| Single-Phase or Unbalance Condition
Copyright © 2011 Francis J. Martino
Three identical motors have repeatedly burned out due to single-phasing or voltage unbalance.
Indications of single-phasing were found to be:
(1) one of the three phase motor windings had burned each time the motors had failed.
(2) a discoloration due to heat was found on one terminal of one motor which was inspected.
An existing contactor which was also inspected showed no pitting or uneven wear on the three power contacts.
The facility electrician indicated that no other equipment in the building has experienced single-phasing.
The cause of the single-phasing has not been determined.
A current unbalance of less than 50% will not be detected by a typical differential overload relay. A large unbalance will be seen by the motor as a single-phasing condition in which one winding will draw excessive current.
A phase unbalance of more than 2% is generally considered in the industry as being detrimental to the motor and correction is therefore necessary. Look for and correct load unbalance on the system.
Unbalance protection relays are available with adjustable trip points from 2 to 10% unbalance.
In the application described here, it appears that the single-phasing condition is intermittent or there is an unbalanced condition that is small enough to remain undetected but large enough to cause the winding to overheat. The overheated winding will run for many hours before the insulation breaks down and burns.
If the unbalance exists for long periods of time then an unbalance protection relay will not allow the motors to run during those periods. Any extended period of time of unbalance will therefore cause production to be lost. With the cause and duration of the unbalance remaining unknown, it is possible that so much production will be lost that it will be more economical to allow the motors to burn.
An alternative to a phase unbalance relay is a variable frequency drive that will normally operate with a three phase input. The drive will continue to operate and give a balanced three-phase output when single-phasing or unbalance conditions occur from the power source.
Thus, the drive will provide ride-through when the input single-phasing or unbalance problem occurs, allowing production to continue without damaging the motors.
The drives will have the following features:
oversized to allow for the increased input current draw when operating on single phase,
programming to disable the unbalance shutdown feature,
the ability to ride-through the single-phasing or unbalance and present a balanced condition to the motor.
A separate AC input line reactor will be provided for each drive to reduce DC ripple on the internal DC bus that will be caused by asymetrical firing of the drive input devices when an unbalance condition occurs (the AC reactor will be rated for a nominal 3% if used with the Model iS7 drive which has a built-in DC bus link reactor, or rated for a nominal 5% if used with the LSIS Model iP5A or Cerus Model P-Series Drive. The reactor will be in a separate Type 1 ventilated enclosure.
Frank Martino
Power Quality and Drives LLC
(203) 217-2353
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