wtf is a digital motor?

[scrat]

http://en.wikipedia.org/wiki/Switched_reluctance_motor

The thing described in this link is near as dammit to the device I referred to in my post above.

It's a glorified Induction motor,with the rotating field supplied by a dc switching circuit instead of the ac mains.

VK6ZGO

Switched reluctance are synchronous motors, while induction are asyncronous. The latter require rotor currents (induced by the same principle as in a transformer), while the first work because of the inductance difference along the rotor axes, so only require the right current vector in the stator windings (so could be more efficient).

[vk6zgo]

The only way an electric motor can operate,unless it has permanent magnets mounted on its rotor,is from induced

currents in the rotor.

They may be eddy currents,as in the aluminium disc of a classic Kw/H meter,but they are still induced currents.

VK6ZGO

[scrat]

No, anysotropy of the rotor is another way it can generate torque without electrically conductive rotor.
There are reluctance motors and hybrid (e.g. Interior Permanent Magnet) that work on that principle (different inductance or reluctance on orthogonal directions).
There's a component for the torque which is (Ld-Lq)*Id*Iq, where "d" and "q" stand for the direct (aligned to the rotor minimum inductance axis) and quadrature components.
The system follows the point of minimum reluctance, which is where the energized coil is aligned with one of the rotor teeth in a switched rel motor.

[Zero999]

The only way an electric motor can operate,unless it has permanent magnets mounted on its rotor,is from induced 

No.

If you read the Wikipedia article I linked to, you discover a motor can work without permanent magnets or a wound rotor.

If you find that hard to understand make a simple linear motor, wind a coil round a straw with a nail  loosely fitted in the middle and you'll discover the nail is pulled into the coil when it's energised.

[vk6zgo]

The only way an electric motor can operate,unless it has permanent magnets mounted on its rotor,is from induced 

No.

If you read the Wikipedia article I linked to, you discover a motor can work without permanent magnets or a wound rotor.

If you find that hard to understand make a simple linear motor, wind a coil round a straw with a nail  loosely fitted in the middle and you'll discover the nail is pulled into the coil when it's energised.

OK,do that,then after you have proved a solenoid will draw a nail into it,remove the nail.


 Is it still magnetic?

Where did the magnetic field come from?

Would it work with a piece of ferrite material?

Note:-I never said you have to have a wound rotor,in fact,the example I gave  was of an aluminium disc

rotating in a magnetic field.They do work,there is one in most meter boxes.


OK,I see where you are coming from:-

"If the induced magnetic field is from induced currents,why does a permanent magnet attract a non magnetised piece of iron?"

Right?

So if we can induce a magnetic field in that case without induced currents,why not in a motor?


Well,one thing is,that a conductor moving through a magnetic field will have a current induced in it,anyway.

The magnetic field is continually changing,so the  polarity of the induced magnetic field  & hence the alignment of the atoms

making up the material will change.I suggest that this constitutes a current.

OK,I'm not a Physicist,so I'm not able to determine whether magnetic realignments constitute a current,but I will leave you with

these suggestions:-

Try making a motor with a ferrite rotor,& an insulator as a shaft


Try a motor with the whole rotor assembly made from an insulator

My guess is neither of them will work.

VK6ZGO

[vk6zgo]

Sorry,Hero999,most of my comments in the last posting should more properly be directed at Sprat!

You will note,however,that in all of my earlier postings,I referred to an "Induction Motor".

I never implied that these type of motors had wound rotors,in fact the  majority do not.

I incorrectly implied that all electric motors  ran due to induced currents---that of course is untrue,as DC /Universal motors

& slip-ring AC motors directly energise the armature windings.

I still maintain that :-

Those motors which do not have a wound,directly energised armature depend upon induced currents in the rotor to function.

(Note:- I have avoided the term armature,in favour of rotor in previous posts,leaving armature to refer to the wound device.)

Non- magnetic materials will work in an induction motor,non- conductive materials will not.

VK6ZGO

[Rufus]

I still maintain that :-

Those motors which do not have a wound,directly energised armature depend upon induced currents in the rotor to function.

You are wrong. Reluctance motors are synchronous. The rotor stays aligned with the rotating magnetic field created by the stator. It doesn't go 'through' any magnetic field to have currents induced.

[tecman]

I still maintain that :-

Those motors which do not have a wound,directly energised armature depend upon induced currents in the rotor to function.

