Rated torque
Under the rated voltage and rated load, the electromagnetic torque generated on the motor shaft is called the rated torque of the motor.
Starting torque
When the voltage is applied to the motor in the stopped state, the torque generated by the motor is called the starting torque. The starting torque represents the starting ability of the motor, which is related to the starting mode (such as star-delta starting, frequency conversion speed regulation starting, etc.), and the direct starting squirrel-cage type is generally 0.8 to 2.2 times the rated torque. Usually the starting torque is more than 125% of the rated torque. The corresponding current is called the starting current, which is usually about 6 times of the rated current.
For a DC motor, its starting torque is large, so its starting current is also very large, so it cannot be started directly. But for the AC motor, its starting torque is not very large, so its starting current is not very large, so it can be started directly. The torque of the AC motor is small when it starts, so it cannot start with load.
Maximum torque
The junction point where the motor torque enters the unstable region from the stable region. That is to say, if the load torque is greater than the maximum torque of the motor, the output torque of the motor will become smaller and enter a stalled state.
Stall torque
After entering the locked-rotor state, the speed is zero, and the torque that the motor can output at this time is the locked-rotor torque.
static torque
The torque at which the stator locks the rotor when the motor is energized but not turning. Usually, maximum torque>stall torque>rated torque. The ratio of the maximum torque to the rated torque is called the overload factor of the motor.
The maximum torque multiple and the stall torque multiple are indeed two important performance indicators to measure the performance of the motor, but the bigger the better. The larger the maximum torque multiple, the motor will have the ability to overload the limit, but at the same time it is also a very important assessment of the volume and materials of the motor.
It is beneficial to have a larger stall torque multiple, especially large motors generally have a relatively large moment of inertia. If the stall torque multiple is relatively large, the motor will start more quickly and rotate more freely. But the stall torque multiple cannot be as big as possible. The larger the two torque multiples, the starting current of the motor will generally increase a lot, and the impact on the power grid will also be greater. Therefore, when selecting a motor, it is necessary to select a suitable value according to the actual working conditions and keep a certain value. Just enough margin. In general, select 1.8 to 2.2 for the stall torque multiple. Select the maximum torque multiple from 2.0 to 2.8 (depending on the size of the motor).
Due to the existence of the static torque of the motor, the motor must be powered off before operating the motor, otherwise the gears of the motor gearbox will be easily damaged when the motor is operated forcibly. Also: in order to reduce the rotational speed of the motor output shaft, many gears are combined into a gearbox. Each gearbox has a limit torque, which is called the damage torque. If the torque generated by excessive external force acts on the gearbox, it will cause damage to the gear.
