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Induction motors

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Alternating current {AC) induction motors are widely used in the oil and gas industry because of their simplicity, reliability, and low cost.

Overview

Induction motors are either single-phase or three-phase. In an induction motor, the actual rotor speed always is less than that of the rotating magnetic field. Fig. 1 shows a typical induction motor and labels its three basic parts:

  • Stator
  • Rotor
  • Enclosure

Stator construction

The stator and the rotor are electrical circuits that perform as electromagnets. The stator is the stationary electrical part of the motor. The stator core of a National Electrical Manufacturers Association (NEMA) motor is made up of several hundred thin laminations that are stacked together to form a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core.

Each group of coils and the steel core it surrounds form an electromagnet. Electromagnetism is the principle behind motor operation. The stator windings are connected directly to the power source.

Rotor construction

The rotor is the rotating part of the electromagnetic circuit. The most common type of rotor is the "squirrel cage" rotor, so called because it is reminiscent of one of the exercise wheels found in the cages of pet rodents.

The squirrel-cage rotor consists of a stack of steel laminations that has evenly spaced conductor bars around its circumference. The stacked laminations form the rotor core. Aluminum is die-cast in the slots of the rotor core to form the series of conductors around the rotor’s perimeter. Current flow through the conductors forms the electromagnet. The conductor bars are mechanically and electrically connected with end rings. The rotor core mounts on a steel shaft to form a rotor assembly.

The wound rotor is another type of induction-motor-rotor construction. A major difference between the wound rotor motor and the squirrel-cage rotor is that the conductors of the wound rotor consist of wound coils instead of bars. These coils are connected through slip rings and brushes to external variable resistors, as shown in Fig. 2. The rotating magnetic field induces a voltage in the rotor windings that increases the resistance of the rotor windings. This increase in resistance allows less current flow in the rotor windings, which decreases motor speed. Conversely, decreasing the resistance allows more current flow, and so increases motor speed.

Wound-rotor induction motors are used in applications of certain types of pump, in mine hoists, mills, and applications where speed reduction is required in the drive application.

Stator coil arrangement

The schematic in Fig. 3 illustrates the relationship between the stator coils. The coils operate in pairs. This example uses six coils, a pair for each of the three phases. The coils are wrapped around the soft iron core material of the stator. These coils are referred to as motor windings. Each motor winding becomes a separate electromagnet. The coils are wound in such a way that when current flows in them, one coil in a pair is a north pole and the other a south pole. For example, if A1 were a north pole, then A2 would be a south pole. When the current reverses, so does the polarity of the poles.

References

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See also

Electrical grounding

Electrical distribution systems

Power factor and capacitors

Alternating current motors

Electrical systems

Synchronous motor

Motor specifications

NEMA motor characteristics

Alternating current motor drives

Motor enclosures

PEH:Electrical Systems