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Should I Varnish Dip My Motor Every Time? - HECO

May 23, 2018

I am often told “I want that motor varnish dipped too”, but is re-dipping a reconditioned motor really good for the motor? Using proper repair practices motors can be reconditioned and repaired many, many times. Putting varnish over varnish continually is not always in the best interest for that motor in the long term.
Continually varnish dipping the same winding, over and over again, without after a while performing a complete strip and rewind, can obstruct the air vent openings in the armature or stator and significantly reduced the airflow causing an increase to the winding temperature. There is a rule of thumb that says “For every 10° C increase in the motor operating temperature reduces the insulation winding life by 50%”. Motors are engineered and designed to maintain an optimal temperature using a specific cooling system. In that system many considerations are made to adequately cool that motor such as:

  • motor construction
  • type of cooling fan on the motor or armature,
  • insulation class
  • type of motor enclosure
  • vents within the stator laminations or armature.

If one of the cooling considerations are altered the motor may not operate at the design temperature, it will operate hotter and subsequently reduce the life of the motor.

The purpose for the varnish dip is twofold. One purpose is to electrically insulate and protect the windings/coils from ingress of contaminates. The second purpose is to mechanically secure the coils from movement. The initial dip and bake (or VPI – Vacuum Pressure Impregnation) at the time of a green winding is the most important and when done properly will last for many years.

During reconditioning, windings can be re-varnished dipped if there are electrical or mechanical reasons to do so. Understand that there are times a motor should be dipped and there are times where it shouldn’t be. Use the advice of your repair vendor, don’t just varnish dip to varnish dip.

Posted in Equipment Management, Predictive

Photo of a varnish treatment on a motor