Star to Delta

Hi all, I'm trying to rewire a "new" three phase motor from Star to Delta. to enable me to run it from an inverter. I have stripped down the motor and found what I believe to be the star point. Prior to cutting the the conductors at the star point I measured the winding resistances from the three terminals in the junction box. There was approx 44 Ohmes between an two of the three terminals. After cutting the conductors at the star point I attempted to measure the resistance between the cut tails and the connections in the junction box. There was no resistance to be measured, is that correct ?

I refer you back to a post on 8/11/19, Star to Delta.

It is all explained there.

No! The first measurement indicates that each winding resistance is 66 ohms, so that is what you should see in the second measurement assuming that you connect your meter to one terminal and to the "tail" of the winding to which it is connected.

Hi Peter

I would have thought you should be able to read the resistance of each of the 3 windings, 

Hold 1 lead on 1 terminal in the connection box and find the other end by trial at the point where you split the star point. 

Emgee

Edited By Emgee on 15/11/2019 11:22:13

Edited By Emgee on 15/11/2019 11:39:37 edited out untrue information

Edited By Emgee on 15/11/2019 11:43:03

First reading was a winding in parallel with 2 in series, hence would be 2/3 of the resistance of single winding. Hence expect 3/2 times = 66 ohms.

Emgee I have tried the above. Connected my meter to all terminals in the junction box and all individual cut tails in the motor. No resistance measured. Reconnected the three tails together then get the 44 Ohms between any two terminals in the junction box ???

Peter, make sure you have removed the insulated coating from the ends you are trying to measure.
note my correction above.

Emgee

The tails are totally clean but I cannot detect any resistance from any individual winding ?

The way I read the original post the 44 ohms was between any two terminals with the motor in star. Given that the star point is buried it presumably doesn't appear at a terminal. Therefore the 44 ohms is measured across two windings in series. So each winding should be 22 ohms.

Andrew

Presumably the insulating coating must have been removed if there is continuity when the 3 tails are reconnected!

Oops! I was thinking delta. Quite right Andrew.

Do you mean the resistance is zero or infinity, just to be clear?

A reading of 44 ohms between any two terminals in the connector box of a star connected motor means each winding is 22 ohms. If the motor was in delta then the 44 ohms reading would be from a coil resistance of just under 30 ohms due to 2 in parallel and one in series being 15 + 30 roughly. Since the aim is to change from star to delta this means the coils are 22 ohms.

Martin C

Failed the fastest finger first challenge!

Edited By Martin Connelly on 15/11/2019 12:06:28

Not sure how to put text with photos, will get there in the end.

Peter, does the picture show the star connection point or the tails to windings connections ?

Emgee

The chocolate block is where all three conductors were joined together, I assumed this must be the star point ? Would there be another point in the motor were all three conductors where joined ?

Peter

The terminal plate being 3 pole would indicate the motor is single speed 3 phase, but maybe it's a 2 speed motor but only connected for single speed running.
It's a mystery, hopefully someone will have an explanation.

Eemgee

Peter, if you send me a PM with your phone number I'll give you a call regarding the motor. John

Did further checks this afternoon, found the conductors at the star point looked clean and free of insulation but must have had a thin coating of varnish on them. Used some fine wet and dry to totally clean the copper conductors. I was then capable to measure the individual resistances. Three new wires soldered, heat shrunk and routed to the terminal chamber........Too long off the tools !