The MCF8315A uses motor resistance, motor inductance and the motor Back-EMF constant to estimate motor position in closed loop operation. The MCF8315A has capability of automatically measuring motor parameters in offline state, rather than having the user enter the values themselves. Offline measurement of parameters takes place before normal motor operation. Variation in motor parameters can impact the accuracy of motor position estimation and therefore the motor commutation and rotation. The reccomendation is to estimate the motor parameters before motor startup to minimize the impact caused due to possible parameter variations.

The MCF8315A has Motor Parameter Extraction Tool (MPET) which estimates the motor phase resistance, the motor phase inductance and the motor Back-EMF constant. Go to “Control Configuration - Motor parameter extraction tool (MPET)”.

Step 1: Enable Measure motor resistance [MPET_R], Measure motor inductance [MPET_L] and Motor BEMF constant [MPET_Ke].

Step 2: Enable MPET Start command [MPET_CMD]. Make sure the motor is stationary before enabling MPET start command.

Step 3: After successful motor parameter extraction, copy the estimated parameters to the respective shadow registers by enabling MPET write to shadow register flag.

Figure 3-5 shows the phase current waveform during motor parameter measurement. Figure 3-6 shows the IPD current waveform during R, L and Ke measurement. Bottom half of Figure 3-6 shows the IPD current waveform during R and L measurement. R is measured during the rising of phase current and L is measured during the falling of phase current. After R and L measurement, motor spins in open loop. Once the speed reaches MPET open loop speed reference [MPET_OPEN_LOOP_SPEED_REF], motor is coasted. BEMF voltage of all three phases are measured and Ke is calculated.