SPRUHJ1I January 2013 – October 2021 TMS320F2802-Q1 , TMS320F28026-Q1 , TMS320F28026F , TMS320F28027-Q1 , TMS320F28027F , TMS320F28027F-Q1 , TMS320F28052-Q1 , TMS320F28052F , TMS320F28052F-Q1 , TMS320F28052M , TMS320F28052M-Q1 , TMS320F28054-Q1 , TMS320F28054F , TMS320F28054F-Q1 , TMS320F28054M , TMS320F28054M-Q1 , TMS320F2806-Q1 , TMS320F28062-Q1 , TMS320F28062F , TMS320F28062F-Q1 , TMS320F28068F , TMS320F28068M , TMS320F28069-Q1 , TMS320F28069F , TMS320F28069F-Q1 , TMS320F28069M , TMS320F28069M-Q1
Low speed operation assumes that the back EMF voltage coming back from the motor will be very small. Since the FAST algorithm requires the phase voltages as inputs to the algorithm, it is encouraged to have the maximum number of ADC bits per volt. For example, if the maximum input voltage to the system is 400 V, then it is highly recommended to have a voltage feedback that only goes up to that voltage, so when low voltage is present in the motor's back EMF (during low-speed operation) the maximum number of bits are present in the ADC converter output so that the estimation has more information to support the motor model.
In order to illustrate the importance of having the maximum number of bits per volt, consider the TMDSHVMTRPFCKIT Version 1.1 for the voltage feedback as shown in Figure 15-2.
This circuit with jumpers J1, J2, J3 and J4 open (see section M5 of the PCB), would give us a maximum Vfb-Bus of 3.3 V when DC-BUS is at 409.9 V. This means that this setup provides the best ADC resolution for motors with input of 409.9 V. However if the motor's operating voltage is only 100 V, then the ADC resolution to represent the motor's internal voltage will be very small, in fact, around ¼ of what it could be. If the motor's operating voltage is known to be 100 V, then the above circuit should be changed so that a maximum Vfb-Bus of 3.3 V is present in the ADC pin when DC-BUS is 100 V.
Scale voltage feedback circuit to maximize ADC 3.3V input range.
Changing the voltage feedback value affects two parameters configured in user.h. For example in the above schematic, with J1, J2, J3 and J4 open, the following parameters in user.h are defined:
#define USER_ADC_FULL_SCALE_VOLTAGE_V(409.9)
#define USER_VOLTAGE_FILTER_POLE_Hz(375.5)
This can be derived as follows (J1, J2, J3 and J4 open):
With J1, J2, J3 and J4 shorted, the following parameters in user.h are redefined:
#define USER_ADC_FULL_SCALE_VOLTAGE_V(112.2)
#define USER_VOLTAGE_FILTER_POLE_Hz(383.8)
This can be derived as follows: