Confused with the speed control of BLDC motor using PWM

[sarathisme]

Hi folks,
I am currently learning the bldc motor drive. I am not understanding how changing the terminal voltage across the motor affects the motor speed. As per my knowledge the speed depends only on the frequency at which we are energizing the coils. But using pwm we are not changing the rate at which the three coils are energized , but we are only giving a reduced excitation to the coils. Then how the speed gets affected? I am not getting the logic behind it. Please anyone provide me with a correct explanation.

 

[srizbf]

basic theory is a DC motor speed is controlled by varying armature voltage(or field excitation).
In BLDC , the the average voltage (DC) is
(Ton/T)*Vin.
By varying the Ton(PWM) you are varying the speed.

 

[KlausST]

Hi,

Sensorless commutation? Or do you use sensors?

Klaus

 

[sarathisme]

Thankyou. That I understood. But still something is missing. I need a correct explanation - we are energising the subsequent phases at regular periods rt? The rate at which we are energising the coils is not varying , yet the speed gets varied. Expecting a more detailed explanation (not in terms of equations). @srizbf , thanks for the reply.

sensored commutation . using hall signals

 

[KlausST]

Hi,

sensored commutation . using hall signals

Are you familiar with sensored commutation?

In short.
* Voltage is applied to a coil
* Current is rising according coil inductance and applied voltage
* current generates a magnetic field
* magnetic field generates a force to the permanent magnet
* magnetc force generates a torque
* shaft rotates
* ...until the next hall sensor gets activated
* then the same loop starts with another coil (this is called commutation)

****
Now assume the voltage is PWM´d
--> the effective voltage at the coil is less than before
* the time for the current to rise increases (everything becomes slower)
* ..magnetic field is delayed, magnetic force is delayed, torque is less and delayed, rotation is slower
* and the next hall senser gets activated delayed..

--> less voltage --> less current --> less torque --> less speed

Klaus

 

[sarathisme]

@Klaus Thanks for the explanation. Now I got it. :thumbsup:

 

[sarathisme]

Hey .. I have one more querry.
I have designed and setup the whole circuitry for the bldc motor drive. Thecontrl is implemented using FPGA.But it is not working.
I just want to know if the hall signal output from the motor will give a constant signal output if the motor is unexcited,. (By just connecting the Vcc and ground) . .
BLDC model no is 42BL61.004 Rev.A from Delta line.