# 3 phase motor?

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#### winnie

##### Senior Member
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?

It doesn't need doubles of 3, but you want to maintain symmetry and doubles of 3 is a good way to do this.

An induction motor will usually have slots rather than salient poles. You want an even phase angle increase from one slot to the next.

The phase angles that you have to work with are those supplied from your source, their inverse, and ones you construct by combining source phases (meaning coils from 2 phases sitting in the same slot).

One could imagine having slots or salient windings where the phase angle increases by 60 degrees per. This gives 6 slots per magnetic pole pair.

Or you could go up by 120 degrees per, giving 3 slots per pole pair. Past 180 degrees per slot and you actually reverse direction rather than going faster.

For any other number of slots per pole, you are either combining phases in a single slot (eg 5 turns of phase A plus 10 of phase b) or doubling things up, or intentionally introducing error.

The motor you posted a link to intentionally introduces phasing errors to get the desired pole count.

Hope that helps.

Jon

#### paulengr

##### Senior Member
It doesn't need doubles of 3, but you want to maintain symmetry and doubles of 3 is a good way to do this.

An induction motor will usually have slots rather than salient poles. You want an even phase angle increase from one slot to the next.

The phase angles that you have to work with are those supplied from your source, their inverse, and ones you construct by combining source phases (meaning coils from 2 phases sitting in the same slot).

One could imagine having slots or salient windings where the phase angle increases by 60 degrees per. This gives 6 slots per magnetic pole pair.

Or you could go up by 120 degrees per, giving 3 slots per pole pair. Past 180 degrees per slot and you actually reverse direction rather than going faster.

For any other number of slots per pole, you are either combining phases in a single slot (eg 5 turns of phase A plus 10 of phase b) or doubling things up, or intentionally introducing error.

The motor you posted a link to intentionally introduces phasing errors to get the desired pole count.

Hope that helps.

Jon

You have to look around the rest of the web site but it’s not induction at all or salient pole. It’s PMDC. At least that’s the descriptions. Lots of vague information.

#### winnie

##### Senior Member
You have to look around the rest of the web site but it’s not induction at all or salient pole. It’s PMDC. At least that’s the descriptions. Lots of vague information.

Agree, very vague.

I was using saliency in the meaning of a physical object sticking out. It is not a salient pole rotor, in this motor there were saliencies on the stator and permanent magnets on an external rotor.

But the same principle of wanting the same phase difference from one saliency or slot to the next applies, and the irregular phase spacing will produce all of the problems you described.

Jon

#### paulengr

##### Senior Member
Agree, very vague.

I was using saliency in the meaning of a physical object sticking out. It is not a salient pole rotor, in this motor there were saliencies on the stator and permanent magnets on an external rotor.

But the same principle of wanting the same phase difference from one saliency or slot to the next applies, and the irregular phase spacing will produce all of the problems you described.

Jon

Just thinking about it and the rotating pattern used in Halbach motors it almost looks like interpoles used in DC motors. It might be a way of creating some of the same effect but hard to visualize without the rotor layout too. But it’s just so far removed from any standard design it’s hard to say.

#### mbrooke

##### Batteries Included
Yes, and good communication helps you get someone to see your mental image in their mind.

But I can't put anything into words without a picture.

#### mbrooke

##### Batteries Included
It doesn't need doubles of 3, but you want to maintain symmetry and doubles of 3 is a good way to do this.

An induction motor will usually have slots rather than salient poles. You want an even phase angle increase from one slot to the next.

The phase angles that you have to work with are those supplied from your source, their inverse, and ones you construct by combining source phases (meaning coils from 2 phases sitting in the same slot).

One could imagine having slots or salient windings where the phase angle increases by 60 degrees per. This gives 6 slots per magnetic pole pair.

Or you could go up by 120 degrees per, giving 3 slots per pole pair. Past 180 degrees per slot and you actually reverse direction rather than going faster.

For any other number of slots per pole, you are either combining phases in a single slot (eg 5 turns of phase A plus 10 of phase b) or doubling things up, or intentionally introducing error.

The motor you posted a link to intentionally introduces phasing errors to get the desired pole count.

Hope that helps.

Jon

Can you describe salient poles vs slots and why most motors today don't have solid windings around an "I"?

#### mbrooke

##### Batteries Included
It doesn't need doubles of 3, but you want to maintain symmetry and doubles of 3 is a good way to do this.

An induction motor will usually have slots rather than salient poles. You want an even phase angle increase from one slot to the next.

The phase angles that you have to work with are those supplied from your source, their inverse, and ones you construct by combining source phases (meaning coils from 2 phases sitting in the same slot).

