11 Questions
What is the purpose of the stator windings in a three-phase alternator?
Produce a three-phase voltage
How is the rotor of an alternator energized to function as an electromagnet?
DC current passing through a coil in the rotor
What is the mechanical device responsible for rotating the rotor of an alternator called?
Prime mover
In the context of an alternator, what are armature windings typically designed as?
Short (fractional) pitched and distributed
Which factor determines the frequency of the voltage generated in an alternator?
Rotor speed and number of poles
According to the provided information, how is the frequency of the generated voltage related to rotor speed?
$f = \frac{N}{120P}$
What is the primary purpose of mechanically coupling the induction motor and synchronous generator?
To transfer mechanical power from the induction motor to the synchronous generator
What is the role of the tachogenerator and tachometer in the setup?
To measure the rotational speed of the synchronous generator
How is the field of the synchronous generator supplied?
From a separate variable dc supply
What is the purpose of connecting the induction motor directly to the 3-phase, 415V ac supply?
To drive the rotation of the induction motor and synchronous generator
What is the purpose of the ammeters in the setup?
To measure the output current of the synchronous generator
Study Notes
Alternator Construction
- A three-phase alternator has three sets of stator windings, spaced 1200 apart, to produce a three-phase voltage.
- The rotor contains a coil with a dc current that produces an electromagnet.
- The dc current for the rotor can be injected directly from a dc source using brush and slip ring or by a brushless dc exciter.
Alternator Components
- The mechanical device that rotates the rotor is called the prime mover.
- The alternator stator is usually called the armature.
- Armature windings are often short (fractional) pitched and distributed for better performance.
Frequency of Induced Voltage
- The voltage induced in the phase windings of an alternator are of the same frequency.
- The frequency of the generated voltage (f) depends on the rotor speed (N) and the number of rotor poles (P).
- The frequency is related to the rotor speed by the equation: N = 120f/P.
Experimental Procedure
- Mechanically couple the three-phase induction motor and synchronous generator with a timing belt.
- Connect the tachogenerator with the alternator shaft and the tachometer.
- Connect the induction motor, synchronous generator, ammeters, and power supply outputs with leads according to the diagram in Figure 1.
- The induction motor is directly connected to a 3-phase, 415V ac supply.
- The field of the synchronous generator is supplied from variable dc supply terminals 7 and N.
Test your knowledge about three-phase alternators, stator windings, rotors, and prime movers. Learn about how the rotating magnetic field produces three-phase voltage and the different methods of injecting DC current into the rotor.
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