Simplified Synchronous Machine with zero mechanical input

Hello,

I am trying to simulate a simplified synchronous machine linked to the ground as shown in the picture below:

I gave Pm=0 and E=0 as an input. Logically, I expected to obtain w=0 because if Pm=0 that mean that the rotor is not turning. But in fact I obtain w=1 pu as shown in the figure below:

Can you please provide me with an explanation about this phenomenon ?

Answer 1

[AR]

Hi @Pamela,

I was able to see a similar output when I reproduced the model by setting both Pm and E to zero.This result can be explained by how synchronous machines are modelled in simulation environments.

When Pm (mechanical power) and E (internal voltage) are set to zero:

• The machine does not receive any mechanical input or excitation.
• However, the rotor speed (wm) remains at 1 pu because, in per-unit systems, this value represents the synchronous speed of the system rather than the actual speed of the rotor due to input power.
• Unless there is a disturbance or a change in reference, the model assumes the machine is synchronized with the grid and maintains the base speed.

As a result, even with zero input, the simulation keeps wm at 1 pu, indicating that the rotor continues to spin at synchronous speed, not that it is physically being driven by mechanical power.

This behavior is typical in simplified models where:

• The synchronous speed is used as a reference.
• It is not directly linked to the mechanical power input when no disturbances are present.

I hope this clarifies the phenomenon you observed.