# Configuration of an Induction motor

#### kokkie_d

Joined Jan 12, 2009
72
Dear All,

Is there a test to find out which configuration an induction motor is in; i.e. star or delta?

We have an induction motor of which I am told its in delta configuration nut I have doubts and am wondering if there is a way I can verify this?

Sincerely,

#### VoodooMojo

Joined Nov 28, 2009
505
the individual windings are usually labeled A at one end and B at the other.
the number 1, 2, or 3 notes the number of the winding.

So A1 and B1 are the two ends of the same winding.

In STAR, all the A ends OR all the B ends are connected together.
In effect: B1 to B2 to B3. The other ends (A1, A2, A3) are connected to their corresponding L1, L2, and L3 terminals.

In DELTA, B1 is connected to A2, B2 is connected to A3, B3 is connected to A1.
Supply L1 to A1; supply L2 to A2 and supply L3 to A3

edit:
I neglected to mention that some motors have windings labeled 1A 2A, 1B 2B, and 1C 2C.

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#### strantor

Joined Oct 3, 2010
5,543
Thats for a 6 wire motor; there are also 9 wire motors, and 3 wire motors. I assumed OP was referring to a 3 wire motor, or else wouldn't be asking the question as the diagram is usually inside the cover.

#### kokkie_d

Joined Jan 12, 2009
72
Thanks but I know how to wire the motor for different configurations. Problem is that I have a motor with 3 high current wires coming out and no schematics, diagrams, labels, numbers etc. just three wires.
Is there a way to determine what the wiring is inside? I had a look at trying to figure it out by doing dc tests to determine resistance as seen by power supply and then calculatins etc but am not getting anywhere with that.

#### VoodooMojo

Joined Nov 28, 2009
505
if you can determine which wires are from which windings,
STAR will have the resistance of TWO of the windings and DELTA will have the resistance of only ONE of the windings.
The resistance readings are taken from any two of the line in (L1, L2 and L3) terminals

#### kokkie_d

Joined Jan 12, 2009
72
Following L1-L2 through a dc test:

In star indeed will measure across 2 resistors in series in delta it will measure across 1 resistor in parallel with the other two in series. since delta would be internally (which I can't see) connected. But since I do not know which configuration I'm looking at (black box) I only see the total resistance but that doesn't tell me anything about the internal configuration.

#### strantor

Joined Oct 3, 2010
5,543
if you can determine which wires are from which windings,
STAR will have the resistance of TWO of the windings and DELTA will have the resistance of only ONE of the windings.
The resistance readings are taken from any two of the line in (L1, L2 and L3) terminals
yeah but how would you know if the resistance you are reading corresponds to one winding or two?

I had this question come up a long time ago and never got to the bottom of it, so I'm very interested.

#### strantor

Joined Oct 3, 2010
5,543
hey i just had an idea. put some DC from any one winding to another winding, then read the voltage on the 3rd winding. if it is half the applied voltage then it's star.

EDIT: crap, no it would still be 1/2 the applied voltage if it was delta also.

EDIT2: Ok, read the resistance from winding A to B, then A to C, then join B & C together and read the resistance from A to BC. if the resistance from A to BC is HALF what it was from A to B & A to C, then it is delta. if the resistance from A to BC is 75% of what it was from A to B & A to C then it is star.

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#### steveb

Joined Jul 3, 2008
2,436
Tricky question.

I'll think about it more, but I don't think you can tell what the configuration is by simple resistance measurements unless you know the coil resistance of the isolated motor windings beforehand.

A possible trick is to do some kind of pulse transmission time measurement through the coils. If you send a fast voltage pulse into one terminal and see a dual pulse come out the other two terminals, it is probably a delta connection because there are two paths, with one path twice as long as the other. A star connection has only one path from terminal to terminal.

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#### strantor

Joined Oct 3, 2010
5,543
Tricky question.

I'll think about it more, but I don't think you can tell what the configuration is by simple resistance measurements unless you know the coil resistance of the isolated motor windings beforehand.
Yeah i think you're right. I just opened up one of my 9wire motors and tested out my little "flash of genius" in post #8 (edit2) in both wye and delta and that didn't work either.
I thought about adding a resistor between two of the phases that would offset it's balance by a certain degree for either configuration, but I can't figure it out; making my brain hurt.

#### Kermit2

Joined Feb 5, 2010
4,162
I believe a motor connected internally as 'STAR' will require FOUR wires coming out.

