Re: [CAD_CAM_EDM_DRO] Re: capacitor

mariss92705 wrote:

> Jon,
>
> As speed increases, the motor "falls behind" or develops a phase lag.
> This is identical to the phase lag current has relative to voltage in
> an inductor.
>
> During deceleration this phase lag decreases. As it does, the
> drive "sees" an increasing voltage because current and voltage become
> more in phase.
>
> Once this "in phase" voltage exceeds the power supply voltage,
> reverse current begins to flow if the rate of deceleration is high
> enough.
>
> The servomotor situation is considerably simpler. Since the motor
> cannot develop a "back EMF" greater than the power supply voltage, it
> cannot return any current to the power supply (apart from the
> relatively insignificant L*I^2/2 component). To do so would require a
> Kv generated voltage in excess of what is available from the supply.

Disregarding inductive effects and the PWM action of the servo amp,
that would be correct. But, the motor IS inductive, and most modern servo
amps ARE of the switching type, so the long pulses placing opposing
voltage across the armature will build up large currents in the windings.
When the PWM amp reverses the polarity, which it will do as long
as it is still operating at less than 0 or 100% duty cycle, the inductive
energy will be released with a voltage above the back EMF for that
speed. Adding inductance in series with the motor will increase this
effect. I have 600 uH in series with the motors on my amps. These are
part of the output filter network, which keeps the 100 KHz PWM carrier
from becoming an illegal broadcast station, but it also stores several
times more inductive energy than typical motor armatures. I know that
your amps, for cost control reasons, don't have an output filter, and that
is one of the reasons my amps are NOT cost competitive. This effect
is going to be more pronounced in larger motors anyway, and in motors
with lower resistance armatures.

Jon