VFD and Motor Insulation Requirements 3

[Lovison]

I appreciate everyones feedback but I still have a few open ended questions.

1. If a VFD is being used and the motor is not inverter duty should you consider a minimum class of insulation
and would it help?

2. We know there is Class B, F and H insulation classes available. We also know that <3200' elevation B should suffice in most cases. >3200' F is needed to get a B rise at altitude. >3200' H to get a F rise. But would this apply on submersibles?

3. Most submersibles come with a 1.15 SF which I surmise means it also has F rise?

4. Temperature extremes with pumpage adversely affect the motor cooling properties that can reduce the rise proportionally. I am seeing more submersbiles that are being used on VFD arrangements. I've also been told that all submersibles (3HP to 150HP) are already built to cover inverter duty operations. Have any of you found that to be different than what I've been told?

5. Most submersibles are sealed by oil chambers and mechanical seals. At least the ones I've been working on.
If the outer (lower seal) fails the oil is contaminated and it changes the oils continuity thereby setting off an alarm
feature indicating water entry. However, I've experienced a situation where the motor's inner seal failed and sucked the oil up into the winding areas. My question is will oil entry into the motor cause the motor to go to ground? Or does the motor cause fatigue to the insulation thereby causing the oil to flash? Some of the motor's I've seen have been chared and pools of oil within the chamber.

6. There seems to be a vacuum inside these motors and I don't know why it forms but one must take care when opening up a submersible during repair. I wonder if this vacuum
is sucking the oil up through the inner seal workings?
Have any of you experienced this occurrence?









Wayne E. Lovison
service-parts@naglepumps.com

 

[UKpete]

I have a few comments to chip in:

1. Inverter operation can affect the insulation in two ways: increased temperature rise due to harmonic losses, and also risk of partial discharge due to high voltage spikes (particularly with IGBTs and long leads) that can lead to premature failure. It is possible to get wire enamel which has a higher tolerance to both, described as inverter rated.

2. No it won't apply to submersibles. The increased temperature at altitude only occurs because of the lower cooling air density, not relevant with other forms of cooling.

3 and 4 - sorry can't help (I don't know what is meant by 1.15 SF)

5. I don't have any submersible experience though I have worked on sub-sea induction motors for ROVs (remotely operated vehicles), typically 75kW, 3.3kV, 60Hz 1800rpm. These are pressurized with standard hydraulic oil - internal pressure is maintained equal to outside pressure (using a simple pressure equalizer mounted on the motor) to keep the sea water out. These motors operate quite happily with a standard VPI insulation system, although internal losses are higher due to oil churning. The oil in your motors should not cause any grounding (after all, oil is used in transformers too).

6. It's not at all obvious to me why the motor should have an internal vacuum.

 

[teco]

lovison:- most motors today are wound with a class &quot;f&quot; insulation system with a class &quot;b&quot; temp rise (economics dictate this to be cost effective) , however the only time temperature rise should be taken into consideration with submersible pumps is when the water temp exceeds 40 deg &quot;c&quot;.
oil contamination of the winding should not deteriate a class f insulated machine providing there is no contamination of the oil.
an internal vacuum can be created within the machine by means of differential temperature ie:- air temp inside the stator when the motor is under load and as the motor cools a vacuum is formed.this vacuum has no detrimental effect on the mechanical seals or &quot;o&quot;rings unless they are worn or damaged.
ukpete is corret in 1,2,5 of his assesment.
hope this will help.
kind regds.

 

[Lovison]

Thank you both the data has helped. Wayne E. Lovison
service-parts@naglepumps.com