[SOLVED] BLDC pwm speed control

[ashugtiwari]

Hi All,
Im using small ebmpast 24V small BLDC motor to write test code for motor speed control.
I found that controlling the commutation speed affects the motor speed as well (well thats obvious). So, my question is, when i change the PWM duty cycle, the speed does not change at all but speed is solely controlled by commutation speed.
is PWM only used to reduce the I2R losses in motor coil or it actually controls the speed? In my case, PWM when decreased slowly from 100%, the speed does not change (decrease) but after certain minimum duty cycle (below some lower threshold), motor stops.
I read so many articles online for BLDC motor control buti could not understand why this happens. (im not so smart :roll

Thanks
Ashutosh

 

[ArNvC]

So are you monitoring the speed using a tacho? Can you sense the BEMF?

 

[ashugtiwari]

output of 3 mosfet half bridges goes to three phases of BLDC, and im taking one phase line to monitor BEMF.
but after some understanding and experimenting, reading, i figured this that, the speed variation is proportional to load, so if PWM is on, it actually controls the torque required to make motor rotate. And as u decrease duty cycle the average motor current to motor decreases which in turn decreases torque. The speed varies if motor is loaded. if loaded heavily, motor speed variation is apparent and if loaded lightly, the variation in speed is as small as 1rpm.
And this is true for BLDC motor, not sure of other types of motors. In a way, motor power input is equal to torque multiplied by speed. Changing the load changes the torque and speed such that power is constant.
A motor with no load, changing the torque, changes speed slightly which makes motor running at low power.

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output of 3 mosfet half bridges goes to three phases of BLDC, and im taking one phase line to monitor BEMF.
but after some understanding and experimenting, reading, i figured this that, the speed variation is proportional to load, so if PWM is on, it actually controls the torque required to make motor rotate. And as u decrease duty cycle the average motor current to motor decreases which in turn decreases torque. The speed varies if motor is loaded. if loaded heavily, motor speed variation is apparent and if loaded lightly, the variation in speed is as small as 1rpm.
And this is true for BLDC motor, not sure of other types of motors. In a way, motor power input is equal to torque multiplied by speed. Changing the load changes the torque and speed such that power is constant.
A motor with no load, changing the torque, changes speed slightly which makes motor running at low power.