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View Full Version : Servo and conventional motor...what's the difference?



Splint
04-20-2004, 12:39 PM
Hi,
I was wondering if someone could point me in the direction of a page that explains the difference between a servo motor and a conventional motor. I have some servos ready for a project which may be under powered for the application (I'm yet to fully crunch the numbers) and what I'm trying to find out is whether it is possible to take the encoders off my motors and adapt them to a bigger more powerful motors from some type of electrical appliance. My other question most motors which I could readily get hold of would normally run in a single direction at a set speed, would a motor like this be capable of doing the job of a servo? Due to the high cost of motion control equipment here in Australia and the cost of freight from other countries I thought I might investigate this avenue to at least find out if it can be done.
Thanks
Splint

Chagrin
04-20-2004, 03:17 PM
Yes, that can work, but it's not "ideal" and that's where servo motors fit in. A designation of "servo" given to a DC motor is fairly vague; basically you just have a regular motor that is designed to pack more torque into a lighter armature for better responsiveness. Cheaper DC motors just don't have the focus on weight and power that a servo motor has -- heck, try a site like www.baldor.com and look for ratings for torque or intertia on a general purpose DC motor -- you won't find them, because it's not relevant to a general purpose motor.

Sure, you can definitely go ahead and use appliance motors instead of servo motors and it will work -- just not as well. The real important part is whether or not it works well enough for you (and whether or not you have a lot to risk in the time and materials it would take to test it out).

The simple definition of the difference between conventional and servo motors? Economy.

Al_The_Man
04-20-2004, 04:26 PM
Originally posted by Splint
Hi,
I was wondering if someone could point me in the direction of a page that explains the difference between a servo motor and a conventional motor. I have some servos ready for a project which may be under powered for the application (I'm yet to fully crunch the numbers) and what I'm trying to find out is whether it is possible to take the encoders off my motors and adapt them to a bigger more powerful motors from some type of electrical appliance.
Thanks
Splint
Splint, I gather you mean a universal motor? when you say conventional motor.
I think you will be sorely disapointed if you try a servo application with these, You say the servo's you have are underpowered, have you considered a reduction of some kind, as many DC servos are rated up to 3000 rpm. ratio would depend on what final feed rate was required by your application.
As to removing the encoders, it depends how they are mounted, some are attached to the rear motor shaft and often you can destroy the glass disk when you try and remove, others have a coupling and these are usually easy to remove.
Al

ESjaavik
04-20-2004, 04:51 PM
Hi Splint,
A brushed DC servo runs with the same torque/amp in both directions. Most appliance motors have skewed brushes to give more power, but then they will give less power running in the direction opposite to the designed one. Definately very bad for a servo motor. The amplifier would have to compensate for this, a very "interesting" design.

But there are brushed DC servo motors without encoders, and they would do fine. They are typically used in applications where the position (or speed) sensor is after the motor. Or on very old computer tape stations there are some powerful and fine motors. In that case there is no position sensing, it's used to set up the tension in the tape. The sensing is by the tape being sucked by air down into a pocket, and the length of tape in the pocket is the feedback. These tape stations runs in both directions, and therefore use motors with symmetrical characteristics.

HuFlungDung
04-20-2004, 10:10 PM
Esjaavik,

Thanks for that tidbit of info. I was just wondering about my cordless drill yesterday, noting that it ran faster in forward than in reverse. I thought maybe it was dying on me :)

Splint
04-21-2004, 01:26 AM
Thanks for your help guys. You've certainly cleared a few things up for me. I have considered gearing down the motors as they appear to be able to run up to 6000 rpm, but I really need to search around and find some formulas to calculate the torque required to move the mass of the gantry driven by a ballscrew. At this stage I am still looking at different ideas for designing a gantry style router. I was hoping to build something with a work envelope of about 500mm by 500mm by 1200mm predominately to be used for foundry pattern making but I could reduce the dimentions to reduce the weight of the gantry. For anyone interested the figures below are not the exact figures for the motors I have but they are from the same family of Electrocraft 543 series motors and should do as a substitute since I have not been able to get the figures from any other source.

Electro-Craft servo motors
Cont. stall torque (oz-in) 50
Peak Torque(oz-in) 350
Max. voltage (V) 60
Max speed (rpm) 6000

Mechanical

oz-in-sec2 0.0062
kt Torque constant 11.8
Ke Voltage Constant 8.6
Winding resistance (OHM) RA 1.04
Max pulse current (A) 29
Armature Induct. (mH) 3.3
Body Length(inch) 5.0

Thanks
Splint

Al_The_Man
04-21-2004, 10:01 AM
Splint, There are quite a few free programs that can be downloaded from some of the motor manufacturers site, http://www.electromate.com/technicalsupport/?c=kollmorgansoftware for one, I believe there is one on the Parker-Hannefin site also.
It can certainly get you close.
Al
ED: that url did not seem to post right so look on the Kollmorgan site if you have a problem.