Pulse Width Modulation

Posted on Feb 4, 2014

The control of electric motors is something which interests nearly everyone involved with Meccano model building. Every model has its own motor requirements with regard to the space available, the power of the motor, its speed, whether it must stop and start frequently, and the need for reduction gearing. On the face of it, simple methods of contr

Pulse Width Modulation
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ol are perfectly adequate, with a regulated voltage supply, a simple on/off switch, and the means to reverse the motor. Speed can be controlled with a wire-wound potentiometer (variable resistor) or a circuit such as the Darlington Pair Speed Control. In reality, these methods can provide very unrealistic results. The main problem is poor starting performance, the motor tending to jump almost instantly from a stationary position to what is often more than half speed. The main cause of this seems to be the starting characteristic of the motor itself which when under load seems reluctant to start. A motor has a relatively low resistance when it is stationary. As the speed control is advanced, the current through the motor increases, but the voltage across the motor remains quite low. The speed control therefore has to be well advanced before the voltage and power fed to the motor are high enough to overcome its reluctance to start. As the motor speed and the load on it changes, there are changes in its internal resistance. Speed regulation is not very good under these circumstances, particularly at low speed. A good analogy is bicycle riding. You peddle (exert energy) and then coast (relax) using your momentum to carry you forward. As you slow down (due to wind resistance, friction, road shape) you peddle to speed up and then coast again. The `duty cycle` is the ratio of peddling time to the total time (peddle + coast time)....

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