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In DC or BLDC motor control.. The standard is to PWM the H-Bridge switches to control the motor current .. Why not to PWM the source voltage (V-Motor) and just switch the H-Bridge switches (in lower frequency domain) as needed .. I know .. it may be not economical due to an extra switching element .. I just wanna know the effects of this way regardless the economics ..

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The question mentions DC & BLDC as if a single answer is expected for both. However there are two completely different situations.

In the case of a brushed DC motor, you could control the speed by PWM of the source voltage and use the H-bridge just for direction control. That would work, but there would be no electronic protection against excess current due to accelerating too quickly or overloading the motor. You could ramp the voltage up to control accelerating current and implement some kind of electronic shut-off for overcorrect, but you would end up wanting an active current limit. You could implement that with the source PWM, but that would amount to an armature voltage control with an overriding current loop. The end result would be a control system with more power components, a control scheme as complicated or more complicated than usual and poor performance.

The case of the brushless motor, you would be feeding PWM DC to a six-step inverter and starting with an open-loop, three-phase permanent-magnet synchronous motor control. You would have the same over-current protection problems as described tor the brushed motor and more difficulty in designing a remedy for the problem. In addition, the performance would poor. Here again, you have added complexity and subtracted performance.

Historical Note

The following block diagram shows how a PWM source voltage brushless motor controller (mostly for induction motors) was implemented about 50 years ago. About 45 years ago, that design was obsolete.

enter image description here

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Changing the supply voltage is the same as feeding in a lower voltage, say half the voltage means half the current, means 1/4 the energy turning the motor,

Vs halving the PWM value for the motor at the same voltage, gives half the energy turning the motor, You also have things like beat frequencies, where the H bridge switching may not overlap cleanly with the supply, making things even uglier.

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  • \$\begingroup\$ "half the voltage means half the current" That is not true. A motor is not a resistor. It will draw the current required to do what is asked of it. Applying half voltage means asking for half speed. If the load needs the same torque for half speed, the motor will draw the same current as it does for full speed. \$\endgroup\$
    – user80875
    Apr 16, 2020 at 13:49
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Why not to PWM the source voltage (V-Motor) and just switch the H-Bridge switches (in lower frequency domain) as needed .. I know .. it may be not economical due to an extra switching element

You are right. The main reason for not chopping the power supply separately is that it needs an extra switching element, which is not required if the H-bridge or brushless commutation controller has a suitable PWM input.

However sometimes it doesn't, and then chopping the power supply is another option. This is often done on brushless fans, which have a very simple commutation system that cannot easily have PWM applied to it. The PWM switch may be applied externally to a 2 wire fan, or inside the fan controller (which permits continuous tach output). This pdf from Analog devices shows how it can be done:-

enter image description here

Low frequency PWM basically just turns the whole fan on and off too fast for it to speed up or slow down much. This may only work at relatively high PWM ratios, is non-linear, and has poor efficiency due to the high surge current that occurs each time the fan is turned on.

enter image description here

With high frequency PWM the effect is equivalent to the same average DC voltage, and speed is controlled linearly down to perhaps 10% of full speed. The inductance of the motor coils smooths out the current so efficiency is not impacted.

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