Followup -- How do you test a DC servo motor

I learned something cool today. If I move a light switch on a wall "up" the light goes on. If I move it "down" the light turns off. I'm getting very good at turning lights on and off now.

If you're building a new machine, no question. If you're looking for a spare part, not true. Most people building their own machine use smaller servos, not all.

Karl

If you had kids, you would know that it is a great discovery, that fascinates children at the age of two, or there abouts.

I wondered what the difference was between a brushless AC and brushless DC servo motors were. The AC servo motor are supposed to be wound for a sinusoidal waveform and the DC are supposed to be wound for more of a trapezoidal profile. I don't know if this is true or not maybe.

RogerN

Brushless DC motor has a commutator, and polyphase AC motors do not need one.

Also, in a brushless DC motor, there are permanent magnets on the stator.

Ig, A brushless DC motor does not have a commutator, hence the term "brushless". The commutation is done electronically by the controller that drives the motor. That is one of the advantages of them, no brushes to wear out, no carbon dust, etc.

-Al

There is no difference, it's just that some mfrs choose to use different names. Brushless DC, if it's actually used, would be a particularly bad description -- there's no DC involved in driving the motor. The terms "brushless servo" or "AC servo," while not entirely descriptive, do not contain any contradictions. As I implied in the other thread, a more complete description would be "polyphase permanent magnet sysnchronous motor with angular position feedback." But that doesn't just roll off the tongue like "brushless" does.

I've been looking at 3 phase brushless motors for RC aircraft recently and am curious about driving these motors,how they work. If the coils are energised in sequence will the motor rotate? For example, if three switches were arranged equally spaced around a spinning shaft with one switch actuating lobe and connected to power up the coils on one of these types of motors would the motor spin at the same rate as the shaft? Thanks, Eric

So you're on par with a 2 year old?

Tell us more about the commutators in brushless motors.

Yes, I still like to take things apart to see what they do, etc. It does not seem to do me any harm. Some people lose that interest as they age, and I consider that to be unfortunate. If that is the case with you, I am sorry about that.

Brushless motors have an electronic commutator that supplies current to different windings of armature, to make the rotor turn. The rotor has permanent magnets that interact with the armature windings. An induction polphase motor does not have such magnets.

By the way, those Fanuc servo motors, are unlikely to be brushless.

Is "electronic commutator" new code talk for fixed electrical connections aka wire?

But it has commutators if it's run off an electronic controller, right? But it doesn't have commutators if there's no electronics?

Are we to conclude all motors have commutators?

You may want to find a commutator on a three phase induction motor.

I bought some surplus brushless servo motors surplus years ago. They have 3 phases and a resolver. At the time a resolver to digital converter was over $400 so I didn't purchase one. I programmed a pic microcontroller to output PWM sin signals 120 deg per phase, used them to drive half H-bridges, and test ran my brushless motors as open loop stepper motors. The motors ran great this way. If I come up with an application where I need them I'll look for some Pac Sci drives or similar on eBay.

So, Iggy, if you need to test some brushless motors just program a PIC or similar as a 3 phase PWM micro stepping stepper driver, wire to drivers and you should be good to test.

RogerN

How does "brushless" and commutator work together? I thought you had to have brushes contacting the commutator.

RogerN

How do they get power to the windings on the armature without brushes?

RogerN

I think you are confusing DC brush motors with DC brushless motors. Brushless motors don't have brushes, brushes are required to carry power to rotor windings unless you're talking induction motors. So, what is the difference between brushed DC servo motors and brushless DC servo motors?

RogerN

No problem.

you said it yourself, I'll just use an "electronic commutator" and bam, my induction motor now has a commutator.

easy!

It seems like it would, but I doubt it would be very smooth or efficient. The motors do have more in common with steppers than with brushed DC motors, but as I understand it, steppers use some funky tricks to increase the number of poles. The similarity between the two is more apparent when you consider a stepper can be set up to run as a low speed synchronous motor -- Superior Electric labels their steppers "Slo-syn."

You can run a 3 phase brushless motor as a synchronous motor from a 3 phase source, but it'll suffer some of the same limitations as a stepper. If overloaded it'll stall and probably won't recover, and may or may not be able to start without a supply that ramps up the drive frequency. I've worked on machinery that had synchronous motors that didn't have a ramped start. They started with a bang, occasionally shearing the teeth off a timing belt in the process.

A brushless servo amp gets around these problems by monitoring rotor position with dedicated sensors and adjusting its output to the 3 phases to maintain torque or speed (depending on whether the amp is in torque or velocity mode) proportional to the amp's command input. Beyond that, the amp is just a black box to me.

I mentioned in the earlier thread that with the drive set up properly, you can run a brushless motor, open loop, with a VFD.