Re: Stepper/Servo sizing

Bill,

Good evening. They are completely different animals and must be
treated as such.

Step motors:

A step motor output power is independent of speed. Since power is
speed times torque, this means every time you double motor speed,
torque is halved.

If you were to graph a step motor's speed-torque curve (+Y=torque,
+X=speed), it would be the 1/X function. It is this speed-torque
relationship that gives steppers great torque at low speeds and
miserable torque at moderate to high speeds.

DC servomotors:

A DC servomotor speed-torque curve is a straight line from the
motor's no-load speed to the motor's stall torque on the same graph.

The power output curve is parabolic and peaks at 1/2 the motor's no-
load speed. That is the point where speed times torque is at a
maximum.

Application:

Start with how fast do you want to go and what thrust you want at
that speed. From that you can calculate how much power (Watts) are
involved and based on that, size the motor to the application.

Let's say you want to go 240 IPM and want 500 lbs of thrust at that
speed. Then:

Watts = (IPM times Lbs) / 531 -or- (240 X 500) / 531 = 226W

That is something a decent size 23 seromotor can do.

A typical "decent" size 23 servomotor might have the following specs:

60 in-oz continuous rated torque
60VDC rated voltage
Kt = 13.3 in-oz per amp
Kv = 10V per 1,000 RPM
R = 2 Ohms

To get the 60 in-oz of torque will require 4.5 Amps, (60 in-oz / Kt =
4.5A). The RPM at that torque will be (60V - 4.5 times 2 Ohms) / Kv =
5,100 RPM

So we have 60 in-oz at 5,100 RPM. The torque seems small compared to
a step motor but look at the speed at which this torque is developed!

Next is the reduction ratio. Let's say you have a 10-pitch, (10 TPI)
leadscrew. To move at 240 IPM the leadscrew would have to turn at
2,400 RPM (IPM times TPI = RPM). The motor meanwhile is turning 5,100
RPM, so the reduction ratio would be 5,100 / 2,400 or 2.125 to 1.

Cautions:

This motor has a stall torque of 400 in-oz (60V / 2 Oms = 30A. 30A
times 13.3 in-oz/A = 400 in-oz). This torque and any other torque
greater than the 60 in-oz rated continuous may be used for very short
periods of time only. The 60 in-oz rating is based on what heat the
motor can safely dissipate without damage; in this case 40.5W (4.5A
times 4.5A times 2 Ohms). At 400 in-oz the motor would generate a
whopping 1,800 Watts!

Mariss

--- In CAD_CAM_EDM_DRO@y..., wanliker@a... wrote:
>
> Mariss,
> I am attempting to modify an EMCO Compact 5 CNC Lathe, the original
motors
> are probably around 75 in/oz, from what I heard they work much
better with
> about a 200 in/oz stepper. However I want to use the G320's with a
servo,
> how do I size the servo? The lathe has ball screws and somewhere
around a
> 2:1 reduction belt drive, very slow.
>
> What is the relationship between steppers and servos when trying to
size
> these things.
> Please also post your answer on the list.
> Thanks,
> bill.
>
>
>
>
> [Non-text portions of this message have been removed]