Speed control for dc motor (electric
motor) can be implemented using open loop or closed loop. Closed loop
controller, also known as servo controller, or a feedback control,
gives the best performance since the loop will maintain the actual
speed to follow the reference. In simple application, an open loop
controller can be enough if the load is constant, such as in fan
control (PC fan speed control). This dc motor control circuit uses PWM
(pulse width modulation), gives a better efficiency than using linear
driver. Here is the schematic diagram of this PWM motor controller.
The circuit uses the very popular 555
IC, but here the circuit is configured in unusual way. The PWM output
is taken from the discharge pin, an open collector point that should be
pulled high using a Vcc-connected resistor, while the original output
pin (pin 3) is functioned to discharge the timing capacitor. The
recommended supply voltage for 555 IC is between 9-12 volt, and you can
connect the +V motor to this 555 supply voltage if the motor use 9-12
volt. The frequency of this PWM controller is about 144Hz, and you can
change the frequency by changin the C1 capacitor, use smaller caps for
higher frequency. The motor current can be as high as 37A if you allow
the MOSFET to rise its case temperature to 100 C, and you must provide
enough heat sink to maintain the temperature below this point. It is a
good idea to provide protector diode in parallel with the motor to
short the back-emf generated by the DC motor that might cause a voltage
spike that could burn the MOSFET. This simple pwm control gives you a
variable speed drives for motion control application. [Circuit
schematic diagram source: dprg.org]
