Hello,

I am going to implement on mcu following pwm for three phase voltage source inverter controlling the induction motor.

The pwm will have two operation modes:

01) asynchronous mode where a/d conversions of phase currents are not synchronized with the time base of the pwm. In this mode the carrier frequency is a random variable with uniform distribution in given interval. The reason why the carrier frequency is random is reducing the acoustic noise of the motor.

02) synchronous mode where a/d conversions of phase currents are synchronized with the time base of the pwm. In this mode the carrier frequency is fixed.

In both of the above mentioned modes of operation the period with which the a/d conversions are invoked is fixed.

I am going to implement this in following manner. I will have one timer operating in free running edge aligned mode. Overflow of this timer will start a/d conversions by start of conversion signal. Then I will have another timer operating in center aligned mode which will create the pwm time base. In first operating mode the adc timer and pwm timer operating independently of each other. For transition from the first operating mode into the second operating mode I need some synchronization signal comming from the adc timer into the pwm timer which will ensure that the new up counting of the pwm counter occurs at the same time as the a/d conversion is started. My idea was to use the start of conversion signal comming from the adc timer and create logic and operation with a flag containing synchronous operating mode request. Result of this logic and will be checked by the pwm timer at the beginning of each carrier period. In case the result is logic 0 the pwm timer up counting can occur immediately otherwise the pwm timer up counting will wait for start of a/d conversion.

I am not sure whether this approach to the transition between both operation modes is correct. Does anybody see any potential problem in this approach? Thanks for any ideas.

#pwm, #motor control

Hello Steven,

Are you having a feed back controlled 3 phase AC generation? If so are you comparing the zero-crossings of the phases as your feed back loop?

If so you should trigger your ADC conversions from zero-crossings of one of the phases. In order to keep the whole system you can synchronise everything in single clock source. The base period of this clock you can vary within +/-1% uniformly / or randomly so that noise can be reduced.

Regarding your question you can have synchronised mode of A/D conversion which is better suitable for your application. In asynchronous mode if there is a drift then it can lead to instability.

Regards,

BV Ramesh.

Hello BV Ramesh,

thank you for your reaction. My feedback signals consist of two stator phase currents, mechanical speed and dc bus voltage. I need such a hardware which ensure to me switching between both the asynchronous and synchronous mode of operation ideally only changing bit value in some configuration register. I am going to use the FPGA for building this hardware. Maybe I am attempting to reinvent the wheel and there is already some hardware (maybe ADC and PWM peripherals of some MCU) which is able to fulfill my requirements.