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INDUCTION MACHINE Presentation Transcript
1.INDUCTION MACHINE
2.Three-phase induction motors are the most common and frequently encountered machines in industry
simple design, rugged, low-price, easy maintenance
wide range of power ratings: fractional horsepower to 10 MW
run essentially as constant speed from no-load to full load
Its speed depends on the frequency of the power source
not easy to have variable speed control
requires a variable-frequency power-electronic drive for optimal speed control
simple design, rugged, low-price, easy maintenance
wide range of power ratings: fractional horsepower to 10 MW
run essentially as constant speed from no-load to full load
Its speed depends on the frequency of the power source
not easy to have variable speed control
requires a variable-frequency power-electronic drive for optimal speed control
3.Construction
An induction motor has two main parts
a stationary stator
consisting of a steel frame that supports a hollow, cylindrical core
core, constructed from stacked laminations (why?), having a number of evenly spaced slots, providing the space for the stator winding
An induction motor has two main parts
a stationary stator
consisting of a steel frame that supports a hollow, cylindrical core
core, constructed from stacked laminations (why?), having a number of evenly spaced slots, providing the space for the stator winding
4. 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 (wound-rotor) » 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 (squirrel-cage)
Two basic design types depending on the rotor design
squirrel-cage: conducting bars laid into slots and shorted at both ends by shorting rings.
wound-rotor: complete set of three-phase windings exactly as the stator. Usually Y-connected, the ends of the three rotor wires are connected to 3 slip rings on the rotor shaft. In this way, the rotor circuit is accessible.
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 (wound-rotor) » 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 (squirrel-cage)
Two basic design types depending on the rotor design
squirrel-cage: conducting bars laid into slots and shorted at both ends by shorting rings.
wound-rotor: complete set of three-phase windings exactly as the stator. Usually Y-connected, the ends of the three rotor wires are connected to 3 slip rings on the rotor shaft. In this way, the rotor circuit is accessible.
5.Construction
6.Rotating Magnetic Field
Balanced three phase windings, i.e. mechanically displaced 120 degrees form each other, fed by balanced three phase source
A rotating magnetic field with constant magnitude is produced, rotating with a speed
Balanced three phase windings, i.e. mechanically displaced 120 degrees form each other, fed by balanced three phase source
A rotating magnetic field with constant magnitude is produced, rotating with a speed
7.Synchronous speed
8.Rotating Magnetic Field
9.Principle of operation
10.Induction motor speed
At what speed will the IM run?
Can the IM run at the synchronous speed, why?
If rotor runs at the synchronous speed, which is the same speed of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating magnetic field will not cut the rotor. So, no induced current will flow in the rotor and no rotor magnetic flux will be produced so no torque is generated and the rotor speed will fall below the synchronous speed
When the speed falls, the rotating magnetic field will cut the rotor windings and a torque is produced
At what speed will the IM run?
Can the IM run at the synchronous speed, why?
If rotor runs at the synchronous speed, which is the same speed of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating magnetic field will not cut the rotor. So, no induced current will flow in the rotor and no rotor magnetic flux will be produced so no torque is generated and the rotor speed will fall below the synchronous speed
When the speed falls, the rotating magnetic field will cut the rotor windings and a torque is produced
11.Induction motor speed
So, the IM will always run at a speed lower than the synchronous speed
The difference between the motor speed and the synchronous speed is called the Slip
So, the IM will always run at a speed lower than the synchronous speed
The difference between the motor speed and the synchronous speed is called the Slip
12.The Slip
13.Induction Motors and Transformers
Both IM and transformer works on the principle of induced voltage
Transformer: voltage applied to the primary windings produce an induced voltage in the secondary windings
Induction motor: voltage applied to the stator windings produce an induced voltage in the rotor windings
The difference is that, in the case of the induction motor, the secondary windings can move
Due to the rotation of the rotor (the secondary winding of the IM), the induced voltage in it does not have the same frequency of the stator (the primary) voltage
Both IM and transformer works on the principle of induced voltage
Transformer: voltage applied to the primary windings produce an induced voltage in the secondary windings
Induction motor: voltage applied to the stator windings produce an induced voltage in the rotor windings
The difference is that, in the case of the induction motor, the secondary windings can move
Due to the rotation of the rotor (the secondary winding of the IM), the induced voltage in it does not have the same frequency of the stator (the primary) voltage
14.Horse power
Another unit used to measure mechanical power is the horse power
It is used to refer to the mechanical output power of the motor
Since we, as an electrical engineers, deal with watts as a unit to measure electrical power, there is a relation between horse power and watts
Another unit used to measure mechanical power is the horse power
It is used to refer to the mechanical output power of the motor
Since we, as an electrical engineers, deal with watts as a unit to measure electrical power, there is a relation between horse power and watts
15.Equivalent Circuit
16.Rotating Magnetic Field and Slip (cont’d)
17.Power losses in Induction machines
18.Power flow in induction motor
19. Power relations
20. Torque-speed characteristics
21.Comments
The induced torque is zero at synchronous speed. Discussed earlier.
The curve is nearly linear between no-load and full load. In this range, the rotor resistance is much greater than the reactance, so the rotor current, torque increase linearly with the slip.
There is a maximum possible torque that can’t be exceeded. This torque is called pullout torque and is 2 to 3 times the rated full-load torque.
The induced torque is zero at synchronous speed. Discussed earlier.
The curve is nearly linear between no-load and full load. In this range, the rotor resistance is much greater than the reactance, so the rotor current, torque increase linearly with the slip.
There is a maximum possible torque that can’t be exceeded. This torque is called pullout torque and is 2 to 3 times the rated full-load torque.
22.The starting torque of the motor is slightly higher than its full-load torque, so the motor will start carrying any load it can supply at full load.
The torque of the motor for a given slip varies as the square of the applied voltage.
If the rotor is driven faster than synchronous speed it will run as a generator, converting mechanical power to electric power.
The torque of the motor for a given slip varies as the square of the applied voltage.
If the rotor is driven faster than synchronous speed it will run as a generator, converting mechanical power to electric power.
23.Maximum torque
24. Maximum torque
Rotor resistance can be increased by inserting external resistance in the rotor of a wound-rotor induction motor.
The value of the maximum torque remains unaffected but the speed at which it occurs can be controlled.
Rotor resistance can be increased by inserting external resistance in the rotor of a wound-rotor induction motor.
The value of the maximum torque remains unaffected but the speed at which it occurs can be controlled.
25.Determination of motor parameters
Due to the similarity between the induction motor equivalent circuit and the transformer equivalent circuit, same tests are used to determine the values of the motor parameters.
DC test: determine the stator resistance R1
No-load test: determine the rotational losses and magnetization current (similar to no-load test in Transformers).
Locked-rotor test: determine the rotor and stator impedances (similar to short-circuit test in Transformers).
Due to the similarity between the induction motor equivalent circuit and the transformer equivalent circuit, same tests are used to determine the values of the motor parameters.
DC test: determine the stator resistance R1
No-load test: determine the rotational losses and magnetization current (similar to no-load test in Transformers).
Locked-rotor test: determine the rotor and stator impedances (similar to short-circuit test in Transformers).
26.DC test
The purpose of the DC test is to determine R1. A variable DC voltage source is connected between two stator terminals.
The purpose of the DC test is to determine R1. A variable DC voltage source is connected between two stator terminals.
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