Figure 4.5 Elementary two-pole cylindrical-rotor field winding.
Most of the world’s power systems are three-phase systems. With very few exceptions, synchronous generators are three-phase machines.
A simplified schematic view of a three-phase, two-pole machine with one coil per phase is shown in Fig. 4.6 (a)
Fig. 4.6(b) depicts a simplified three-phase, four-pole machine. Note that a minimum of two sets of coils must be used. In an elementary multipole machine, the minimum number of coils sets is given by one half the number of poles.
Note that coils (a,a) and
can be connected in series or in parallel. Then the coils of the three phases may then be either Y- or
-connected. See Fig. 4.6(c).
Figure 4.6 Schematic views of three-phase generators: (a) two-pole, (b) four-pole, and
(c) Y connection of the windings.
The electromechanical torque is the mechanism through which a synchronous generator converts mechanical to electric energy.
When a synchronous generator supplies electric power to a load, the armature current creates a magnetic flux wave in the air gap that rotates at synchronous speed.
This flux reacts with the flux created by the field current, and an electromechanical torque results from the tendency of these two magnetic fields to align.
In a generator this torque opposes rotation, and mechanical torque must be applied from the prime mover to sustain rotation.
The counterpart of the synchronous generator is the synchronous motor.
Ac current supplied to the armature winding on the stator, and dc excitation is supplied to the field winding on the rotor. The magnetic field produced by the armature currents rotates at synchronous speed.
To produce a steady electromechanical torque, the magnetic fields of the stator and rotor must be constant in amplitude and stationary with respect to each other.
In a motor the electromechanical torque is in the direction of rotation and balances the opposing torque required to drive the mechanical load.
In both generators and motors, an electromechanical torque and a rotational voltage are produced which are the essential phenomena for electromechanical energy conversion.
Note that the flux produced by currents in the armature of a synchronous motor rotates ahead of that produced by the field, thus pulling on the field (and hence on the rotor) and doing work. This is the opposite of the situation in a synchronous generator, where the field does work as its flux pulls on that of the armature, which is lagging behind.
Induction Machines
Alternating currents are applied directly to the stator windings. Rotors currents are then produced by induction, i.e., transformer action.
Alternating currents flow in the rotor windings of an induction machine, in contrast to a synchronous machine in which a field winding on the rotor is excited with dc current.
The induction machine may be regarded as a generalized transformer in which electric power is transformed between rotor and stator together with a change of frequency and a flow of mechanical power.
The induction motor is the most common of all motors.
The induction machine is seldom used as a generator.
In recent years it has been found to be well suited for wind-power applications.
It may also be used as a frequency changer.
In the induction motor, the stator windings are essentially the same as those of a synchronous machine.The rotor windings are electrically short-circuited.
The rotor windings frequently have no external connections.
Currents are induced by transformer action from the stator winding.
Squirrel-cage induction motor: relatively expensive and highly reliable.
The armature flux in the induction motor leads that of the rotor and produces an electromechanical torque.
The rotor does not rotate synchronously.
It is the slipping of the rotor with respect to the synchronous armature flux that gives rise to the induced rotor currents and hence the torque.
Induction motors operate at speeds less than the synchronous mechanical speed.
A typical speed-torque characteristic for an induction motor is shown in Fig.4.7.
Questions & Answers
A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
