In Search of A Better G0704 Spindle

[JimDawson]

These NEMA42 steppers are spec'ed at 4230 in-oz.

If you can get more than 600 RPM out of that stepper I would be surprised.

Isn't a BLDC motor similar to a stepper without the "teeth"? Don't they both have permanent magnets?
Mark S.

They are similar in that they both have permanent magnet rotors. But the BLDC is a 3 phase motor, where the stepper is a 2 phase motor. There are some other differences in construction also.

An AC servo is a BLDC motor with an encoder on it to feedback the position data to the controller/drive.

By definition a servo is any closed feedback loop system. Anything else is open loop, where the controller has no idea what the controlled device is actually doing. The heating and/or air conditioning system in your house is a servo system. Also your car is a servo system where you are the controller.

 

[MontanaAardvark]

If you can get more than 600 RPM out of that stepper I would be surprised.

They are similar in that they both have permanent magnet rotors. But the BLDC is a 3 phase motor, where the stepper is a 2 phase motor. There are some other differences in construction also.

An AC servo is a BLDC motor with an encoder on it to feedback the position data to the controller/drive.

By definition a servo is any closed feedback loop system. Anything else is open loop, where the controller has no idea what the controlled device is actually doing. The heating and/or air conditioning system in your house is a servo system. Also your car is a servo system where you are the controller.

Jim, pardon me, but this terminology confuses the heck out of me. BLDC stands for "Brushless DC", right? An AC servo is an AC motor, not DC. So how are they the same? From what I can tell, there are both single phase and 3 phase BLDC motors, so is nobody using the single phase?

One of the things I'm stumbling over as I try to learn this stuff is interpreting the words I see, so at this point I'm easily confused.

 

[JimDawson]

It is confusing. A BLDC motor is a 3 phase AC motor with a permanent magnet rotor. The drive input can be DC, single phase AC, or 3 phase AC depending on the system. But the output from the drive to the motor is 3 phase. For instance, the Milwaukie Fuel series of battery drills have BLDC motors in them, the motor itself is 3 phase. All of the magic is done in the controller.

Commonly there are 2 main parts to an AC servo drive. First the rectifier section to convert the input AC to DC, then the DC is converted to a variable frequency 3 phase output to the motor. This is true for VFDs also. But a VFD does not have the close control that a servo drive does. With modern VFDs, this line is beginning to blur a bit.

There is a lot of confusion surrounding the term ''servo'', it is misused a lot. As you know, it is not a servo system without a closed loop. A BLDC motor could be purpose built to be installed in a servo system, and hence the inaccurate term ''servo motor''

 

[spumco]

I find that trying to understand every single variable in motor/drive terminology has rendered me senseless. As Jim wrote, it's confusing. What I've done - in my mind - is categorize the motors (likely used in hobby CNC applications) to types with significant differences.

1. Stepper (permanent magnets, lots of poles, used for non-feedback positioning in most cases, low max RPM)
2. Induction (typical 1 or 3phase motor driving spindles and so forth. Big drill press kind of thing)
3. BLDC (permanent magnet, windings energized with DC at highish voltage, commutated using hall effect sensors, not used for positioning. Higher RPM and greater power density per weight or volume than Induction or stepper)
4. Servos
4A. DC servos (brushed DC motor with encoder for feedback. Used for positioning. Old-school CNC axis servos.)
4B. AC servos (brushless permanent magnet, windings energized by magic, used for positioning and/or constant RPM, has an encoder for feedback to the drive enabling commutation and precise positioning.

Yes, there are overlaps in function and terminology but I don't have the reserve brainpower to identify the nuances. How much HP, and at what RPM? What's the max RPM, and is the torque curve fairly flat? Can I position with it, and do I need a magic box just to spin it up? How much does it cost?

With modern VFDs, this line is beginning to blur a bit.

For example, my VFD can position my induction motor due to the encoder, but it'll never be as precise as a true servo simply because the encoder has a much coarser resolution than is typical on 'servo' packages. That, and the rotor weighs a ton compared to a permanent magnet servo motor of the same nominal power - it just can't start or stop as fast as a lightweight servo rotor.

On the other hand, you don't see 400hp servos (magnets are the limiting factor in high-HP applications), but you do see induction motors in the hundred (or thousands) horsepower range. Pump motors at a municipal water plant come to mind.

-S

 

[MontanaAardvark]

I found a technical note from a company that makes controller chips for motors, Monolithic Power Systems. It discusses a lot of these things with diagrams, waveforms and such. I'm finding it helpful. It's incomplete and doesn't include servo motors.

It's at http://www.monolithicpower.com/Port.../appnotes/Brushless DC Motor Fundamentals.pdf