Three phase motors?

Aren't selsyn motors effectively three-phase motors? They have an extra 'exitation' winding, and are delta-connected whereas I presume most such controllers presume a wye connection. They also have very little torque, as they're meant mainly to drive a dial/display pointer. I got some many years ago at a hamfest, but have never hooked them up. But they're the closest thing I know of to "small" sub-horsepower

3-phase motors.

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It's a terminology issue. "Brushless DC" motors are really AC motors. Below 1KW or so, the term "Brushless DC" is usually used. Above 1KW, similar motors are called AC motors. This reflects a disconnect between the big-motor industry and the small-motor industry.

All such motors are multiphase, and are almost always three phase.

This leads to the wierd terminology "three phase brushless DC motor", a phrase you can search on.

John Nagle

Thanks,

I had thought it was something along those lines.

I just took apart an old CPU cooling fan. It definitely had a controller board probably generating the appropriate signals. It has four coils though - would this still be three phase? What is the extra coil for?

Thanks for the info so far!

--buddy

That's a good question, I'v never taken one apart, as all the ones I have still work and I'd rather use them. Are there three coils at

120 degrees and the fourth one somewhere else? Or are they arranged as four coils at 90 degrees (which would be a "4-phase" motor)? Hard drive platter motors (I have many non-functioning HD's) are also three-phase motors, but as John writes above, such motors are usually called "brushless DC", partly because the "3-phase controller" is considered part of the motor - the controller is often inside the motor casing so it actually "looks like" a DC motor.

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They are all separated by 90 degrees and appear identical.

I was wondering if it was a four-phase motor in that case, or perhaps one of the coils served some other purpose....

Or it could be two phase, I suppose, with opposite coils having the same polarity?

--buddy

Hi Buddy,

The spindle motor on a floppy is actually a brushless 3 phase motor, and runs on 5 or 12VDC, as are the spindle drives on a number of tape machines and DVD players. Try pulling a 3 and 1/2 floppy drive apart if your curious...

The 3 phases are generated on chip with feedback either from the back-emf or from hall sensors as the rotor is actually a permanent magnet...

If you're trying to wrap your head around it, think of it as a fancy stepper motor, with 3(or 6) coils instead of 4, and sine wave(AC) switching instead of square/boolean switching.

The only challenge for using those in robotics is that most of the controller chips only go in one direction.

Thanks, TJ

As an illustration, here is the General Electric AC6000 CW locomotive, the most powerful locomotive in the world:

There's a three phase AC motor (or "brushless DC servomotor") on each axle, fully position servo controlled to prevent slip. All wheels on the locomotive rotate together, under computer control, as if geared. This maximizes traction. Works between multiple locomotives, too.

That's how big you can take the servomotor concept.

John Nagle Team Overbot

stepper motors are small 3 or more phase motors requiring a controller

brushless DC is a 3 phase motor that runs off a controller

the controller fires the coils sequentially, 'dragging' a permanent magnet rotor around...or vice versa, the 'rotor' is fixed and has the coil set, which rotates the case...an 'in wheel' motor is born!

used in model aircraft, robotics, computers etc.

not much like a 3 phase plant motor at all