Case and Rotor questions for Simscape PMSM motor models.

Both images use the same PMSM motor block to model the system and add a simscape sensor block to measure the current and rotor velocity. For the output of the PMSM motor block, I was wondering if there is a difference between grounding it to R(Rotor) and grounding it to C(Case) as described in the help. I'm experimenting with it, but I can't figure it out. Is there any documentation or explanation as to why grounding to C reverses the output of the sensor, or why a 3-phase voltage input would make a difference?

Answer 1

[Hitesh]

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:

https://www.mathworks.com/help/simscape/ref/mechanicalrotationalreference.html

https://www.mathworks.com/help/sps/ref/pmsm.html