i need to know the thw controller switching frequency of my motor controller

Hi Banu, 

Thank you for posting in the BLDC motor drives forum!

The PWM frequency should be chosen after considering all the tradeoffs and choosing a frequency that takes all these tradeoffs into account.

The first tradeoff to consider is current ripple. When you apply a PWM signal with a particular duty cycle, the motor driver will provide a voltage to the coils of the motor for the duration of the on time of the PWM, then that voltage is removed for the remaining time until the PWM signal goes high again and the process repeats. Because the coils of the motor are inductive, and an inductor resists instantaneous change in current, then the current through the motor will begin to increase when the voltage is applied across the coils, and when the PWM signal goes low then the current through the motor will begin to gradually decrease over time based on the inductance and resistance of the motor. If the PWM frequency is high, this doesn't allow as much time for the current in the inductor to decay, so that results in a more constant current (smaller current ripple). In order to help calculate the PWM frequency you need for your design you will want to know what is the minimum allowable amount of current ripple you can have in your system. If you expect the current to operate at around 4A peak (Ipeak), and if the max amount of current ripple you are willing to tolerate is 100mA (Iripple), then you need to choose a high enough PWM frequency so that the current ripple doesn't exceed 100mA. Using the line-to-line inductance (L) and resistance (R) of the motor you can calculate this using the equation below:

Although a higher PWM frequency results in smaller current ripple, there is another tradeoff to consider because the higher the frequency of the PWM the more switching losses occur since the MOSFETs are being switched on and off more frequently. This will result in higher power loss as a result. 

Another thing to consider is that the audible range is from around 20Hz to around 20kHz. So if your PWM frequency falls within that range there could be some noise tradeoffs to consider.

Hope that helps,

Anthony 

Hi Banu,

If you are wanting to calculate the PWM frequency you should use for your motor control, then the number of pole pairs in general doesn't effect the calculation of the PWM frequency. The PWM frequency must be a much higher frequency than the commutation frequency (10x or greater), but in general the PWM frequency (based on the considerations and calculations in my previous post) will result in a high enough frequency such that the number of poles of the motor doesn't result in a large enough change in the commutation frequency (compared to the PWM frequency) to be an issue. 

If you are wanting to determine the commutation frequency of your controller, this calculation is based on the desired RPM of the motor, and the number of poles of the motor. The formula is:

commutation frequency = ((RPM speed) x (number of poles of the motor)) / 120. 

The commutation frequency is the number of electrical commutation cycles needed per second to achieve a desired RPM of the motor. 

Let me know if you have any further questions,

Anthony