Material for V blocks for salient pole generators ? 3

[edison123]

A 10 Pole, 600 RPM, 50 MW hydro generator was dismantled today.

In between the two poles, two V shaped anchor blocks are fastened to two studs from the rotor rim. These are used to anchor the pole coils against rotating circumferential forces.

In all earlier salient pole machines, I have seen this anchor V block made of non-magnetic materials like aluminium, phosphor bronze or non-magnetic stainless steel.

In this case, these V blocks of magnetic mild steel.

Will not a magnetic V block in the inter-polar space act as a magnetic shunt and create a leakage path between poles ?

I would appreciate your inputs.

 

[spargher]

You are right
We normally use alluminium for this king of V blocks

But I also think that the blocks are in the external side of the pole coils, so the losses should be very little because the main magnectic flux path is through the stator core

 

[edison123]

Thx spargher.

This is the first time I have across V blocks of magnetic steel and my doubts flux shunting still remain.

 

[motorspert]

Edison
You are correct, they will provide a flux path back to the next pole. Remember though, that to "get to" the v block from the pole body the flux has to cross the field coil and any insulation which just won't happen. The more common leakage circuit is from the pole tip (or shoe) down to teh vblock and across to the adjacent pole. There will still be an airgap here that the flux will have to jump which means the flux will travel the easier path through the stator and on to the next pole. Remember also that the v-block will only be a small percentage of the total pole length, so any leakage flux would be negligable.

Richard

 

[edison123]

Thx Richard. I was kinda hoping that you, aolalde et. al. will jump in on this. You caught my drift.

Yes, I am concerned about this pole tip - v block - pole tip leakage path. When compared to the stator-rotor air gap (over 29 mm), this air gap (pole tip - v block - pole tip)isn't that different. May be tomorrow, I will actually measure this air gap.

My concern is, if the leakage flux is negligible, why most manufacturers have used non-magnetic V blocks in old machines ?

 

[UKpete]

This is a guess, but maybe it was used for superior strength, or (less likely perhaps) because the thermal expansivity of aluminium is high and may result in the V block pressing into the winding when it gets hot (I understand this can be a common failure point for rotor winding insulation though I haven't seen this personally).

In terms of cross sectional area, the flux path via the V block is surely very small compared to the area of the stator flux path. In other words, the V block could magnetically saturate (and therefore be incapable of carrying further flux) without having a significant effect on the very large flux crossing the airgap.

By the way, could the material be a weakly magnetic stainless steel? Just throwing a few ideas in for what it's worth.

 

[edison123]

Thx UKpete.

Definitely, the V block material is magnetic mild steel.

I noticed one other curious thing.

The pole to pole connections are anchored by a non-magnetic stainless steel clamp.

Strange logic ??