Typical Torque-Speed Curve of a Cage Induction Motor
Types of Torques
Locked Rotor or Starting or Breakaway Torque
- The Locked Rotor Torque or Starting Torque is the torque the electrical motor develops when it starts at rest or zero speed.
- A high Starting Torque is more important for applications or machines that are hard to start, such as positive displacement pumps, cranes etc. A lower Starting Torque can be accepted in applications as centrifugal fans or pumps where the start load is low or close to zero.
- The Pull-up Torque is the minimum torque developed by the electrical motor when it runs from zero to full-load speed (before it reaches the break-down torque point)
- When the motor starts and begins to accelerate, the torque in general decreases until it reaches a low point at a certain speed – the pull-up torque – before the torque increases until it reaches the highest torque at a higher speed – the break-down torque – point.
- The pull-up torque may be critical for applications that need power to go through some temporary barriers achieving the working conditions.
- The Break-down Torque is the highest torque available before the torque decreases when the machine continues to accelerate to the working conditions.
Full-load Torque or Braking Torque
- The Full-load Torque is the torque required to produce the rated power of the electrical motor at full-load speed.