Re: [CAD_CAM_EDM_DRO] Gecko burned - now motor types

Jon Elson <elson@...> wrote:

R Rogers wrote:

>Thank you for a very informative response. So essentially the brushes in a DC servo motor are a much more direct method in which to switch the circuits within the motor.
>Now, if we used a chopper type drive for a servo (designed for a servo obviously i.e. higher current at idle). would it greatly improve the holding torque? Just a rambling thought.
>
>

No, actually, a servo generally requires very LITTLE current at idle.
If a force attempts to
deflect the motor, the servo loop detects the movement and applies
whatever current is
needed to resist the movement.

/// I see. It knows the deflection via the encoder and the drive supplies the current to overcome it.

A servo generally can provide the same
performance
as a stepper system with a much smaller motor. Such a motor may have a peak
torque rating 1/2 to 1/4 that required of a stepper. The reason is that
most stepper
systems are massively over-specified in terms of holding torque so that
they can deliver
sufficient torque at the high speed end of the range. The stepper's
torque falls off
dramatically as the speed increases. The servo generally has little to
no loss of torque
at speeds most stepper motors can't achieve.

One of the reasons for this is that in the conventional stepper motor,
each phase winding
must go through a complete current cycle every 4 steps, or 50 times a
revolution.
On a DC brush servo, the current only reverses in two sets of armature
windings
every time the commutator segments pass a brush. If the motor has 30
armature
windings and commutator segments, then only 1/15th of the windings
reverse current
direction at a time. And, every armature coil reverses current flow
only twice per
revolution! Also, only the iron of the armature is exposed to the flux
reversal
as it turns, the field circuit (either permanent magnet or wound field)
has constant
flux, and so energy isn't expended to reverse that flux. This
dramatically reduces the
effective inductance of the armature windings.

Jon



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