Re: [CAD_CAM_EDM_DRO] Servo/Spindle motor ammeters?

Abby Katt wrote:

>Hi Guys,
>
>Just searched my archives of the list and found no direct references to
>this, so thought I would post here as it seems a pretty obvious question
>(though I suspect my range of answers will surprise me, as usual, about
>how obvious items are rarely so - especially in CNC!)
>
>I was at the Mach 2006 expo and saw a lot of the "real-thing" CNC mills,
>personally I thought Mach3 looked way better than many of them (don't
>flame me! :), but I did notice a feature standard to just about every
>mill and not on the average hobbiest CNC. They had load bar-graphs for
>the spindle motor, and the servo motors.
>
>I want load-meters for my mill! :)
>
>So I was wondering, can I just get away with 3 DC ammeters placed
>between the power of each servo driver,
>

A 0 - x amps DC meter on the power input to the drive will show (roughly)
the watts required. An ammeter in series with the motor will show motor
torque, but it needs to be a zero-center meter, ie. minus X amps - 0 -
plus X amps

> and one AC ammeter in series
>with my (AC) spindle?
>

The AC ammeter will not measure power to the motor, it will measure line
current. AC motors at idle draw a LOT of current with near zero power
factor.
Many motors draw about 75% or rated load current at idle (my Bridgeport
draws about 3.2 A at idle and 3.5 A at rated load, for instance), but
the phase
angle of that current changes dramatically. But, an ammeter will not show
that.
An ammeter on the input to a VFD would give a more representative
indication.

Jon