Negative torque pump: Pressure at port A must be greater than or equal to Minimum valid pressure.

Hello everyone,

I am working on a hydraulic circuit that is driven by a motor operated by a Frequency Drive(VFD). (to simpify the model and avoid sharing confidential data, I used Test sequence with a negative torque output to simulate the VFD output) The motor control of the VFD creates some oscillations reaching negative torque values in the generated motor torque resulting in errors of the fixed displacement pump and pipe, eventually stopping the simulation.

Error:An error occurred during simulation and the simulation was terminated

Caused by:

['HydraulicCircuit/Solver Configuration']: At time 3.500000, one or more assertions are triggered. See causes for specific information.

Pressure at port B must be greater than or equal to Minimum valid pressure. The assertion comes from:

Block path: HydraulicCircuit/Fixed-Displacement Pump (TL)

Assert location:

o (location information is protected)

Pressure at port A must be greater than or equal to Minimum valid pressure. The assertion comes from:

Block path: HydraulicCircuit/Pipe (TL)

Assert location:

o (location information is protected)

o (location information is protected)

I am looking for a way to make the hydraulic circuit resistant to these sudden pressure drops as they will certainly occur due to PID regulator settings of the VFD.

Answer 1

[Vedant Shah]

Hi @Simon,

The simulation error occurs because the motor torque input to the fixed-displacement pump contains rapid oscillations and brief negative spikes generated by the VFD control signal. These sudden torque reversals forced the pump flow to change faster than the hydraulic network could physically respond, causing instantaneous pressure drops below the minimum valid pressure. The pipe and pump blocks therefore triggers assertions and terminates the simulation to prevent non-physical results.

To stabilize the model, a "Rate Limiter" block can be inserted in the torque command path between the VFD signal source and the pump input. This limits the rate of change of torque so that it varies gradually rather than instantaneously. Additionally, a "Saturation" block can be used to restrict torque to a realistic operating range and prevent negative torque commands that the physical system would not experience under normal operation.

Refer to following model which does not give any error:

For more information, refer to the following documentations:

https://www.mathworks.com/help/releases/R2024b/simulink/slref/saturation.html

https://www.mathworks.com/help/releases/R2024b/simulink/slref/ratelimiter.html

[Simon]

Hello @Vedant Shah

I have tried to change the model according to you suggestion and it works like a charm!

I have tried to use the "Saturation" block but the "Rate Limiter" is a game changer. I just need to "play" with the slew rate parameters and it will be perfect.

Thank you very much for your assistance!

Have a wonderful day :)