What is a pulse width modulation for DC motors really?

I have studied the Navy related materials on pulse width modulation but I'm not clear as to what they really are or what they're supposed to indicate. From what I read, the output of PWM doesn't have a sufficient current carrying capabilities to drive a motor. It's added in a circuit to stabilize the output current.

The textbook is being vague about pulse width modulation that it's frustrating me: "Pulse width modulation is the method by which a fixed frequency and amplitude is varied in duty cycle by an analog signal." The book then describes that inputs to a PWM are a clock pulse and a DC level. I'm not clear where this clock pulse comes from.

What I need to get at before studying PWM circuits is what does a duty cycle indicate? What happens to the pulse width when the load increases? How does a DC voltage get converted into pulse width?

• Have you had a chance to play around with a 555 yet? – Ignacio Vazquez-Abrams Dec 9 '14 at 7:13
• Yes, we had a lesson on 555 and a scheduled test but it got cancelled – NavyColors_Blue Dec 9 '14 at 7:15
• That's a shame, because investigating a 555 will answer most of your questions. I recommend that you do so independently. – Ignacio Vazquez-Abrams Dec 9 '14 at 7:15
• That's a rather badly phrased textbook. – pjc50 Dec 9 '14 at 9:47
• @newbie, A PWM signal is typically used as an output from a microcontroller to the input of a transistor. Power for the DC motor is provided through the transistor. The duty cycle of the PWM varies to control the voltage on the output side of the transistor and thus, the speed of the DC motor. – semaj Dec 9 '14 at 16:22

Take a look at this: -

Here are two PWM waveforms; with with low duty cycle and one with high duty cycle. Take note that the average value of the signal equates to a DC level (dotted line). This could be achieved by controlling (say) a motor with a variable resistor but the motor current would flow thru the variable resistor and dissipate lots of power as heat.

Alternatively, if you rapidly apply high pulses and low pules instead, the average value remains the same (as when using a variable resistor) but because a semiconductor switch is being used the power dissipation in that switch is theoretically zero. this is because that switch is either open circuit or short circuit.

So you need a clock signal and a dc input. The dc input represents that desired average DC output you want on your PWM. Using a 555 can achieve this but there are tons of methods. PWM, as a raw signal from a 555 needs amplifying by power transistors if you are driving a motor.

That's the basic PWM system and an addition is to add a control loop around the system to regulate things.

PWM simply stands for Pulse Width Modulation. It can be done at almost any power level, from microwatts to megawatts. This means LEDs or motors (of all sizes) to incandescent lamps to .... The drive capability depends on the driver circuit, not the fact of whether or not it is PWM.

Duty cycle (the number) ranges from 0 to 1. The number indicates the fraction of time it is ON. That is, a PWM signal with a duty cycle of 0.1 is on 10% of the time and off 90% of the time.

The duty cycle only changes if it is "told" to change. If there is a feedback network that maintains RPM, then the duty cycle will change as the motor load changes (to keep the speed constant). If there is no feedback, then the duty cycle will not depend on the load.

A DC voltage gets converted into a PWM signal with a switch. The switch can be a little FET (for low power) or a big BJT or Power MOSFET or IGFET, or maybe even many transistors connected in a way to turn on and off fast plus handle the power requirements.

Hope this helps - Jim