Tricky Y common point excavation

This is another Mains AC project involving rejigging a motor for use on 240V 3 Phase AC.
Unless you really know what you are doing I strongly recommend not doing this.
The information is intended for those that are just interested and/or comfortable at doing this sort of work.

I was given an old and very rusty 8", 3/4HP 415V 3P GMF grinder that was Meggering fine with no shorting, and thought I would have a crack at seeing if I could get it to run under 240V 3Phase, and get it operating as a variable speed grinder.

Just getting it apart was an effort as there was a fair bit of rust involved - sorry no photos of it at that stage

To get it working efficiently under 240V the motor needs to be converted from Y to ∆ but finding the common point for the 3 windings proved rather tricky.

The inputs were clear enough but the only sign of a common point was a thickly varnished black/brown fibreglass sheath coming out from underneath one set of windings and diving in between two other windings. the sheath was firmly gooped by varnish to some thickly varnished fabric covering another set of coils.

I peeled back some of the sheath and sure enough there were three wires in there and I guessed they were connected together at one end, but the question was which end?
I tried to pry back the coils but they were so thickly varnished I had no chance. After many minutes of tweezer/haemostat/fine scissors work I managed to lift a 1 cm length of all three wires about a millimetre away from the varnished fabric underneath.

Then the question was which end of these 3 wires do I tug on to pull out the common point?

I started by gently waggling each side of the buried wires. Long term waggling is not good as the copper wires can work harden and (as Bob Thomas and I discovered a few months back) eventually snap.

Eventually I could trace one wire on one side coming from down inside the stator so I tugged firmly on the other end and sure enough, out popped the common point.
The red arrow points to the coil the common point was buried under.
https://www.woodworkforums.com/attach...d=324422&stc=1

Things were relatively easy after that

Here are the new and old connections tied down and ready for the HT silicone.
It is still Meggering fine so it was OK to silicone and now I have to wait for that to dry off.

https://www.woodworkforums.com/attach...d=324423&stc=1

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After a busy day horse chauffeuring yesterday I managed to get some shed time to get back to this motor.

I had a small window of time on sunday and was eager to see it to would work so I briefly connected it up in ∆ mode.

My test VFD is set up with a slow (20s) ramp time and I always have a power meter in line when I test these things out so I can monitor what is going on.

As the motor accelerated it started making a suspect whining sound and as the current crept over 5 A :oo: (free running should be less than 1A) I stopped the motor before it fried itself.

This is a sign of unbalanced currents in the ∆ connection usually caused by a faulty (open) connection but I didn't have a any more time to follow up so I had to ;eave it.

This morning I checked the continuity of the individual coils and they were all still 13.0 ohms so I connected it up in Y mode and it ran fine (0.1 A current draw).

Then I reconnected it in ∆ mode - same problem as on sunday.

So out with the multimeter to see if I could find the problem and sure enough the resistance across each set of coils (should in delta be 8.7 ohms [1/(1/13 + 1/26)] was either 13 or 26 ohms which means there was loss of contact somewhere around the delta loop but it could not be internal to the coils as the motor was running fine on Y. I didn't stop to find out where it was, I replaced all the crimped connectors, replaced all nickel plated bolts/buts and washers with brass ones, and sure enough 8.7 ohms was then being measured across all the coils and it now runs very smooth and sweetly (0.1A current draw).

Lesson here is, take your time and check the connection resistances to make sure they all add up before applying the power! I usually do this but just got too eager - I was lucky not to damage the motor.

It's been a while since I set up a proper VFD, but from memory they often have a current limit/fault setting, at least the Danfoss and ABB ones did (I think).

Don't know if the Yum Cha units offer the same feature? I noticed the NowForever VFD on my 6040 CNC has *very* few features compared to the aforementioned units.

My workshop has 3ph available, but I'm interested in converting a machine to run off a 240 delta VFD setup. The closest I've come so far is when I replaced the pool pump, but 3ph pool pumps were much more expensive than their single phase counterparts.

Quote:

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Originally Posted by RustyArc

It's been a while since I set up a proper VFD, but from memory they often have a current limit/fault setting, at least the Danfoss and ABB ones did (I think).
Don't know if the Yum Cha units offer the same feature? I noticed the NowForever VFD on my 6040 CNC has *very* few features compared to the aforementioned units.

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The do have current limits and the dozen or so times they really needed to kick in, they have.
One was during my motor testing of a 3HP motor on a 4HP VFD and it cut out after the VFD had been running at more than 4.5HP.
The other time was when I did not check the factory settings before first trying the VFD, and a 2HP VFD tried to reach 400Hz driving a 1HP 50 Hz motor. - it just turned itself off at about 150Hz.
Another time was trying to drive a 4HP compressor motor with a 4HP VFD - for it just couldn't do it and turned itself off.
The one on my 3HP linisher has tripped more often than I liked, on rapidly changing the frequency - now I know to change the frequency slowly and then no probs.

