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Pulse Width Modulated (PWM) Controller for 12 Volt Motors

Motor Speed Control (Power Control) - continued
With the advent of solid state electronics in the 1950's and 1960's and this technology becoming very affordable in the 1970's & 80's the use of pulse width modulation (PWM) became much more practical. The basic concept is to keep the voltage at the full value (in this case 12 volts) and simply vary the amount of time the voltage is applied to the motor windings. Most PWM circuits use large transistors to simply allow power On & Off, like a very fast switch. This sends a steady frequency of pulses into the motor windings. When full power is needed one pulse ends just as the next pulse begins, 100% modulation. At lower power settings the pulses are of shorter duration. When the pulse is On as long as it is Off, the motor is operating at 50% modulation.

Several advantages of PWM are efficiency, wider operational range and longer lived motors. All of these advantages result from keeping the voltage at full scale resulting in current being limited to a safe limit for the windings. PWM allows a very linear response in motor torque even down to low PWM% without causing damage to the motor. Most motor manufacturers recommend PWM control rather than the older voltage control method.

PWM controllers can be operated at a wide range of frequencies. In theory very high frequencies (greater than 20 kHz) will be less efficient than lower frequencies (as low as 100 Hz) because of switching losses. The large transistors used for this On/Off activity have resistance when flowing current, a loss that exists at any frequency. These transistors also have a loss every time they "turn on" and every time they "turn off". So at very high frequencies, the "turn on/off" losses become much more significant. For our purposes the circuit as designed is running at 526 Hz. Somewhat of an arbitrary frequency, it works fine. Depending on the motor used, there can be a hum from the motor at lower PWM%. If objectionable the frequency can be changed to a much higher frequency above our normal hearing level (>20,000Hz) .

Note that I am using the terms "full power", instead of "full speed". Although we tend to think in terms of motor 'speed' both methods discussed are really varying the "power" available to the motor. The actual speed of the motor is the result of the load curve of the application. An axial fan has basically a linear load curve. As power goes up, speed will increase linearly. A centrifugal fan has a velocity squared load curve. To double the speed the power will have to be four times as great. The point is that we are not really controlling speed as much as power to the motor. So depending on the application, speed will not always be directly in relation to the PWM%.

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