Asynchronous machine's rotor speed is negative

The rotor speed of the induction machine is negative and I cannot explain the reason for this as the torque should be in a working range.

I am using the default model 04: 50 HP, 460 V, 60 Hz, 1780 RPM with an initial slip of 1.

V_RMS = 460 V with 60 Hz.

My calculation:

So in my calculation a load up to 200 Nm shoud be fine.

I appreciate any help. Thank you!

[Sahas]

Hi @Markus König,

Can you share the Simulink model file so I can check some things out and help you out better?

Answer 1

[Shivam Gothi]

Hello @Markus König,

Hello @Markus Konig,

As per my understanding, you are trying to simulate a three-phase asynchronous machine (Here, configured as an induction machine) through the DOL starting method. The load torque is entered correctly as per the machine rating but the speed is going negative.

To address this issue, consider the below given torque-speed characteristics of an induction machine. (Note: the values are assumed hypothetically because the machine parameters are not given). Two different load-torque characteristics (“L1” and “L2”) are also plotted. You can see that at slip=1 (speed = 0), the load-torque should be smaller than the starting torque of the induction machine so that it accelerates and attains a positive speed.

In your case, you are directly starting from a load torque of 200Nm (shown by load-torque characteristics “L1” in the above figure). Therefore, at zero speed, your motor will start rotating in negative direction because load-torque is greater than the mechanical torque. As a result, there is no stable operating point and motor keeps on increasing the speed in negative direction.. (Note: the operating point is found by the intersection of load torque-speed characteristics and machine torque-speed characteristics.)

Whereas, if you gradually ramp-up your mechanical load torque to the value of 200Nm (as shown by load-torque characteristics “L2”), then the motor will accelerate and attain a stable operating point.

Below given is the screen-shot of Simulink model that shows how to gradually ramp-up the mechanical load to 200Nm.

Below given figure captures the speed of motor on scope. You can see that it is now accelerating and achieving the rated speed as expected.

I hope it helps!

[Markus König]

Thank you very much, that explains it! :)