Understand the inputs of the asynchronous machine

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K on 2 Sep 2023
Commented: Yash on 14 Sep 2023
I'm getting started with simscape specialized power systems and I don't get the input for the asynchronous machine in motor mode from the documentation.
According to the documentation, if I use the mechanical rotational port (see screenshot p3) I need to connect the ideal torque source to the shaft in motor mode and give a torque as an input, but if I select the mechanical rotational port I need to sepecify the mechanical power in the block parameter of the asynchronous machine, which has to be positive to specify a motor.
My question is the following: why do I need to give the torque as an input with the torque source as well as the mechanical power if I'm using a Squirrel-cage preset model? I mean the output torque and rotational speed are already defined by the model depending on the nominal power, pol pair, voltage, rotor and stator parameters... If I have a preset model and give a mechanical power as an input wouldn't that be an overdefined problem? the same goes if I give a torque and a mechanical power as an input at the same time.
I tried a simple model (screenshot in p1) with a 50Hz 400V voltage source and a mechanical torque 149200W. The measured torque is 0Nm when it should be 958,2Nm=149,200kW*9550/1487U/min and the rotational speed is 1500U/min (screenshot p2) when it should be 1487U/min by 50Hz (considering the slip due to induction).
Any answers?
Thank you!

Answers (1)

Yash on 11 Sep 2023
I understand that you are facing issues while simulating an asynchronous machine in Simulink.
Regarding the usage of the mechanical rotational port, you are correct that you need to connect an ideal torque source to the shaft. This torque source represents the mechanical load connected to the machine. However, in the block parameters of the asynchronous machine, you don't need to specify the mechanical power directly. The mechanical power is calculated based on the torque and the rotational speed.
If you are utilizing a Squirrel-cage preset model, the output torque and rotational speed are determined by the model itself, considering the machine parameters and the electrical input. In this case, you shouldn't provide both a torque input and a mechanical power input simultaneously. It would indeed result in an overdefined problem.
Upon reviewing your sample model, it appears that the mechanical torque value is very high. This might have caused the unexpected behaviour of the output. Additionally, I have noticed that the input at the mechanical rotational port is not provided correctly, as per the documentation. I have made the necessary corrections to address these issues, resulting in the desired results. Refer to the image below:
For more details, refer to the documentation link provided below:
I hope this helps you address the issue.
K on 13 Sep 2023
thank you so much for the explanation. It helps a lot.
So if I give an input torque while using a squirrel model, the mechanical power will be ignored right? but the mechaical power field can't be left blank, which would be better, the default value or 0W.
If I don't specify an input torque while using a squirrel model then the mechanical power is a must here, and depending on the sign of the mechnical power the machine will operate in generator or motor mode right? I tried and it doesn't make a differance wether it's positive or negative.
I'm trying to connect an induction motor (squirrel) with a synchronous generator using the mechanical rotational port to supplement an island grid.
My electrical load from the island grid defines the electromagnetic torque of the generator, which defines the electromagnetic torque of the motor.
The electromagnetic torque of the generator should be like this (before adding the motor the generator's input was the speed 1500rpm)
After adding the squirrel motor with the electromagnetic torque of the generator as an input *(-1) I have a similar electromagnetic torque of the generator except the positive oscillation (which is wrong)
and the electromagnetic torque of the motor is constant (which is also wrong)
I need the rotaional port S because I'll be adding other mechanical parts but I don't seem to get how it works, and changing to sign of the mechanical of the generator's input doesn't change anything. Connecting the two shafts without the torque input gives me a worst result.
thank you
Yash on 14 Sep 2023
You should keep the default value in the mechanical power option when using the Simscape Asynchronous Machine block. The operating mode of the machine, whether it functions as a generator or a motor, does not depend on the sign of the mechanical power.
If you are interested in changing the operating mode, you need to modify the input connections to the machine. The figure below (taken from the documentation) indicates how to connect an Ideal Torque Source block from the Simscape library to the machine shaft to represent the machine in motor mode or in generator mode when the rotor speed is positive:
For more detailed information, I recommend you to refer to the documentation provided in the link: https://www.mathworks.com/help/sps/powersys/ref/asynchronousmachine.html
I hope this explanation clarifies any confusion.

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