X
Advertisement
pulse width modulation

Pulse Width Modulation

There are many different ways to control the speed of motors but one very simple and easy way is to use Pulse Width Modulation. But before we start looking at the in’s and out’s of pulse width modulation we need to understand a little more about how a DC motor works.

Next to stepper motors, the Permanent Magnet DC Motor (PMDC) is the most commonly used type of small direct current motor available producing a continuous rotational speed that can be easily controlled. Small DC motors ideal for use in applications were speed control is required such as in small toys, models, robots and other such electronics circuits.

A DC motor consist basically of two parts, the stationary body of the motor called the “Stator” and the inner part which rotates producing the movement called the “Rotor”. For D.C. machines the rotor is commonly termed the “Armature”.

Related Products: Motors AC Motors

Generally in small light duty DC motors the stator consists of a pair of fixed permanent magnets producing a uniform and stationary magnetic flux inside the motor giving these types of motors their name of “permanent-magnet direct-current” (PMDC) motors.

The motors armature consists of individual electrical coils connected together in a circular configuration around its metallic body producing a North-Pole then a South-Pole then a North-Pole etc, type of field system configuration.

The current flowing within these rotor coils producing the necessary electromagnetic field. The circular magnetic field produced by the armatures windings produces both north and south poles around the armature which are repelled or attracted by the stator’s permanent magnets producing a rotational movement around the motors central axis as shown.

2-Pole Permanent Magnet Motor

permanent magnet dc motor

As the armature rotates electrical current is passed from the motors terminals to the next set of armature windings via carbon brushes located around the commutator producing another magnetic field and each time the armature rotates a new set of armature windings are energised forcing the armature to rotate more and more and so on.

So the rotational speed of a DC motor depends upon the interaction between two magnetic fields, one set up by the stator’s stationary permanent magnets and the other by the armatures rotating electromagnets and by controlling this interaction we can control the speed of rotation.

The magnetic field produced by the stator’s permanent magnets is fixed and therefore can not be changed but if we change the strength of the armatures electromagnetic field by controlling the current flowing through the windings more or less magnetic flux will be produced resulting in a stronger or weaker interaction and therefore a faster or slower speed.

Then the rotational speed of a DC motor (N) is proportional to the back emf (Vb) of the motor divided by the magnetic flux (which for a permanent magnet is a constant) times an electromechanical constant depending upon the nature of the armatures windings (Ke) giving us the equation of: N ∝ V/Keϕ.

rheostat motor control

So how do we control the flow of current through the motor. Well many people attempt to control the speed of a DC motor using a large variable resistor (Rheostat) in series with the motor as shown.

While this may work, as it does with Scalextric slot car racing, it generates a lot of heat and wasted power in the resistance. One simple and easy way to control the speed of a motor is to regulate the amount of voltage across its terminals and this can be achieved using “Pulse Width Modulation” or PWM.

As its name suggests, pulse width modulation speed control works by driving the motor with a series of “ON-OFF” pulses and varying the duty cycle, the fraction of time that the output voltage is “ON” compared to when it is “OFF”, of the pulses while keeping the frequency constant.

The power applied to the motor can be controlled by varying the width of these applied pulses and thereby varying the average DC voltage applied to the motors terminals. By changing or modulating the timing of these pulses the speed of the motor can be controlled, ie, the longer the pulse is “ON”, the faster the motor will rotate and likewise, the shorter the pulse is “ON” the slower the motor will rotate.

In other words, the wider the pulse width, the more average voltage applied to the motor terminals, the stronger the magnetic flux inside the armature windings and the faster the motor will rotate and this is shown below.

Pulse Width Modulated Waveform

pulse width modulation waveform

The use of pulse width modulation to control a small motor has the advantage in that the power loss in the switching transistor is small because the transistor is either fully “ON” or fully “OFF”. As a result the switching transistor has a much reduced power dissipation giving it a linear type of control which results in better speed stability.

Also the amplitude of the motor voltage remains constant so the motor is always at full strength. The result is that the motor can be rotated much more slowly without it stalling. So how can we produce a pulse width modulation signal to control the motor. Easy, use an Astable 555 Oscillator circuit as shown below.

pulse width modulation circuit

This simple circuit based around the familiar NE555 or 7555 timer chip is used to produced the required pulse width modulation signal at a fixed frequency output. The timing capacitor C is charged and discharged by current flowing through the timing networks RA and RB as we looked at in the 555 Timer tutorial.

The output signal at pin 3 of the 555 is equal to the supply voltage switching the transistors fully “ON”. The time taken for C to charge or discharge depends upon the values of RA, RB.

