Epoxy VPI System Vacuum Levels

[BigMotorGuy]

In regard to form wound induction motors, are there any drawbacks to drawing a higher than recommended vacuum(recommended by the VPI equipment manufacturer)? For example a recommended vacuum level of 3 torr is typical for using epoxy resin. Is going to less than 1 Torr going to cause any problems? Will it really help in getting better impregnation of the coils?

 

[Motorwinder]

I've found by hanging a coil in the tank, the only way to get better penetration is by going double cycles. It seems the pressure cycle is where you get the penetration. Our tank usually goes to 4 torr. I don't see where it would do any harm going to higher vacuum. It would seem better.

 

[BigMotorGuy]

Thanks Motorwinder........that was my thoughts as well. We currently double dip all of our motors and with our spec of 3 Torr, our pump has trouble maintaining it at times. Though I think we do need a rebuild on the pump, I was considering going to a higher capacity vacuum pump that is capable of getting down to maybe 1 Torr so as to insure to meet our spec. Just have never heard of any down side to pulling higher vacuum levels.

 

[rhatcher]

You should base your cycle times and vacuum/pressure levels on the varnish manufacturer's recommendations and not the VPI tank manufacturer. Each varnish has specific qualities that require specific treatments. Also, I have heard that increased vacuum level or increased vacuum time may cause problems with some varnishes (de-polymerization?) but I cannot qualify that statement with facts or direct experience.

That being said, your specification should be based on establishing reliable varnish penetration for the different voltage coils that you handle (2300V, 4000V, 6600V) based on the different types and/or different layers of insulation that are required for each voltage. Treating sample coils, as described by motorwinder, is the only way to know that your process times are good.

That being said, keep in mind that 'sample coils' are typically placed in a fixture that clamps them between plates of steel (or some other hard impermeable substance) to simulate being installed in a stator slot.

Finally, I would agree with Motorwinder that the pressure cycle is more important to penetration than the vacuum cycle. However, that may be dependent on the varnish type that you are using. Again, I would say that the varnish manufacturer should be the definitive source for determining your cycle times.