# 3 phase motor?

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#### mbrooke

##### Batteries Included
Going by this diagram, how would this particular motor function? Would the rotor be pulled around toward the stator in an elliptical or off center orbit while spining slower? Would there be postive, negative or zero sequnce current in the squirrel cage rotor? What about the stator? Or would the combined magnetic field of the red and green phase equal the blue phase when said blue phase is at its peak in the sine wave?

I could be wrong, but I don't see a symmetrically rotating magnetic field...

#### oldsparky52

##### Senior Member
You are playing over my education level, but I believe I see what you are asking and I am interested in the answer also.

#### mbrooke

##### Batteries Included
You are playing over my education level, but I believe I see what you are asking and I am interested in the answer also.

In short, think of a single phase motor. In theory north and south fields are of equal magnitude 180* apart. Though I could be veering off as I'm just comparing what I believe are apples to apples...

#### oldsparky52

##### Senior Member
Is this where you got that drawing?

Yes

#### paulengr

##### Senior Member
The drawing looks like the original 2 phase Tesla induction motor except it’s three phase. Eiffel and others greatly improved the torque pulsation issue first by adding coils in pairs and second by adding the third phase.

What you have there is a three pole motor. A really, really odd thing. It will work and have an unusual RPM but without balanced poles it’s going to vibrate.

#### mbrooke

##### Batteries Included
The drawing looks like the original 2 phase Tesla induction motor except it’s three phase. Eiffel and others greatly improved the torque pulsation issue first by adding coils in pairs and second by adding the third phase.

What you have there is a three pole motor. A really, really odd thing. It will work and have an unusual RPM but without balanced poles it’s going to vibrate.

Ok- but why can't it be wound ABC,ABC,ABC instead of the current ABA,CAC,BCB?

#### paulengr

##### Senior Member
Ok- but why can't it be wound ABC,ABC,ABC instead of the current ABA,CAC,BCB?

You’re right. It’s actually one pole. I believe that interleaving the coils is to help the torque pulsations. They will cancel some of the torque though. I just don’t see any good ideas.

#### mbrooke

##### Batteries Included
You’re right. It’s actually one pole. I believe that interleaving the coils is to help the torque pulsations. They will cancel some of the torque though. I just don’t see any good ideas.

Same, I don't see any good ideas. I'm confused as a tornado.

#### paulengr

##### Senior Member
Same, I don't see any good ideas. I'm confused as a tornado.

Translated, this guy found some really old drawings and is experimenting but probably never read a book on motor design. I’m impressed by what he did but Tesla was similar...it took several others to take his ideas and make them practical. Look at his 1887 patents and you’ll see what I mean.

#### mbrooke

##### Batteries Included

Translated, this guy found some really old drawings and is experimenting but probably never read a book on motor design. I’m impressed by what he did but Tesla was similar...it took several others to take his ideas and make them practical. Look at his 1887 patents and you’ll see what I mean.

So what is the correct way to wind this motor? I'm getting the notion that the pole number isn't even right?

#### paulengr

##### Senior Member
So what is the correct way to wind this motor? I'm getting the notion that the pole number isn't even right?

It’s an odd one. The rotor is external and i believe it is intended as a permanent magnet motor, not AC. The others refer to Halbach PM motors for instance. Not much detail but it keeps referring to model airplanes. I think the idea is the props are connected directly to the external rotor.

#### winnie

##### Senior Member
What I see is an approximation to a winding that is not possible because of symmetry constraints.

You are given 4 electrical poles, 9 salient poles, and 3 phase drive. In other words the magnetic field cycles N S twice over 9 mechanical structures but you only have 3 different AC phase angles to use.

In an ideal case to get 4 magnetic poles, each saliency would get AC 80 degrees away from the adjacent saliency. But what you have are 3 AC sources 120 degrees apart, plus their inverse (since you can wind in either direction).

This winding is a three phase approximation to the ideal 9 phase winding.

 Salience 1 ideal 0° as built 0° 2 80 120 3 160 180 4 240 240 5 320 0 6 40 60 7 120 120 8 200 240 9 280 300

#### mbrooke

##### Batteries Included
What I see is an approximation to a winding that is not possible because of symmetry constraints.

You are given 4 electrical poles, 9 salient poles, and 3 phase drive. In other words the magnetic field cycles N S twice over 9 mechanical structures but you only have 3 different AC phase angles to use.

In an ideal case to get 4 magnetic poles, each saliency would get AC 80 degrees away from the adjacent saliency. But what you have are 3 AC sources 120 degrees apart, plus their inverse (since you can wind in either direction).

This winding is a three phase approximation to the ideal 9 phase winding.

 Salience 1 ideal 0° as built 0° 2 80 120 3 160 180 4 240 240 5 320 0 6 40 60 7 120 120 8 200 240 9 280 300

Can you explain more into it? A need a bit of a break down in regards to symmetry. Are you saying that a 3 phase motor needs doubles of 3?

I'm getting this from your chart:

#### mbrooke

##### Batteries Included
I take it like this, 3 phase motors can only have 6, 12, 18, ... windings?

#### paulengr

##### Senior Member
I take it like this, 3 phase motors can only have 6, 12, 18, ... windings?

They don’t have to but you get torque issues if you don’t have pairs (N and S). That’s why in AC poles are always even...2, 4,6...

Sent from my iPhone using Tapatalk

#### mbrooke

##### Batteries Included
They don’t have to but you get torque issues if you don’t have pairs (N and S). That’s why in AC poles are always even...2, 4,6...

Sent from my iPhone using Tapatalk

What about vibration? What would happen if the motor was built without pairs?

#### paulengr

##### Senior Member
What about vibration? What would happen if the motor was built without pairs?

You get massive vibration issues. The pole is pulling on or pushing the rotor at an angle. We can visualize this as two torques. One is tangential to the rotor and pulls it the way we want. The other is perpendicular and just puts force directly on the bearing. With 2 poles 180 degrees apart (or any symmetrical arrangement) the forces on the shaft are cancelled and all we get is tangential torque.

So without that symmetry the rotor rotated but pushes hard against the bearing so you will have very high 1x and to some degree 2x vibration, and most likely large bearing harmonics as well.

Plus efficiency losses.

I just can’t imagine a good reason for doing this. Even vibration motors don’t have assymetrical poles.

#### mbrooke

##### Batteries Included
Thats what I suspected originally! Its amazing how the human brain can picture something without ever seeing, experiencing or testing it out

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
Its amazing how the human brain can picture something without ever seeing, experiencing or testing it out
Yes, and good communication helps you get someone to see your mental image in their mind.

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