The capacitor charges up through the network RA but is diverted around the resistive network RB and through diode D1. As soon as the capacitor is charged, it is immediately discharged through diode D2 and network RB into pin 7. During the discharging process the output at pin 3 is at 0 V and the transistor is switched “OFF”.

Then the time taken for capacitor, C to go through one complete charge-discharge cycle depends on the values of RA, RB and C with the time T for one complete cycle being given as:

The time, TH, for which the output is “ON” is: TH = 0.693(RA).C

The time, TL, for which the output is “OFF” is: TL = 0.693(RB).C

Total “ON”-“OFF” cycle time given as:  T = TH + TL  with the output frequency being ƒ = 1/T

With the component values shown, the duty cycle of the waveform can be adjusted from about 8.3% (0.5V) to about 91.7% (5.5V) using a 6.0V power supply. The Astable frequency is constant at about 256 Hz and the motor is switched “ON” and “OFF” at this rate.

Resistor R1 plus the “top” part of the potentiometer, VR1 represent the resistive network of RA. While the “bottom” part of the potentiometer plus R2 represent the resistive network of RB above.

These values can be changed to suite different applications and DC motors but providing that the 555 Astable circuit runs fast enough at a few hundred Hertz minimum, there should be no jerkiness in the rotation of the motor.

Diode D3 is our old favourite the flywheel diode used to protect the electronic circuit from the inductive loading of the motor. Also if the motor load is high put a heatsink on the switching transistor or MOSFET.

Pulse width modulation is a great method of controlling the amount of power delivered to a load without dissipating any wasted power. The above circuit can also be used to control the speed of a fan or to dim the brightness of DC lamps or LED’s. If you need to control it, then use Pulse Width Modulation to do it.

62 Comments

Join the conversation!

Error! Please fill all fields.

What's the Answer *

  • S
    Sandeep Dubey

    PLEASE give some post on Protection circuitry in Electrical system

  • P
    Pavani

    Can you explain pulse modulated devices

  • p
    paul

    Clear and concise.
    How do you determine the size of components to run 12 v dc motors that draw 10-20 amps?

    • Wayne Storr

      You start with the switching transistor capable of switching 12V at 20 amps, such as the 2N3772 or the TIP35A, etc, and work backwards.

  • L
    Lachlan

    What are Ra and Rb

  • R
    Ranido edelito

    How to asemble pwm

  • Soff

    Hi,
    Are ‘this pwm speed controller’ and buck converter the same? Putting aside the regulating system, they both work by varying the pulse width to get the desired Vout, right? Also, they both have Vout < Vin. Am I correct in this?

  • P
    Paul Schiloski

    I have a question concerning the following calculations:

    The time, TH, for which the output is “ON” is: TH = 0.693(RA).C
    The time, TL, for which the output is “OFF” is: TL = 0.693(RB).C

    Where does the constant 0.693 come from?

    • Wayne Storr

      Capacitor, C charges and discharges between 2/3Vcc and 1/3Vcc.

      RC time constant is given as: ln(1/(1-(1/3Vcc / 2/3Vcc))) = ln(1/(1-0.5)) = ln(2) = 0.693. (0.7 is near enough)

      t(off) = 0.693(Rb)C and t(on) = 0.693(Ra + Rb)C so time period T = t(off) + t(on) = 0.693(Ra + 2Rb)C

      therefore, frequency f = 1/T = 1.44/((Ra + 2Rb)xC)

      More here: 555 Oscillator Tutorial

  • B
    Bade Mahesh

    I want mini project related to electronics ……………

  • a
    asfandyar tahir

    i want the circuit diagram of PWM in proteus

  • e
    eduardo

    what if i use a 12v Dc power supply, and a motor of 6A of current ? do i’ll have a overheat?

    • R
      Ryan

      use CMOS transistor instead of standard NPN.
      -> pull one out from ATX computer power supply, most of them will have 15 Amp @ 60V and they are perfect switches, better then NPN used in the above drawing.
      Also consider adding low pass filter (noise filter) for the IC to avoid any back-feeding from the electric motor and possibly damaging the 555 IC

    • Doktor J

      eduardo, if you use the 2N3055 shown in the diagram, in the TO-3 package (the big metal can one with the diamond-shaped metal pad) it should be fine with 6A. I might recommend adding a heatsink though (cheap TO-3 heatsinks can be had on eBay, and since the transistor is rated at 15A you shouldn’t need a big one).

Looking for the latest from TI?