ML4425 Inverter driver IC for BLDC control

[treez]

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
http://pdf.datasheetcatalog.com/datasheet/fairchild/ML4425.pdf

 

[cl10greg]

Here are my thoughts for reading the datasheet and experience.

1.) My interpretation is the VCO/TACH is based on the Back EMF based on page 3 of the data sheet
2.) That would be a correct assumption but doing a verification test is always ideal.
3.) This capacitor is used pretty much as a soft start and would mitigate your inrush current so it would be ideal to use this for circuit and pulse protection.
4.) I am not sure what you mean by chuntering
5.) There is most likely a hardware restriction on the acceleration as you pointed out. The feedback from the motor plus the switching capabilities and ramping profile given to the transistors.
6.) Again I am not sure what you are really asking on this one. Reading the datasheet you have 3 choices for the speed. Speed comp will set the compensation of the control loop. Speed set is your voltage to set the speed. Speed feedback is the EMF sampling circuit
7.) My guess is that it would still spin but just at a lower speed assuming you have ample voltage to overcome the initial requirements of the motor to start. But depending on the motor and the internal structure of the driver, a lot of different things could happen.

 

[treez]

on page 3 of the datasheet it says of the "VCO/TACH" pin.....

The
output frequency is proportional to
the motor speed when the back-
EMF sensing loop is locked onto
the rotor position

...does this mean that there is no output signal on the VCO/TACH pin until the motor is in "LOCK"...ie at nominal speed and commutating at the right instants as called for by the Back EMF signals.

Also, does the SPEED feedback loop just adjust the commutation duty cycles....?....after all, the actual ultimate motor speed is simply governed by the commutation frequency of the bridge...do you agree?

 

[treez]

Does the Speed loop of the ML4425 IC regulate the PWM duty cycle of the inverter and the switching frequency of the bridge, or just the PWM duty cycle?

ML4425 datasheet:
http://pdf.datasheetcatalog.com/datasheet/fairchild/ML4425.pdf