Picking a 3 phase motor star point - practice

[Gremmy]

Evening all,

I’ve a few three phase machines I’ve been eyeing up which are not dual voltage motors and I only have single phase.

I like the idea of VFDs as they are relatively low cost and the soft start and speed control are appealing.

So thought I’d have a practice at finding the star point on an old Brooks motor that came off my wadkin AGS10 which I changed for a single phase.

I don’t know if the motor even runs, it looks a bit charred?!?

If the process is relatively straightforward I will have more confidence on any new 3 phase purchases that hardwired star and avoid an expensive trip to motor repairer.

I don’t want to go chopping in as I wouldn’t be doing that on an actual purchase that I want to run.

I’m thinking 9 o’clock?

 

[Fitzroy]

That’s a blooming mess! Much harder to tell with all the discolouration, I’d say at the 10/11 position, under the sheathing that is double wide.

F.

 

[guineafowl21]

Looks a nasty colour. Start by identifying where the original phase connections enter, then you can rule those positions out. I wouldn’t worry too much about chopping that ‘orrible black tape - you can re-stitch later.

First off, measure resistance between existing phase wires, so you can confirm the star point has half that value.

 

[Trevanion]

I wonder if the black colour is the motor being over-greased in the past and then baked on?

 

[Gremmy]

Thanks all, I’ve got Tuesday off so will have a dissection then, starting at the 10/11 area.

Trevanion - you could well be correct on the “too much grease”. When I pulled the bell ends off, there were lumps and lumps of orange grease, the consistency of fresh potters clay.

 

[OldWood]

Gremmy - I'll watch your progress with interest. I did this on the motor for a Graduate lathe some 15 years ago and don't recollect have that putting me under stress.

However the 3 ph. Cooksley 12" P/T I bought recently has caught me out in that the thicknesser drive is 2 speed implemented by switching the motor from 2 to 4 pole, and as I don't know which will work better with a VFD, I am going to have to find both star points and convert both sets of coils. The main motor is a Brookes one with suitable jumpers thank goodness.

Your grease story amuses me as the thicknesser gear box has a small screw near its base as the oil level indicator. The filler plug is accessible once in the P/T, but the level screw isn't. Like you the motor is a bit oily - nothing so bad though! Anyway maintenance had been maintained, but whereas the capacity should be on estimate no more than a quarter litre, I emptied near enough 1.5 litres of heavy oil out of it.

I haven't found much guidance on the web for doing the star/delta conversion, though as I write that I am remembering that the US call this Y/Delta so perhaps I need to extend my search.
Rob

 

[Gremmy]

An eagerness came over me and I’ve been down in the shed.

Success on finding the star point!
Found first time at 10/11 position,
Thanks Fitzroy! The once material tape holding the windings had turned rock solid and all I could do was break it off by teasing with wooden sticks and carful use of a cutter.

I’m actually curious to see if this motor runs.


I should have a VFD arriving tomorrow. Given the poor state, before I dig out the multimeter, soldering iron and heat shrink, would this be fool hardy to try and run purely to test out?

 

[Fitzroy]

Once a motor is wound and secured with string/cord the whole lot is varnished/coated so it’s normal for it to be rock hard. After you’ve soldered the new connections on, insulated them, and secured in place it’s sensible to varnish/coat it all again.

F.