Re: [CAD_CAM_EDM_DRO] Re: Servo Question

mmurray701 wrote:

>Hi,
> The numbers are defenatly right, its possible theirs a mistake at
>the source. They were taken from:
>http://www.baldor.com/products/specs.asp?
>1=1&page=1&catalogonly=1&catalog=MTE-2250-
>AMACN&product=Servo+Motors&family=DC+Servo%7Cvw%5FServoMotors%5FDC
>
>

Yup, there's an error all right!

Quoting from the same data :
Torque Constant
(Lb-In/Amp & N-m/Amp): 1.0125/0.114407

If the Kt is 1.0125 Lb-In/Amp, and the

Peak Current: 18.5

then, the peak torque must be 1.0125 * 18.5 = 18.73 Lb-In.
the 300 Lb-In is just rediculous. You'll also note that the
ratios of the peak/cont ratings are not the same when expressed
in Lb-In and N-M, which is impossible.

>I'm currently using 200 oz steppers on a mini mill. I'm occasionally
>loosing steps. I'd like some more top speed and a little extra
>torque. I'm not seeking a huge increase in speed or torque, but
>primarily just the advantages of a closed loop system not loosing
>position. I want to be able to walk away and be confident its not
>going to loose position do damage from a crash or something.
>
>So to replace 200 Oz steppers what size servo should I be looking
>at? My current rapids translate into 600 RPM. Would be great if I
>could run the lead screws at about 1000 rpm and still have a little
>extra torque. So if I was using a 5000 rpm motor (5:1 reduction) id
>need it to have around 40 oz-in continious torque? Or could I get
>away with something smaller? Kind of makes sense that something
>smaller would work since its not needing to provide 40 oz
>continiously.
>
>

Note that your steppers are generating NOWHERE near 200 Oz-in at 600
RPM. Steppers
are rated at standstill, and the rating is called HOLDING torque.
Servos have
constant torque up to the point where the driver/power supply can't
provide any more
voltage, at which point the torque falls off sharply. So, you should
never push the
speed right up to where the motor's back-EMF approaches the power supply
voltage.
The motor in question has a Kv of 12, so at 5000 RPM, the back EMF is 60 V.
You'd want to run a power supply pretty close to the 80 V limit to get
this to work.
Note, the motor has a 2 Ohm resistance, so at 18 A, the IR drop across
the motor
would be 36 Volts, alone, in ADDITION to the back EMF. So, to achieve peak
torque at 5000 RPM, you'd need 96 V applied at the motor terminals!

In extremely heavy industrial service, you should not count on the peak
torque
rating of the motor. But, in less severe home-shop sort of service, you
can make
demands in that area for short durations. Limiting this motor to 3 A is not
really practical. You might want to limit it to 10 A or so, however. I
might
suggest a 4:1 belt reduction ratio for a trial. Assuming 1000 RPM on
the screws,
you'd need 4000 motor RPM, or 48 V of back EMF. Assuming a current limit of
10 A, the total voltage required would be 48 + 10 (Volts) * 2 (Ohms) = 68 V.
Again, to have some margin for power supply dips under load and IR drop in
the servo drive, a 75 V power supply might be a good idea.

10 A would give you approximately 18 In-Lb, or 18 * 4 = 72 In-Lb at the
leadscrew. 72 In-Lb is 1152 In-Oz, which should make your table accelerate
like a rocket!

>Basicly what would be a good motor (that will run off Gecko 320's)
>to replace 200 oz steppers?
>
>

Well, if you really only want 200 Oz-In, then the Baldor motors you
specify are
going to be a bit of overkill. You might look for a smaller motor.
Actually,
for a mini mill, you'd better limit the torque, or have a big supply of Acme
nuts ready to replace the ones that burn out!

Jon