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Several questions about torques of the electric motor

Date:2023-10-20   Author:XINDA MOTOR
The output torque of the motor in normal operation varies with the load, but generally the output torque of the motor refers to the rated output torque M at rated power and rated speed, M=9550N/n. That is to say, the motor power is constant, the speed is constant, and the torque is constant. The motor output torque is constant under rated conditions, but the starting torque is a multiple of the rated torque. Generally, three-phase asynchronous motors, such as Y and Y2 series motors, are about 2 times. Relevant data can be found from the motor samples, which represents the starting ability of the motor with load. For example, the starting multiple of a special high-slip motor is much higher. Another example is Winding motor: For example, the YR series is even higher. You can check the relevant samples.

Rated torque: Under rated voltage and rated load, the electromagnetic torque generated on the motor shaft is called the rated torque of the motor.

Starting torque: The moment when voltage is applied to the asynchronous motor in a stopped state, the torque generated by the asynchronous motor is called starting torque. The starting torque represents the starting ability of the motor, which is related to the starting method (such as star-delta starting, variable frequency speed regulating starting, etc.). 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 the rated current.

For DC motors, the starting torque is particularly large, so the starting current is also very large, so it cannot be started directly. Of course, this is for large DC motors. Small DC motors including permanent magnets are exceptions. For AC motors, the torque is not very large, so the current is not very large, and they can be started directly. Of course, AC motors have small starting torque, so they cannot be started with load.

Maximum torque: the junction point where the motor torque enters the unstable zone from the stable zone. 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.

Locked-rotor torque: After entering the locked-rotor state, the rotation speed is zero. At this time, the torque that the motor can output is the locked-rotor torque.

Static torque: The torque at which the stator locks the rotor when the motor is energized but not rotating.

Generally, maximum torque > locked-rotor torque > rated torque. The ratio of the maximum torque to the rated torque is called the overload coefficient of the motor.

The maximum torque multiple and the locked-rotor torque multiple are indeed two important performance indicators for measuring motor performance, but bigger is not always better. The greater the maximum torque multiple, the motor will have the ability to overload the limit, but at the same time, the size and materials of the motor are also a very important assessment.

It is beneficial to have a larger locked-rotor torque multiple, especially for large motors that generally have a relatively large moment of inertia. If the locked-rotor torque multiple is larger, the motor will start faster and rotate more freely. However, the locked-rotor torque multiple cannot be as large as possible.

The larger the two torque multiples are, the starting current of the motor will generally increase a lot, and the impact on the power grid will be greater. Therefore, I would like to remind everyone that when selecting a motor, you should choose an appropriate value based on the requirements of the actual working conditions. Just keep a certain margin. Under normal circumstances, the locked-rotor torque multiple is selected from 1.8 to 2.2. The maximum torque multiple is selected from 2.0-2.8 (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 performing special operations on the motor. Otherwise, the gears of the motor gearbox may be easily damaged during forced operation. Also: In order to reduce the speed of the motor output shaft, many gears are combined into a gearbox. Each gearbox has a limit torque, which is called damage torque. If the torque generated by excessive external force acts on the gearbox, it will cause damage to the gear.