Re: [CAD_CAM_EDM_DRO] nema 34 motors on BP series 1 conversion

Hi there.

I had considered these for my Bridgeport, but decided against them.
They're a little underpowered for a machine that size (unless you don't
mind 6 IPM rapids :).

The 660 is only 176 in-oz continuous (11 in-lb). The motors I
eventually found (also from eBay) are from a CNC retrofit for a
Bridgeport, and they're 29 in-lb continuous. I checked Anilam's
website, and they only sell 31 or 41 in-lb motors for retrofits now.

He (clangenf) also sells the E690 motor, which is twice the power - 22
in-lb. I think he wanted $40 more per motor for those. That would
definitely be worth it, IMO.

I don't know the technical differences between the Geckos and Rutexes -
I went with Geckos partly on reputation, and partly on philosophy. (I
like the fact that Mariss releases some designs as "open source") To
get full speed from these motors, you would need a higher voltage drive
because of the high resistance and high Ke. A Gecko will give you full
torque, but at a lower max speed.

Here are the ratings that clangenf sent me: (I left out the midrange
motor, because he doesn't sell them)
Same for both motors:
120V max terminal voltage
1750 RPM max operating speed
5.5 in-lb or 88 oz-in/A torque constant
62V/kRPM back EMF constant (or voltage constant, Ke)

Different parameters:
E-660 E-690 Parameter
176 352 continuous stall torque, oz-in
7.8 4.3 terminal resistance, ohms
2.0 4.0 continuous current, A
10.0 20.0 max current, A

taking the numbers for the larger motor, this is what you'll get from a
Gecko, at 80V (assuming no other losses):
Max current = 80V / 4.3 ohms = 18.6 A
Max. torque at 0 speed: 18.6A * 5.5 In-Lb/A = 102.3 in-Lb (as your motor
catches on fire - this is ~1.5 kW of power dissipated in the motor)
Max no load speed: 80 / 62 kRPM = 1290 RPM
Max speed with full torque: 1013 RPM (4A * 4.3 ohms = 17.2 V, subtract
from supply voltage of 80 to get 62.8V, which gives 1.0129 kRPM at 62V/kRPM)

So - the bigger ones would be OK, if you use between 2:1 and 4:1 drive
on a 5TPI ballscrew. The smaller ones would probably need between a 4:1
and 8:1 drive ratio.

I'm no expert though - just going through the same trials as you are.
As I said, I did find my motors on eBay as well - 3 29 in-lb motors for
$325, with 1000CPR encoders (though one was bad, and I had to buy some
small motors with encoders to get a cheap replacement) I just had to
keep looking for several months, and keep my sniping finger well flexed :)

- Steve

feck123 wrote:

>Hi...was hoping to get some opinions /advice on whether it is worth
>it to use these motors from e-bay-thay are nema 34 mount-or keep
>trying to find some others/bigger ones ?I do not know where to find
>any other reasonably priced motors for my bridgeport cnc
>conversion...also,what would you recommend for the drives? rutex or
>the geckos? IF there is anybody that sells a whole setup that would
>be awesome but ANY help is greatly appreciated!! thanx Mike
>
>http://cgi.ebay.com/ws/eBayISAPI.dll?
>ViewItem&category=71400&item=3841702374&rd=1&ssPageName=WDVW
>