INDUCTION MOTOR�

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• BRANCH-E & TC ENGG
• SUBJECT-ELECTRICAL MACHINE
• CHAPTER-6-INDUCTION MOTOR
• TOPIC- INDUCTION MOTOR
• BRANCH-E&TC
• SEM-4^TH
• FACULTY-Er.Gurupada Mishra (LECT. ELECTRICAL ENGG DEPARTMENT)
• AY-2021-2022, SUMMER-2022

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Structure

• The stator is the outside stationary part of the motor.
• The rotor is the inner rotating part.
• In the animation:
• Red represents a magnet or winding with a North polarization,
• Green represents a magnet or winding with a South polarization.
• Opposite, red and green, polarities attract.

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Operation

• As the rotor reaches alignment, the brushes move across the commutator contacts and energize the next winding.
• In the animation:
• The commutator contacts are brown,
• The brushes are dark grey.
• A yellow spark shows when the brushes switch to the next winding.

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Just Like dc Machines, ac Machines also consist of

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• Stator, and

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• Rotor.

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AC Machines Construction

• The outer stationary steel frame enclosing a hollow, cylindrical core.
• A large number of circular silicon steel laminations with slots cut in the inner circumference.
• Three phase windings mutually displaced by 120° are wound in these slots.
• The greater the number of poles, the lesser is the speed and vice-versa.
• Three phase supply induces rotating magnetic field.
• Air gap between the stator and rotor ranges 0.4mm to 4mm, determines the power of the motor

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The Stator:

• Squirrel Cage is the most common form of rotor:

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• Laminated cylindrical core with parallel slots at the outer periphery
• Copper or aluminium bars are placed in the slots
• All the bars are welded at each end by metal rings called “End rings”
• End rings are sometimes castellated to facilitate cooling.
• It is not connected to the supply and operates on the transformer principle
• Advantages: This is a simple and robust construction
• Disadvantage: Low starting torque as it is not possible to

add external resistance.

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The Rotor is the inner rotating section

• Wound
• Laminated cylindrical core
• Has star connected three phase winding
• Open ends are connected to three separate insulated slip rings
• External resistances are connected to increase the starting torque

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The fundamental principle of operation

Is:

• The generation of a rotating magnetic field,
• This causes the rotor to turn at a speed that depends on the speed of rotation of the magnetic field

Fundamental Principle of Operation

A uniform rotating magnetic field is produced in the air gap between the rotor and stator by applying balanced 3 phase supply.

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Principle of Operation

• The stator supports windings a-a, b-b and c-c, which are geometrically spaced 120◦ apart.

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• Therefore, the currents generated by a 3-phase source are also spaced by 120◦.

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Principle of Operation

The phase voltages referenced to the neutral terminal, would then be given by the expressions

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• The coils are arranged so that the flux distribution generated by any one winding is approximately sinusoidal.

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• Since the coils are spaced 120◦ apart, the flux distribution resulting from the sum of the contributions of the three windings is the sum of the fluxes due to the separate windings.

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• Thus, the flux (in a three-phase machine) is a rotating vector in space, with constant amplitude.

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• Hence, A stationary observer on the machine’s stator would see a sinusoidally varying flux distribution.

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• Since the resultant flux is generated by the currents, the speed of rotation of the flux must be related to the frequency of the sinusoidal phase currents.

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• The number of magnetic poles resulting from the stator winding configuration is two. However, it is possible to configure the windings so that they have more poles.

In general,

• The speed of the rotating magnetic field is determined by the frequency of the excitation current, f , and

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• By the number of poles present in the stator, p, according to the equation

where n_s (or ω_s)

is usually called the synchronous speed.

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• The stator magnetic field rotates in an AC machine, and
• therefore the rotor cannot “catch up” with the stator field and is in constant pursuit of it.
• The speed of rotation of the rotor will therefore depend on the number of magnetic poles present in the stator and in the rotor.

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• The magnitude of the torque produced is a function of the angle γ between the stator and rotor magnetic fields

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• The number of stator and rotor poles must be identical if any torque is to be generated.

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It is important to generate a constant electromagnetic torque to avoid torque pulsations

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Pulsations could lead to undesired mechanical vibration in the motor itself and in other mechanical components attached to the motor (e.g., mechanical loads, such as spindles or belt drives).

Three Phase Currents

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Rotating Magnetic Field

Assume that the current waveforms are as in the top Figure.

• At the moment t = 0:
• Red phase current is at positive maximum
• Yellow and Blue phase currents are both at negative half-maximum.

Each of these currents produces a magnetic field. These fields interact to form the net field shown in the first sequence in the Figure.

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The magnetic field resembles that associated with a two pole bar magnet. As a consequence the machine is called a 2-pole motor.

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Rotating Magnetic Field

As time increases the current distribution changes:

• The red current falls;
• The yellow current becomes less negative eventually becoming positive and
• The blue current approaches negative maximum.

As these changes take place the net field, which maintains a constant magnitude, rotates clockwise

Hence, the second sequence shows the position after 1/3^rd cycle (120 electrical degrees):

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• The yellow current is at positive maximum and
• Red and Blue current are both at negative half-maximum.

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At this time, the field has rotated 120° from its original position.

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Rotating Magnetic Field

• After 2/3^rd cycle (third sequence) the field has moved a total of 240° and after one complete cycle (last sequence) the field has returned to its original position.
• The net field rotates at what is called the synchronous speed, n_s.
• This speed in revolutions per second is equal to the frequency, f, in hertz (Hz) or cycles per second, of the stator currents.

