Why VFD Duty Motor Frame Size is Higher Than Normal Motor

Why VFD duty motor Frame size come in higher size:

All must know VFD duty motor Frame size come in higher size than normal motor, in motor name plate why the manufacture is writing ‘vfd duty’ motors. In order to operate a motor with vfd, you have to look winding insulation parameter. Because of while reducing the speed of the motor Simultaneously, the lower frame size reduces the stator cooling air, due to this sudden increase in temperature leads to winding failure. This is only for TEFC (Totally enclosed Fan cooling system)

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What is TEFC:

A totally enclosed machine with cooling augmented by a fan, driven by the motor itself. Blowing external air over the surface through the cooling passages, if any, incorporated in the machine.
Speed is the main heart of such motor. But in vfd we operate the motor at any speed so that we need two special design process

  1. Insulation quality
  2. Increase in frame size
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What is Vacuum Impregnated Winding:

Vacuum Pressure Impregnation (V.P.I) is a process by which a fully wound electric apparatus stator or rotor is completely submerged in a Resin. Through a combination of dry and wet vacuum and pressure cycles, the resin is assimilated throughout the insulation system. Once thermally processed, the impregnated windings become a monolithic and homogenous structure. The resulting benefits are higher dielectric strength, increased mechanical strength, greater thermal inductivity, and superior protection against the ingress of water, chemical and containments.

Purpose of Vacuum Impregnated Winding :

To withstand high temperature, because increase in temperature leads to reduces the dielectric strength of the insulation. So that we need special type of winding called Vacuum Impregnated Winding.

Increase in Frame size:

Increase in frame size, which increases the size of the motor. There is the change to get sufficient cooling for the motor with increased body space.


If you purchase a motor 90 KW with frame size of 280L but VFD duty motor will come with the frame size of 315L for the same rating.

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Thats why VFD duty motor Frame size come in higher size….
While ordering a motor for vfd application , you should mention it for ‘VFD application’ like that.

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