Torque is an important parameter of an electric motor, and torque under different conditions reflects different motor performance. Motor torque includes rated torque, starting torque, locked-rotor torque, and maximum torque. Today, Ms. will share some knowledge with everyone.
Motor torque and performance
Rated torque
The torque generated on the motor shaft under rated voltage and rated load is called the rated torque of the motor.
Starting torque
The torque generated by a stationary motor at the instant voltage is applied is called the starting torque. The starting torque characterizes the motor's starting capability and is related to the starting method (such as reduced voltage starting, AC motor variable frequency speed control starting, wound-rotor motor series resistor starting, etc.).
For direct-start squirrel-cage motors, the starting torque is generally 0.8 to 2.2 times the rated torque. Under normal circumstances, the starting torque should be more than 1.25 times the rated torque, and the corresponding starting current should be 5 to 6 times the rated current.
Large DC motors have particularly high starting torque, resulting in very high starting current. Therefore, large DC motors should not be started directly; instead, they should be started with reduced voltage. Small DC motors and permanent magnet motors are exceptions.
Maximum torque
Maximum torque is the boundary between the stable and unstable torque regions of a motor. If the load torque exceeds the maximum torque, the motor's output torque will decrease, and it will enter a stalled state. At this time, the current will be very high, and the motor will burn out.
Stall torque
When a motor enters a stall state, its speed is zero, and the torque that the motor can output at this time is the stall torque.
Relationship of torque in different states
The maximum torque is greater than the rated torque, but the stall torque of an asynchronous motor may be greater than the maximum torque or less than the rated torque. The former is due to the difference in motor parameters under stall and maximum torque conditions, while the latter is determined by the design based on actual operating conditions.
The ratio of maximum torque to rated torque reflects the motor's overload capacity.
Maximum torque and stall torque are two important performance indicators for measuring motor performance, and should be selected appropriately according to actual operating conditions.
The greater the maximum torque, the stronger the motor's overload capacity, but the motor's size and materials will also increase.
A higher stall torque is beneficial, especially for large motors, which generally have a large moment of inertia. A higher stall torque allows the motor to start more quickly and rotate more smoothly. However, a higher stall torque is not always better. Excessive stall torque can cause harmful shocks to the equipment, significantly increasing the motor's starting current and causing a major impact on the power grid.
Therefore, when designing a motor, it is necessary to select appropriate values based on actual operating conditions and retain a certain margin. Generally, the stall torque multiple is selected as 1.8-2.2. The maximum torque multiple is selected as 2.0-2.8 (depending on the size of the motor).
