Induction Motors

AC induction motors are ideal for most industrial and commercial applications because of their simple construction and less number of parts which reduce the maintenance cost. Induction motors are frequently used for both constant-speed and adjustable speed drive (ASD) applications.

The two basic parts of an induction motor are the stationary stator located in the motor frame and the rotor that is free to rotate with the motor shaft. Today's motor design and construction are highly refined. For example, stator and rotor laminations have been designed to achieve maximum magnetic density with minimum core losses and heating. The basic simplicity of this design ensures high efficiency and makes them easily adaptable to a variety of shapes and enclosures.

A three-phase induction motor can best be understood by examining the three-phase voltage source that powers the motor. Three-phase currents flowing in the motor establishes a rotating magnetic field in the stator coils. This magnetic field continuously pulsates across the air gap and into the rotor. As magnetic flux cuts across the rotor bars, a voltage is induced in them, similar to a voltage induced in the secondary winding of a transformer. As the rotor bars are part of a closed circuit (including the end rings), a current begins to circulate in them. The rotor current in turn produces a magnetic field that interacts with the magnetic field of the stator. Since this field is rotating and magnetically interlocked with the rotor, the rotor is dragged around with the stator field.

When there is no mechanical load on the motor shaft (no-load condition), the rotor almost manages to keep up with the synchronous speed of the rotating magnetic field in the stator coils. Drag from bearing friction and air resistance prevents perfect synchronism. As the load increases on the motor shaft, the actual speed of the rotor tends to fall further behind the speed of the rotating magnetic field in the stator. This difference in speed causes more magnetic lines to be cut, resulting in more torque being developed in the rotor and delivered to the shaft mechanical load. The rotor always turns at the exact speed necessary to produce the torque required to meet the load placed on the motor shaft at that moment of time. This is usually a dynamic situation, with the motor shaft speed constantly changing slightly to accommodate minor variations in load.

The rotor consists of copper or aluminum bars connected together at the ends with heavy rings. The construction is similar to that of a squirrel cage, a term often used to describe this type of AC induction motor.

The squirrel cage induction motor is most commonly used because of the following reasons

  • Simplicity
  • Rugged construction
  • Low Manufacturing Cost

Motors rated from 0.1 kW to 2500 kW in low voltage range can be used with Soft Starters. To select the right Soft Starter please check our design center section of the website.

for the motors ranging from 25kW onwards in 3.3/6.6/11 kV range, the HFSR soft starters is the best match.