Three phase motor capacitor wiring help

[Onoff]

Thanks, that looks like what I am after.
I am going to dig into this tmrw and do some reading. A forum member kindly sent me some motor books which will be a great help too.

Book 16, page 49, fig 3.7 shows a circuit with a relay of the type I think @m_c is referring to. Has a start button you let go of as soon as the motor is up to speed.

You could even put a cheap (like a tenner from e.g Banggood) tacho on the motor to measure the rpm and give a digital readout.

 

[Agroshield]

Have a look at the diagram with the yellow background about half way down this page - Static Phase Converter [Pros and Cons] - Electric Problems

I'd guess the two identical capacitors will have been permanently connected to the false phase (L1-L3 and L2-L3 in the diagram), then the odd capacitor has been connected via the contactor and time delay relay.

Determining the relative size of the capacitors does seem to produce different answers. In the diagram immediately below the one you show, they have 5, 12.5 and 80 uF. Very close to the diagram is a downloadable paper from researchgate, where they have 60, 55 and 30 uF.

What is certain is that the biggest cap. is the start cap. The only similarity between the two above is that one (L2-L3) is approx. twice the other (L1-L3).

I do not know the correct answer but I am not convinced that equal caps. on L1-L3 and L2-L3 is the correct answer.

 

[Edward Teach]

This is an interesting read,
I hope the results are posted and explained,
I run a 3 phase motor on single phase, with 1 capacitor for the third phase,
But the motor only runs for maybe 10-15 seconds at a time,
It powers a hydraulic motor, on my Omer Concord 25 car ramp.

 

[cheb]

I found this site helpful: https://www.js-technik.de/en/faq/detail/groupid/4/id/75

 

[Smouser]

This is an interesting read,
I hope the results are posted and explained,
I run a 3 phase motor on single phase, with 1 capacitor for the third phase,
But the motor only runs for maybe 10-15 seconds at a time,
It powers a hydraulic motor, on my Omer Concord 25 car ramp.

I hope I can work it all out .

If I do, I will surely post some photos and explanations.

 

[waddycall]

I have two three phase machines (drill and 8” grinder) running permanently on single phase with capacitors. Not noticed any power issues. This book is what I used

Electric Motors in the Home Workshop (Workshop Practice): Amazon.co.uk: Cox, Jim: 9781854861337: Books

Buy Electric Motors in the Home Workshop (Workshop Practice) by Cox, Jim (ISBN: 9781854861337) from Amazon's Book Store. Everyday low prices and free delivery on eligible orders.

www.amazon.co.uk

 

[m_c]

Running a 3ph motor using capacitors to create a dummy phase is quite hard on a motor, especially if the capacitors are faulty or not correctly sized. Given its age I would not like to run an old motor like that permanently using capacitors, maybe a short test to see if the motor is working.

The price of vfds has come down so much that its the most sensible way to run something simple like that saw, plus you can set it up so the vfd stops the blade nearly instantly.

The issue with using a VFD, is the winding insulation on a motor that age won't be VFD tolerant. VFDs result in high voltage transients through the windings, which can cause old insulation to break down. It may never be a problem, or it may destroy the motor quite quickly.
At least with capacitors the biggest issue is purely putting too much voltage through the false phase due to poorly selected capacitors, unless you add a sine wave filter to a VFD to kill any transients.

 

[m_c]

Determining the relative size of the capacitors does seem to produce different answers. In the diagram immediately below the one you show, they have 5, 12.5 and 80 uF. Very close to the diagram is a downloadable paper from researchgate, where they have 60, 55 and 30 uF.

What is certain is that the biggest cap. is the start cap. The only similarity between the two above is that one (L2-L3) is approx. twice the other (L1-L3).

I do not know the correct answer but I am not convinced that equal caps. on L1-L3 and L2-L3 is the correct answer.

Capacitor sizing is never perfect, as the actual capacitance needed depends on the motor, the voltage, and the motor load.
Generally you'll end up with the false phase being a bit high under no load, with the voltage dropping low once a load is applied.

The run capacitances vary exactly where the false phase develops peaks power, and if badly out will result in the motor running rough, but so will the wrong capacitance in general.
A big old large motor with lots of inertia, it's not likely to be that noticeable, but a light weight modern motor, it could be very noticeable.

 

[Smouser]

The issue with using a VFD, is the winding insulation on a motor that age won't be VFD tolerant. VFDs result in high voltage transients through the windings, which can cause old insulation to break down. It may never be a problem, or it may destroy the motor quite quickly.
At least with capacitors the biggest issue is purely putting too much voltage through the false phase due to poorly selected capacitors, unless you add a sine wave filter to a VFD to kill any transients.

I have read that somewhere too. I am going to try and get the capacitor setup this weekend.

 

[Smouser]

No update on the wiring yet. I wanted to check the overall condition of the motor first.

I opened the electrical connection box part on the motor to check the state of the wiring.

I found a little surprise.

It was PACKED with sawdust. I have never seen anything like it.

The wiring terminals were pretty rusty too. I will get them cleaned up.

The backside of the terminals with the wires going to the windings.

The wring insulation looked fine. No crumbling or anything like that.
Wire made by Pirelli General Southampton.

After I cleaned all the sawdust off, I opened the rear of the motor.

Slightly dusty

Looking a bit better. The bearing was fine. There was enough grease in there to protect the bearing from damage. I cleaned the old grease out and replaced it with new grease.

I then opened the front of the motor.

Not as bad as the rear. I must have got some of the dust out with my shop vacuum and airline when I did the back.

