TIDUF60 December 2023
Voltage feedback is needed in the FAST estimator to allow the best performance at the widest speed range, the phase voltages are measured directly from the motor phases instead of a software estimate. The eSMO relies on software estimation values to represent the voltage phases without using the motor phase voltage-sensing circuit. This software value (USER_ADC_FULL_SCALE_VOLTAGE_V) depends on the circuit that senses the voltage feedback from the motor phases. Figure 2-36 shows how the motor voltage is filtered and scaled for the ADC input range using a voltage feedback circuit based on resistor dividers. The similar circuit is used to measure all three of both compressor and fan motors, and dc bus.
The maximum phase voltage feedback measurable by the microcontroller in this reference design can be calculated as given in Equation 55, considering the maximum voltage for the ADC input is 3.3 V.
where
With that voltage feedback circuit, the following setting is done in user_mtr1.h:
//! \brief Defines the maximum voltage at the AD converter
#define USER_M1_ADC_FULL_SCALE_VOLTAGE_V (404.1292683f)
The voltage filter pole is needed by the FAST estimator to allow an accurate detection of the voltage feedback. Make the filter low enough to filter out the PWM signals, and at the same time allow a high-speed voltage feedback signal to pass through the filter. As a general guideline, a cutoff frequency of a few hundred Hz is enough to filter out a PWM frequency of 5 to 20 kHz. Change the hardware filter only when ultra-high-speed motors are run, which generate phase-voltage frequencies in the order of a few kHz.
In this reference design the filter pole setting can be calculated with Equation 57:
The following code example shows how this is defined in user_mtr1.h:
//! \brief Defines the analog voltage filter pole location, Hz
#define USER_M1_VOLTAGE_FILTER_POLE_Hz (416.3602877f)