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Rewiring a 3 phase motor to Delta

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Smouser

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Hi all,

I recently got a three phase motor with a machine that is 950RPM (uncommon) but unfortunately it is only wired for 380/440v.

I am looking to rewire it so that I can use a VFD @ ~240v.

Would anybody be able to offer advice/guidance on where to look for the star point from a photo of the motor?

IMG_20210406_111009.jpg


IMG_20210406_171537.jpg


IMG_20210406_172505.jpg
 

Spectric

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A two pole motor runs at 2850 Rpm, a 4 pole runs at 1425 Rpm so you must have a 6 pole motor which runs at 950 Rpm.

I can see the phase wires but the star point will be within the windings, could be within the black sleeving that is tied down. It is not a job that I would undertake and would take it to a motor rewinder who could do this and ensure all insulation is Ok afterwards.

Also you need an inverter that has at least 1.75 times the required output power, both Siemens and ABB produce safe invertors with all the required protection so long as you set it up correctly.
 

Smouser

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Thanks.

Does it make any more complicated or difficult because it is a 6 pole motor?

I really want to have a go at this (recognising that I could mess the motor up, but hopefully not).

I know of no motor rewinders near me and google does not produce anything either. Hence me wanting to have a go at it.
 

guineafowl21

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I did a very similar motor a while back, fitted to a Multico mortiser:


I agree with Spectric - look around 12 o’clock in the second picture.
 

Smouser

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Ok!

After staring at the motor for a long while the only place that looked like it had 3 wires going to it that was possibly joined was at the opposite side of the motor to the three existing wires that comes out of the motor and goes to the connection plate (other side of the motor of the previously posted photos).

Can anybody confirm that this is the Star point?
Should there only be one place in a motor where three wires connect to each other?

It was the only connection I opened up, did I strike it lucky?

IMG_20210406_212508.jpg

IMG_20210406_212626.jpg
 

Fitzroy

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Unwind the three wires then check the resistance across the three coils. If you’ve found the star point the resistance of each will be similar.
 

Spectric

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Think of a pole as being a magnetic pole, so for a six pole motor you will have three pairs of magnetic poles each pair at 180° so a total of Six but repeated as it is three phase so 18 poles all in.

Now if you follow a phase wire to it's first coil then the other end of that coil will go into another coil, skiping two and eventually you will find the last coil and it is this wire leaving that final coil that will be connected to the ends of the other two phase coil circuits, these are what need bringing out of the motor to give you the six required. Now connect as a Delta, three interconnections of two wires each that will be your supply phase connections.
 

Spectric

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If you think about how they are wired then all the coils are in series, one series of six coils for each phase. This means there will only be two wires at these points and so as Fitz has said just carefully unsolder these three wires and then measure the resistance between each and one of the existing phase wires and they should all be almost the same. If so you need to attach a length of wire to each, double insulate with heatshrink and bring them out with the other three wires but ensure they are not floating around or going to get in the way of the rotor or anything else.
 

Smouser

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Unwind the three wires then check the resistance across the three coils. If you’ve found the star point the resistance of each will be similar.
I am on it! Back in 20 minutes or so.

Think of a pole as being a magnetic pole, so for a six pole motor you will have three pairs of magnetic poles each pair at 180° so a total of Six but repeated as it is three phase so 18 poles all in.

Now if you follow a phase wire to it's first coil then the other end of that coil will go into another coil, skiping two and eventually you will find the last coil and it is this wire leaving that final coil that will be connected to the ends of the other two phase coil circuits, these are what need bringing out of the motor to give you the six required. Now connect as a Delta, three interconnections of two wires each that will be your supply phase connections.
I am afraid I don't understand completely what you mean (my fault, not yours), but I will try Fitzroy's suggestion and see what the result is.
 

Spectric

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This procedure is for tracing that star point, if you have found it then you can skip, but this is relevant

If you think about how they are wired then all the coils are in series, one series of six coils for each phase. This means there will only be two wires at these points and so as Fitz has said just carefully unsolder these three wires and then measure the resistance between each and one of the existing phase wires and they should all be almost the same. If so you need to attach a length of wire to each, double insulate with heatshrink and bring them out with the other three wires but ensure they are not floating around or going to get in the way of the rotor or anything else.
Any shorts within the windings will be detected by the VSD and it will show a fault providing it is not just a cheap asian one in which case anything can happen.
 

Smouser

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This procedure is for tracing that star point, if you have found it then you can skip, but this is relevant



Any shorts within the windings will be detected by the VSD and it will show a fault providing it is not just a cheap asian one in which case anything can happen.
Thanks for your help. I am learning slowly. I have read your posts and think I understand it now. However, in practice it is pretty difficult to follow with the wires so tightly packed/hidden under each other/hidden under insulation.

I have a Bosch Rexroth EFC3600 VFD (which I use for my pillar drill). I will set that up to test the motor and then think of buying another VFD.


I was wondering if my VFD maybe have 2 profiles. Then I could wire the VFD to a female plug and put a male plug on the pillar drill and donkey saw and use one VFD for both machines.

Obviously not at the same time and only one will be plugged in at any one time.

I will just have to make sure I change the profile to the correct motor before I start up.
 

guineafowl21

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Well done! Another way to confirm the star point is to leave it connected and measure resistance between the existing phase wires.

Then separate the point and measure between those phase wires and each of the star point wires. The resistance should be half.

It’s not considered good practice to wire a socket to the VFD output, in case you activate the VFD with no load attached which can damage it. The load should be hard wired. An alternative would be a changeover switch similar to those used to switch between mains and generator power for a house.

As for two profiles on the VFD, even if you could, is that a good idea given the ‘i diot factor’? It’s all too easy when focussed on a job to make silly mistakes with complicated switching arrangements. For the safety of your VFD and motors, it may be better to just get another VFD.
 

Spectric

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There is also another issue and that is the setup, the VFD should always be connected to the load and your on/off switch between the supply and VFD. Now the setup is specific to the machine it is connected to. If you want to run multiple three phase machines then you either have an inverter for each machine or get a digital convertor which can run multiple machines provided you do not exceed its load rating. With the convertor you do not get variable speed but you get 400 Vac three phase and can run more than 3Kw loads. Going back to the setup, we used to save all our VSD configuration files onto toughbooks so that if one went down and needed to be replaced it was a simple process to reinstate the config file afterwards, the VFD can have many parameters.
 

Smouser

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Success!

The motor is running! Hoover motor first run

I just used my existing inverter to quickly test. Now it is time to get another inverter and get some sort of switch on the machine as the current switch is 400v only.
I need some sort of a switch because the saw turns itself off when the cut is finished and that cut off switch will need to be wired in too.

I am well chuffed.

I also updated the motor plate ;)

IMG_20210407_180541.jpg


Thank you to all who helped and contributed, I could not have done it on my own.
 

Fergie 307

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When looking for a vfd you will see that a lot now come with a removable control panel and cable, so you can mount the controls conveniently away from the actual unit. This can be pretty useful. As Guineafowl21 says your vfd will have inputs for forward, stop, reverse and so on. Just work out how your switch functions and make appropriate connections to the stop input on the vfd to achieve the same thing.
 

Mick p

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Thanks.

Does it make any more complicated or difficult because it is a 6 pole motor?

I really want to have a go at this (recognising that I could mess the motor up, but hopefully not).

I know of no motor rewinders near me and google does not produce anything either. Hence me wanting to have a go at it.
Try Solent rewinds and repair in hilsea Portsmouth
 
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