ryan7
Newbie level 2
Hello,
I was going through application note AN843, it uses sine table
This table is for angles between 270 and 90 degrees
;Sine table for the inverter.
;*******************************************************************************
TABLE code 0x0100
;below table is from 270 eg. to 90 deg @ 10 deg. resolution; for 20MHz, PR2 = F9, Timer2 1:1 prescale
sine_table db 0x0,0x3,0x07,0x10,0x1B,0x28,0x38,0x4A,0x5C,0x70,0x83,0x96,0xA7,0xB7,0xC4,0xD0,0xD8,0xDD,0xDE
I know how to obtain the hex values, and from that value how to calculate the corresponding duty cycle
But since AN843 is a V/F control of 3 phase induction motor,the voltage to hertz ratio should be kept constant, therefore the voltage (hence the duty cycle) should vary depending on the values of the speed (the frequency of the sine wave)
I am having trouble understanding how does the offset pointer knows for a specific value of frequency, which value should it point in the sine table
Please if it is not to much to ask refer to page 6 last paragraph of application note AN843
''After every Timer0 overflow interrupt, the value pointed
to by the offset registers on the sine table is read. The
value read from the table is scaled based on the motor
frequency input, by multiplying by the frequency input
value to find the ratio of PWM, with respect to the maximum
DC bus. This value is loaded to the corresponding
PWM duty cycle registers.''
After every Timer0 overflow interrupt - means for each value of frequency
the value pointed to by the offset registers on the sine table is read - the specific value on the table from which the duty ratio corresponding to the frequency will be calculated(as the V/Hz ratio need to be kept constant)
it is this value that i want to know how it is obtained
Any help would be much appreciated
Sorry if my question sounds trivial
B,regards
I was going through application note AN843, it uses sine table
This table is for angles between 270 and 90 degrees
;Sine table for the inverter.
;*******************************************************************************
TABLE code 0x0100
;below table is from 270 eg. to 90 deg @ 10 deg. resolution; for 20MHz, PR2 = F9, Timer2 1:1 prescale
sine_table db 0x0,0x3,0x07,0x10,0x1B,0x28,0x38,0x4A,0x5C,0x70,0x83,0x96,0xA7,0xB7,0xC4,0xD0,0xD8,0xDD,0xDE
I know how to obtain the hex values, and from that value how to calculate the corresponding duty cycle
But since AN843 is a V/F control of 3 phase induction motor,the voltage to hertz ratio should be kept constant, therefore the voltage (hence the duty cycle) should vary depending on the values of the speed (the frequency of the sine wave)
I am having trouble understanding how does the offset pointer knows for a specific value of frequency, which value should it point in the sine table
Please if it is not to much to ask refer to page 6 last paragraph of application note AN843
''After every Timer0 overflow interrupt, the value pointed
to by the offset registers on the sine table is read. The
value read from the table is scaled based on the motor
frequency input, by multiplying by the frequency input
value to find the ratio of PWM, with respect to the maximum
DC bus. This value is loaded to the corresponding
PWM duty cycle registers.''
After every Timer0 overflow interrupt - means for each value of frequency
the value pointed to by the offset registers on the sine table is read - the specific value on the table from which the duty ratio corresponding to the frequency will be calculated(as the V/Hz ratio need to be kept constant)
it is this value that i want to know how it is obtained
Any help would be much appreciated
Sorry if my question sounds trivial
B,regards