You are wrong. Reluctance motors are synchronous. The rotor stays aligned with the rotating magnetic field created by the stator. It doesn't go 'through' any magnetic field to have currents induced.

Reluctance motors are synchronous, but can "slip" rotation without potentially damaging current, as with a PM motor.

paul

[vk6zgo]

Whatever!

VK6ZGO

[Leo Bodnar]

here go the carbon emissions thing again

Watching a good engineer diving straight into heavyweight marketing bullshit is upsetting.
Pretty much akin to an attractive and smart woman slipping into prostitution.  

[Zero999]

Would it work with a piece of ferrite material?

It certainly will, the rod inside the solenoid doesn't have to be conductive to be pulled into the coil.

I still maintain that :-

Those motors which do not have a wound,directly energised armature depend upon induced currents in the rotor to function.

(Note:- I have avoided the term armature,in favour of rotor in previous posts,leaving armature to refer to the wound device.)

Non- magnetic materials will work in an induction motor,non- conductive materials will not.

A switched reluctance motor is not an induction motor. The rotor doesn't have to be conductive in order for it to work but it has to be magnetic and by that I mean attracted to a magnet not a permanent magnet. The rotor could be made of non-conductive ferrite and it would still work. If the rotor were to be made from non-magnetic aluminium, it would not work as a switched reluctance motor, the rotor might rotate but it wouldn't be synchronous, it will only turn by induced flux, like an induction motor.

A switched reluctance motor is a synchronous machine because the rotor directly tracks the rotating magnetic flux.

[scrat]

@VK6ZGO:
I just confirm what Hero is saying.
About the "insulator made" rotor, unfortunately it is impossible in thre real world, since almost all ferromagnetic materials are weak conductors.
However, rotor (induced) currents are strongly unwanted in synchronous PM, REL or hybrid motors (which do not have slip rings). This is for at least two reasons: "cold" rotor means more efficiency, and control is only based on the rotor PM flux/anysotropy direction vs stator current "direction" (which is mechanically locked to the shaft) giving more accurate torque control (while induction motors' rotor flux is just estimated).
Induced currents are usually blocked by using laminated and grain oriented iron materials (http://en.wikipedia.org/wiki/Electrical_steel.

[vk6zgo]

Yes! You were right,& I was wrong!

I found a reference which explained it to my satisfaction.

The sticking point with me was that I couldn't see how when the current was switched to the stator coils,

the change in magnetic field between zero & the full field would not cause induced currents in the rotor.

The magic words in the reference were "Laminated  rotor",which convinced me that there would be minimal

induced current in the rotor,& hence your explanation was the correct one.

Of course,Laminated rotors are used in brush type motors to minimise induced currents in the rotor core ,which are

regarded as losses,so I should have remembered that.

The motor I repaired the drive board for may have been either a switched reluctance,or an induction motor,as

my main concern was fixing the electronics.


VK6ZGO

[Zero999]

Yes! You were right,& I was wrong!

It's good you've swallowed your prided any admitted it, other people I know never admit it when they've made a mistake.

I found a reference which explained it to my satisfaction.

Could you please post a link? It may help other people who've also had difficulty understanding it.

[onemilimeter]

I built a switched reluctance drive (the motor and converter) before. It's a synchronous motor. I can confirm that the explanation by Hero999 and scrat are correct.

Induction Machine:
- Two magnetic fields (one is produced when stator winding is energized, another one is produced when induced current flows in rotor conductors) --> Lorentz Force Law --> torque


Switched Reluctance Machine:
- One magnetic field (produced when stator winding is energized) --> torque production is based on the principle to minimize the reluctance of the magnetic circuit --> reluctance torque
- Require rotor with saliency.


By the way, Mr Dyson visited our department two days ago! I should have asked him the advertisement about "zero carbon emission"...

[Leo Bodnar]

The whole vacuum cleaner business is a sham.  Using 2kW of power to pick up a few milligrammes of dust and transfer it into a bag is ridiculous.

Then Mr Dyson steps out and self-proclaims he is genius because he can get rid of the bag and use brushless motor to power the same old crappy technology. 

Loads of spin and glitzy looking plastic enveloping the old inefficient century-old technology.

[Simon]

yea, Mr Dyson is now in my books as a con man. showing off what is current technology as his own personal invention. The guy needs to get with the program

[Mechatrommer]

thats the problem with people who know shit about ee and try to talk about one. i read his story in reader's digest (friend's, not mine!). he's an artist, like the rest of the artist's who tried to make money.