Re: [CAD_CAM_EDM_DRO] Servo drivers ?

turbulatordude wrote:

>It looks like there are a lot of lower priced DC motors that might be
>used with a servo driver, but it also seems these are lower voltage.
>
>The drivers I found so far are Jon's PICO , the Gecko and a Rutex.
>
>IMSERV has a $2,000 package for drivers but that is out of my league.
>
>I'm not sure how to frame the question, but are we just basically
>looking for watts for power ?
>
>

Well - I can't give you any information from experience, but if you want
some of my book (web) knowledge, read on.

You need to be concerned with watts, volts, and amps - but for different
reasons.

I'll put in approximate values from my motors, with Geckodrives.
The Gecko can do 80V, 20A.

Motors are Baldor, 28in-Lb continuous.
Kt = 3 in-Lb / A
Ke = 40V/kRPM
Max continuous current = 9.2 A
Max voltage = 150V
Max speed = 2500 RPM
Peak current = 37 A

Max Voltage: This will determine how fast the motor can go. So, for my
motor with 40V Ke, and a Gecko at 80V, the maximum theoretical speed
I'll ever get from the motor will be 2000 RPM (assuming no losses, and
totally unloaded motor). The calculation is 80V / 40V/kRPM = 2.0 kRPM.
The Gecko is the limiting factor here. The high voltage Rutex drives
would let the motor be the limiting factor, at 2500 RPM (and 100V back EMF).

Max Current: This will determine the maximum torque that the motor can
deliver. Servos are rated with a torque constant (Kt), which tells the
amount of torque per amp of current. If you have a motor that can take
10A continuous, then any drive that can supply 10A continuously
(regardless of the operating voltage) will be able to get full torque
from the motor. The total voltage across a servo is (neglecting losses)
the back EMF, and the IR drop across the winding resistance. So, the
winding resistance will cause a reduction in maximum speed proportional
to the amount of torque you're getting from the motor. For my motors,
the Geckos will happily pump the max. continuous current into them, and
I have roughly double that in reserve, for high acceleration
situations. Again, the high-current Rutexes could drive the motor at
its full peak current, giving a total of 112 in-Lb of torque(!), whereas
the Geckos will be limited to only 60 or so (poor me :) ).

Max Power: This has to do more with heatsink / cooling design. A
poorly designed drive may have max current of X, and max voltage of Y,
but may dissipate too much power to be useful for a particular motor (in
a particular application). The actual power dissipation in the drive
will depend on the design, but I think they're usually a semi-constant
value, plus something proportional to current. So, if you plan to do a
lot of acceleration (and this includes deceleration), then a good
thermal design and proper heatsinking is important.

>For a plasma cutter, the gantry will move pretty quickly with both
>rapid accel and decel, and have to drag around the hoses.
>
>

Accel = decel = torque. If you need to change the momentum of big hunks
of metal, you need torque.

>Is there a rough guideline of which type driver is best for certain
>machines ?
>
>

If so, I'd love to have seen it before I bought mototrs for my Bridgeport :)

>Kinda like the Xylotex is great for Sherlines and Geckos can handle
>Bridgeports ?
>
>
>Dave
>

- Steve
(and I almost sound like I know what I'm talking about :) )