Browsing: Induction Motors

Universal Motor

A dc series motor specifically designed for ac operation is usually referred to as a universal motor. A universal motor is wound and connected just like a dc series motor. That is, the field winding is connected in series with the armature winding.

Construction

The construction of universal motor is very similar to dc series motor except that whole of its magnetic path is laminated. The laminated stator is necessary because the flux is alternating when motor is operated from ac supply. The armature is of wound type and similar to that of a small dc motor. It essentially consists of a laminated core having skewed or straight slots and a commutator to which the leads of armature windings are connected. The compensating winding is used to reduce the reactance voltage present in the armature when motor runs on a.c. supply.Universal motor

Working

When a series motor is connected to a dc source, the current is unidirectional in both the field and the armature windings. Therefore, the flux produced by each pole and the direction of the current in the armature conductors under that pole remain in the same direction at all times. Hence, the torque developed by the motor is constant.

When the motor is connected to an a.c. supply, the same alternating current flows through the field and armature windings. The field winding produces an alternating flux φ that reacts with the current flowing in the armature to produce a torque. Since both armature current and flux reverse simultaneously, the torque always acts in the same direction. It may be noted that no rotating flux is produced in this type of machines; the principle of operation is the same as that of a d.c. series motor.

Characteristics

The operating characteristics of an a.c. series motor are similar to those of a d.c. series motor.

  • The speed increases to a high value with a decrease in load. In very small series motors, the losses are usually large enough at no load that limit the speed to a definite value (1500 – 15,000 r.p.m.).
  • The motor torque is high for large armature currents, thus giving a high starting torque.
  • At full-load, the power factor is about 90%. However, at starting or when carrying an overload, the power factor is lower.

Applications

Universal Motors are used in vacuum cleaners, mixer grinders, portable drills and domestic sewing machines, etc.

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Principle of Operation of Three Phase Induction Motor

When three phase supply given to the stator of a three phase induction motor, a rotating magnetic field is set up which rotates at synchronous speed.

principle of operation of three phase induction motor

  • The stationary rotor conductors cut the revolving magnetic field.
  • According to Faraday’s law of electromagnetic induction an emf is induced in the rotor conductors.
  • As rotor conductors are short circuited and current start flowing through the rotor conductor.
  • Due to flow of this current a magnetic field is set up in rotor conductor.
  • A resultant field is set up, which exerts force on the rotor conductors and rotor starts rotating.

Why the rotor does not rotates at Synchronous speed?

The rotor tries to attain the synchronous speed but rotor fails to do so.

  • If the rotor attains the synchronous speed then the relative speed between rotating magnetic field and rotor will be zero,.
  • If this cause no emf will be induced in the rotor conductors.
  • If there is no emf is induced in the rotor conductor, no current will flow through the rotor
  • No magnetic flux will set up in rotor.
  • Hence no torque is produced in rotor. That is why, an induction motor never runs at synchronous speed. It is always runs at a speed less than synchronous speed.
  • An induction motor is also called synchronous motor.
  • The principle of operation of this motor depends upon electromagnetic induction. Hence, the name induction motor.

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Industrial Applications of Induction Motors

More 90% induction motors are used in industrial applications. Based on these characteristics following applications are suggested. In agriculture induction motors are invariably used. The other applications are as follows:

Textile mills

In a textile mill, following operations are carried out, cotton to slivers, spinning, weaving and finishing. The process of obtaining slivers from cotton involves, removal of seeds and other impurities from cotton, transformation into laps and finally into slivers. After this follows spinning where a continuous yarn of sufficient strength is produced which is wound on bobbins. The process of weaving consists of joining two sets of threads, one extends throughout the length of the fabric and the other one has its threads going across. This process is carried out in a loom. The process of finishing consists of bleaching, dyeing, printing, and packing. Loom motors require high starting torque, frequent starting and stopping and low temperature rise under full load condition. Therefore, totally enclosed fan cooled, high torque squirrel cage induction motors are used. Spinning motors require moderate starting torque and smooth acceleration, so as to have straight and unbroken yarns. Normally a 4 or 6 pole squirrel cage induction motor is used.

Cement mills

The raw material required consist of limestone, silica, alumina and ferric oxide. The lime stone is crushed and a required amount of iron ore is added and then they are grounded in a grinding mill. The powder from this grinding mill is homogenised in silos, by passage of air from bottom through the medium and is then fed into the kiln, from where cement clinker at high temperature is obtained. This clinker is then air cooled in special type of coolers. The motors used for crushers is the slip ring induction motor having 200 to 250% pull out torque.

 Coal mining

 For drilling in coal mines, usually induction motors with flame proof enclosures are used. Sometimes high frequency (150 to 200 Hz) motors are used. This is done to reduce the size of the motor for the same power ratings. For raising coal or raising or lowering men and some equipments usually wound rotor induction motor is used. While applying brakes, initially d.c. rheostatic braking is used but finally for dead stops, mechanical brakes are used. For large size mines, usually d.c. motor with Ward-Leonard Ilgner system is employed for mine winders.

Lifts

 For lifts normally slip ring induction motors having high starting torque, high overload capacity and pull out torques, are used.

 Lathe, grinding and milling machine

 Here the machine tools require constant operating speed and low starting torque and therefore, d.c. shunt motors or squirrel cage motors are usually used.

Cranes

 For cranes or hoist, the requirements are high starting torque and frequent switching operations. For this d.c. series motor is used. In case of a.c. supply, slip ring induction motors are used where speed control is obtained by rotor resistance.

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