Hello,
We are using the ML4425 3 phase inverter controller IC for driving a BLDC for water pumps which form an irrigation system for Tulip Fields.
All we need is for the pump motor to spin at 8000 RPM.
We will bypass the ML4425’s current sense by grounding the “ISENSE” pin…we will then control the current which feeds the BLDC/Inverter by controlling the output current of a buckboost converter that’s upstream of the BLDC/Inverter.
As you can see, we simply increase the current from the Buckboost converter to the inverter until the BLDC spins at 8000RPM.
I have read the ML4425 datasheet, but still have the following questions about the ML4425 operation……
1….Does the “VCO/TACH” pin give pulses which originate from the received Back EMF signals, or from the actual IGBT drive signals?
2….If I just want to drive the pump at 8000 RPM, then surely I can just put whatever voltage corresponds to 8000RPM onto the “SPEEDSET” pin, and then I will be guaranteed that the BLDC will spin at 8000 RPM? (-because the bridge transistors will then be switched at the frequency which gives 8000RPM?)
3……Do we have to use the “CRR” pin to give a timed ramp to the acceleration of the BLDC….or can we just leave this “CRR” pin unconnected and go from the “align” phase to the required speed straight away?
4……If we do leave the “CRR” pin unconnected as in question 3, then will we run the risk of the motor chuntering backwards and forwards due to the bridge transistors not commutating at the right time in the Motors spin position?
5……With the “sensorless” Back EMF sensing method used in the ML4425, is their a limit to the acceleration? since acceleration will mean switching the bridge at times before the Back EMF signal is calling for the switching to occur?
6……Which pin on the ML4425, is the one that gets used to receive the speed demand voltage…is it the “SPEEDSET” pin.
7……If I switch the bridge transistors at 200Hz (this gives 8000rpm in a 3 phase BLDC) , and at maximum duty cycle, then supposing that the DC Link voltage is not enough for the BLDC to spin at 8000 RPM……what then happens?..does the motor stall?...does the motor start wildly oscillating back and forward?..…does the motor just spin at less than 8000RPM, and just “slip” round?
ML4425 datasheet