When the motor shaft is fixed in place and energized, the current generated is called the stall current. Generally, AC motors, including frequency-controlled motors, are not allowed to stall. According to the external characteristic curve of an AC motor, a stall current will generate a "reversal current" that can burn out the motor.
The locked-rotor current and the starting current are numerically equal, but the duration of the 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 over time, with the decay rate related to the time constant of the motor. However, the locked-rotor current of the motor does not decay over time, but remains constant.
From the perspective of motor state analysis, we can divide it into three states: starting, rated operation, and stopping. The starting process refers to the process of the motor being energized and its rotor changing from a stationary state to a rated speed.
Regarding motor starting current
Starting current is the current that flows when a motor's rotor transitions from a stationary to a running state the instant it is energized under rated voltage conditions. It represents a change in the rotor's motion, i.e., a change in its inertia, and therefore the corresponding current is relatively large. During direct starting, the starting current of a motor is typically 5 to 7 times its rated current. Excessive starting current can have a significant negative impact on both the motor itself and the power grid. Therefore, for large and medium-sized motors, soft starting and other methods are used to limit the starting current to about twice the rated current. With continuous improvements in motor control systems, various starting methods such as frequency conversion starting and reduced-voltage starting have largely solved this problem.
Regarding motor stall current
As the name suggests, stall current is the current measured while the rotor is stationary. Motor stall current is a situation where the motor still outputs torque when the speed is zero, which is usually mechanical or man-made.
When a motor is overloaded, the driven machinery malfunctions, the bearings are damaged, or the motor experiences rotor rubbing, the motor may fail to rotate. When a motor is stalled, its power factor is extremely low, and the stall current is large; prolonged stalling can potentially burn out the motor windings. However, stall tests are necessary to assess certain motor performance characteristics; these tests are performed during both type testing and inspection of the motor.
The locked rotor test mainly measures the locked rotor current, locked rotor torque, and locked rotor loss at rated voltage. By analyzing the magnitude of the locked rotor current and the three-phase balance, the rationality of the motor stator and rotor windings, as well as the magnetic circuit composed of the stator and rotor, and some quality problems can be reflected.
During motor type testing, the locked-rotor test measures multiple voltage points, while during factory testing, only one voltage point is selected. Generally, the test voltage is chosen as one-quarter to one-fifth of the motor's rated voltage. For example, if the rated voltage is 220V, 60V is selected as the test voltage, and if the rated voltage is 380V, 100V is selected as the test voltage.
