Hi,

As I find nearly all the small mills I'm interested in use DC brush motor for the spindle, I wonder if it would be a good idea to replace it with AC servo motor? Also, does any one know how much torque the spindle should provide for cutting soft metal and much much for steel?

Many thanks
David

If you wana use a AC servo then you will need a AC servo drive to run it. Any servo with ~5k rpm and ~750 watts will do.

If the cost is not a problem, (as I was designing a servo driver myself and the motor itself is not much more expensive than a brushed one,) does brushless motor offer other benefits besides the quieter operation and maintenance free? Since the purpose of using a variable speed motor is to get rid of the gear box, (well, a sort of,) I also wondered if a brushless motor would give higher torque at low speed?

The two popular forms of brushless motors are DC brushless and AC sinusoidal, both motors are constructed virtually identical, the difference is in the commutation,
with AC sinusoidal considered superior, Most have similar torque curves to DC brushed motors, with max torque at zero rpm. and remaining fairly flat or slope down slightly to max speed.
The AC motor is usually slightly more cost also.
Al.

Hi,

As I find nearly all the small mills I'm interested in use DC brush motor for the spindle, I wonder if it would be a good idea to replace it with AC servo motor?

When you use the term "good idea" that leaves us open to all sorts of possibilities.

Th first thing to realize is that the DC motors work. How long they will work and what their real power output is is another matter. So is it a good idea to replace something that isn't broken yet?

Now we can improve things by putting a better motor on the unit, but do you need that improvement and will the machine mechanics allow you to take advantage of that improvement. Having more spindle power may not mean anything productivity wise if the machine can't take advantage of it mechanically. Now you will likely gain the advantage off a wider range of speed adjustability, that is good. But I suspect you will not be able to get rid of the gearing with a motor and drive that fits into place easily on these smaller machines.

Is it a good idea? That is up to you. Many of use would first spend his money on CNC the machine.

Dave


Also, does any one know how much torque the spindle should provide for cutting soft metal and much much for steel?

You will have to look up the required formulas or tables and determine what power is required for the likely feed rates that you will be able to achieve on your machine. The power required depends on how much material you are removing and how fast that is happening. From power you can determine torque.


Many thanks
David

Dave

AC motors can be plagued with 60 cycle speed fluctuation ripple. DC motors, especially those with large pole counts and skewed armatures, are much smoother running by comparison.

This is why on some grinders fitted with AC motors, you do NOT run tight drive belts to the grinding wheels.

Even with large wheels that act as flywheels, there is a hum or buz that can be seen in the finish. This is due to the speeding up and slowing down of the motor as the phase ripple magnetically 'pulses' the armature. The same does not occur with a loose belt (this absorbs/dampens the phase ripple) or a true DC motor that is NOT PWM driven to affect speed control.

We found a similar situation when we did some VFD work to regulate speed in AC motors. The VFD would rattle the daylights out of the cages in the ball bearings - it drove us nuts until we figured out what was going on. The same thing occcured with PWM drives but to a lesser degree depending on pole count on armature and/or frequency of the PWM drive (the higher the better).

As soon as you cut power, the buzz/rattle stopped with the AC motor. Turn it on and the cages rattled like a can of marbles. Fit a DC motor to the same system, smooth as glass and very quiet.

I won't argue the "performance benefits" of AC motors over DC. However, there are some instances where you simply can't beat a DC motor for providing smooth pulse free power to some noise and/or finish critical systems.

AC motors can be plagued with 60 cycle speed fluctuation ripple. DC motors, especially those with large pole counts and skewed armatures, are much smoother running by comparison.

Similar efforts have been made to improve AC motors at low speed. That is one reason why I highlighted that the drive and motor combo can make a difference in its behavior at low speed.

The other issue is that many of the cheap AC drives on the market are just that cheap. The motor basically operates open loop, this combined with an operating frequency it wasn't designed for leads to poor performance at low speeds.

The right motor and drive can improve things dramatically but these drives are very expensive.

This is why on some grinders fitted with AC motors, you do NOT run tight drive belts to the grinding wheels.

Even with large wheels that act as flywheels, there is a hum or buz that can be seen in the finish. This is due to the speeding up and slowing down of the motor as the phase ripple magnetically 'pulses' the armature. The same does not occur with a loose belt (this absorbs/dampens the phase ripple) or a true DC motor that is NOT PWM driven to affect speed control.

Interesting! My experience with grinders is thin at best.


We found a similar situation when we did some VFD work to regulate speed in AC motors. The VFD would rattle the daylights out of the cages in the ball bearings - it drove us nuts until we figured out what was going on. The same thing occcured with PWM drives but to a lesser degree depending on pole count on armature and/or frequency of the PWM drive (the higher the better).

The noise and issues caused by the PWM is another issue all together. Really old drives with slow PWM circuits could be very difficult to live with if you had to hear them all day.


As soon as you cut power, the buzz/rattle stopped with the AC motor. Turn it on and the cages rattled like a can of marbles. Fit a DC motor to the same system, smooth as glass and very quiet.

I won't argue the "performance benefits" of AC motors over DC. However, there are some instances where you simply can't beat a DC motor for providing smooth pulse free power to some noise and/or finish critical systems.

One thing I failed to point out in the discussion about timing belts is that they can be a problem even for axis drives. One of my tasks in the distant past was teaching new equipment mechanics in the fine art of keeping Diamond turning machines it proper operation. The adjustment of the timing belt tension to the leadscrews on the axis drives was critical. These axises where very high gain systems and many a surface finish issue was solved by proper adjustment of the axis drive timing belts.

This was not however a spindle drive. My experience timing belt driven spindles is very limited. I've seen some machines where timing belts have been used to drive spindles (specialty grinders) with success. That may simply be the result of lower acceptable quality standards. For whatever reason though I've not seen any lathes with timing belts driving the spindle directly. The interesting thing there is that your average V-Belt isn't that smooth of a power transmission device either.

In the end I think it is a question of expectations and maybe a bit of skill in setting up a timing belt drive. A number of people have successfully implemented timing belt driven spindles for these small lathes. The important thing is that if it works for you and the way that you use your machine, then that is all that really needs to happen. For some people it is also important to keep the spindle synchronized with the motor which timing belts do nicely.

dave

This subject seems to have got off the original track, as the original poster was refering to AC SERVO motor as a spindle motor, not induction motors.
An AC sinusoidal servo motor is basically a synchronous motor, in other word the P.M. rotor will faithfully follow the rotating field resulting a very smooth rotation, resulting in high efficiency, high power and high torque over a wide range of rotation.
Even so the ordinary 3 phase induction motor has a lot more advantage over the more common 1 phase motor, where the 60hz freq. Goes through zero 120 times a second.
With 3 phase induction motors, the actual ripple frequency is 120hz, but has the advantage of not passing through zero due to the phase overlap there is always field current at some level.
Believe it or not, the ‘Windings’ on a three phase induction motor Rotor are often constructed skewed.
The AC servo also has the advantage over DC brushed of much higher rpm capability.
Al.