Hi,
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.
Craig