Single Phasing 3-Phase Motors 1

[wt1155]

What will the net result of damage to a plant full of 3-phase motors that have one leg of the service interuppted for a 4 - 6 hour period of time?

 

[Mobius44]

It depends on how the motor's are protected in the example plant, the motor's might overheat from the phase loss and need to be rewound or replaced. The motor-driven equipment might also create related problems.

Modern electronic motor overloads include phase loss protection which would trip and protect the motor.

-Sean

 

[sreid]

I general, overheating motors. Some of the motors will act as generators (for the open phase)for other motors. The motors should trip on over heat (or smoke) one at a time.

 

[dpc]

The motors will run hotter than they should and can be damaged due to overheating. In theory, good overload relays should trip the motor before damage occurs, but that has not been my experience, particularly with older bimetallic overloads. I have seen mills where nearly every motor failed after an extended period of operating with one phase out.

 

[controlsdude]

"I have seen mills where nearly every motor failed after an extended period of operating with one phase out. "
dpc


I would think the noise would be so loud for a single phasing motor that someone would notice immediately. I find it hard to believe that the overloads would not kick the motor out.

I remember before I started up material handling systems I was working in a shop and a motor was single phasing on a saw that was wired incorrectly and the noise was so loud.

 

[dpc]

Believe it. And put in loss-of-phase protection. I would NEVER trust a bimetallic overload relay to protect a motor against single-phasing. I seen too many failed motors. The overload relay does not accurately replicate the negative sequence current impacts in the motor.

 

[jraef]

I agree with dpc. Some IEC overload manufacturers promote the "differential trip" feature of their overload relays as providing protection against single phasing. But in my experience by the time they trip the damage is done, mostly because of the negative sequence heating effects. The differential trip feature is better than nothing, but should NOT be relied upon as a primary means of detecting phase loss.

And motors that are single phasing don't necessarily make a lot of extra noise, at least not to the point of being noticeable over the machinery noise in many manufacturing environments.

 

[wt1155]

Thank you all for your responses. They have been enlightening. Would we be able to 'restore' any of the damaged motors by having them rewound?

 

[itsmoked]

Unless they are over about 20hp, 50 in some locals, it is often much less expensive and gets you new bearings/etc. to buy new motors.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com