Comparing Miniature DC Motors and Gearboxes (cont.)
Impact on Stall Torque depending on Calculation Method Employed
Graph 2 illustrates the effect of the first three methods of Calculating Stall Torque. Keep in mind that exactly the same Nominal Voltage was used. These 3 curves are for the identical motor at exactly the same nominal Voltage! Which one is correct?
Frankly it could be argued that none of them are!
significant difference!
The bottom line is the Faulhaber motor can actually be more powerful than Vendor X’s motor and it is “rated” for significantly higher continuous torque levels, due mainly to the superior rotor construction and temperature rating.
Questions to Ask a Vendor
Clearly, starting at the datasheet makes sense but more importantly understand what the ratings mean between vendors. In many cases you will have to pose the question directly to the vendor, especially if you have simply downloaded a data sheet from their website.
Frankly, in general, this important information is not usually published. There is more information on reliability published on a carton of light bulbs bought at the home Depot than is generally published for sub-fractional dc motors and gearboxes!
Is the maximum speed and torque a rating for a certain amount of brush life or is it a thermal limit? Or a mechanical limit!
If it is for brush life and you are designing a fin control system for a shoulder-launched rocket you may be able to significantly up the motor’s power because the total operating time will be relatively short. Who needs a motor “rated” for 3,000 hours MTBF if the total operating time is 5 minutes or less?
How DC Motors are Rated?
There can be a multitude of criteria on how to rate a motor. Since output power is a function of the Voltage squared even relatively small increases in rated Voltage for a particular motor will result in a large increase in power.
In comparing the two subject motors you will notice that Motor Vendor X “rates” their motor for a significantly higher no load speed. So it will indeed have a significantly higher power. Graph 3 shows the comparison when the no load speeds are equalized.
The FAULHABER approach for Graphite Commutated Coreless motors is to “rate” the motor for 3,000 hours or greater MTBF under controlled test conditions at room temperature. If the target life was 1,000 hours minimum the same motor would be “rated” for a higher voltage and be significantly more “powerful”.
Leveling the Playing Field
Graph 3 illustrates what happens when the no load speed is increased for the Faulhaber motor so that it is the same as Vendor X. Now the stall torque, using Vendor X’s method, shows the Faulhaber motor has higher stall torque. And when perusing the published specifications we find that Vendor X’s motor is rated for a maximum coil temperature of 85°C whereas the Faulhaber is rated for scorching 125°C. That is a
Are the thermal ratings for nominal values or are they worst case? If they are nominal values be careful. A 12% tolerance on winding resistance and torque constant means that worst case you will be dissipating more than 27% more heat for the same torque load.
In the case of mating gearboxes exactly the same types of questions should be asked.
If you operate it within its ratings how long would you expect the gearbox to last? What happens if you exceed the maximum input speed? Does the gearbox catastrophically disintegrate or will the life be somewhat less?
Why does the Faulhaber Group’s 10 mm planetary gearbox have a maximum recommended input speed of 5,000 rpm whereas Vendor Y’s 10 mm planetary is rated for 10,000 rpm and Vendor X’s is rated for 8,000 rpm! All 3 planetary gearboxes have exactly 0.1 Nm of maximum rated continuous torque, by the way!
In general be suspicious of similarly constructed and dimensioned motors from different vendors that have significantly different values for such parameters as thermal resistance. The laws of thermodynamics and physics have not changed the last time I checked. Likely the “specification” methodology is different or there is a heat sinking disparity. Or an exciting new thermally conductive material has been utilized.
In the end testing is imperative. But by asking educated questions and understanding what the “ratings” mean your selection process will be more focused and your testing will have fewer disappointments.
Most importantly never forget the old adage ‘caveat emptor’!
