2 Attachment(s)
Converting 3 phase star to delta
Hi all,
Last week I purchased a 3 phase 2.2Kw induction motor on ebay for $50. Actually I knocked a further $10 off the price and got it for $40. It was brand new, flange mount and about the power rating I wanted. Actually 1.5Kw would have done nicely but for the money I figured I could live with it. From the name plate I got the impression that it was wired as Star 415V but it had all the internal connections inside the terminal cover to change to Delta as required. So I took a punt!
Attachment 236361
Well, I got it home and imediately took the cover off only to find there were only the usual 3 wires inside. So, it was indeed Star connected BUT changing to delta would not be as easy as first thought.....
Attachment 236362
So, based on the front cover I draw up a quick diagram of what it was and what I needed it to be.
Based on this, I was looking the part of the windings that had 3 wires joined together, this being the centre of the star connection. These would need to be disconnected, extended and brought out into the terminal block so that a delta connection could be formed.
Disassembling the motor was straight forward. I removed the rotor as well in order to give myself some more room and in any case I really had no idea where I would find this connection, it could have been at the rear or front of the motor.
The rear end windings of the motor contained about 6 pieces of fibreglass type insulation at various places. Three of these are where the existing wires are soldered to the windings. I didn't want to needlessly cut the binding string or remove a piece of the insulation only to find it was not the connection I was after so I spent some time examining, bearing in mind I had never done this before.
Eventually I was confident I found the correct piece of insulation that protected the star connection as I was confident it had 3 wires going in. As far as I was aware, there is only one place in a star wired motor where 3 wires connect and its in the middle of the star. Removal of the insulation revealed 3 wires soldered together. That was certainly easier than I thought it would be.
I then separated the 3 wires and through a continuity test, worked out and numbered the wires 4,5 and 6 according to my diagram. 1 and 5, 2 and 6, 3 and 4. The DC resistance of the motor when Star connected was about 6.8 ohms between connections, the DC resistance between 1 & 5, 2 & 6, and 3 & 4 was about half that at 3.4 ohms, as expected.
After labeling the 3 new connections (Love my label machine!) I soldered insulated wire to them. The insulated wire I used was a high temperature silicon insulated wire rated to 200 degC. I only needed a small amount, so when I rocked up at Middys and asked for 1 metre of 1.5 sqmm of the stuff they threw it at me and said don't worry about the cost! So if anyone needs some, I have about 50cm left.
These 3 ends were then routed with the existing wires into the terminal box and terminal eyes were crimped on and heatshrink added.
After making the wiring neat again, I then spray a rather generous quantity of circuit board laquer over the windings where the new wiring had taken place. After waiting for it to dry, I then added further heat shrink insulation on the 3 connections. I was a bit hesitant to use normal heatshrink but I figured it's rated to 135degC and I'm thinking that if the wire gets that hot then theres something majorly wrong. In any case, even if the heatshrink does fail, there is laquer as well and most of the joins are sitting on fibreglass insulation anyway.
Finally it's a matter of connecting the terminals together correctly on the terminal block according to my diagram. The motor is now configured as a 3 phase 240V delta connection. It can at any stage be changed back to star by connecting according to the diagram on the nameplate.
I have since connected it up to my VFD and it runs really well. It's a 1440 rpm 4 pole motor that will happily run beyond 3000 rpm (I took it to 4000 rpm for a few seconds just to see) and at 10Hz it does about 300rpm. At 10Hz the motor still has too much torque to stop with your hand. The motor is made in Brazil by Kohlbach. It's pretty well made and has NSK bearings. For $40 I think it's a pretty good deal!
Cheers,
Simon