JEPPIAAR ENGINEERING COLLEGE
DEPARTMENT OF ELECTRICAL AND ELECTRONICS
131307-ELECTRICAL ENGINEERING-IMPORTANT QUESTIONS WITH ANSWER KEY
1. EXPLAIN IN DETAIL ABOUT THE CONSTRUCTION OF DC GENERATOR WITH ITS CONSTRUCTION PARTS?
2. EXPLAIN IN DETAIL ABOUT THE CHARACTERISTICS OF DC MOTOR?
3. EXPLAIN THE 3 POINT STARTER IN DETAIL
1. EXPLAIN IN DETAIL ABOUT THE CONSTRUCTION OF A TRANSFORMER?
2. EXPLAIN THE EQUIVALENT CIRCUIT IN A TRANSFORMER?
3. EXPALIN THE TRANSFORMER TESTS IN DETAIL?
1. CONSTRUCTION OF AN INDUCTION MOTOR
Synchronous Motors at low Speeds
On the other hand, in the construction of small synchronous motors to run at low angular velocities, it is extremely difficult to find space for the numerous conductors and for the exciting or field coils, which must produce as many ampere-turns as in the case of large motors. For this reason non-synchronous motors are more convenient for low rotative speeds.
The author has been able, however, to produce motors of low power (a few hundred watts) which have moving iron and have a very high number of poles (as many as 50 for example), by utilizing inductiontype excitation, the magnetic circuit being closed exteriorly, as shown in Fig. 2, in such a way as to allow all the space needed for the exciting coils.
These coils can then be replaced by permanent magnets, thus producing motors which run without excitation, at speeds sufficiently low to be synchronized by hand, and which can render useful service, in certain applications, such as for oscillographs. For this purpose the author preferably employs a small horseshoe magnet that is made to revolve around a stationary armature having a number of poles which is a multiple of 6. It is possible, in this way, to obtain very stable synchronous rotation of a revolving mirror without expending more than 1.5 to 2 watts.
To understand the behaviour of induction motors, it is helpful to understand their distinction from a synchronous motor. A synchronous motor always runs at a shaft rotation frequency that is an integer fraction of the supply frequency; the synchronous speed of an induction motor is the same. It can be shown that ns in rpm is determined by
where f is the frequency of the AC supply in Hz and p is the number of magnetic poles per phase. Some texts refer to the number of pole pairs per phase; a 6 pole motor would have 3 pole pairs. In this case, P, the number of pole pairs, takes the place of p in the equation
2. Explain the construction of reluctance motor?
Reluctance motors operate on the principle that forces are established that tend to cause iron poles carrying a magnetic flux to align with each >other. One form of reluctance motor is shown in cross section in the figure. The rotor consists of four iron poles with no electrical windings. The stator has six poles each with a current-carrying coil. In the condition represented in the figure, current has just been passed through coils a and a′, producing a torque on the rotor aligning two of its poles with those of the a-a′ stator. The current is now switched off in coils a and a′ and switched on to coils b and b′. This produces a counterclockwise torque on the rotor aligning two rotor poles with stator poles b and b′. This process is then repeated with stator coils c and c′ and then with coils a and a′. The torque is dependent on the magnitude of the coil currents but is independent of its polarity. The direction of rotation can be changed by changing the...