Why does Simulink BLDC model block reports 1/10 of given Rotor-induced back emf

Using the ready-to-use model "BLDC_back_EMF" and entering a value of 9V for the property Rotor-induced back emf (settings tab Rotor) the scope block is reporting 0.9V peak-peak (1rpm, simulation time 1sec, input speed 2pi); this behaviour is consistent for possible units mV, V and kV) and independent of the number of pole-pairs which is correct.

Anything which i didn't understand?

Answer 1

[Joel Van Sickel]

Hello KNeu,

the back EMF is dependent on motor speed. If you spin your motor faster, you will see the the output voltage increase. What value did you specify you for the back EMF measurement? In addition to that, back EMF is not the same as output voltage. You also have to take into account the current and impedance of the machine.

Regards,

Joel

[KNeu]

Hello Joel,

Thks for answering quickly; yes I am aware that an increased speed results in an increased value of the back EMF; means also that the back EMF is at his maximum value if speed reaches the no-load speed value (or the current drawn due to the applied torque is getting down and results in the no-load current I0);

As given in my question the value of the Rotor-induced back emf property has been set to 9V, the measured back-EMF results in 0.9V.

I have attached a screenshot of the used model for your reference.

[Joel Van Sickel]

can you send me a screenshot of your motor parameters?

[KNeu]

Hello Joel,

Please notice attached screenshot.

[Joel Van Sickel]

Hello KNeu,

ok, you are running at 600 RPM, this means that it is at 10 Hz, or 10*2*pi rad/s. Your input speed is only 2pi rad/s which is 1/10 the rated speed, so it makes sense you are only seeing 1/10 the voltage.

Regards,

Joel

[KNeu]

Hello Joel

clear. Didn't noticed that the value provided in the parameter Rotor speed used for back emf measurement links to the parameter Rotor-induced back emf. My fault.

[KNeu]

to conclude :

back-emf constant ke = 9.6V/600rpm = 9.6V / (10*2*pi rad/s) = 0.1527 Vs

measured back-emf voltage E = omega * ke = 2*pi rad/s * 0.1527Vs = 0.96V