THREE-PHASE INDUCTION MOTORS

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THREE-PHASE INDUCTION MOTORS: Principle of Operation – Construction and types of Rotor – Torque equation – Torque-Slip characteristics – Maximum torque – Equivalent circuit – Phasor diagram – Circle diagram - Starters – Speed control – Crawling and Cogging – Electrical Braking – Double Cage Rotors – Induction Generators.:

THREE-PHASE INDUCTION MOTORS : Principle of Operation – Construction and types of Rotor – Torque equation – Torque-Slip characteristics – Maximum torque – Equivalent circuit – Phasor diagram – Circle diagram - Starters – Speed control – Crawling and Cogging – Electrical Braking – Double Cage Rotors – Induction Generators.

PowerPoint Presentation:

18-Jan-14 3 AC Machines Three-Phase Induction Motors. Synchronous Machines. Single-Phase Motors. Stepper and Brushless DC Motors.

PowerPoint Presentation:

18-Jan-14 4 INDUCTION MOTOR ADVANTAGES : 3-phase induction motors are simple & rugged. Its cost is low & it is reliable Its has high efficiency. Its maintenance cost is low Its is self starting motor Suitable for all environments like coal mines & chemical factories Disadvantages It is essentially a constant speed motor and its speed cannot be changed easily. Its starting torque is inferior to d.c. shunt motor.

Introduction:

Introduction The three-phase induction motors are the most widely used electric motors in industry . They run at essentially constant speed from no-load to full-load. However , the speed is frequency dependent and consequently these motors are not easily adapted to speed control. We usually prefer d.c. motors when large speed variations are required.

PowerPoint Presentation:

An induction motor works on transforming action. The stator works as the primary while the rotor works as the secondary. It is also called asynchronous motor. It is also called Rotating Transformer .

Principle of Operation:

Principle of Operation

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( i ) When 3-phase stator winding is energized from a 3-phase supply, a rotating magnetic field is set up which rotates round the stator at synchronous speed Ns (= 120 f/P). (ii) The rotating field passes through the air gap and cuts the rotor conductors, which as yet, are stationary . Due to the relative speed between the rotating flux and the stationary rotor, e.m.f.s are induced in the rotor conductors. Since the rotor circuit is short-circuited, currents start flowing in the rotor Conductors.

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(iii) The current-carrying rotor conductors are placed in the magnetic field produced by the stator. Consequently, mechanical force acts on the rotor conductors . The sum of the mechanical forces on all the rotor conductors produces a torque which tends to move the rotor in the same direction as the rotating field. (iv) The fact that rotor is urged to follow the stator field (i.e., rotor moves in the direction of stator field) can be explained by Lenz’s law. According to this law, the direction of rotor currents will be such that they tend to oppose the cause producing them. Now, the cause producing the rotor currents is the relative speed between the rotating field and the stationary rotor conductors. Hence to reduce this relative speed, the rotor starts running in the same direction as that of stator field and tries to catch it.

Construction:

Construction A 3-phase induction motor has two main parts ( i ) stator and (ii) rotor. The rotor is separated from the stator by a small air-gap which ranges from 0.4 mm to 4 mm , depending on the power of the motor .

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18-Jan-14 11

Construction:

18-Jan-14 13 Construction A 3-phase induction motor has two main parts Stator Rotor Stator The stator of an induction motor consists of Stator Frame, Stator core, 3 phase winding, two end covers, bearings etc.

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Stator Stamping Tooth Slots

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Stator has three main parts: Outer Frame – It is the outer body of the of the motor. It protects the inner part of the machine. Stator Core – Built up of high grade silicon steel. Carries the alternating magnetic field. Stator winding – Has a three phase winding.

Construction:

18-Jan-14 17 Construction Stator core is a stack of cylindrical steel laminations. Slots are provided on the inner periphery. Three windings are space displaced by 120 0 electrical. Large size motors use open slots. Small size motors use semi closed slots. The air gap length should be as small as possible

Construction:

18-Jan-14 18 Construction Stator frame provides only mechanical support to the stator core. The stator winding for a definite number of poles . Greater the poles, lesser the speed and vice versa. Rotor Squirrel-cage rotor Phase-wound rotor

Construction:

18-Jan-14 19 Construction Squirrel-Cage Rotor

Squirrel cage rotor:

Squirrel cage rotor It consists of a laminated cylindrical core having semi closed circular slots at the outer periphery. Copper or aluminum bar conductors are placed in these slots and short circuited at each end by copper or aluminum rings called short circuiting rings. The rotor winding is permanently short circuited and it is not possible to add any external resistance.

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The rotor slots are not parallel to the shaft but skewed to – Reduce humming . Provide smoother torque for different positions of rotor. Reduce magnetic locking of stator and rotor.

