Re: [CAD_CAM_EDM_DRO] Re: What makes a motor a servo? calculating power

Mariss was comparing power output which in my opinion is not really
relevant. Steppers and servos behave differently and it is easy to say
that steppers are better or servos are better by picking situations that
particularly suit each type of motor.

The whole point is that you don't normally fit your machine requirements
to a motor, you fit your motor to the machine. Steppers and servos have
different design criteria. For steppers you need four values:
1) Maximum torque (usually at max cutting speed)
2) Speed at which you need this torque
3) Torque at rapid speed
4) Rapid speed
These are all at the lead screw.

For instance I decide that when cutting I need a lead screw torque of
600 oz-in at 200 rpm and 100oz-in at a rapid speed of 800rpm.

As the speed is low we might as well gear the stepper down 2:1 so the
stepper needs to produce 300oz-in at 400rpm and 50oz-in at 1600rpm.
For reliability we need to add a 2x factor of safety to these figures so
we get 600oz-in at 400rpm and 100oz-in at 1600rpm. Now start looking
through the stepper torque curves to find one that will fit these
values. You will probably find you need something like a 800oz-in or
more stepper to fulfil the criteria.

Looking through the data sheets you may find a stepper that has more low
speed torque than you need but not enough high speed torque. In that
case you can tweak the reduction ration and work out the numbers again.

For servos you need just two values
1) Maximum cutting torque
2) Maximum speed.

First work out the reduction ratio. This is simply the max motor speed/
max screw speed. Assuming our motor has a max speed of 4000rpm the
reduction ration is 4000/800 = 5:1. The servo needs a continous torque
rating of 600/5 = 30oz-in.
Again you may find a motor with more torque and lower speed or
vice-versa. In that case you tweak the ratio to suit.

The servo will be capable of producing more peak power than the stepper
but who cares? As long as it fulfils the critera for your machine that
is all that matters.

In practise there are more things to think about than this, for instance
I haven't considered the desired acceleration, which can open another
can of worms.

Les


lcdpublishing wrote:

>I too am getting more confused on this issue.
>
>Why is it that the servo starts out at 60 in-Oz and the stepper starts
>out at 600 in-oz.
>
>To me, that is like comparing 2 cylinder engine to a V8 engine.
>Unless there is something I am not seeing, why not run a direct
>comparison.
>
>600 in-oz stepper against a 600 in-oz servo?
>
>60 Volts @ 6 Amps to 60 Volts 6 amps
>
>Or whatever the criteria. To me, comparing a 60 In-Oz Servo to a 600
>In-Oz stepper motor make no sense at all.
>
>
>

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