1. THREE-PHASE INDUCTION MOTOR
CHAPTER 7
THREE-PHASE INDUCTION MOTOR
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2. It can be considered to be the cheapest motor.
Three-phase induction motors are the most common and frequently encountered machines in industry
It can be considered to be the cheapest motor.
It is rugged and requires less maintenance.
It is simple in design.
It gives reliable operation.
Its efficiency is very high.
It is easy to control
It runs at constant speed from zero to full load
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3. CLASSIFICATION OF INDUCTION MOTOR
Depending on the rotor construction, induction motor can be classified into two categories:
Squirrel-cage induction motor.
Slip-ring induction motor or wound rotor induction motor.
Depending on the number of phases it can be classified as:
Single-phase induction motor
Three-phase induction motor
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4. CONSTRUCTION
The three basic parts of an AC motor are the
rotor, stator, and enclosure.
A stationary stator
consisting of a steel frame that supports a hollow, cylindrical core
core, constructed from stacked laminations, having a number of evenly spaced slots, providing the space for the stator winding
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5. CONSTRUCTION A revolving rotor
composed of punched laminations, stacked to create a series of rotor slots, providing space for the rotor winding
one of two types of rotor windings
conventional 3-phase windings made of insulated wire similar to the winding on the stator
aluminum bus bars shorted together at the ends by two aluminum rings, forming a squirrel-cage shaped circuit.
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6. CONSTRUCTION Squirrel cage type: Wound rotor type:
Rotor winding is composed of copper bars embedded in the rotor slots and shorted at both end by end rings
Simple, low cost, robust, low maintenance
Wound rotor type:
Rotor winding is wound by wires. The winding terminals can be connected to external circuits through slip rings and brushes.
Easy to control speed, more expensive.
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7. PRINCIPLE OF OPERATION
When a 3 phase stator winding is connected to a 3 phase voltage supply, 3 phase current will flow in the windings, which also will induced 3 phase flux in the stator. These flux will rotate at a speed called a Synchronous Speed, ns. The flux is called as Rotating magnetic Field.
Synchronous speed is given by the expression
Where p = is the number of poles, and
f = the frequency of supply
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8. PRINCIPLE OF OPERATION
This rotating magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings
Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound-rotor, and induced current flows in the rotor windings
The rotor current produces another magnetic field
A torque is produced as a result of the interaction of those two magnetic fields
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9. SLIP
The rotor speed of an Induction machine is different from the speed of Rotating magnetic field. The % difference of the speed is called slip.
Where; ns = synchronous speed (rpm)
nr = mechanical speed of rotor (rpm)
If the rotor runs at synchronous speed, s = 0. If the rotor is stationary, s = 1
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10. f is the supply frequency, E2 is the standstill e.m.f.
Effect of slip on rotor frequency
Effect of slip on rotor induced e.m.f.
Effect of slip on rotor resistance,
reactance and impedance
Where fr is the rotor frequency, Er2 is the rotor emf, R2, X2 and Z2 is the rotor resistance, reactance and impedance respectively.
f is the supply frequency, E2 is the standstill e.m.f.
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11. TORQUE EQUATION OF A THREE-PHASE INDUCTION MOTOR
The torque of a three-phase induction motor depends on the following factors:
Rotor current (I2)
Power factor of the rotor circuit
Flux which links with the rotor (φ) or
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12. Torque Equation for Induction Motor in Running Condition
Where,
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13. TORQUE SLIP CHARACTERISTICS
When slip s = 0, Nr = Ns. Under this condition, the motor stops. So the torque (T) at this value of s is zero. This shows that the torque slip characteristics starts from the origin.
For smaller values of slip, the torque is directly proportional to the slip.
For larger values of slip, the torque is inversely proportional to slip.
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14. TORQUE SLIP CHARACTERISTICS
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15. RATIO BETWEEN FULL-LOAD TORQUE AND MAXIMUM TORQUE
Where Tfl is the full load torque Tmax is the maximum torque sfl is the slip at full load and sm is the slip at maximum torque.
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16. RATIO BETWEEN STARTING TORQUE AND MAXIMUM TORQUE
Where Tst is the starting torque Tmax is the maximum torque, sm is the slip at maximum torque.
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17. EFFECT OF ROTOR RESISTANCE ON TORQUE–SLIP CHARACTERISTIC
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18. Equivalent Circuit of Induction Motor
Equivalent circuit with Rotor open circuited
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19. Equivalent Circuit of Induction Motor
Equivalent circuit with Rotor short circuited
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20. Equivalent Circuit of Induction Motor
Equivalent circuit with electrical equivalent of mechanical load
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21. No-load Test
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22. No-load Test The motor is allowed to run freely.
This test determines the rotational and core loss of the motor .
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23. Block Rotor Test
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24. Block Rotor Test In this test, the rotor is blocked from rotating.
This test gives the total winding resistance and leakage reactance of the motor.
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25. STARTING OF THREE-PHASE INDUCTION MOTOR
There are several methods of starting squirrel-cage induction motors, which include: i) Direct on-line starting. ii) Autotransformer starting iii) Star–delta starting
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26. Direct on-line (DOL) starter
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27. Autotransformer starter
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28. Star-delta starter
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29. SPEED CONTROL
From the stator side, the speed of an induction motor can be controlled by the following methods: i) V/f method of controlling the speed of induction motor ii) Speed control by changing the supply voltage. iii) Speed control by changing the number of poles. iv) Speed control by changing the rheostat connected with the stator terminals. From the rotor side the speed of an induction motor can be controlled by the following methods: i) Speed control by changing the rheostat connected with the rotor terminals. ii) Cascade control
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30. BRAKING
The braking methods for induction motor are as follows: i) Regenerative braking of induction motor ii) Braking by plugging iii) Dynamic braking
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