Re: [CAD_CAM_EDM_DRO] Servos, a quick question

info.host@... wrote:

> Hello listies,
>
> I was having a look on the internet at servos to see how much they'd be instead of steppers around the 100 - 150 oz/in range. I found Magmotor http://www.magmotor.com/catalog/c38/index.html and even though I'm not overly interested in buying servos from them I was wondering about them. If you look down to the C38-200 it says it will deliver 1 30oz/in constant torque and 700 at it's peak. But it's amperage at constant torque is 6.7, is that general of servos?

The difference is that in servos, the torque is directly proportional to current, the speed is
directly proportional to speed. So, at zero torque, the motors need be given zero current.
When moving slowly, at modest torque, the voltage is very low, the current is low, and so
a PWM type servo amp will draw VERY little current from the main power supply, and deliver
very little power to the motor, as it doesn't need it. At high torque, low speed, the current may
be high, but the voltage will be very low, so the current drawn by the amp will still be low.

The stepper, meanwhile, draws both a fair bit of current and a lot of power ALL the time, just
to hold position.

> That they need comparably a lot more amperage than steppers? I remember seeing that this sort of stepper would only take 2amps. Do servos use a lower voltage and a higher amperage? At peak, if I was running a four axis machine with those motors, I would have to be supplying 160amps of controlled power.

Absolutely NOT! 160 AMPS??????? That's enough for a 50 TON Cincinnatti gantry bed mill!
If you would need 3 axes, and need all 3 to produce peak torque at peak speed, then you would
need to have a power supply that could deliver (700/130) * 6.7 = 36 A peak * 3 motors = 108 A.
But, that is an extremely odd condition, and could only be maintained for a fraction of a second,
or the table will go sailing through the wall! Most high acceleration takes place at LOW speed,
and the PWM drive reduces current draw from the power supply, because very little voltage
is needed. The PWM drive takes power in (at a high voltage) and puts power out (minus
small internal losses) generally at a lower voltage. If the motors are running at 1/10 of rated
speed, then the current into the amp will be roughly 1/10 of the current out!

I would recommend a 15 A continuous output DC power supply, with a generous bank
of filter caps to store sufficient energy for the peak loads. That should be completely
sufficient.

> 26.8 amps constant, doesn't that make the control quite expensive? These are two pole servos, are 4 poles smoother running like 3 phase power compared to single?

No. A 4-pole DC motor may be configured to produce more torque at a lower speed,
but that is about all the difference. They both can run very smoothly, if designed for
servo positioning work.

Jon