Wire Size Change in Electric motor 3

[tehribo]

Hello!

The first formula which I read in EASA's "AC Motor redesign" book seems very strange to me. You can check it the attached file or:

CM2 = CM1 X (T1/T2),

My question is, ok if I change the lenght (turns) and the area of the wire. I am changing also the resistance of the winding, correct?
So I am changing the parameters of the whole motor?

So is this formula true?

Thank you!

P.S in the formula :

CM2 = circular mil area per turn of the new winding
CM1 = circular mil area per turn of the original winding
T1 = turns per coil of the original winding
T2 = turns per coil of the new winding

http://eldvigateli.com

- Electric motors Bulgaria

 

[waross]

The strength of a DC magnet coil. The following general relationships are valid within limits.
The strength of a magnetic coil depends on the number of Amp-turns. If you double the number of turns you double the resistance. Hence the current drops in half. End result, the same number of Amp turns and the same strength magnetic field.
The strength of a DC magnetic field is more influenced by the cross sectional area of the wire than the number of turns.
BUT.
If you double the number of turns the inductance increases by square factor or four times. This may have implications for any AC coil where the number of turns is changed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

Yes, the formula is correct. Changing the number of turns in the motor changes the horsepower output. Reducing the number of turns increases the motor horsepower and causes proportional increase in motor current. Increasing the number of turns has the opposite effect.

Reading the section on 'horsepower change' should help your understanding.

 

[tehribo]

Thank you very much for your answers!

So if I understood you correct...The following example is true?

If I have to rewind a motor with original data:

1 wire X 0,90 mm diameter, 100 turns

it will not change if I made it with

1 wire X 1,00 mm diameter, 90 turns

?

http://eldvigateli.com

- Electric motors Bulgaria

 

[electricpete]

Thank you very much for your answers!

So if I understood you correct...The following example is true?

If I have to rewind a motor with original data:

1 wire X 0,90 mm diameter, 100 turns

it will not change if I made it with

1 wire X 1,00 mm diameter, 90 turns

If you change nothing else, that would increase the flux density by about 10%. That may or may not be acceptable.

...It will also decrease stator leakage impedance (on the order of 20% I think since reactance is proportional to square of turns) and therefore increase locked rotor current (probably on the order of 10% assuming rotor and stator leakage reactances are about the same). That may or may not be acceptable.

What your posted page says is "the circular mil determined in this manner will make the slot fill of the new winding the same as the original winding". Slot fill is not the only thing you have to look at.


=====================================
(2B)+(2B)' ?

 

[tehribo]

My english is not good enough or my knowledge, but I still cannot get it

Maybe the first sentence from the scanned page is the explaination?

"Whenever the turns in a coil are changed, the wire size usually changes to provide the same slot fill"

maybe I should read (translate) it as:

If (for some reasonower change,voltage change and etc) you have to change the turns in a coil, you will also have to change the wire size to provide the same slot fill

Thank you in advance for the future answers!

http://eldvigateli.com

- Electric motors Bulgaria

 

[rhatcher]

tehribo,

You said

If I have to rewind a motor with original data:

1 wire X 0,90 mm diameter, 100 turns

it will not change if I made it with

1 wire X 1,00 mm diameter, 90 turns

If you mean that the motor original rating will not change then you are incorrect. If you change the number of turns then the motor kW rating will change. The change you suggest will give increased kW rating, increased no-load current, increased load current, and increased motor heating.

I also found this confusing when I first studied this book. What the book does not say is that changing the wire size using this formula is only done after calculating a change in the number of turns to change motor kW rating, voltage, or poles. First, you calculate the number of turns to change, then you use this formula to calculate the change in wire size. I am not sure why they put this calculation first in the book since it is confusing.

If you check Section 3, page 9, you will see the calculation for changing the number of turns to change the kW rating of the motor. If you want the motor to keep the same kW rating then you must not change the number of turns.

It sounds like you are trying to rewind a motor to keep the same rating but the 0.9mm wire is not available. If this is true, you can substitute a different wire size without changing the number of turns.

For a larger wire size such as 1.0mm, make a sample coil (100 turns of 1.0mm wire) and check to see if it fits in the slot. If the coil fits then no other change is needed. If you have a spool of 0.95 wire then this size is more likely to fit. In either case, the motor will have the same rating after rewind as it did before.

You can also substitute a slightly smaller wire such as 0.85mm if the wire cross sectional area is enough to carry the motor current.

Another option is to use two smaller wires in place of one 0.9mm. For this, you must make sure that the two smaller wires have the same cross sectional area as the single large wire. For example, a 0.67mm and a 0.6mm in parallel will have similar cross section to a single 0.9mm. Usually two smaller wires will easily fit in place of one large wire so a sample coil to check the slot fit is not required.

For substituting smaller wire(s), the wire chart in Table O, pages 120-121 will be helpful in determining cross sectional area for calculating ampacity.


Let us know if you need more help.

 

[rhatcher]

tehribo

You made the following statement while I was typing my previous answer:

If (for some reasonower change,voltage change and etc) you have to change the turns in a coil, you will also have to change the wire size to provide the same slot fill.

This is a correct understanding. Good job!

 

[tehribo]

Thank you very much everybody for the patience and your answers!

http://eldvigateli.com

- Electric motors Bulgaria