Finding star point

A friend in the village has acquired a 60s vintage Wadkin cabinet saw, a very nice bit of woodworking kit. It has a 2hp 3ph Brook motor which is star connected.

I was wondering if anyone here has specific experience of finding the star point on these motors please? In particular just how much dismantling is needed to get at it.

Edited By John Haine on 13/02/2023 13:35:49

Once you have the motor on the bench and using a scriber put scratch marks across both end shields to stator, two lots one end and single the other. Theses marks will help you when reassembling the motor, then its a matter of very carefully dismantling the motor. Its not at all difficult to dismantle a motor, its the reassembling, getting it to run smooth, hence the scratch marks. Brook often have 4 threaded through bolts with a 2BA thread each end. Be very careful when withdrawing the rotor not to scratch the stator windings. Very recently (maybe last week) some one on HERE has posted a series of pictures on how he located the STAR point, in fact there been several lots. You are looking for a destinctive lump, often at the non drive end, using a sharp knife open it up. If you have got it wrong look else where, its some times a bit difficult to locate . You can of course go to a rewind shop, they will know for certain. You will need some cotton tape, cotton string, three lengths of flexible wire and some air drying varnish, all available on the favourite purchasing site. Do remember to identify the new ends, but as always, there is away should you forget. John

Thanks John, that's helpful.. I know the basics though have never tried it. Do you have any specific experience with a Brook motor please?

Hello John,

I've done this twice, once on an American made motor of 1/2 HP, the second time was on a Chinese 3/4 HP motor, both single phase. I used Jim Cox's excellent WPS book for guidance and got it right both times.

Do be sure you are using high temperature varnish when soaking the windings and binding things up

Best of luck Brian

Sometimes it is a case of playing it by ear. Even motors from the same manufacturer might differ slightly.

Perhaps remove the two end bells and the rotor, take a good photo of both ends, clearly identifying which is which, and post them here if the star point is not obvious.

If it is old and has been used for woodworking, a very good clean and blow out will do it a power of good.

In addition to the items mentioned above, you will need some insulation sleeve. Standard heatshrink is not really the right stuff. The siliconised fibreglass sleeving is good. Similarly, the new tails you connect are better if made from heat resistant wire. If you can find an old electric cooker or oven, the wires and sleeving used in it are ideal. A source for new material is commercial catering spares/repairs places.

I did it on a RJH Ferret grinder, can't remember make of motor but British. I was warned off from using it on vfd by the manufacturers, can't remember why, something to do with stray currents comes to mind. However I converted it to run off a capacitor setup, which worked fine. The advice was to silver solder new tails after finding and separating the star point, must be to do with potential heat. I chickened and used soft solder, perhaps crimping with a proper tool would be OK

As I recall things the Brook I did had the star point easily visible and pretty much outside the winding lacing. Foot and paint style seemed similar to yours so probably of same era.

Older motors seemed to have less prominent joint settled down into the windings and well laced. Presumably a vibration thing.

Clive

Thanks Clive, that's encouraging!

Finally remembered, Steinmetz connection, one capacitor described in workshop practice book. Can scan relevant pages if interested.

Back again John, The conversion article Star/Delta I referred to, was dated 5/10/2015 it contained excellent pictures, unfortunately I don't know how to get to retrieve it, perhaps some one will help out. John

Star point

was this the thread you had in mind John?

Mike

Thanks for all the info, lots to go on. I've also asked a local rewind place for a quote, but overall it looks like a fairly straightforward job to do myself. And I think we are definitely intending to use a VFD rather than a capacitor Kluge.

Usually the star point is easy to find because it is tied on top of the windings after the motor is wound and then tested. The winder will leave all of the winding ends accessible, test them individually, then connect, insulate and tie them in to the winding before laquering/encapsulating so that they don't move and rub through the insulation. The star point will usually be and obvious junction of 3 winding ends if it is not fully taped over or appear as a large bulge if it is.

Just wondered why you thought it was star connected? Yes, 400V, but could also be delta connected at 400V?

Look at the three leads, do they join with one or two winding wires? If one, star, if two, delta.

It is star connected.

Place I worked had a big circular saw that you started up in star connection then switched to delta. I presume it was to reduce starting current on a high inertia (big circular saw) load.

2hp is a small motor that would normally be quite happily started direct on and typically wound for 400V star, when motors get larger star delta becomes useful and the motors may be wound for star at 690V and delta at 400V. Electronic soft start units are probably very competitive with a contactor based star delta starter these days. As larger motors will not be run from a single phase input VFD then winding for 230V delta is not very practical as it will not be possible to use star delta starting.

Mike