The motor shaft is fixed so that it does not rotate, and the current is energized. At this time, the current is the locked rotor current. General AC motors, including frequency modulation motors, are not allowed to stall. According to the external characteristic curve of the AC motor, when the AC motor is locked, a “subversion current” will be generated to burn the motor.
The locked-rotor current and the starting current are equal in value, but the duration of the motor starting current and the locked-rotor current are different. The maximum value of the starting current appears within 0.025 after the motor is powered on, and it decays exponentially with the passage of time. , the decay speed is related to the time constant of the motor; while the locked-rotor current of the motor does not decay with time, but remains constant.
From the state analysis of the motor, we can divide it into three states: starting, rated operation and shutdown. The starting process refers to the process of changing the rotor from static to rated speed state when the motor is energized.
About motor starting current
The starting current is the current corresponding to the change of the rotor from the static state to the running state at the moment when the motor is energized under the condition of rated voltage. It is the process of changing the motion state of the motor rotor, that is, changing the inertia of the rotor, so the corresponding current will be relatively large. When starting directly, the starting current of the motor is generally 5 to 7 times the rated current. If the starting current of the motor is too large, it will have a large adverse impact on the motor body and the power grid. Therefore, for large and medium-sized motors, the starting current will be limited to about 2 times the rated current by means of soft starting. The continuous improvement of the motor control system and various starting methods such as variable frequency starting and step-down starting have better solved this problem.
About motor stall current
Literally, it can be understood that the locked rotor current is the current measured when the rotor is kept stationary, and the motor locked rotor is a situation where the motor still outputs torque when the speed is zero, which is generally mechanical or artificial.
When the motor is overloaded, the driven machinery fails, the bearing is damaged, and the motor has a sweeping failure, the motor may not be able to rotate. When the motor is locked, its power factor is extremely low, and the locked rotor current is relatively large, and the motor winding may be burned out for a longer time. However, in order to test some performances of the motor, it is necessary to perform a stall test on the motor, which is carried out in both the type test and inspection test of the motor.
The locked-rotor test is mainly to measure the locked-rotor current, the locked-rotor torque value and the locked-rotor loss at the rated voltage. Through the analysis of the locked-rotor current and the three-phase balance, it can reflect the stator and rotor windings of the motor, as well as the stator and rotor. The rationality of the composed magnetic circuit and some quality problems.
During the motor type test, there are many voltage points measured by the locked-rotor test. When the motor is tested at the factory, a voltage point will be selected for measurement. Generally, the test voltage is selected according to one-fourth to one-fifth of the rated voltage of the motor, such as When the rated voltage is 220V, 60V is uniformly selected as the test voltage, and when the rated voltage is 380V, 100V is selected as the test voltage.
Post time: May-09-2022