One could imagine having slots or salient windings where the phase angle increases by 60 degrees per. This gives 6 slots per magnetic pole pair.

Or you could go up by 120 degrees per, giving 3 slots per pole pair. Past 180 degrees per slot and you actually reverse direction rather than going faster.

For any other number of slots per pole, you are either combining phases in a single slot (eg 5 turns of phase A plus 10 of phase b) or doubling things up, or intentionally introducing error.

The motor you posted a link to intentionally introduces phasing errors to get the desired pole count.

Hope that helps.

Jon

Let me digest all this. It helps. Can you describe salient poles vs slots and why most motors today don't have solid windings around an "I"? Is it purely for for as you describe an even phase angle increase? This concept I can kind of grasp, but how does physics allow that two windings of different phases sit in the same slot? How isn't there a problem with sequence components or local circulating current?

Sorry for the double post, the edit time is now 5 minutes.

#### winnie

##### Senior Member
Let me digest all this. It helps. Can you describe salient poles vs slots and why most motors today don't have solid windings around an "I"? Is it purely for for as you describe an even phase angle increase? This concept I can kind of grasp, but how does physics allow that two windings of different phases sit in the same slot? How isn't there a problem with sequence components or local circulating current?

Sorry for the double post, the edit time is now 5 minutes.

1) Saliency vs smooth: While we tend to think of the rotating field in terms of 'N' and 'S' 'poles' moving around, what you really have is a smooth distribution of magnetic flux across the airgap of the motor. You want this flux distribution to smoothly rotate. Physical poles sticking out create fixed structures in the flux distribution, exactly what you don't want in a rotating field. These fixed structures are better suited to situations where you have a fixed magnetic field structure, such as in the _rotor_ of a synchronous machine, where the saliencies rotate in perfect synchronism with the rotating magnetic field.

A machine with slots is 'smoother' but still has fixed structures in the form of the actual slots. You will hear the term 'slot harmonics' used to describe the effects of the fixed iron structure on the rotating magnetic field.

2) When you have coils from different phases in the same slot, they absolutely do interact and this can cause problems. You have to fall back to symmetry to prevent these problems. In a normal 3 phase machine you will have some slots that have conductors from different phases, but if you were to follow for example an 'A' phase circuit you would find the same number of 'turns' shared with B phase as with C phase. The net result is that the interaction of A and B gets balanced by the interaction with A and C, and the same pattern holds for all of the A, B, and C phase circuits. So the motor is built to be balanced and symmetric and you don't have circulating currents. Now you take a look at an individual slot. It might contain say 20 'A' conductors giving a phase angle of 0°, or 20 'c' conductors giving a phase angle of 60°. But between these two slots you will find a slot with 10 'A' conductors and 10 'c' conductors, and that slot has an effective electrical phase angle of 30°.

Again, if you don't have balance between all of the phases, you will have circulating current issues...

-Jon

#### mbrooke

##### Batteries Included
1) Alright, but I've seen what I'm assuming are old induction motors with large "I" slot windings. In fact most old motors from the 1900s are like that.

2) Can you post any examples of this? I remember motor schematics where some slots where all A phase, another a mixture of A and B but unsure of the ratios.

Make sense as I thinking about it.

What about semi abnormal conditions like voltage regulators or abnormal ones like an open phase? Does salient make any difference?

#### paulengr

##### Senior Member
1) Alright, but I've seen what I'm assuming are old induction motors with large "I" slot windings. In fact most old motors from the 1900s are like that.

2) Can you post any examples of this? I remember motor schematics where some slots where all A phase, another a mixture of A and B but unsure of the ratios.

Make sense as I thinking about it.

What about semi abnormal conditions like voltage regulators or abnormal ones like an open phase? Does salient make any difference?

The more typical case of salience being used is in what is often called advanced motors. These are very high efficiency motors that don’t operate on what you would consider “typical” designs. As far as I know they are all synchronous designs except doubly fed wound rotors. These are reluctance rotors, salient pole rotors, and permanent magnet synchronous motors (BLDC). There are no circulating electrical (squirrel cage) currents in the rotor. They all create torque via some kind of magnetic field effect. The upside is efficiency. The downside is they are significantly more expensive and pretty much all have to be operated with a VFD. The salient pole motor in particular is just a slotted rotor but kind of the opposite of a squirrel cage because there is no intentional electric circuit, just a magnetic one.

You can google these terms. We started with salient poles in motor engineering class 25 years ago because it’s very easy to demonstrate motor theory with it, then moved on to synchronous then induction.

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
I found this:

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