DC Motor Driven By PWM

In the picture above, Oscium’s iMSO and an iPad Mini work together as an oscilloscope for measuring the voltage applied to the electric motor of a helicopter’s main rotor. There are a couple of progress driven factors that make playing with remote control helicopters, airplanes, boats and trucks more fun and less expensive than they were even ten or fifteen years ago. Microprocessors double in capability as transistor density increases according to Moore’s Law. Electronics and transducers are smaller today and retain many of the capabilities (strength, power, etc.) of their larger, space hogging and power-consuming predecessors. For those of us smitten with exploring the world of electronics, these models are miniaturized versions of complex systems comprised of analog, digital, and RF circuitry. The model helicopter pictured above is a one-way radio with a digital receiver and microcontroller. Wow!

Crazy Looking Signal 

Look at the waveform displayed on the oscilloscope’s screen – it’s the analog voltage applied across the terminals of the main rotor’s DC motor. Here are a couple of things to notice:

  1. Pulses - Like most miniature or battery powered motors, this motor is driven with a Pulse Width Modulated (PWM) voltage instead of a nominal DC voltage. Battery preservation and power efficiency are primary concerns for designers of battery powered “things”. The PWM method of controlling the motor’s drive voltage is an efficient alternative to a resistive divider between the power source and the load. A variable resistor, like a digital potentiometer, that sets the motor’s voltage would consume precious power as heat through a waste resistor. PWM can be set so the required equivalent DC voltage is applied to the motor without wasting power, thus preserving run times between charges.
     
  2. Spikes - Close inspection of the waveform shows ringing or spikes with the rising and falling edges. Going back to the first semester electrical engineering course work, remember what happens when you apply a time varying current to an inductive load. The result? Voltage spikes! Now, the amplitude of the unmitigated spikes is proportional to the change in current with respect to time. The spikes we see in the waveform are minor (ie. non-destructive) because the current is small to begin with and there are probably provisions in the electronics to accommodate a switching current. I remember when households were lit with 100-watt incandescent light bulbs. When I’d switch the light off I could see a spark in the switch behind the little plastic plate. That spark occurs because I switched the current from ON to OFF in a short amount of time. Think about it: 100 watts from 110 VAC; that’s about one amp through the switch; more if there are a couple of light bulbs on the same circuit. As a kid (I would certainly NEVER try anything like this now – I’ve moved on to things that cost much more than a light bulb) I could burn out a bulb by cycling the power through the switch, repeatedly, until the filament in the bulb succumbed to a spike on the line.
     
  3. EMF - The last effect I’ll mention is so subtle that it often goes unnoticed. Electro Motive Force (EMF) or back current also plays a part in the waveform we see on the oscilloscope. Motors are inductive loads to the power supply. They usually have very little series resistance (for efficiency). To a DC source, a motors look almost like a dead short. A twelve volt supply, for example, would try sourcing three amps to a motor that presented a four ohm resistive load. At three amps through four ohms, the little motor would be dissipating close to 36 watts. The motor would likely burn up if there was no EMF. The EMF is generated from a magnet rotating inside an electric field (from Physics, remember). As it turns out, the EMF current flows in the opposite direction as the current from the battery. When the motor reaches its designed RPM, the two currents will sum together at something less than the current from the source, limiting the sourced current in effect), and thus reducing the total current through the little motor.​

Here’s a 30 second video clip of the live signal and helicopter spinning up the main rotor. Watch how the waveform changes with time as the motor spins.