Energy efficient variable speed drives empowered with torque estimation

The induction motor (or asynchronous motor), which is the most widely used Alternating Current (AC) motor in industry, always relies on a small difference in speed between the stator-rotating magnetic field and the rotor shaft speed called slip to induce rotor current in the rotor winding. As such, the induction motor cannot produce a torque at synchronous speed, where slip ceases to exist. A motor when allowed, will provide a torque up to its maximum limit to rotate its load; especially under locked rotor or heavily loaded equipment scenarios. The value of the maximum torque limit is different for each AC induction motor and it is directly dependent on the motor characteristics. In some cases, such as conveyors, motorized lathe machines, or direct-coupled motor drives, if the motor is allowed to exercise maximum torque, it can lead to irreversible damage to the equipment. This paper presents the design of a Variable Speed Drive (VSD) system having the possibility of measuring the rotor speed and controlling the torque of an asynchronous motor to be within specified torque limits.