Clarification on BLDC Motor Model Parameters in PSIM
Dear Altair Support Team,
I am a student currently working with the BLDC motor model in PSIM, and I have encountered a few aspects that I find challenging to understand (Images below) :
- Alpha (α) Parameter: I initially thought that the alpha parameter represented the phase commutation delay which impacts directly the touque smoothness. However, upon adjusting its values, I observed changes in the slope of the phase voltage waveforms rather than in the torque smoothness (for alpha beetwen 0 and 30. Could you please clarify the exact role and physical significance of the alpha parameter in the BLDC motor model?
- Vrms/Vpk ratio and the angle alpha Datasheet Table: I am curious about how this table was constructed. Specifically, were these parameters obtained through experimental measurements, simulations, or calculations?
- Relationship Between Shaft Time Constant, Friction Coefficient, and Moment of Inertia: I am trying to grasp the physical interpretation of the relationship among these three parameters.
Thank you very much for your assistance.
Best regards,
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Hi Toto,
A.
The angle alpha determines the shape of the phase back EMF, as described in the online help and illustrated below.
For instance, when alpha = 0, the back EMF takes the form of a square wave. This shape is defined by the motor's design. It's important to note that alpha is not directly specified in the parameter dialog. Instead, it is implicitly defined by the two parameters: Vpk/krpm and Vrms/krpm.
B.
The relationship between the Vrms/krpm and Vpk/krpm ratios and the angle alpha can be determined through simulation. To do this, a circuit can be built using three trapezoidal line-to-phase signals that represent the back EMFs of the three phases. By measuring the resulting line-to-line voltage, both Vpk and Vrms values can be obtained. By varying the angle alpha, different Vpk and Vrms values can be recorded and compiled into a table.
Attached are two test circuits that measure Vpk and Vrms at 1000 rpm for alpha = 30° and alpha = 0°. If you compute the corresponding ratios and compare them with the table, you will find that they match: for a ratio of 0.7448, alpha = 30°, and for a ratio of 0.8154, alpha = 0°.
C.
Shaft Time Constant (τm) describes how fast the motor's shaft speed responds to torque changes, and is the time it takes for the motor to reach 63% of the final speed after the torque change
Friction Coefficient (B) is the torque that resisting the shaft's rotation and is proportional to the shaft's angular velocity.
Moment of Inertia (J) describes the rotational inertia of the motor's shaft and the connected load. It quantifies how much torque is needed to change the angular velocity of the rotating system.
The relation of these three physical quantities is shown in the following equation:
τm = J / B
Attached Files:
Best regards,
Vasileios Vlastos
Power Electronics Intern