Hi, I have been lurking for about a year as a guest but have only just signed up as a forum member. I am based in UK and have a 12/1 JKSon Listeroid clone coupled to a 7.5 KVA ST generator situated in an engine shed about 20 metres from my house at the bottom of the garden and running on WVO (the liquid stuff) and old chip fat (the solid stuff). Whilst it is still all coming together, the plan is to provide CHP for my house. The power is grid tied and the hot water (thermosyphon engine coolant and exhaust gas heat exchanger) passes through to my house radiator circuit via a flat plate heat exchanger. I have about 200 hours running experience so far.
I have a couple of ST generator heads (only ONE is coupled to the engine at any one time). Both are remarkably similar in materials and construction (even down to the same braid and knot used to tie cables to the rotor spindle. I have a few questions about these ST heads and I hope that you knowledgeable folk will be able to answer them and enlighten and help me… I will ask each question in separate threads to try to keep the topic concentrated ….. so here goes with number 1:
Knock when turning the rotor slowly by hand
One of my 7.5 KVA ST alternators has been in quite hard use for about 200 hours from new. The front (fan) bearing has started to make a whining noise and I reckoned that it was time to put a decent set of bearings in to replace the Chinese originals. Turning the rotor slowly by hand after disconnection from the prime mover displayed a roughness in the bearings but also a kind of soft non-metallic knock as the rotor was either moved slowly continuously or too and fro. I thought this needed investigating too.
The bell housings came off relatively easily (hammer and a block of wood on the fan end bell housing and a large span puller on the rear end bell). Both bearings were left on the rotor shaft. The rear bearing came off OK with a simple puller with a bit of grunt but the front bearing was a pig and required a hydraulic puller and some very fine arms to get behind the bearing but not foul the fan. Both bearings had unacceptable roughness (but very little real play), and were packed with the dark, thick “yak fat” grease resembling axle grease through which the balls cut a track. These bearings have now been replaced with fully sealed pukka SKF bearings.
Each of the 4 windings on the rotor are carried on formers (I would call them bobbins) that resemble a yellow / cream plaster type material. These formers are located between the rotor shaft and the laminated pole cores. Each core appears to be secured to the shaft by 2 large cap screws. The formers, I think, should be held firmly in place by some angled metal brackets at each end of the former, axially in line with the shaft which I assume to be an interference fit between the former and a milled out pocket section of the shaft – although what actually retains them I do not know. These angled brackets appear to be firmly held in place.
The copper windings on the formers themselves appear to be well wound with enough lacquer to stop any movement of the enamelled copper wires; but not enough for my satisfaction so I will apply a lot more to make REALLY sure they cannot move.
There is no obvious movement of individual pole core laminations or the cores as a whole. They appear to be tight and secure.
Each of the 4 formers have about 5 mm (1/4 inch) of axial movement and about 1.5 – 2 mm of side to side movement about the pole core. There is NO vertical movement. I am assuming this movement is what is causing the soft knocking sound. The movement occurs under the weight of the formers and windings when the rotor is rotated. My guess is that when spinning, centrifugal forces stop the axial movement but I would expect the magnetic forces to cause the formers to be constantly chattering and twisting in the side to side lateral movement. Having said this, I see no evidence of powder from the gradual erosion of the plaster like former material caused by chattering that I would expect to see if it was actually happening.
So my questions are…
1. Should I be concerned about the movement of these formers ?
2. What should I do to arrest the movement ?
3. What is supposed to firmly hold the formers and stop them from moving ?
4. What, if any, damage may have been caused by the movement and how can I check for it ?
5. Is there anything else that might be causing the dull, soft, hollow knocking sound ?
Many thanks for wading through a very long first post.
Regards,
Ian