ADC(Vfb-U, Vfb-V, Vfb-W) in HVACI + PFC v2.1 with TMS320f28335

Andrey.B.

Other Parts Discussed in Thread: CONTROLSUITE

Hello everyone

I am working on Motor control kit (HVACI +PFC v2.1). In all of the example projects there is no usage of voltage feedback(Vfb-U). But I want to use them in my project (because it seems to me more reliable seeing real process especally in case of probable faults)

Declaration of DC bus voltage in one of example projects:

volt1.DcBusVolt = ((AdcMirror.ADCRESULT7)*0.00024414)*0.909; // DC Bus voltage meas.

// (ADCmeas(q12)/2^12)*(3.0V/3.3V) (HVACI_Sensorless.c)

As I understand I must do the same in phase voltage measuring and multiply it on Base peak phase voltage to get real values.

#define BASE_VOLTAGE 236.174 // Base peak phase voltage (volt) (HVACI_Sensorless-settings.h)

But I don`t understand why do we use this coefficient 0.909 (3.0V/3.3V) ? In discription of f28335 ADC there is no reference voltage of 3.3 V(only 3 V) and no other visible voltage sources between voltage devider in phase and input of ADC in controller(sorry I don`t know why but can`t include any picture ).

Also I found in ControlSuite file names HV-DMC-Calculations.xls.(in documentation HVACI +PFC v1.7 ) There I found another Vfb Max =409.9 which calculates as VfsvADC / Gfb(coefficient of voltage devider)

R19+R20	1.12E+06	ohms	Gfb	0.008
R6	9.09E+03	ohms

VfsvADC	3.3	volts	Gadc	1241
ADC res	12	bits

Left Shift	3	bits	Gshft	8

Vfb-U Max	409.90	volts	q	6

Please help to understand finaly how to calculate correctly real value of phase voltage from ADC and why do we consider 3.3 V here?

Best regards,

Andrey

over 8 years ago

0 Ramesh Ramamoorthy over 8 years ago

TI__Mastermind 27240 points

Andrey,

Base values are defined based on what the CPU (or ADC) reads as max value from the hardware. In this case, the hardware is set up to generate 3.3V at 409V. The ADC in 28035 operates at 3.3V and hence the feedback is set up that way. However, the ADC in 28335 operates at 3.0V and therefore, when the feedback sensing is at 3V, the ADC will read max value. Since a common software code is used for both 28335 and 28035 CPUs, this scaling of (3.0/3.3) is needed for 28335 projects.

If you intend to use 28335 CPU, then scale the feedback max to 3.0V instead of 3.3V and in that case you don't need this scaling.

rgds,

ramesh

0 Andrey.B. over 8 years ago in reply to Ramesh Ramamoorthy

Intellectual 335 points

Ramesh,

Thank you fo reply.

As I understrand, in case of 28335 maximum observing voltage will be:

VfsvADC / Gfb = 3 / 0.008=375 V

and final expression will be:

volt1 = ((AdcMirror.ADCRESULT)*0.00024414)*375 ; // in Volts.

But in this case how "BASE_VOLTAGE 236.174 // Base peak phase voltage (volt)" correspond

with previouse calculations?

It doesn`t suit neither 3 nor 3.3 range, and no other values (exept maximum observing values) can`t be used as base.

Best regards,

Andrey

0 Ramesh Ramamoorthy over 8 years ago in reply to Andrey.B.

TI__Mastermind 27240 points

Because of the scaling, BASE_VOLTAGE is preserved.

rgds,
ramesh

0 Andrey.B. over 8 years ago in reply to Ramesh Ramamoorthy

Intellectual 335 points

But what for is this value (236.174) necessary? BASE_VOLTAGE 236.174 is not maximum observing voltage so it can`t be used for convertion
from relative units to real voltage in Volts.
Only 375 V in case of 3V ADC or 409V in case of 3.3V ADC can be used as BASE values.

Best regards,

Andrey

0 Ramesh Ramamoorthy over 8 years ago in reply to Andrey.B.

TI__Mastermind 27240 points

It represents phase voltage base, which is obtained by 409/sqrt(3). For the project you are referring to, if BASE_VOLTAGE is irrelevant, then ignore it.

rgds,
ramesh

0 Andrey.B. over 8 years ago in reply to Ramesh Ramamoorthy

Intellectual 335 points

Here in HVACI + PFC v2.1 according to it`s schematic Vfb-U,Vfb-V,Vfb-W signals measure phase voltage directly . So as I understand we don`t need to convert 409 to 409/sqrt(3) or 375 to 375/sqrt(3) (in case of 3V ADC) . We would do this conversion only in case of linear voltage measuring.
So final expression for phase voltage will be:

volt1 = ((AdcMirror.ADCRESULT)*0.00024414)*375 ;// in Volt

Is it true?

Best regards,

Andrey

0 Ramesh Ramamoorthy over 8 years ago in reply to Andrey.B.

TI__Mastermind 27240 points

This will be true for dc bus voltage.

0 Andrey.B. over 8 years ago in reply to Ramesh Ramamoorthy

Intellectual 335 points

Of course, I forgot about offset
So,
((AdcMirror.ADCRESULT)*0.00024414 - constant offset)*375
Is it correct for phase voltage now?

0 Prakash Kumar Thulasi Kumar over 8 years ago in reply to Andrey.B.

Expert 2015 points

this is how I calculate voltage values...
Voltage = (ADCRESULT - OFFSET) * Gain
OFFSET = ADCRESULT when Voltage value is 0V.
Gain = (Maximum Voltage Value / ADCRESULT when Voltage value is maximum)
Hope this helps!

0 Andrey.B. over 8 years ago in reply to Prakash Kumar Thulasi Kumar

Intellectual 335 points

Thank you all for help.

Best regards,
Andrey

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ADC(Vfb-U, Vfb-V, Vfb-W) in HVACI + PFC v2.1 with TMS320f28335