Phase wound Rotor (or) Slip ring rotor:

18-Jan-14 24 Phase wound Rotor (or) Slip ring rotor It is also called SLIP RING ROTOR Consists of a laminated core having semi closed slots at the outer periphery and carries a 3-phase insulated winding. The rotor is wound for the same number of poles as that of stator. The three finish terminals are connected together forming a star point and the three star terminals are connected to three slip rings fixed on the shaft.

Construction:

18-Jan-14 26 Construction Wound rotor The rotor winding is uniformly distributed.. Three leads are connected to the slip rings. Carbon brushes are pressed on the slip rings.

Production of Rotating Magnetic field:

Production of Rotating Magnetic field When a 3-phase winding is energized from a 3-phase supply, a rotating magnetic field is produced. This field is such that its poles do no remain in a fixed position on the stator but go on shifting their positions around the stator. For this reason, it is called a rotating magnetic field

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It follows from the above discussion that a 3-phase supply produces a rotating field of constant value (= 1.5 fm, where fm is the maximum flux due to any phase).

Torque-Slip Characteristics:

Torque-Slip Characteristics

Speed Control of Induction Motor:

Speed Control of Induction Motor

Supply Voltage Control or Stator Voltage Control:

Supply Voltage Control or Stator Voltage Control

Disadvantages :

Disadvantages A large decrease in voltage is required for relatively small decrease in speed . This large decrease in voltage results in increase current . Due to increased current ,the motor may get overheated

By changing supply frequency:

By changing supply frequency The speed control of an induction motor using variable frequency supply requires a variable voltage power source(by changing the prime mover speed). This method is very rarely used (electrically driven ships)

Pole changing method:

Pole changing method The number of stator poles can be change by Multiple stator windings Method of consequent poles Multiple stator windings: In this method the stator is provided with two separate windings which are wound for two different pole numbers. One winding is energized at a time. This method is less efficient and more costly , and therefore, used only when absolutely necessary.

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Method of consequent poles In this method a single stator winding is divided into few coil groups. The terminals of all these groups are brought out. The number of poles can be changed with only simple changes in coil connections. In practice, the stator winding is divided only in two coil groups. The number of poles can be changed in the ratio of 2:1.

Rotor resistance control:

Rotor resistance control

Speed control by cascading :

Speed control by cascading

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Operation Motor-1 may be operated to run separately from the supply Motor-2 may be allowed to run separately from the supply Cumulative cascade Differential cascade

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Starters- Introduction Most large induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges . To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed. The high starting current will produce severe a voltage drop and will affect the operation of other equipment. It is not desirable to start large motors direct on line (giving full voltage to the stator). Normally with motors beyond 5 HP, starters are provided. For reduction in the starting current, a lower voltage is applied to the stator, especially for the squirrel cage induction motors. Full voltage is only applied when the motor picks up speed.

Starting methods of induction motors :

Starting methods of induction motors D.O.L (Direct on line )starter Star delta Starter Auto transformer starter Rotor resistance starter

D.O.L (Direct on line )starte:

D.O.L (Direct on line ) starte It is simple and cheap starter for a 3-phase induction motor. The contacts close against spring action. This method is normally limited to smaller cage induction motors, because starting current can be as high as eight times the full load current of the motor . An isolator is required to isolate the starter from the supply for maintenance. Protection must be provided for the motor. Some of the safety protections are over-current protection, under-voltage protection, short circuit protection, etc.

Auto transformer starter:

Auto transformer starter

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Operated by a two position switch i.e. manually / automatically using a timer to change over from start to run position. In starting position supply is connected to stator windings through an auto-transformer which reduces applied voltage to 50, 60, and 70% of normal value depending on tapping used. Reduced voltage reduces current in motor windings with 50% tapping used motor current is halved and supply current will be half of the motor current. Thus starting current taken from supply will only be 25% of the taken by DOL starter. For an induction motor, torque T is developed by V2, thus on 50% tapping, torque at starting is only (0.5V) 2  of the obtained by DOL starting. Hence 25% torque is produced. Starters used in lager industries, it is larger in size and expensive. Switching from start to run positions causing transient current, which can be greater in value than those obtained by DOL starting.

Rotor resistance starter:

Rotor resistance starter

PowerPoint Presentation:

This starter is used with a wound rotor induction motor. It uses an external resistance/phase in the rotor circuit so that rotor will develop a high value of torque. High torque is produced at low speeds, when the external resistance is at its higher value. At start, supply power is connected to stator through a three pole contactor and, at a same time, an external rotor resistance is added. The high resistance limits staring current and allows the motor to start safely against high load. Resistors are normally of the wire-wound type, connected through brushes and slip rings to each rotor phase. They are tapped with points brought out to fixed contactors. As the motor starts, the external rotor resistance is gradually cut out of circuit ; the handle or starter is turned and moves the three contacts simultaneously from one fixed contact to the next. The three moving contacts are interconnected to form a start point for the resistors. To ensure that the motor cannot be started until all rotor resistance is in circuit, an interlock is fitted which prevents the contactors from being closed until this condition is fulfilled.

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