Again the bearings were fine, just regreased them with new grease.

That is it for the night. I hope to try and dive into the wiring tmrw.

 

[Bornfree]

Roller bearings they don't make them like that anymore

 

[Smouser]

I have been wondering about the motor's wiring.

The plate on the motor stated it is a 200v motor. However, the previous owner ran the motor on three-phase commercial power.
As far as I know, the UK does not have 240v three-phase power so I would assume it was running on 420v three-phase power (it was not running from a generator).

Surely the motor would have burnt out by now if it was only rated for 200v?

This brings me these photos:

I believe the motor is currently wired in STAR (which would make it ~400v) and the motor could be wired in DELTA (which it probably was from the factory making it 200v and thus the nameplate indication).

I can think of only one other (unlikely) possibility: It was wired in STAR from the factory and it is 200v. If you were to wire it to DELTA it would be ~100v ish.

What do you guys think?

 

[m_c]

I'd guess the 200V rating is for Delta connected.

The easy way to find out, is once connected up, is to monitor the current draw. If the supply voltage is too high for the motor voltage, excess current will flow even off load.

 

[Smouser]

I'd guess the 200V rating is for Delta connected.

The easy way to find out, is once connected up, is to monitor the current draw. If the supply voltage is too high for the motor voltage, excess current will flow even off load.

Oh great info. I will investigate that further.

 

[hermetic]

The motor looks very much like it has been rewound, the insulation and the varnish type is not what you would expect to see in a motor of that age. The plate states that the motor is mesh connected, and yet there are not enough wires present at the terminal box for a mesh connected motor. It looks like it has been run on single phase and the starter relay would be either a starting relay which drops one of the capacitors out, or just switches the whole plot on! helpfully, with all the wiring in blue, it is hard to tell The straightforward capacitor system of creating three phases will get it turning if connected in Delta, but only a test will tell if it is producing enough power to turn the sawbench from standstill. Personally I doubt it, but you have nothing to loose (apart from the cost of purchase) by trying a VFD because the motor looks to be a more modern wind, and given you have no 3 phase, if she dies, she dies!

 

[Smouser]

The motor looks very much like it has been rewound, the insulation and the varnish type is not what you would expect to see in a motor of that age. The plate states that the motor is mesh connected, and yet there are not enough wires present at the terminal box for a mesh connected motor. It looks like it has been run on single phase and the starter relay would be either a starting relay which drops one of the capacitors out, or just switches the whole plot on! helpfully, with all the wiring in blue, it is hard to tell The straightforward capacitor system of creating three phases will get it turning if connected in Delta, but only a test will tell if it is producing enough power to turn the sawbench from standstill. Personally I doubt it, but you have nothing to loose (apart from the cost of purchase) by trying a VFD because the motor looks to be a more modern wind, and given you have no 3 phase, if she dies, she dies!

Very interesting!

Thank you for the reply. I am on the brandy now but will investigate further tmrw!

 

[Agroshield]

The motor looks very much like it has been rewound, the insulation and the varnish type is not what you would expect to see in a motor of that age. The plate states that the motor is mesh connected, and yet there are not enough wires present at the terminal box for a mesh connected motor.

Your point about the rewinding is well made and well reasoned. It is a possibility.

However, I think you need to expand on your statement that there are not enough wires present for a mesh connection. His photo of the back of the terminal block clearly shows three red wires and three black wires going into the motor. They would be the ends of the three coils. The photo of the front of the terminal block shows a wire on the RHS bridging three terminals, forming the star point and the three incoming phases on the LHS. So what wires do you think are missing?

It would be more usual to use brass bus bars (bridging strips) either to form the star connection or for the mesh, but the lack of these is not very strong evidence of anything at all. The mesh statement on the nameplate is clearly original, and if it has been rewound, the rewinders are not always bothered to re-stamp the plate if something they do changes the motor's characteristics.

 

[brightspark]

if 6 wires come out it should be dual voltage

 

[doubleboost]

 

[hermetic]

Your point about the rewinding is well made and well reasoned. It is a possibility.

However, I think you need to expand on your statement that there are not enough wires present for a mesh connection. His photo of the back of the terminal block clearly shows three red wires and three black wires going into the motor. They would be the ends of the three coils. The photo of the front of the terminal block shows a wire on the RHS bridging three terminals, forming the star point and the three incoming phases on the LHS. So what wires do you think are missing?

It would be more usual to use brass bus bars (bridging strips) either to form the star connection or for the mesh, but the lack of these is not very strong evidence of anything at all. The mesh statement on the nameplate is clearly original, and if it has been rewound, the rewinders are not always bothered to re-stamp the plate if something they do changes the motor's characteristics.

Yes, having looked again ithe wire does connect all three terminals (I thought it just connected two, which was puzzling) the motor is in star, and would need connecting in delta for use on a VFD, unless you got a VFD with a 415v output. For full power running on artificial 3 phase the "Brooks book" recomends using a pilot motor which is started first, and runs unloaded, in order that the remaining motors produce their full torque on startup, which is required with a sawbench. My point about is not being a mesh motor, which would have 12 wires (six coils) at the terminal box was given as evidence that it has been rewound. A mesh connected motor with six coils instead of the normal three was often used for dual voltage or dual speed running, it is a more complex version of delta connection. If you can get a copy of the brooks book "Installation and maintenance of electric motors" it has the circuit diagram for the pilot motor artificial 3 phase system using a pilot or slave motor. Also there is a couple of Youtube videos by Andy Reynolds which explain it.

and

Phil;