Induction machines Flashcards
What is the difference between Td and Tout and what are their Eqns.?
- Td is the internal torque generated by the internal electrical to mechanical power conversion
- Td = pd/wm=(1-S)Pg/(1-S)wsync=PAG/wsync
- Tout is the torque at the terminals of the motor including the friction losses
- Tout= pout/wm
What are all the rules for Td?
- Td=PAG/Wsync=3I22*R’2/S/Wsync
Draw the graph of the torque speed charactristics and label it?
Describe the torque speed charactristics curve and explain what we can understand from it?
- The torque at synchronous speed is zero (NS)
- There is a torque called pullout torque that is the maximum torque that can’t be exceeded and is 2 to 3 times higher than thefull load torque
- There is a linear region where the resistance is much greater than the reactance so the torque increases linearly with the slip
- The starting torque is higher than the full load torque
- If the rotor is driven faster than synchronous speed it will run as a generator
What are the conditions for the motor operation of the induction machine?
- 0
- PAG=+ve
- Pd=+ve
What are the conditions for the generator operation of the induction machine?
- An external prime mover is used to rotate the rotor at speed higher than synchronous speed
- S<0
- PAG=-ve
- Pd=-ve
What are the conditions for the brake operation of the induction machines?
- Used to stop moving shafts by switching the two phases of the stator to switch the direction of tha magnetic field.
- 1
- PAG=+ve
- Pd=-ve
What is the Eqn of the slip at maximum torque?
What is the Eqn of the maximum torque?
Or we can use the eqn of the starting torque and substitute S by Smax
How is the line voltage speed control method achieved and what are its properties?
- Can be done by using an autotransformer which is expensive
- Or by adding a resistance to the stator winding which would decrease the effeciancy
- Adding a reactance to the stator winding which would decrease the PF
- Decreasing the Supply voltage decreases the maximum torque
- Only used for speeds less than the base speed
- Provides limited range of speed control
- Only used for small driving fans
What are the properties of the Rotor resistance speed control?
- Achieved by adding a resistance to the rotor circuit
- Only used for speeds lower than base speed
- Can only be used with wound rotor machines
- Decreases effeciancy and increases losses
- Wide range of speed control
- Increasing Ra decreases nm
- Maximum torque is kept constant
- Cheap
What are the properties of supply frequency speed control method?
- Changing supply frequency changes the synchronous speed so the curve can be shifted to right or left
- As the frequency increases nm increases
- Motor speed can be controlled from 5% to 200% the base speed
- Wide range of speed control
- Can be used with squirel cage and slip ring machines
- expensive
- complicated
What is the Eqn of Tstart and at which conditions?
At S=1
What are the general properties of the induction machines?
- Mostly used as generators
- called induction because the volt is induced in the rotor winding rather than being connected by a wire
- Doesn’t need Dc field current to operate the machine
- Generators only used for the wind turbine
What are the advantetges of the induction machine that makes it widely used?
- Easy to be maintained
- Self exciting
- Rugged device
- Cheaper than synchronous machines
- Singly excited as power is only supplied to stator
- Wide range of power