Unrealistic terminal voltage on Synchronous Machine Round Rotor (Simscape) when driven with a real-world DC Field Voltage

Hi everyone,

I am observing unexpected scaling behavior when modeling a 5 kW, 400 V, 50 Hz synchronous machine using the Synchronous Machine Round Rotor block from Simscape Electrical (Fundamental Parameters mode).

When attempting to excite the rotor using a real-world DC voltage source connected directly to the field terminals (fd+ / fd-), the internal parameter scaling of the Park (d/q) transformation creates a massive discrepancy between the physical terminal values and the model's internal reciprocal system.

Here is the setup and the observed problem:

• Machine Target Specs: 5 kW, U_rated = 400, f = 50 Hz, p = 1, driven mechanically at nominal speed 3000 RPM
• Rotor Setup: I_fd = 10A, no-load field current required for rated terminal voltage, R_fd = 4.8 Ohm
• Expected Behavior: Applying a 48 V DC source to fd+/fd- should result in an I_fd of 10 A and induce the nominal line-to-neutral peak voltage of 326 V, approx. 230 V RMS at the open-circuit stator terminals.
• Actual Behavior:

1. If a 48 V DC Voltage Source is connected to fd+/fd-, the resulting field current drops to a tiny fraction of an Ampere (approx 0.001 A), and no stator voltage is induced.
2. If a Controlled Current Source set to 10 A is used instead, the stator side behaves 100% correctly (approx 326 V_peak phase voltage). However, the voltage measured across the fd+/fd- terminals skyrockets to over 460 kV.

Checking the Base Values log of the block reveals the underlying transformation split:

• Field circuit base values (Non-reciprocal system): V_base = 48 V, I_base = 10A (Physical values).
• Rotor circuit base values (Reciprocal system): V_base = 7804.88 V, I_base = 0.800781A (Stator-referred dq-system).

Because the physical block pins (fd+/fd-) interface directly with the stator-referred reciprocal Park transformation, the effective input impedance seen at the terminals scales with the square of the turns ratio (ü^2). As a result:

• An applied 48V source sees an effective internal resistance of R_fd = approx 46.8 kOhm, collapsing the current.
• Forcing 10 A into the terminals forces U_fd = 10A * 46.8kOhm to approx 468 kV

Is there a recommended way within Simscape Electrical to parameterize the block so that physical field voltages 48V DC can be applied directly to fd+/fd- without having to manually pre-scale R_fd by the reciprocal base ratio (ü^2)?

Is the documentation stating "Field circuit voltage required to produce rated terminal voltage" intended only for signal-based Simulink library blocks rather than Simscape domain physical connections?

Any insight into the preferred modeling workflow for real-world exciter circuits driving this Simscape block would be greatly appreciated!

Answer 1

[nws]

What you are seeing is expected for this block, because the fd+/fd- ports in the Synchronous Machine Round Rotor block are handled in the machine’s stator-referred reciprocal d/q formulation rather than as direct physical rotor winding terminals in native 48 V, 10 A units. As a result, a real 48 V DC source cannot usually be connected directly without referring/scaling the exciter to the block’s internal base system; for reference, please see the MathWorks documentation for the block here: https://www.mathworks.com/help/sps/ref/synchronousmachineroundrotor.html.