On 08/11/2011 08:11 AM, Dave wrote:
>>> Wonder why they headline the power rating of the
>>>
> motor, rather than the torque of the motor, since torque is what we are
> mostly concerned with when it comes to the use of the motor?
>
> It is about the same thing as rating a motor in horsepower. At a certain
> power - Torque and RPM are interchangeable to some extent via gearing.
> Most smaller servo motors tend to run fast - 3000 rpm in the case of that 400
> watt motor so usually gearing or a belt reduction is required to take
> advantage of the
> power available. The best thing to look at are the speed-torque curves. I
> don't know if there is a link to them from the Machmotion website. I found
> them on the TECO website before.
> That way you can determine exactly how much torque you can get out of the
> motor at a given speed. As Andy said though, the torque curve is pretty much
> flat on these motors all the way out to their rated rpm and then it falls
> off. Those motors will actually spin at speeds greater than the rated speed
> but at reduced torque.
>
> If you compare the torque - speed curves of a brushless industrial servo
> motor/drive to a stepper motor drive, you will find the differences are
> startling.
>
> If you want to do a comparison just for reference - look up the speed torque
> curves for the Automation Direct servos - at AutomationDirect.com and
> compare them to a stepper motor - you can find some speed torque curves at
> Kelinginc.net.
>
> Here are some links:
> A Stepper motor.. the bottom is in Pulses per second - half stepping so I
> think that is 400 pulses per rev for most bipolar stepper motors.
> http://kelinginc.net/KL34H295-43-8BT.JPG
>
> http://www.automationdirect.com/static/specs/sureservolow.pdf
> Look at page 2 in this PDF file for some speed torque curves.
>
> That speed-torque curve on the Keling website is really a best case situation
> since they are using a high voltage stepper drive in that speed-torque curve.
>
> Most people use lower voltage drives on steppers - like the common Gecko
> drives.
>
> Mariss of Geckodrive has said before that in general if you need more than
> 200 watts of power in an axis you should consider a servo drive.
> I believe I have seen Parker docs that indicated that they can get as much as
> 400 watts of power out of a stepper but to do that they use a high voltage
> stepper drive.
>
> If you look at the speed-torque curves of the stepper motor you will see why
> steppers do their best at lower speeds.
>
> Dave
>
Dave,
I knew about the properties of the torque fall-off of steppers, and
that servos generally kept their torque at higher rpms, obviously much
better than steppers. i was just wondering why motors were listed with
power ratings rather than torque, since torque is more along the lines
with how we determine what we need from a driven motor than power. Must
be like you said, HP, especially in motor vehicles seems to be a selling
point, kinda like "billet" aftermarket auto parts. To me, anyway, I
would much rather have the torque ratings up front, and in an industrial
environment, where somebody is going to use that product to drive a
machining operation, I would think the integrator would much rather look
at what really needs to be looked at, not the eyewash.
But, whadda I know about effective marketing. ;-)
Mark
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