AC servo motor vs DC servo motor

I have seen many AC servo motors listed for sale. It seems like everyone is using DC servo motors. I know that by reversing polarity the DC's will reverse. What (aside from the obvious DC and AC) is the difference between the AC and DC servo motors?

The drivers available and the cost of those drivers. The DC driver's are very reasonable.

A couple of points are that DC motors have brushes that wear, are generally higher inertia, they are however usually easier to match in the way of motor to amplifier, due to AC motors requiring a matching commutation method, AC are lower inertia and have no brushes to wear, for a detailed expl. go here http://www.mae.ncsu.edu/courses/mae7.../00_motors.pdf
Al.

And there are also brushless DC servos...which also require commutation...

I like to think of a brushless ac or dc servomotor as a step motor with really big steps. They are a lot like induction motors. A DC motor will work with most anyone's controller, but the brushless motors are much more particular. I have a lot of incompatible drives and motors in the basement right now. Got to do something about that.

Brushless motors are really dominating the machine tool industry right now. There are a lot of advantages.

I've never put together a CNC system, but I do know that not only the drive, but also the control scheme is very different for AC and DC servo systems, with AC being the more "complicated".

The DC motor uses very simple duty-cycle-controlled PWM to control the motor the speed of the motor, whereas the AC motor uses sinusoidal PWM, where the speed control is in the frequency of the PWM -- enter the variable frequency drive.

AC motors used for positioning applications often use the "Vector Control" scheme. I don't know much about it, but here's a paragraph from "Electric Motor Drives" by R. Krishnan.

"Separately excited dc drives are simpler in control because they independently control flux, which, when maintained constant, contributes to an independent control of torque. This is made possible with separate control of field and armature currents, which, in turn, control the field flux and the torque independently. Moreover, the dc motor control requires only the control of the field or armature current magnitudes, providing a simplicity not possible with ac machine control. By contrast, ac induction motor drives require a coordinated control of stator current magnitudes, frequencies, and their phases, making it a complex control."

I don't claim to fully understand the implications, but I understand that controlling the phase of a signal (AC drives) is far more involved than controlling the magnitude of a signal (DC drives)

Hope this helps.

Chad

Almost all commercial drives (ignoring some sold into the very low cost hobby market) vary a PWM signal to control the motor. This is true for steppers, brushed dc, and brushless. There are a lot of different motors that are called brushless, and the design of these motors are still evolving. I think as time goes on, there is going to be more and more hobby activity with brushless motors just because that's what's going to be on the surplus market.

Brushed DC has the advantage that the speed is proportional to the voltage. Both brushless and stepper motors have to be commutated, that means there is more than one winding and the voltage has to be varied between them in order to pull the rotor around. There are many, many commutation schemes with brushless motors which is why they are such a complicated subject.

I bought a whole box of 10 brand new Copley Controls DC Brushless amps on Ebay, about 10c on the $, they are great as I can use them equally well for DC brushed as well as DC brushless motors.
Al.

Al,

I just saw a brushless dc motor for the first time today -- my brother's one came in the mail. The way he was explaining it to me was that it had 3 separate coils that had to be driven individually. If this is correct, it would lead me to believe that a brushless dc amp capable of controlling a single brushless dc motor would potentially be capable of controlling 3 brushed dc motors. Something tells me there is a gap somewhere in this logic however.

I guess I'm asking if your amp has some additional capability to drive brushed dc motors, or if it is an inherent capability that is a result of how brushless dc motors are driven.

Chad

Your correct, the motor is basically wound as a three phase motor, of the three phases out, U,V,W. only U & V are used to feed a DC brushed motor and the switch that normally selects the hall switching angle is set to 60deg. Of course the DC motor does not use the hall inputs.
If a brushless amp has the 120/60deg selector then it can usually run a brushed DC motor.
Due to most being H bridge output, one section of the bridge is not used.
Al.

I find the idea of a 3-phase (brushless) DC motor almost confusing -- but I get the impression that each "phase" is used to drive a coil that is responsible only for 360/3 = 120 degrees of mechanical rotation. Thus, by applying PWM to each of the three coils in proper sequence, you can drive the full 360 degree rotation and control the speed of the motor.

I noticed on my brother's motor that while a brushed DC motor tends to have very distinct "stops" (i.e. with the motor off, spin the shaft by hand and it has very noticeable points where the shaft likes to stop -- conceptually similar to a stepper motor in that regard), the brushless motor seemed not to have this behavior. I'm supposing it has something to do with the mechanical interface of the brushes on the DC motor.

I may have answered my own question, but what is the benefit of brushless DC motors over their brushed counterparts (aside from reduced maintenance). They appear to be quite "fat" little motors. Is the benefit related to the more "continuous" nature of rotation that makes them "smoother" when used in a servo system?

Chad

Actually when running, the DC brush-less exhibits cogging at low speed due to the resolution of the commutation whereas a good quality DC brushed with skewed rotor is smooth down to zero (when running).
The AC sinusoidal motor which is constructed identical to the DC brush-less has the smoothness of a DC brushed.
This is why if you intend using a DC brush-less motor without gearing at very low speeds, you will see a step like pattern of rotation.
The advantage, apart from less maintenance, of a DC brush-less or AC servo is the fact that the windings are on the stator, so cooling is much more effective.
Although the DC brush-less and AC sinusoidal motors appear identical, manufacturers call them DC brush-less because there is only two windings powered at a time compared to a true AC servo where the three phases are conducting.
Al.

Thanks for all the great info on these motors.

Quote:

---

Although the DC brush-less and AC sinusoidal motors appear identical, manufacturers call them DC brush-less because there is only two windings powered at a time compared to a true AC servo where the three phases are conducting.

---

So is a sinusoidal PWM signal used to drive DC brush-less motors or does it use constant-amplitude PWM?

Chad

This will probabally explain it in more detail.
Al.

Al,

Thanks for the file. That clarifies a few things. I never realized how significantly different brushed and brushless DC motors really are.

Chad

Hi to all,
I am new on CNC Zone and to CNCs also. My query is about the details of the DC servo motor (with brushes). Mine is not working and taking jerks. Can some one please tell me what the matter may be.
thanks

You need to find out if this is because of the controller or anything external to the motor.
If you test the motor on a steady DC source, power supply or automotive battery etc, this should give you some indication.
Test in both directions, if the motor has a tach, take a dc reading while running.
Al.

I have checked, it is because of the motor and it has inbuilt tachometer also.

If the motor acts the same way with an external power supply then I would check the brushes and commutator first.
Al.

i have checked the motor with external supply and also its brake and both are fine. the motor is working perfectly with external supply. how can i check the tachometer? cause when i attach the motor with machine then the tacho comes in loop and the problem begins.
thanks for the advices