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# PWM to control speed of dc motor

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#### muheeb16

##### Member level 3
The input to the switching transistor is pulse type waveform and the motor is attached at the collector.Now either the current flows through the motor (fully on) or it doesn't (fully off).Why doesn't this create vibrations?And why is there a concept of average voltage involved (when the instantaneous voltage does fall to zero)?

The input to the switching transistor is pulse type waveform and the motor is attached at the collector.Now either the current flows through the motor (fully on) or it doesn't (fully off).Why doesn't this create vibrations?And why is there a concept of average voltage involved (when the instantaneous voltage does fall to zero)?

Wrong circuit analysis. The motor voltage doesn't fall to zero. There is firstly motor EMF (a constant voltage according to the present speed) and secondly flyback voltage ("back emf") of the armature inductance during switch-off, opposite to the normal motor voltage.

Due to the latter, you need a flyback diode for a single PWM switch, otherwise the switch will break down and be possibly damaged.

Motor PWM is usually operated at a frequency high enough to avoid torque vibrations. Preferably the motor inductance will already smooth the current waveform.

The rotating part of a motor is heavy which causes momentum. The momentum smoothes the high frequency PWM pulses causing the vibration to be very small.

Make a motor that is a light as a feather and feed it low frequency pulses. It will vibrate like crazy!

The momentum of a DC motor causes it to be a generator when the PWM voltage is turned off and it will have the same polarity and voltage as the on-voltage of the PWM.
I just tried it. I measured the voltage across a DC brushed motor then disconnected its battery and its voltage stayed at the same polarity and slowly dropped as the motor slowly slowed down. The voltage polarity did not reverse and the voltage did not suddenly drop to zero.

Hi,

just my comment:

Preferably the motor inductance will already smooth the current waveform.
This is right. And torque is more related to current than to voltage.
* The inductance will smoot the current, and
* the mechanical inerita of the motor will additionally smooth the vibrations.

Speed is more related to average voltage.

****
btw:
*Too high frequency may wear out the carbon brushes of the motor.
* But low frequency may generate audible noise.

Klaus

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