For a wye-connected motor, the motor phase resistance refers to the resistance from the phase output to the center tap, R_{PH_CT} (denoted as R_{PH_CT} in Figure 8-7).

Figure 8-7 Wye-Connected Motor Phase Resistance

For a delta-connected motor, the motor phase resistance refers to the equivalent phase to center tap in the wye configuration. In Figure 8-8, it is denoted as R_Y. R_{PH_CT} = R_Y.

For both the delta-connected motor and the wye-connected motor, the easy way to get the equivalent R_{PH_CT} is to measure the resistance between two phase terminals (R_{PH_PH}), and then divide this value by two, R_{PH_CT} = ½ R_{PH_PH}.

Figure 8-8 Delta-Connected Motor and the Equivalent Wye Connections

The motor phase resistance (R_{PH_CT}) must be converted to a 7-bit digital register value Rm[6:0] to program the motor phase resistance value. The digital register value can be determined as follows:

1. Convert the motor phase resistance (R_{PH_CT}) to a digital value where the LSB is weighted to represent 9.67 mΩ: Rmdig = R_{PH_CT} / 0.00967.
2. Encode the digital value such that Rmdig = RMValue[3:0] << RMShift[2:0].

The maximum resistor value, R_{PH_CT}, that can be programmed for the DRV10982-Q1 device is 18.5 Ω, which represents Rmdig = 1920 and an encoded Rm[6:0] value of 0x7Fh. The minimum resistor the DRV10982-Q1 device supports is 0.029 Ω, R_{PH_CT}, which represents Rmdig = 3.

For convenience, the encoded value for Rm[6:0] can also be obtained from Table 8-3.