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Short video of the machine running.

It's made of three AAA batteries in series giving a total of 4.5V to the DC motor, these are connected through a potentiometer to control the speed. The DC motor is connected to a gear that runs a music box playing Fur Elise, but its too fast, and when speed is reduced, it can't make the gears go around. Off my head it seems there are several methods to get this fixed, I am asking here to get some tips on the best.

Possible solutions:

  1. Change gearing (what gear setup should I use?)
  2. More powerful DC motor
  3. Change batteries
JYelton
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bogen
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  • DC motors run much too fast for this application to be used directly. A more powerfull (lower impedance) battery will only make the motor run faster, a higher-voltage battery will do likewise, or maybe burn out the motor. A PWM speed control might work, but IMO the only real solution is a geared motor, with an RPM after the gear that fits your need. – Wouter van Ooijen Aug 04 '14 at 06:59
  • I'd start trying to align those gears, that can help dramaticaly. – Vladimir Cravero Aug 04 '14 at 08:50
  • Increasing torque, but not speed, is impossible unless you are driving some kind of weird non-linear mechanical load. Imagine you are pushing something across a desk: can you push it harder, without pushing it faster? – Phil Frost Aug 04 '14 at 10:55

2 Answers2

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The very first thing you need to do is to clean up the drive so the gears mesh properly with no wobble. One way to do that is to drill and ream the existing bore of the larger spur gear to accept a bushing with an OD which will give a slip fit in the spur gear's new bore and an ID which will give a slip fit around the music box movement's input shaft. There also needs to be a radial hole in the bushing which will allow the spur gear's set screw to secure the spur gear to the shaft with the bushing interposed between them.

Since what you have already has enough torque to drive the badly misaligned gears as well as the variable load of the music box movement, it seems pointless to change the motor or the gearing when all you need to do is to control the motor speed - without compromising its torque - by using a pulse-width modulated (PWM) motor driver.

In essence, what'll happen with PWM is that you'll use the 4.5 volt source to drive the motor, maintaining its output torque, but you won't leave it on all the time.

For example, if the motor turns at 60 RPM with 4.5 volts across it 100% of the time, it'll turn at 30 RPM with 4.5 volts across it 50% of the time but because you're not limiting the voltage into the motor with a resistor or suchlike, during whatever time it's on it'll output full torque, just what you want.

A simple PWM circuit in hardware is shown, following, with the duty cycle adjustable between about 1% and 99%, and the frequency adjustable by changing the value of C1.

The LTspice circuit list is here if you want to play with the circuit.

enter image description here

EM Fields
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0

Wouter is usually right :-) - but let's try anyway.

Power = Torque x Speed ... 1 or
Torque = Power/Speed ... 2

So if you want to increase the torque but not the speed then fro, (1) above, you need more power. At constant speed the motor will tend to draw" a certain current leading to a given power level and you tend to be in a fatal loop.

From the video it seems that the loading varies with position in the tune (probably related to number of music box fingers being plucked but may also be variable stiction at different points.

One solution is to operate the motor from MORE voltage BUT to speed regulate the motor so that it can produce more power than needed on all occasions BUT you prevent it from doing so except when it needs more power to maintain speed. One way to regulate speed is to use a toothed wheel or striped disk to produce tachometer pulses and then increase or decrease drive voltage as the pulse rate decreases or increases. This is similar to what is done professionally but may be more complex than you wish to get involved in.

A method that MAY produce improved results is to operate the motor from a significantly increased voltage via a series resistor such that in normal operation the voltage drops to about the level required for lower speeds but if the current drops the motor voltage increases as there is less voltage drop across the resistor at lower current. This approximates a constant current feed and the higher the voltage and the larger the resistor value the closer it comes to a current feed. Care needs to be taken not to apply more voltage than the motor can tolerate.

Easiest is, as you say, either a geared motor or a more powerful motor, but the series resistor plus higher voltage supply may be worth trying.

Russell McMahon
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