In the case of this motor the VFD I was testing it with was a 3kW unit so the 3 limit was almost certainly too high for the motor. I should have either programmed it for a lower current limit or used a lower power VFD.

Quote:

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My workshop has 3ph available, but I'm interested in converting a machine to run off a 240 delta VFD setup. The closest I've come so far is when I replaced the pool pump, but 3ph pool pumps were much more expensive than their single phase counterparts.

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They sure are very interesting gizmos but not for the average DIYer.

This is almost a re-run of the above posts.
A more recent GMF grinder being converted from Y to ∆.
First step is to find the common or Y point and this one was also very tricky to find.

What I usually look for is some insulation on top of the coils that is not related to the connecting wires from the outside of the motor but this time there was not a hint of anything.
The grey insulting paint was a right PITA because it would not move unless it was heated.
So I started digging under the insulation which required a hot air gun and my coffee machine cleaner picking tools.
I actually use a SMD hot air gun because you can get very precise heat control and a nice low airflow so that it doesn't fry everything in the vicinity

Prying off the insulation
https://www.woodworkforums.com/attach...d=350821&stc=1

AH HA! There it is , buried under those coils.
https://www.woodworkforums.com/attach...d=350820&stc=1
More heat and prodding and picking dragged that hard nylon tube out and buried inside it there it was the start point.

This is the first star point I have see that is just crimped together.
In fact I can see a reduced quality in a number of areas between this and the older GMF i converted last year.
https://www.woodworkforums.com/attach...d=350819&stc=1

Here we are, new connectors silver soldered in and everything all tucked up ready for the HT silicone
https://www.woodworkforums.com/attach...d=350818&stc=1

Thanks for the pictorial, I might be trying this myself soon :-)

Sent from my D6653 using Tapatalk

Hi Bob, you have done a few of these conversions now, I've got a grinder that's turned into one of those "when I get back to it jobs" atm I've got the ends of the coils apart which were only twisted together, I thought they would have been soldered at least, I'm curious to know of all that you have done, Was the common point only twisted together?
In your first pic it looks like the three wires are only twisted together.



Pete

Quote:

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Originally Posted by pjt

Hi Bob, you have done a few of these conversions now, I've got a grinder that's turned into one of those "when I get back to it jobs" atm I've got the ends of the coils apart which were only twisted together, I thought they would have been soldered at least, I'm curious to know of all that you have done, Was the common point only twisted together?
In your first pic it looks like the three wires are only twisted together.

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This about the 20th one I have done and they have all been copper bead welded except for this one. If you look closely at the photo of the first one you will see what looks like a little copper ball. It's not solder because not even a butane torch would melt it. Copper bead balls also look more uniform in shape an d colour than a braze or solder.

Yep, the light reflecting off the centre of the bead made it look like a hole and so it looked like it was just twisted together, ok sounds like the proper thing to do is melt a little bead on the ends, cheers for that Bob.


Pete

As per usual, this is for information only - these modifications can be very dangerous especially without all the right gear and knowledge to test that everything is safe.

This motor was tricky not because I had to pry deep in around in the coils to find the Y point but because the motor was so small.
The motor is from the built in coolant pump on my Hercus Mill. and is about the size of a large coffee mug.

Powerwise, using V*I its 415V x 200 mA = 83W but being small its highly inefficient and the motor name plate says its output power is 40W or 1/20 HP. Not quite as small a motor from Anorak Bob's tool and cutter grinder which was 1/30th HP but its down there is the same league.

The motor does not have a fan but as it's a coolant pump some of the 43W of excess heat under full load will be carried away by the coolant itself.

Finding the Y point was quite easy as inside the motor there were 4 obvious junctions and 3 of these led to wires going to the outside of the motor for mains connection so the 4th one make "Y" in the photos was the one to try.

Even though its a small motor there is plenty of room around the outside of the coils to reroute new wiring etc. Sometime this can be a right PITA.
Attachment 437014

Some light hot air gun application to soften the resin allowed me to pry the junction insulation away from the main coil and cutting the surrounding insulation away there it is.
The wires are so fine and the twisting used to bring them together can work harden them so untwisting means they are easier to break, or while cutting them with the scissors whilst cutting away the surrounding insulation.
That's a little copper bead joining the three wires together.
Attachment 437015

And teasing the 3 wires apart you can see how fine they are
Attachment 437016

Then its a matter of connecting new wires to each one of these 3 wires, restitching the wires down, covering up the junctions with some insulation and the bringing the new wires out to the connection box of the motor.
In most motors because of heating issues I would use silver solder connections and fibreglass insulation, but this motor is so small that regular solder and ordinary heat shrink should be OK
The old loosen wires and new wires are then tied/stitched back down to the coils and some heat resistant silicone is used to hold everything in place. The silicone is probably not needed on such a small motor but I had it on hand anyway.
Attachment 437017

It's going to be tight inside the connection box because apart from being a small box, instead of 3 wires coming from the insides of the motor there will now be six.