OriginalMitsta":17kowr4o said:
Andy,
Spoke to a motor rewinding company regarding the conversion from star to delta. Their response was basically that as the motor was so old, there was a high probability that whilst re-wiring the motor, they may experience a wire break meaning that a full rewind would be needed. This would be prohibitively expensive.
I also contacted Brook Crompton who told me that the startup current would be 600 - 650% of full load current (meaning that I wouldn't be able to use the standard 32A MCB garage ring mains for this and would have to run a dedicated cable to a 40A MCB. Cheers
I can understand the reticence of the rewirers, it can get a bit brittle in there.
BTW, even with the new motor and VFD, you may still need a C type breaker as mentioned by Deema and myself. Inrush is only fleeting but can be disconcerting when the MCB keeps tripping and everything looks fine.
As your going to get a VFD and probably going to dump the original motor, maybe you want to try this first
Only if your comfortable, it may mean you don't need to buy a new motor and the original works with your new VFD
On your old motor, you should have 3 external wires, U1, V1 and W1 If labelled) one for each winding. Each connects to a different phase L1, L2 and L3 from your 3 phase supply each like this.
This is how I converted my old Brook motor on my milling machine to delta. First take the end off and have a prod about for the star point, it is likely to be near the surface as it joins the 3 windings together as per the previous diagram
I don't know if I was lucky but I found it straight away, it was a lumpy bit where 3 wires are soldered together covered in a bit of sleeving.
You now cut the star point into 3 individual wires and get the meter out and perform some tests.
First, checking for continuity and finding out which external end belongs to which internal end. Get your meter out and find which internal wires belong to which externals wires, there should be a one to one relationship
A healthy motor should be have a resistance in the low tens of ohms and all be similar. Label them U, V and W as per convention. Then check for winding shorts, with all the internal and external wires separated, you shouldn't detect any continuity between U, V and W.
If that is all hunky dory, now solder on a cable to each internal winding wire, the winding wire is covered in insulating varnish that needs scraping off. Cover the joints with fine shrink wrap and route the cables externally labelling each end, U1 external, U2 internal for one winding etc.
Now test again and if it is all OK, tidy up the winding coverings and put the end of the motor back on and test yet again.
At this point you will have 6 wires, both ends of 3 windings. I used the existing terminals to create Delta wiring by putting 3 wires on each post instead of two. The following shows two wires on each post before I add the wires from the VFD.
Your trying to imitate the top of the following diagram, the bottom shows how delta is selected on a modern motor with 6 posts. By doubling up the winding wires I did it with the 3 original wiring posts.
SAFETY - make sure the motor is earthed, check continuity between the case and earth with a meter. Also don't apply power with the covers of anything off. If your not comfortable and/or don't have a competent person to check it, don't do it.
Motor under test with the VFD on the bench after a coat of paint , note the cheap plastic box that came with the VFD for on/off and speed control. If you find the motor is going the wrong way the swapping any two phases from the VFD will sort it.
I put the gubbins out of the cheap plastic box, basically two momentary switches and a potentiometer into the original cast alloy box, added a chicken head knob for speed control via the potentiometer and gave it a coat of paint.
On the other hand you may be able to sell the motor as it is and buy the new one
If you want to confirm physical mounting of the new motor, all the dimensions are usually on line.
Cheers
Andy
