Can someone please tell me what this is (electrical)?

No not at all, in fact if it was mine it would be in bits by now 8-)

Greg

Looks like 2 phase motor intended for electronic commutation - maybe to drive a big disk pack.

Lack of cogging doesn't always indicate an induction motor. Put about 12V DC through one winding - if it positively locates to one or more angular positions it's a permanent magnet machine.

Jim

That's true, coreless motors don't cog as the only part that moves is the copper windings, but they generally (always?) have brushes which aren't apparent on the pictures. Greg

That's true, coreless motors don't cog as the only part that moves is the copper windings, but they generally (always?) have brushes which aren't apparent on the pictures. Greg

Greg,

You can get brushless coreless motors which require external electronic commutation. I was thinking that the object might be one of those.

Jim.

How do they manage that?, the whole idea of coreless is that only the windings rotate, no iron at all, hence the low inertia that's essential for fast moving robotics. If the windings rotate surely you need either a commutator or slip rings to connect to them?. We have dozens of these on our production robots and I believe they all have commutators.

I know it's possibly to make brushless alternators using a two step process and rotating rectifiers, in fact it's the norm, but I can't immediately see how that can be applied to a servo motor?.

Greg

Greg,

I may have made a mistake by referring to coreless brushless motors, but makers of coreless motors also produce brushless DC motors. - for example, Faulhaber.

The motors are three phase DC and you can check out the drivers if you click on 'Drive Electonics' on the LH menu and download one of the PDF files.

Jim.

Yes I think coreless and brushless are exclusive, though there are so many ways to make electrical machines I won't rule it out 8-). If it were practical I feel sure they would be used on production robots as the maintenance overhead of having brushes in all those motors is significant.

Greg

Electronically commutated motors are becoming the norm rather than a rarity because this system eliminates the cost and reliability problems of commutator and brushes while still being capable of providing the flexibility and torque characteristics of a commutator machine.

High speed machines use a 2 or 3 phase winding driving a 2 or 4 pole permanent magnet rotor - this can be a conventional cylindrical rotor or an external annular rotor.

There are also pancake varieties

Low speed machines mostly have annular rotors with a large number of poles - 20 or 30 pole pairs are not unusual. Because supermagnet material is used, extremely short magnet length is possible and the radial magnet thickness may be no more than a thin lining surrounded by a soft iron shell.

Pancake or moving coil commutator motors are often used in demanding servo application both because of their high torque to inertia ratio AND the use of multi segment commutators which permit almost uniform torque output irrespective of rotor angular position.

Electronically commutated motors can achieve similar torque to inertia ratio but the simpler (square wave) control systems suffer from some torque ripple. Uniform torque is possible but needs a tailored near sine wave drive system.

Jim

Very true

Which is not coreless, hence my question.

Greg

Most machines in this class have a conventional iron core mechanically part of the exciting winding but coreless designs are common.

Many videotape drum drive motors are in this class. These generally use a rotating central cylindrical permanent magnet surrounded by a soft iron cylinder. A stationary 2 phase exciting winding is wound in the form of an annular cylinder which occupies the air gap between the magnet and the outer soft iron cylinder which provides the magnetic return path.

This is a true coreless design with zero cogging. It can be identified by the usual trick of applying DC to one winding and observing the magnetic "cog" that this generates.

Most floppy disk drive motors use a pancake version of the coreless design. The rotor is a soft iron backed, axially magnetised, permanent magnet disk and the exciting windings are wound in the form of an air cored two phase pancake backed by a soft iron disk which provides the magnetic return path.

Jim

The mystery is at least partially solved. I got it as part of a job lot, the company it came from has just come back to with this:-

"This motor was a prototype for a wheel chair, we do not have the datasheet for it, but it is rated to about 24V, and has a torque of about 20-30 Nm, it is brushless, and is like a standard motor just squashed".

I assume it will need some sort of external electronic driver, judging by the number of wires?

Cheers Tim

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Now that starts to make sense, presumably the rims attached to all those holes. Yes a brushless needs a controller that converts the DC supply into 3 phases and these come in two types, sensorless that uses back emf to detect the rotation so only needs the three wires, and sensor that has some sort of tacho as well. This motor has the latter but as the tacho is of unknown design it may be easy to use a sensorless. Greg

So what is the difference between a brushless DC motor, with external driver/controller, and a brushless AC (servo) motor?

Cheers Tim

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A little more info:

"The driver you need for the wheelchair motor will need to be very high current (About 25A) and brushless and be able to accept Halls as the feedback."

Tim

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I'm sorry but your understanding of the term coreless differs from mine. As I understand it, and certainly when talking about the servo motors in robotic assembly machinery such as used in our production line, the whole point of coreless is that the rotor has to have the absolute minimum mass to allow it to accelerate and decelerate as quickly as possible so the machine can move back and forth at huge rates. This is achieved by making the rotor from nothing more than copper windings, wound on a former that is removed after they have been resin impregnated, thus leaving a tube of windings. This is attached to a shaft via a light weight disc, with a commutator passing current to the windings. The stator consists of magnets and iron that very closely surround the rotor and produce a magnetic field perpendicular to the windings. #

The key point is that no iron or magnets rotate, yet your descriptions talk of a rotating permanent magnet so I can't agree that you're talking about coreless motors. Here's the best pic I could find quickly, from one of the leading manufacturers:

Greg

And here's a picture of a brushless from the same people. Unlike the brushless motors I've used their technology doesn't cog, which answers my own question 8-), and means that yours could indeed be a DC brushless but you should be able to detect strong magnets with a screwdriver!.

Greg

Doh!, here's the pic

Greg

No they're fundamentally different, the brushless has magnets and a three phase AC motor doesn't, you vary the speed of the brushless by changing the voltage and the other the frequency.

Just look for magnets, if it cogs you've found them, if not you have to probe around with a screwdriver looking for them.

Greg