Hi Junghyo,
In a Simscape mechanical rotational network, grounding means you define port’s angular velocity to be zero. So:
- Grounding R means the rotor is fixed. The motor’s case (C) can now rotate.
- Grounding C means the motor housing is fixed — which is what happens in most real-world systems. The rotor (R) rotates with respect to the stator.
Grounding the C port (Case) fixes the motor housing, allowing the rotor to rotate normally, resulting in a standard rotor speed reading. In contrast, grounding the R port (Rotor) holds the rotor stationary and allows the housing to rotate, which reverses the direction of measured rotor speed.
That’s why when you ground the C port, the sensor connected between R and C measures rotor velocity normally. But if you ground R, then the case is rotating in the opposite direction, which gives a reversed velocity reading.
The rotational sensor used here measures angular velocity from R to C. So:
- If R is rotating and C is fixed, the sensor shows positive speed.
- If C is rotating and R is fixed, the sensor shows negative speed (reverse direction), assuming same rotation.
The 3-phase input itself doesn’t change sign or behavior because it’s applied to the electrical terminals (A, B, C) of the motor. However:
- The resulting electromagnetic torque and motion are interpreted relative to the grounded mechanical reference (R or C).
- So, the same electrical input can produce opposite mechanical motion depending on whether R or C is grounded.
For more information "Mechanical Rotational Reference" and "PMSM", kindly refer to the following MATLAB documentation:
