motor might not have had much in the way of a load so it never reached the rating of the over loads.

Now for the sticky part, the POCO has some very good outs on this, first if the outage was caused by an act of god, = no claim, if a machine can be damaged by single phasing, then it is a poor design to not have single phase protection, again not the POCO's fault.

They make single phase detection relays for a reason, if a manufacture choses not to include them in there design it is not the fault of the POCO, single phase problem is a common problem with 3 phase systems, and should be design correctly, many hydraulic pumps can be damaged by running in reverse, and if there is no check valve after the pump they too can start up running backward in a single phase condition if there is enough pressure to turn them and the motor, I have seen this happen, and it will destroy the pump, but this has to be dealt with in the design of the equipment not depending on that there will always be 3 phases.

In this case I would have to side with the POCO.

 

[hurk27]

Also remember many machines that use limit switchs to stop the motor at the end of a stroke will not stop if the motor is running in reverse so it hits the dead end stop and things can break.

 

[Jraef]

Not sure where you are coming up with 40% here. Doesnt the toruqe decrease as the square of the voltage reduction? So in a single phase condition the voltage at the motor would only be 58% of rated and .58^2 = .336 or aprox 34% torque. How did you get 40%?

SWAG. I was being lazy...

 

[Jraef]

Also remember many machines that use limit switchs to stop the motor at the end of a stroke will not stop if the motor is running in reverse so it hits the dead end stop and things can break.

All very good points. I hadn't considered the idea of a spring load, very possible on something with a rake assembly. In fact if the spring exerts more reverse torque than what the motor can produce at single phase power, it's possible it reversed even though it was running at the time of the phase loss. Combine any of that with a lack of a reverse operation mechanical protection and a single phase protection scheme, mechanical damage is very possible. In that case the POCO may have a good case for not being responsible.

 

[dpeter]

I think the "manualy ran by the owner" would be enough to let the POCO off the hook. In my experience a 10 to 1 reduction would be enough to be self braking and a 100 to one most certainly would be and the driven load could not even begin to move the motor either way.

 

[steve66]

I don't understand people who want to sue the POCO for damage to their equipment.

The POCO never promised they would provide power 24x7. They never promised they would deliver perfect voltage with no brownouts, blackouts, surges, spikes, or changes in the frequency, or even reversal of phase rotation. All these things will happen.

How could the POCO ever guarentee perfect power considering all the lightning strikes and thunderstorms, and other things that affect deliery of power?

So why should the POCO be responsible when any of these things causes damage to someone's electrical equipment? In my opinion, they shouldn't be. (And I don't think they are. Sue all you want - but I think they probably have a clause in their terms of services that says they aren't responsible for damage caused by any of that.)

So electrical (and mechanical) equipment should be designed to survive these incidents. If a power outtage caused an ice machine to self destruct, if anyone should be sued, it should be the people who sold the ice machine.

If the damage is due to single phasing, its the customers own fault for not installing a phase monitor relay.

If someone is still worried about the possiblity of equipment damage or loss of business, they should buy insurance.

Just my opinion.

 

[hurk27]

I think the "manually ran by the owner" would be enough to let the POCO off the hook. In my experience a 10 to 1 reduction would be enough to be self braking and a 100 to one most certainly would be and the driven load could not even begin to move the motor either way.

I have seen worm drives back spin a motor if enough pressure is built up in the forward direction, a straight gear reduction drive can easily back spin a motor, I had a problem with a motor worm drive not holding pressure against the cardboard in a box crusher one time and we had to install a brakeing system on the motor to hold it till the cardboard got banded, but if this motor was ever single phase it would have had no problems taking off in the reverse direction.

 

[Mule]

thanks ! all great thoughts and good discussion.........for me, I can't imagine the driven load recoiled back pressure being strong enough to transfer through a 1:100 reducer....shew !!!. AND if it was, how a "lamed" single phased 3ph motor having a enough torque to take that massive pressure in that direction, let alone enough to do this kind of damage against apparent mechanical resistance (thus the damage).

Here's the key thing with me, if the mechanical resistance to reverse direction was so great it destroyed itself, HOW did it ever recoil in the first place against this resistance? Let alone doing the damage with greatly reduced torque from the lame motor? am I wrong here?

I'm of the opinion that his argument is with the manufacture of the machine, since it is relatively new and both its design, and "newness" of mechanical fit and adjustments should not allow such a mechanical failure.

The customer is however talking about installing a phase loss relay now.....

 

[dpeter]

Since this is a somewhat new installation, I would think the installation manual would be available for review. I would look it over carefuly for phase and rotation cautions and instructions. If it states only to check for proper rotation by observing the equipment then it would seem reasonable that running backwards would not be damaging. Sure, it may not work but it shouldn't self destruct. Now if there are instructions as to phase orientaion and warnings about damage occuring if incorrect then that is a slightly different story. In either case the manufacturer should be aware of and have made an effort to avoid or protect from such things.
My guess is that they did and if the owner opperated it contrary to the instructions then good luck getting any releif.
If the company did know or should have know about certain issues and did nothing then maybe you have a shot at compensation.
However it turns out, good luck.