Star point in three phase motor

[Myfordman]

40uF per horsepower to run and an additional 3 to 5 times that in parallel to start. 220k ohms 1 watt resistors in parallel with start and run capacitors for safety discharge.

 

[Myfordman]

I'm concerned that your white tab appears only to have one wire and the red and blue have two each. I think all connections should have the same number of wires.

 

[memzey]

40uF per horsepower to run and an additional 3 to 5 times that in parallel to start. 220k ohms 1 watt resistors in parallel with start and run capacitors for safety discharge.

Thanks bob that’s brilliant. Now why is this information not presented in such a concise and easy to understand way anywhere else on the interwebs? I suspect that those who want to be seen as internet gurus don’t really have the knowledge and those that have the knowledge are too busy using it to waste time with internet gurudom! This motor is 2hp but the lathe I’m interested in buying is 1hp. I might buy 2 x 40uf capacitors to experiment with on this one and if that works out ok, use one on the lathe. If I don’t mind the motor taking a few seconds to get to full speed do you think that will be ok?

I'm concerned that your white tab appears only to have one wire and the red and blue have two each. I think all connections should have the same number of wires.

Eagle eyes there bob! There are actually two wires in each winding however the end of one of the white tabbed wires snapped off when I undid the star point. You might be able to see a bit of it sticking out from the end of the red tabbed connection.
When I measured the resistance for each winding (using the parameters as described by Mike) the reading for each was less than 1 on the scale of the meter but not exactly the same. They varied by about one or two scale notches between them. I assume this is ok?

 

[Myfordman]

40uF per horsepower to run and an additional 3 to 5 times that in parallel to start. 220k ohms 1 watt resistors in parallel with start and run capacitors for safety discharge.

Thanks bob that’s brilliant. Now why is this information not presented in such a concise and easy to understand way anywhere else on the interwebs? I suspect that those who want to be seen as internet gurus don’t really have the knowledge and those that have the knowledge are too busy using it to waste time with internet gurudom! This motor is 2hp but the lathe I’m interested in buying is 1hp. I might buy 2 x 40uf capacitors to experiment with on this one and if that works out ok, use one on the lathe. If I don’t mind the motor taking a few seconds to get to full speed do you think that will be ok?

I'm concerned that your white tab appears only to have one wire and the red and blue have two each. I think all connections should have the same number of wires.

Eagle eyes there bob! There are actually two wires in each winding however the end of one of the white tabbed wires snapped off when I undid the star point. You might be able to see a bit of it sticking out from the end of the red tabbed connection.
When I measured the resistance for each winding (using the parameters as described by Mike) the reading for each was less than 1 on the scale of the meter but not exactly the same. They varied by about one or two scale notches between them. I assume this is ok?

OK I can provide the answer here in a couple of lines but it is the result of experimenting with motors for years that enables me to relate my rule of thumb.
You will also need far bigger starting capacitors. it is not just a matter of speed starting. Without getting the motor up to speed in a few seconds, you will burn it out or blow fuses.
A couple of push buttons and two relays plus the capacitors will make a nice little starter. Just keep your finger on the start button until the motor get up to speed and then release it.

Just in case you were wondering why your motor has two wires per winding, it is simply easier to wind with a pair of thinner wires being more flexible than a single one with the same cross sectional area as the two thinner wires. You quite often see this on 2 hp and above.

 

[Steelspray38]

Hi Everyone, Just signed up after reading this topic. How helpful you all are and patient ..... refreshing
Please excuse me if my approach is incorrect or clumsy, I am an absolute novice to forums !
Ok , having read through the postings , I want to identify the star point. Here comes the dumb question, is the star point also found in the terminal box on top of the motor if the terminals are linked across the 3 posts and if so where. Why .... because I bought an expensive Chinese VFD for a 3HP lathe and comes up with a fault on the VFD with regards to the motor (1962 build). Anyway I want to build a rotary converter based on YouTube UK channel that will let me run other machine tools on 3ph .I should add it's a hobby and I am a retired Engineering machinist .I have managed to run a modern 11Kw motor 3ph using a 240v motor to get it up to speed then it runs Ok though just as a test for a few minutes. I am more than willing to pitch in and share what little knowledge/experience I have
Can anyone please help ? Apologies for the preceding epic

 

[Fitzroy]

If you have six ends in your junction box on the motor then yes the star point is in the junction box, if you only have three then it’s inside the motor body.

Fitz.

 

[OldWood]

There's always some smart a***e who stumbles upon a thread as this some time later and feels it is necessary to put his pennyworth in!!

I am such, and until about a fortnight ago I would have gone with the statement from Fitz on "six ends in the junction box" and not said anything.

But ... having bought a late1960's 3 phase planer/thicknesser which turns out to have a seperate drive motor to the thicknesser rollers, I now know that 6 wires can mean something quite different. In my case the machine has two feed speeds and this is achieved by pole switching the 3 phase motor between 2 poles giving 2900 rpm (the theoretical is 3000 - ie 50Hz x 60 secs, but slippage must occur) - and 4 pole giving 1450 rpm. A 14:1 gearbox gives 200 or 100rpm, and somewhere I have seen what the actual wood feed rate is.

So in this case there are six field coils, rather than the three in a single speed motor. However they are probably best thought of as three coils with a centre tap on each coil. Slightly complex switching is required to change from high to low speed. So your 6 wires in this case are the 3 ends plus the 3 centre taps. If like me you want to use a VFD and correspondingly run in delta configuration at 240v, you would have to have 9 wires so I am facing similar motor dismantlement to find the star point and re-configure it.

This is usually referred to as a Dahlander configuration, and if you really want to get confused, go and read that up. Just to show that this isn't just a flash in the pan invention, Mr Dahlander came up with it in 1896 and for some applications it is still used today because of its reliability and low cost.