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n_s (rev s^-1) = f (in Hz)

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Consider a simple rotor, with one short circuited coil, inserted within the stator:

• Initially, the rotor is stationary.
• The moment the stator supply is switched on currents start to flow and the rotating magnetic field is established.
• The relative motion between the moving field and the stationary rotor conductors induces emf in the stationary rotor conductors (in accordance with Faraday’s Law)

Rotor Slip

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• Current start flowing in the conductors as they are short circuited by the end rings.
• These currents create their own magnetic fields, which interact with the rotating stator field to produce forces on the individual conductors and a net rotor torque

Rotor Slip

• The rotor starts to accelerate lowering the relative speed between the rotating field and rotor conductors.
• This reduces the induced emfs, conductor currents and subsidiary magnetic fields;
• thus decreasing the forces on the conductors and electrical torque on the rotor.

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The rotor continues to accelerate until the electrical torque exactly equals the mechanical load torque on the shaft.

• At this point the rotor is running at a speed slightly slower than the rotating field.
• This small difference in speed is needed.
• In order to create an electrical torque there must be some distortion of the net field, which will only happen when currents flow in the rotor conductors.
• These currents depend on emfs being induced in the conductors, which in turn depend on there being a difference between the speed at which the conductors rotate and that of the rotating magnetic field.

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This difference in speed is expressed as a ratio known as the (per unit) slip.

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Remembering that the rotational speed of magnetic field is known formally as the synchronous speed, the slip is defined as

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For most machines the value of the slip varies between around 0.01 on no-load, (when the only torque required is to overcome friction at the bearings) and 0.10 at full load.

Hence the rotor speed is always less than the stator rotating field speed and the difference is called “Slip”

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Note: For a stationary rotor the slip is 1; Generally the change in slip from no load to full load is 0.01 to 0.1 so the speed of the motor is constant.

What will happen if the rotor reaches the speed of the stator flux?

Is it practically possible?

• No relative speed between stator field and rotor conductor
• No induced current
• No torque

No, Because friction will slow down the rotor

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Where E₂ = emf induced in rotor winding at standstill

s = per unit slip

ω = 2πf (f = supply frequency in Hz)

X₂ = standstill rotor reactance per phase

a = (R₂ = rotor resistance per phase)

Mechanical output power = Torque × Angular velocity of rotor

P_m = T × (2πn)

P_m = 2πTn_s(1-s)

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But n_s = f and ω = 2πf

P_m = Tω(1-s)

Torque

A 3-phase 415V 2 pole 50Hz induction motor has an effective stator : rotor turns ratio of 2:1, rotor resistance 0.15Ω/phase and rotor standstill reactance 0.75Ω/phase. The motor runs at 2900 rev min^-1. Calculate

• a) per unit slip
• b) torque
• c) mechanical output power

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Exercise

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(T/T_m) versus s for various values of a = (R₂/X₂)

Torque-Slip Characteristics

The graphs show that in steady state conditions induction motors with the smallest value of “a” run at practically constant speed over the normal operating range of the machine. Unfortunately, these machines generally have poor starting torques and for a motor to start it is necessary that Starting torque > load torque

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Multi-Pole Motors

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Variable Frequency Supplies

• Previously, it was shown that the synchronous speed of an induction motor is totally dependent on the frequency of the stator currents.

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• As the rotor speed is dependent on the synchronous speed then the former is also dependent on the supply frequency.
• By using an inverter to provide a variable frequency supply the speed can be controlled.

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• Inverters with ratings up to 750 kW can provide control over speed ranges varying from 10:1 to 100:1.
• These are most commonly used in pumping applications, synchronised paper presses and conveyor systems.

A brushless dc motor has:

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• A rotor with permanent magnets, and
• A stator with windings.
• It is essentially a dc motor turned inside out.

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Brushless DC Motors

• The control electronics replaces the function of the commutator and energize the proper winding.

• A brushless ac motor is driven with ac sine wave voltages.
• The permanent magnet rotor rotates synchronous to the rotating magnetic field.
• The rotating magnetic field is illustrated using a red and green gradient.

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Brushless AC Motors

• An actual simulation of the magnetic field would show a far more complex magnetic field.

• The stator windings of an ac induction motor are distributed around the stator to produce a roughly sinusoidal distribution.
• When three phase ac voltages are applied to the stator windings, a rotating magnetic field is produced.
• The rotor of an induction motor also consists of windings or more often a copper squirrel cage imbedded within iron laminates. Only the iron laminates are shown.

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AC Induction Motor

• An electric current is induced in the rotor bars which also produce a magnetic field.

• The rotating magnetic field of the stator drags the rotor around. The rotor does not quite keep up with the rotating magnetic field of the stator.
• It falls behind or slips as the field rotates.
• Here, the slip has been greatly exaggerated to enable visualization of this concept. A real induction motor only slips a few percent.

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AC Induction Motor

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• In this unit we explained the principle of operation of 3- phase induction motor and its applications, with particular focus on Induction Motors:
• Its Construction
• Principles of Operation
• Torque
• Torque vs. Slip Characteristics
• Multi-pole Motors

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• We also briefly highlighted the difference between popular DC and AC Machines:
• Conventional DC,
• Brushless DC,
• Brushless AC
• Induction Motor

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Conclusion

THANK YOU