Sample Paper Of Electrical Machines are give below students can learn and read for the Interview and Semester point of view.
Introduction of Electrical Machine
The basic idea of an Electrical machine comes from the transformarion of Energy from one source to another an Electrical Machine is based on this phenomena.
An electric generator is a device that converts mechanical energy to electrical energy.
Sample Paper Of Electrical Machines given Below:
1 Explain generated emf of rotating electrical machines.
Ans: The EMF generated by Faraday’s law of induction due to relative movement of a circuit and a magnetic field is the phenomenon underlying electrical generators. When a permanent magnet is moved relative to a conductor, or vice versa, an electromotive force is created. If the wire is connected through an electrical load, current will flow, and thus electrical energy is generated, converting the mechanical energy of motion to electrical energy.
2 Explain no load and blocked rotor test of three phase induction motor.
Ans: If we assume that the machine rotor is at a standstill and the armature is excited, then the armature-produced rotating field is moving with respect to the rotor. In fact, the relative speed between the rotating field and the rotor is synchronous speed. For this condition, the rotating field induces a large voltage in the rotor bars. The large voltage causes a large current in the squirrel-case which, in turn, creates a magnetic field in the rotor. The rotor magnetic field interacts with the armature magnetic field, and a torque is produced. If the produced torque is larger than any load torque, the rotor begins to turn. As the rotor accelerates, the speed difference between the rotor and the armature field is reduced. This reduced speed difference (or slip) causes the induced rotor voltage to be reduced, the rotor current to be reduced, the rotor flux to be reduced, and the torque produced by the machine to be reduced. Eventually, the torque produced by the motor equals the torque demanded by the load, and the motor settles to an equilibrium rotor speed. This equilibrium rotor speed must be less than synchronous speed since there must be a slip to produce torque.
3 Explain deep bar and double cage rotor induction motor.
Ans: Double-Cage Rotors An induction motor with two rotor windings or cages is used for obtaining high starting torque at low current. The stator of a double cage induction motor is similar to that of ordinary induction motor. In double cage rotor there are two layers of bars as shown: The outer cage bars have a smaller cross-sectional area than the inner bars and are made of high resistivity materials like brass etc
4 Explain constructional features of single phase induction motor
Ans: The stator of an induction motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimize the distribution of the magnetic field, the windings are distributed in slots around the stator, with the magnetic field having the same number of north and south poles. Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases. Many single-phase motors having two windings can be viewed as two-phase motors, since a capacitor is used to generate a second power phase 90 degrees from the single-phase supply and feeds it to the second motor winding. Single-phase power is more widely available in residential buildings, but cannot produce a rotating field in the motor, so they must incorporate some kind of starting mechanism to produce a rotating field.
5 Name various split phase starting methods of single phase induction motor.
Ans: There are 3 most commonly used methods of starting single-phase AC induction motors:Shaded Pole:This method is commonly used on small AC induction motors, like might be found in an ice cream freezer or electric fan. It involves a physical object acting as a small coil which causes a separate starting magnetic field out of phase with the main field generated by the primary induction coil..
Capacitor Start:This method is commonly used on larger AC induction motors, like might be found in an air-conditioner blower motor or compressor. In a Capacitor Start motor, a second coil offset somewhat from the main coil, is connected in series with a capacitor to cause a phase shift.
“Split-phase” or Switch Phase:This method is commonly used on motors that require a great deal of starting torque, like many washing machines and some dryers. So-called “Split-phase” motors use a more powerful offset coil and capacitor combination that is turned on and off by a centrifugal switch controlled by the rotor speed. When the motor is stopped or turning slowly, the centrifugal switch engages the start winding and provides maximum torque for starting the motor.
6 Explain V-curves of synchronous motor.
Ans: V curve is the graph showing the relation of armature current as a function of field current in synchronous machines. The purpose of the curve is to show the variation in the magnitude of the armature current as the excitation voltage of the machine is varied.
7 Why do cylindrical Alternators operate with steam turbines?
Ans Steam turbines are found to operate at fairly good efficiency only at highspeeds. The high speed operation of rotors tends to increase mechanical lossesand so the rotors should have a smooth external surface. Hence, smoothcylindrical type rotors with less diameter and large axial length are used forSynchronous generators driven by steam turbines with either 2 or 4 poles.
8 Which type of Synchronous generators are used in Hydro-electric plants and why?
Ans As the speed of operation is low for hydro turbines use din Hydro-electric plants, salient pole type Synchronous generators are used. These allow better ventilation and also have other advantages over smooth cylindrical type rotor. Some More Short Questions in Sample Paper Of Electrical Machines
1. What is meant by load angle of an Alternator?
Ans The phase angle introduced between the induced emf phasor, E and terminal voltage phasor , U during the load condition of an Alternator is called load angle.
2 Upon what factors does the load angle depend?
Ans 7KH PDJQLWXGH RI ORDG DQJOH LQFUHDVHV ZLWK LQFUHDVH LQ ORDG )XUWKHU WKH ORDG angle is positive during generator operation and negative during motor operation.
3 An Alternator is found to have its terminal voltage on load condition more than that on no load. What is the nature of the load connected?
The nature of the load is of leading power factor , load consisting of resistance and capacitive reactance.
4 Define the term voltage regulation of Alternator.
Ans The voltage regulation of an Alternator is defined as the change in terminal voltage from no-load to load condition expressed as a fraction or percentage of terminal voltage at load condition ; the speed and excitation conditions remaining same.
5 Why is the MMF method of estimating the voltage regulation considered as the
Ans Compared to the EMF method, MMF method, involves more number of complex calculation steps. Further the OCC is referred twice and SCC is referred once while predetermining the voltage regulation for each load condition. Reference of OCC takes care of saturation effect. As this method require more effort, the final result is very close to the actual value. Hence this method is called optimistic method