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

As I said earlier there are a heck of a lot of ways to make a motor, there are also a heck of a lot of vague ways to describe them 8-). One confusion is that a DC brushless is driven by a controller that effectively generates AC!, so you could describe it as an AC servo motor. You can of course get AC servo motors without magnets, in fact ANY motor can be used with some form of feedback and then be described as a servo motor..

Actually I was answering a question about the differences between a brushless DC motor and a 3-phase AC motor, by which I assumed Tim meant an induction motor, maybe I misunderstood.

Greg

I could probably have phrased it better, but my question was about "AC (servo) motors" & brushless DC, I had in mind the sort which typically looks like this:

Both types seem to be, in effect, boxes fed with synthetic 3-phase AC, so I was curious about what made them different from one another.

Thanks

Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

I think we may well be looking at different naming conventions for the same things. I've found that this is definitely the case for 'coreless' motors, a colleague has confirmed that when talking about servos used in robotics the term coreless is reserved for motors with no rotating iron or magnets, hence low inertia, but in wider circles the term is applied wherever the windings are not around a core so there's no cogging. Language can be surprisingly vague when it matters 8-).

Greg

I think the differences between us are pure semantics - you define coreless construction as the special case in which the aircored winding is the rotating element. I prefer the more general definition - motors in which the exciting windings are not wound on an iron core.

It's interesting to note that the basic construction of the Maxon motor used in your example is exactly the same as the mechanics of a semiconductor commutated videotape drive motor. Both use an annular aircored exciting winding located within a coaxial field magnet system. The Maxon motor anchors the magnet system and lets the windings rotate. The video tape motor anchors the windings and lets the magnets rotate.

I of course agree with your contention that,if minimum inertia is the prime target, rotating winding designs are the clear winner.

Jim

They might be if the electronics are included in the package. The electronics give commutation based on rotor position so that if you apply more voltage the rotor will go faster. However, if the electronics gave a fixed frequency to the coils then the speed would be controlled entirely by the output frequency.

The brushless commutator is missing in Tim's motors, so what he has is a two or three phase, fixed excitation synchronous machine. Feed it with your chosen frequency, control the voltage to give about unity power factor at that speed and load (power factor will be leading for too low a voltage and lagging for too high a voltage) and off you go. If you feed it from a simple invertor then calibrate it by measuring power factor ad the desired operating speed as you wind up the output volts until you get unity (or measure the current until you get a minimum), then tell the invertor to ramp between that voltage and zero as the speed goes to zero. It'll run like an induction motor but without dropping speed as load increases. Once you get past the maximum torque that it can provide, it'll pole slip in an unpleasant way and either blow the fuses, let the smoke out or (if you're driving a lathe with it) frighten the life out of you.

Mark Rand RTFM

All I can say is that with a normal controller for a brushless DC motor you vary the voltage to control the speed. You either vary the DC supply voltage or the controller effectively varies it by PWM. The frequency is not fixed and you have no control over it as the controller commutates according to the motor position, either detected by a sensor or by detecting the back emf from the motor.

Greg

That's what I said in the bit that you snipped :-)

Mark Rand RTFM

A good explanation but, unless you are well experienced, a"simple" inverter can turn into a major undertaking!

If it's a wheelchair motor it's almost certainly a 12V machine and, to economise on the electronics, probably 2 phase excitation. It looks as if it's intended for direct wheel drive which indicates a multipole low speed machine with the magnet system in the form of a thin ring of supermagnet material.

The number of poles can be checked by the DC injection "cogging" test. Probably a large number - maybe 20 or more.

IF these guesses are right it could be driven from a standard stepper motor driver and operate as a giant stepper motor!

Jim

As stated before, it's 24 Volt. I don't actually have a use planned for it, but if there was a chance of finding a way to drive it without major expense it might be saved from the bin and a use might suddenly present itself . I suppose a simple resistance test will detemine whether it is 2- or 3- phase

Cheers Tim Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service