Tuesday, April 28, 2009

Load Characteristics

The induction motor is used to convert electrical energy into mechanical energy. The driven load presents a mechanical load to the shaft of the motor. As the motor is started, it accelerates the driven load from zero speed to the rated full load speed of the motor. As the load accelerates, the torque presented to the motor shaft will vary depending on the design of the machine.
Generally, the load torque is expected to be higher at full speed than at lower speeds. Some applications such as loaded conveyors may require a high breakaway torque to get the load to begin to move from zero speed.





In order to correctly design a motor starting system, it is important to know the load torque curve. The load (machine) design determines the required starting torque. The motor design then determines how much current is required to develop that torque. If the torque developed by the motor is insufficient, the motor can not accelerate the load to full speed.

The load torque can be expressed in Newton Meters, Pound Foot, or as a percentage of the motor full load torque.
For a specific machine, it is best to always work in absolute units such as Newton meters or Pound Foot. This way, if the motor size is changed, the starting characteristics and curves will be changed automatically. When relative units (%) are used, all the values need to be altered to reflect the change in motor Full Load Torque.



Generic or indicative curves can be saved in relative units (%) to enable approximations to be made where absolute details for a specific load are not available.

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