PWM Control.. Why ?

Discussion in 'General Electronics Chat' started by Mazaag, Feb 6, 2006.

  1. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    Hey Guys,

    Referring to this link , , they are using Pulse Width Modulation to control the speed of a motor. The potentiometer controls the reference voltage, and hence the duty cycle of the pulses.

    My question is, why is such a circuit required? Why not simply connect the motor to a Voltage Divider with a pot ? Wouldn't varying the voltage across the motor vary its speed? Or is there something i'm missing ?

    Thanks Guys
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    A simple divider with a pot isn´t used to chnge the speed of a motor, because it isn´t power efficient - the voltage (and thus power) would have to dissipate on the pot. Maybe in the year 1850 they used a large rheostat (adjustable resistor), but today we use PWM, because it doesn´t use any excessive power to control the motor.
    It uses a switch, which is either on or off, but never in linear stage, where would be high power dissipation on the switch.
    Hope this is enough for you, but if you need more, ask.
  3. n9352527

    AAC Fanatic!

    Oct 14, 2005
    ... plus better starting characteristic and low speed operation in present of friction.
  4. hgmjr


    Jan 28, 2005
    Another advantage of using a PWM is that the torque of the motor is preserved over a wider range of operating speeds.

    Placing a variable resistance in series with a motor to control its speed leads to difficulty when the motor is required to deliver torque. As the load increases, greater current is drawn by the motor. As the current needed by the motor increases the voltage drop across the series resistor increases resulting in less voltage across the motor. The end result is that the motor tends to stall very easily under load.

  5. chesart1

    Senior Member

    Jan 23, 2006
  6. toolnuts

    New Member

    Jan 29, 2006
    The prime reason, as someone said , is efficiency. That mainly comes about because the losses in a transformer are directly related to the frequency. A motor is a rotating transformer of sorts. Your talking about the worst case situation , a DC voltage (the lowest efficency).

    The higher the switching frequency the higher the efficency. Also you want it high enough so you don't hear it.

    When you see switching power supplies (or inverters - welders etc..) they are so much smaller and lighter than their linear counterparts because they use the highest switching frequency they can to reduce the transformer iron, and all the related losses (eddy current, etc..)