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Author Topic: Spindle drive specification  (Read 8208 times)

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Re: Spindle drive specification
« Reply #10 on: June 19, 2019, 04:41:49 PM »

Did you successfully Retrofit your Emco Turn with mach3?
Re: Spindle drive specification
« Reply #11 on: June 21, 2019, 07:04:39 PM »
since this thread was started (2011) AC servos have become MUCH more common, including a good supply of second hand
units on EBay. Trying to preserve old DC motors is becoming an expensive proposition despite otherwise good performance.
AC servos offer at least as good torque characteristics and a VASTLY improved range of control modes, including indexing,
torque, velocity and position modes.

Induction motors and a VFD are pretty good, especially on a budget but a servo (AC or DC) will have torque to zero speed
for which an induction motor will overheat rapidly.

My wife left with my best friend...
     and I miss him!
Re: Spindle drive specification
« Reply #12 on: June 21, 2019, 07:09:35 PM »
Thanks a lot for the answer…   You are right AC servos have become more common nowadays
I am working retrofitting two Emco Lathes.
Re: Spindle drive specification
« Reply #13 on: June 21, 2019, 07:36:43 PM »
I'd had little to do with AC servos until I bought one second hand (1.8 kW Allen Bradley) for use as a spindle
motor for my mini-mill. It has been a revelation. I have from University days been familiar with the great torque characteristics
of DC servos, but its the hugely increased range of control options that put DC servos into the shade.

Servos, either AC or DC, have superb torque density, that is they produce more torque for a given size motor than
an induction motor. They can be exceeded, but only at low speeds by large steppers. In addition they have superb
overload characteristics, they continue to work in overload until the onset of overheat whereas induction motors once they hit
their stall torque, seldom more than 50% rated, stall completely. Likewise hit the torque limit of a stepper, reducing at increasing
speed, and it stalls.

Yet another advantage of AC servos, not often used in servo drives, but extensively used in brushless spindle motors is 'field
weakening'. Its where you can enter a mode that induces the rotating magnetic field by the stator has its normal quadrature
(torque producing) component but also an anti-parallel component which in effect redues or 'weakens' the permanent magnetic
field of the rotor and thereby reduces the back EMF allowing for higher speed operation with the same power supply.

You may have  noted that high end production CNC machines will list traverse speeds say of 50m/min but max cutting speeds
of 30m/min. This is the classic means of utilizing field weakening. In the field weakened condition the servo can spin faster
albeit at reduced torque but sufficient to get great traverse speeds. When the servo reverts to its normal non-field weakened
condition it regains its full torque to produce its maximum cutting speed. Great flexibility  can be achieved with field oriented
control that is just not possible with DC servos.

My wife left with my best friend...
     and I miss him!