My question is about the difference between pwm and variable voltage control:
We all know that to control the speed of a DC motor,pulse width modulation is the common technique where a switching circuit connected to the motor,
switch the power from on to off repeatedly respecting a specific duty cycle:
Duty cycle=("on time"/periode)*100
The motor will operate on the resulted average voltage which reflect on its speed.
I was wonderig why not simply connect the motor in series with a rheostat to the power supply and varies the resistance to change the speed(law of addition of voltage)
I read on Wikipedia that this was the traditional way before introducing pwm,but it had some disadvantages related to the power dissipation and others things,anyway Wikipedia did not explain much.
Maybe you can give me a link about the subject
thank you

 

With PWM the electronic switch is either on or off. When on, the voltage across the switch is very low so little power is wasted as heat in the switch. Full supply voltage is applied to the motor, so torque is a maximum. When off, there is very little current through the switch so again little power is wasted as heat in the switch.
With a rheostat a lot of power is wasted heating the resistance of the rheostat. Also, voltage is dropped across the rheostat, so the motor torque is less than maximum. Further, if the motor load changes then the motor current and hence the voltage drop and hence the torque changes.

 

With PWM the electronic switch is either on or off. When on, the voltage across the switch is very low so little power is wasted as heat in the switch. Full supply voltage is applied to the motor, so torque is a maximum. When off, there is very little current through the switch so again little power is wasted as heat in the switch.
With a rheostat a lot of power is wasted heating the resistance of the rheostat. Also, voltage is dropped across the rheostat, so the motor torque is less than maximum. Further, if the motor load changes then the motor current and hence the voltage drop and hence the torque changes.

is there a different between torque and speed