What is motor stall?
VFD Controlled Motor Stall Animation
Motor stall is when an electric motor stops rotating or fails to generate torque while energized. This typically happens when the load on the motor is excessive or the motor cannot overcome the resistance on its shaft.
Stall Explanation:
As the VFD output frequency increases, due to load the motor cannot gain speed and the motor current suddenly rises. In this situation, there is no movement in the load. This is called “stall” and if it lasts too long, the motor/output may burn out or the VFD will trip on error.
As the VFD output frequency increases, due to load the motor cannot gain speed and the motor current suddenly rises. In this situation, there is no movement in the load. This is called “stall” and if it lasts too long, the motor/output may burn out or the VFD will trip on error.
The fact that a motor spins easily with no load doesn't matter; when a load is applied and it cannot generate torque, the motor will become passive and cannot rotate.
Example of stall: Suppose the motor is running at 50 Hz and 380 V. If you increase the frequency to 80 Hz, it can still generate torque. But if you push it up to 190 Hz, the motor will still draw power but cannot produce torque, and stall will occur.
Main reasons for stall in AC motors:
Overloading: If the load is too large, the motor cannot find enough torque to turn it and stops.
High resistance: Too much friction or heavy load can exceed the motor’s capability.
Low voltage: If the voltage is low, there won’t be enough power to turn the load.
Poor lubrication: Insufficient lubrication increases friction and puts stress on the motor.
Mechanical failures: Issues like a bent shaft or broken pulley can also cause stall.
If the load is too great for the motor, “stall” will happen. At that moment, the output torque equals the static torque and there’s no more output power. In this case, the motor will overheat and windings may burn out.
Continuous stall will burn the motor, and your wallet. Windings will be damaged and the motor will be ruined.
Proper motor selection and correct protection are essential. Always run the motor within its rated values and select according to the application.
In controllers without stall protection or with wrong settings, this situation can also damage the drives and circuits.
Additional info: If you supply an AC motor with a frequency above its rated value, the motor will exceed its nominal speed and torque production drops, stall will occur. Again, wrong voltage application also leads to winding damage and failures.
In summary: Motor stall destroys both the motor and the equipment. Proper sizing, regular maintenance, and protection will minimize this risk. In the field, the most common stalls are due to lack of lubrication, incorrect motor selection, and low voltage.
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