Simulation of Induction motor (Asynchronous Machine)

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Hi, I want to plot the output torque and speed of an induction motor. The input for the motor is mechanical torque. Under zero load I see that the output speed curve starts at a non-zero value. I was expecting the output speed of the motor to start at zero and reach its final value with time. Can someone help me understand why this is? Is there something wrong with my model. I've attached my simulink model.
I'm using an Asynchronous machine block connected to three individual ac sources. These ac sources are created by using a function block that generate sine waves. The sine waves are fed into a controlled ac voltage source block which generate three 460V, 60Hz voltage sources that are 120 degrees apart. The reason I'm doing this is because I eventually want to simulate the motor behavior under a varying load. To do this I plan use the function block to vary the frequency.
Any help and suggestions are greatly appreciated! Thank you!

Answers (1)

Joel Van Sickel
Joel Van Sickel on 5 May 2022
Hello Rafael,
unfortunately, when you take a Torque input for that block, it intitializes to a non-zero speed and you can't force it to zero. There are 2 workarounds for this by changing the model, and one by changing starting conditions. If you start by applying zero stimuli to the motor (zero voltage) it should initialize to 0 speed. Otherwise you can change the model. Either you change it to a speed input, and then externally model the mechanics in Simulink (sum the torques, pass them through an integrator, divide by rotational inertia, and include friction if you want, to get speed). Then you can control the initial condition of the speed. Alternatively, you can also set the input port to be a physical connection, and model the mechanics in Simscape, which will also give you control of the initial condition for speed.
This example shows how it is done with the physical port using simscape: You can set initial speed conditions on inertial blocks if you add one of those to the rotational mechanical simscape network. https://www.mathworks.com/help/physmod/sps/ug/mechanical-coupling-of-synchronous-generator-with-exciter-system-using-the-simscape-mechanical-rotational-port.html
Regards,
Joel

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