Microcontroller controls speed of pump motor.....do in hardware instead?

[treez]

Hello,
We have a pump in our Tulip field irrigation system.
All we need is to get the pump motor to spin at 8000rpm.
We have a VIN of 18-32V, and this goes to a variable duty cycle buckboost, then to an three phase inverter which feeds the motor (BLDC) . (the inverter switches at 100% duty cycle all the time...ie there's no fast switching current regulation going on inside the inverter)

It would be very simple for us to get the microcontroller to measure the motor speed, and then increment or decrement the buckboost duty cycle until the pump speed is 8000 rpm.....however, this means using software, and due to the lack of software engineers, and their cost, we just want to do this in hardware. Using NAND logic etc, how can we convert the simple microcontroller code into hardware which will do this job?

 

[chuckey]

When you feed a BLDC motor with a frequency, this will determine its speed. The current the motor takes will be dependent on its load. So straight away you know the frequency of your oscillator. A 50 HZ, 2 pole three phase motor will spin at 3000 rpm -5% (this is called slip and is not present in a BLDC motor), a 4 pole motor, 1500 RPM - 5%. So it depend on how many poles your motor has got. If its a 2 P motor, then it needs, 8000/3000 X 50 HZ = 133 HZ.
So if you power the motor from a 133 HZ 3 phase supply it will rotate at 8K RPM. Now you have to regulate the mean voltage or current to stop burning out the motor coils. So your pulse repetition frequency is 133 HZ ~ 7.5 mS, so in this time you would expect to use 1/2 of this as a current pulse per phase, i.e. a 3.75 mS + pulse followed by a 3.75 - pulse.
these pulse widths will have to be modified by the ratio of (motor rating/ supply voltage), i.e. for a 12v motor and a 18V supply, its 12/18 X 3.75 = 2.5 mS.
Check the motor data for number of poles and drive currents/voltages.
Frank

 

[--BawA--]

you can implement this using opamp as comparators , just amplify the error (setpoint - current value) , and generate the PWM accodingly, also you can use some dedicated ic to perform this task like SG3525.
https://www.onsemi.com/pub_link/Collateral/SG3525A-D.PDF