Typical fractional horsepower shaded pole A.C. gearmotors are abundant in the recyclers world, they are cheap to build, have many applications, and are powerful torque providers. The common, asymmetrical form of these motors has only one winding, with no capacitor or starting windings and or a starting switch, making them economical and reliable. They tend to do one thing only however and thus can be classified as a “dumb” device. Their drawbacks are: being synchronous with line frequency they are not variable enough in speed without sacrificing either torque or R.P.M.'s, are not electrically reversible, requiring mechanical disassembly to change rotation, and are intermittent duty only, if no auxiliary cooling is provided. D.C. gearmotors on the other hand tend to be more expensive, are easily varied in speed, and can be made to perform as a “smart” device as will be explained later. This Instructable will detail how to convert an inexpensive or salvaged A.C. motor as described above into an intelligent and functional direct current type, with all the attendant benefits of a low voltage D.C. version.
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Step 1: Safety First
Mains/ high voltage may be present during this project, ensure total disconnection before handling or modifying components.
Step 2: Disassembly
A representative shaded pole gearmotor is made up of a reduction gearbox of a given ratio, a stator, a rotor, and an end cap/ thrust bearing. As the stator is no longer needed, it is beneficial to salvage the coil bobbin for future use as it usually contains some nice copper wire- the ferrous laminations can also be recycled too so nothing should go to landfill. To proceed, simply study how the core is situated in the frame; most are simply pressed into the receiver end of the assembly, and thus can be driven or pressed out by supporting the “C” frame sides and tapping the core ends alternately until it is free. This usually does not require massive force or any cutting, just a round punch and a hammer will do.
Step 3: The Gearbox Conversion
Spacers were cut from a salvaged printer feed roller which had a hollow core to substitute for the removed stator distance. Their length is determined by the stator's laminated thickness so the rotor with a gear driving shaft end will be repositioned in the proper geometry to engage the main reduction gear. The remaining roller with an encoder wheel attached is to be save
Step 4: The Driver Conversion
I used a printer carriage drive motor, and a saved G2 drive belt to transmit motor power to the gearbox. The rotor now serves as a reduction sheave with a wee bit of flywheel action. Scrap aluminum angle is fabricated to hold the two main components in proper relationship and are affixed to a simple wood base. Slightly oversized holes were drilled in the motor bracket to allow tensioning and tracking adjustment of the belt so it runs true. The new variable speed D.C. gearmotor is now ready for forward, reverse, and continuous speed operation, as well as being portable if so desired.
Step 5: Some Numbers
A no load bench test revealed the following input voltage and output R.P.M. performance. This is an average compilation since several d.c. sources were used to give meaningful responses, and thus is an approximation. A pulse width modulated variable voltage circuit was used for speed control and transformer driven wall- warts or bricks supplied the power. I do not know how well it would operate with switching type power supplies since some are purpose- designed and may not be satisfactory in this application, so that is to be determined. Be advised these findings are particular to the gearbox I used, yours will of course, differ, depending.
05 volts = 18 rpm
10 volts = 42 rpm
15 volts = 60 rpm
20 volts = 80 rpm
Step 6: That “Smart” Thing
Using the salvaged encoder wheel mounted on the output shaft and monitored with a sensor, this humble gearmotor can now be aware of where it's been, where it's going, and where it will stop as well as which direction it is turning or if it's turning at all, and just how fast. Further, it will not be affected by any slippage, backlash, or error in what's happening behind it, only being concerned with what's ahead. Now that's some pretty smart stuff for what began in life as a sole- purpose product, so hook up the 'Duino or 'Pi, cook some code, and have fun.
Step 7: Parting Thoughts
By intention, this build was accomplished with recovered assets exclusively, no out of pocket expenses were incurred. It should be easily replicable with a minimum of tools, time, and workspace, and serve as a driver platform for other projects requiring a precisely controlled motive force.