DRV10983-Q1 Datasheet, PDF (19/72 Pages) Texas Instruments – Automotive, Three-Phase, Sensorless BLDC Motor Driver

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DRV10983-Q1
SLVSD14 – JUNE 2017
8.4.1.2 Motor Velocity Constant
The motor velocity constant, Kt[6:0] describes the phase-to-phase BEMF voltage of the motor as a function of the
motor velocity.
Figure 12 shows the measurement technique for this constant as used in the DRV10983-Q1 device.
PhU
Rm
Ev
Lm
Lm
Eu
Ew
Lm
E_p
T_e
Kt
=
E_p
f_e
E_p
=₁
t_e
Rm
Rm
PhV
PhW
Figure 12. Kt_PH Definition
With the motor coasting, use an oscilloscope to capture the differential voltage waveform between any two
phases. Derive the motor velocity constant used by the DRV10983-Q1 device as shown in Equation 1.
Kt_PH = E_p × t_e
where
• E_p is ½ the peak-to-peak amplitude of the measured voltage
• t_e is the electrical period
(1)
The measured motor velocity constant (Kt_PH) must be converted to a 7-bit digital register value Kt[6:0]
(combination of KtShift[2:0] and KtValue[3:0]) to program the motor velocity constant value. The digital register
value can be determined as follows:
1. Convert the measured Kt_PH to a weighted digital value: Kt_{ph_dig} = 1090 × Kt_PH
2. Encode the digital value such that Kt_{ph_dig} = KtValue[3:0] << KtShift[2:0].
The maximum Kt_PH that can be programmed is 1760 mV/Hz. This represents a digital value of 1920 and an
encoded Kt[6:0] value of 0x7Fh. The minimum Kt_PH that can be programmed is 0.92 mV/Hz, which represents a
digital value of 1 and an encoded Kt[6:0] value of 0x01h.
For convenience, the encoded value of Kt[6:0] may also be obtained from Table 4.
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