Three-Phase Induction Motors


  Three-Phase Induction Motors.pdf (Size: 4.78 MB / Downloads: 16)
Principles of Operation

The electrical section of the three-phase induction motor as shown in Figure
1.2.2 consists of the fixed stator or frame, a three-phase winding supplied from
the three-phase mains and a turning rotor. There is no electrical connection
between the stator and the rotor. The currents in the rotor are induced via the air
gap from the stator side. Stator and rotor are made of highly magnetizable core
sheet providing low eddy current and hysteresis losses.
1.1.1 Stator
The stator winding consists of three individual windings which overlap one
another and are offset by an electrical angle of 120°. When it is connected to the
power supply, the incoming current will first magnetize the stator. This magnetizing
current generates a rotary field which turns with synchronous speed ns.

Rotor

The rotor in induction machines with squirrel-cage rotors consists of a slotted
cylindrical rotor core sheet package with aluminum bars which are joined at the
front by rings to form a closed cage.
The rotor of three-phase induction motors sometimes is also referred to as an
anchor. The reason for this name is the anchor shape of the rotors used in very
early electrical devices. In electrical equipment the anchor's winding would be
induced by the magnetic field, whereas the rotor takes this role in three-phase
induction motors.

Motor Design

The torque characteristics can be largely adapted to the application in threephase
induction motors. Important properties here are a low starting current IA
and high starting torque MA. The torque characteristic and also the size of the
starting current are determined mainly by the type of rotor cage and the shape of
the rotor slot as shown in Figure 1.8.1
A high breakaway torque MA and a small starting current IA can be achieved by
a relatively high ohmic rotor resistance in the starting torque. Basically a more
or less large "current displacement effect" (skin effect) takes place during starting;
this applies to all types of rotor designs. The following designs are distinguished: