Thread: Converting 3 phase star to delta
10th Oct 2012, 10:40 PM #1
Converting 3 phase star to delta
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!
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.....
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!
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10th Oct 2012, 10:52 PM #2
10th Oct 2012, 10:57 PM #3
Nice work, well written and good description of how to go about finding the star point. I'm sure lots of others will find this post very helpful in the future
10th Oct 2012, 10:58 PM #4
10th Oct 2012, 11:00 PM #5
10th Oct 2012, 11:02 PM #6
10th Oct 2012, 11:18 PM #7
Thanks Ray. It's not very often that things go to plan but this worked out well. Very enjoyable, very doable and over within a few hours.
Now the part that I thought would be the easy part has turned out to be a little bit trickier. The motor shaft is 28mm and the original motor for my mill is only 19mm. I was all set to put the rotor in the 3 jaw and dial in the fixed steady and just turn down the shaft as required. EASY!
Well, I have only just realised that the 28mm shaft has what appears to be an M8 thread tapped in it. If I turn it down to 19mm as required then it's only going to have shaft with a wall thickness of about 5.5mm. Not much meat!
Not sure what to do yet.
11th Oct 2012, 05:54 AM #8
Simon, the rule of thumb used is that for a hole you want a minimum of a hole radius around a hole (whether for a flange or a boss). More is better of course but it seems to work. In your case I wouldn't take it below diameter 16. Remember that you will have a pulley over the shaft as well which will act as reinforcing.
(Watch your keyway depth though)
Last edited by Michael G; 11th Oct 2012 at 06:02 AM. Reason: added a bit
11th Oct 2012, 10:32 PM #9
Good job Simon.
If you bring the white leads down to the bottom terminals it would be wired as a 6 wire config motor and can be changed delta or star as per the nameplate diagram.
12th Oct 2012, 07:01 AM #10
Thanks Michael & Andy,
I have since done further testing of the motor, being new to VFD I have enjoyed having a play with different parameters. One thing I have noticed is that while the motor does not get hot after approx 15mins of continuous use (nor should it with not load and only drawing 400 watts) but the rotor or more accurately the shaft tends to heat up a fair bit. I assume it's hear built up from the rotor.
I'm about to run some tests with a non contact thermometer but I would estimate that the shaft gets to about 60 -70 deg C after running for about 15 mins. Is this normal for a motor with non load? The body of the motor does not even get warm. I am running the VFD pretty much as factory settings but obviously have set the max current to 9 amps. Even if running at 50Hz it does the same. I have played with the carrier frequency both high and low and it seems to make no difference either.
Is it normal operating temp for the rotor? I know it's only drawing 400 watts because I have the VFD running through a watt meter.
12th Oct 2012, 07:25 AM #11
well done Simon
but something seems amiss...I dont know the effects of a vsd on a motor but I would'nt have thought the rotor should be getting that warm after just 15mins...45 is hot to the feel...60-70 you'd have trouble holding your hand on it
I assume you are measuring at the front end and not the rear...
what is the temp of the front end shield around the bearing?...could something be wrong with the bearings in their end shields...to much preload?...outer race of bearing rotating in the end shield?
12th Oct 2012, 07:27 AM #12
You beat me to it, I would think bearings are the likely problem.
12th Oct 2012, 09:49 AM #13
Hi Eskimo, Ray,
I can't tell as yet without further investigation as to the source of the heat. The motor has an IP rating of 66 and so has an oil type seal front and rear. While I do not know how old the motor is, it has never been put into service and while I had it apart to re-wire I had noticed that the bearings are in good (new) condition and fitted well. the rotor turns without too much resistance, the only resistance being from the oil seals front and rear.
I'm still not all that familiar with induction motors, I know too well that the windings can heat up if the motor is not run the way it was intended (too much load, not enough cooling etc) but does the rotor heat up due to induced current under less than ideal conditions?
You may well be correct about the heat source not coming from the rotor (I'm hoping) as I do notice that it cools down very quickly (minutes) after stopping. Given the internal mass of the rotor I would think it would take a long time to cool if it's entire mass was the same temperature. Perhaps I need to run it till it gets warm and then dismantle to see if the rotor is uniform temperature of if it's just near the bearings?
Got the house to myself for an hour so I'm going to do some tests.....
12th Oct 2012, 12:40 PM #14
I have done some measurements. The shaft was heating to about 50 deg C while the body of the motor was staying a few degrees above ambient at about 25 Deg C.
I ran the motor for about 20 mins continuous and then expediently dis-mantled it before the heat dissipated. The rotor was pretty much an even temp so it was very difficult to determine where the source was coming from. Once again, the bearings look new as do their seats. I cleaned everything up and this time I took the oil seals out. It is my belief that assembling bearings and shafts with oil seals already fitted can mask a mis-aligned bearing. You seem to loose a lot of feel when a bearing mounted shaft runs through a seal.
So I once again put it back together taking extra special care with bearing seating. Once the motor was assembled I turned the shaft and it free spun with no sign of drag. I left the seals out and powered it up for 10 minutes. After 1o minutes I didn't even need the thermometer, the shaft was barely warm, pretty much same temp as the motor body!
The heat generated must have been from a slightly mis-aligned bearing. It could be the seal generating the heat as I have not yet replaced them but I can't imagine a seal producing that much heat from friction without it destroying itself in a very short time.
So, today I learn a new lesson in machine assembly AND with very little consequence. Nothing was damaged! I like those type of lessons!
There is one other discrepancy which I cannot explain, well not just yet anyway. When the motor was running nusually hot before I sorted out the bearing issue, it ran at 1440 rpm at 50Hz as expected for a 4 pole motor.
I have just remeasured the rpm since I have fixed the bearing issue and it now shows 2990 RPM at 50Hz. It's definately a 4 pole motor, it says it on the plate. Were the bearings so much misaligned that it dragged the motor from it's 2990 synchronous speed down to 1440RPM? I can't imagine it would have, the shaft was still easy to turn by hand. Does a 4 pole Star 1440 rpm motor stay a 1440 rpm motor in Delta?
Perhaps my optical tacho is playing with me?
Anyway, thankyou so much to Ray and Eskimo for turning my attention to the bearings. Being a new motor I discounted that idea but it turns out that I was the cause through my error of re-assembly.
12th Oct 2012, 02:44 PM #15
how did you change a 4 pole motor to 2 pole with out changing anything ...WE (U & I ) need to patent that idea...VFD's are going to be obsolete as soon as WE patent that method..I like the idea of becoming rich
6 pole + 960rpm or so
4 pole = 1440rpm or thereabouts
2 Pole = 2800rpm or so
so what did you ...err we do?
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