Delta only requires 3.

#### steveb

Joined Jul 3, 2008
2,436
I believe a motor connected internally as 'STAR' will require FOUR wires coming out.

Delta only requires 3.
It's not a requirement to have 4 terminals with a star/wye connection because you can still run the motor with a floating neutral.

#### strantor

Joined Oct 3, 2010
5,543
I believe a motor connected internally as 'STAR' will require FOUR wires coming out.

Delta only requires 3.
never seen that.
I've seen plenty of wye configured motors, but never one with 4 wires. I haven't seen it all though.

#### Kermit2

Joined Feb 5, 2010
4,162
Wouldn't leaving a neutral floating require that incoming power be balanced? I would think for safety reasons one wouldn't rely on a power companies ability to maintain the phases. Loss of a phase or a stalled single phase motor nearby could cause a problem in such cases. But, whatever, safety is over-looked in many situations these days, and I've lost count of the changes made to codes that are designed for the sole purpose of money savings in some companies bottom line.

#### kokkie_d

Joined Jan 12, 2009
72
Yeah i think you're right. I just opened up one of my 9wire motors and tested out my little "flash of genius" in post #8 (edit2) in both wye and delta and that didn't work either.
I thought about adding a resistor between two of the phases that would offset it's balance by a certain degree for either configuration, but I can't figure it out; making my brain hurt.
i was thinking about similar lines, will do some calculations tonight. Adding a resistor straight in line with one of the wire does not change anything to the outcome of the readings.

Will do some math around between phases although I am worried it might result in nothing.

I'll probably try Steveb's trick of a pulse. How fast should that pulse be? Any ideas? Are we talking milli, micro or nano (or faster)?

#### steveb

Joined Jul 3, 2008
2,436
I'll probably try Steveb's trick of a pulse. How fast should that pulse be? Any ideas? Are we talking milli, micro or nano (or faster)?
Well, we are talking about the propagation time over the length of wire, which is near the speed of light. So, nano or at least micro is desirable. You will need to leave the output terminal unloaded and use a high impedance scope probe. The scope needs to be reasonably fast too, preferably at least 100 MHz.

#### BillB3857

Joined Feb 28, 2009
2,544
hey i just had an idea. put some DC from any one winding to another winding, then read the voltage on the 3rd winding. if it is half the applied voltage then it's star.

EDIT: crap, no it would still be 1/2 the applied voltage if it was delta also.

EDIT2: Ok, read the resistance from winding A to B, then A to C, then join B & C together and read the resistance from A to BC. if the resistance from A to BC is HALF what it was from A to B & A to C, then it is delta. if the resistance from A to BC is 75% of what it was from A to B & A to C then it is star.

Unless I'm confused (not too unusual) resistance readings should determine Y vs Delta. See the attached..........

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#### steveb

Joined Jul 3, 2008
2,436
Unless I'm confused (not too unusual) resistance readings should determine Y vs Delta. See the attached..........
I think the underlying assumption is that he does not know the resistance of the coils when isolated. Above I said the following.

"... I don't think you can tell what the configuration is by simple resistance measurements unless you know the coil resistance of the isolated motor windings beforehand. "

Clearly, if you know the isolated resistances the problem is trivial.

#### BillB3857

Joined Feb 28, 2009
2,544
Assuming (yes, I know what that means) that all windings are EQUAL, the ratio would remain as I outlined in my drawing. For illustration purposes, I chose 10 ohms per winding. Any value could be plugged in based upon the first reading with the short removed.

(ED)
DUHH.. I just ran the numbers to check the ratio. Not much difference between 1.32 and 1.333333333...Looking at real numbers with known resistance, it throws a curve. I stand corrected. He WOULD need to have some idea of the single winding resistance! Sounds like he may need to pop an end bell.

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#### strantor

Joined Oct 3, 2010
5,543
Unless I'm confused (not too unusual) resistance readings should determine Y vs Delta. See the attached..........
Your little drawing demonstrates exactly what I was trying to say before, and is in fact correct. I confirmed this by wiring a 9-wire motor in delta and wye, but was not able to use the numbers determine if it was in wye or delta. The only thing that could help you find the winding configuration without knowing the individual winding resistance beforehand would a difference in ratio. in both wye and delta the ratio from shorted to unshorted is .75.
6.667 * .75 = 5
20 * .75 = 15