Tiêu đề 1 . UNIVERSITY EXAMINATION MODEL Answer key.pdf ✅

UNIVERSITY EXAMINATION MODEL QUESTION PAPER Course Code: EE205 Course Name: DC Machines and Transformer Answer Key PART A 1. In lap-wound machine, the circulating current will exist if in the emfs generated various parallel paths (equal to the no .equal. The existence of circulating current in lap - wound machine may due to any one or all of the following reason: a. The air gap under different poles may not be the same, due to wear of the bearings or faulty assembly. As a result of it, some poles carry more flux than the other poles and, therefore, different emfs. are generated in the various parallel paths. b. The joints b/w pole cores and yokes or b/w pole - cores and pole - faces may not be identical for all the poles. This result in different reluctance for the various magnetic circuit. Even if the air gap is uniform, the pole flux will be different under the various pole and, therefore, different emfs are generated in the parallel circuits. c. The different reluctance for each magnetic path may also be due to the impurities or imperfections in the materials constituting the magnetic circuit. The circulating current result in overheating of the armature. In order to avoid overloading and sparking at the brushes, it is essential to take some steps so that circulating currents are not handled by the brushes. These steps consists in providing equalizer rings or equalizer connections. An equalizer ring has a low resistance conductor wire, which connects together the points in the armature winding which should be at the same potentials. The function of equalizer ring is to cause the circulating current to flow within the armature winding itself, without letting them pass through the brushes. The function of equalizer rings are (i) to save the brushes from handling the circulating currents (ii) to reduce the magnetic flux unbalance that cause the potential difference in various parallel paths. 2. Good commutation means linear commutation. Two ways to promote good commutation are Resistance commutation and EMF commutation. a) Resistance Commutation: The effect of reactance voltage is minimized by increasing the contact resistance between brushes and commutator. This is achieved by using carbon brushes. b) EMF Commutation: In this method, an arrangement is made to neutralise the reactance voltage by producing a reverse voltage in the coil undergoing commutation. The reversing voltage can be produced by the use of interpoles. As their polarity is same as the main pole ahead, they induce an emf in the coil undergoing commutation which opposes the reactance voltage. KTUQBANK.COM
3. Motor armature current is decided by the load. On light load or no load, the armature current drawn by the motor is very small. In case of a d.c. series motor, since the field windings are in series with armature, Φ α Ia and on no load as Ia is small. Hence flux produced is also very small. According to speed equation, N α 1/Φ as Eb is almost constant. So on very light load or no load as flux is very small, the motor tries to run at dangerously high speed which may damage the motor mechanically. This can be seen from the speed-armature current and the speed-torque characteristics that on low armature current and low torque condition motor shows a tendency to rotate with dangerously high speed. This is the reason why series motor should never be started on light loads or no load conditions. For this reason it is not selected for belt drives as breaking or slipping of belt causes to throw the entire load off on the motor and made to run motor with no load which is dangerous. 4. 5. The ordinary or commercial efficiency of a transformer is defined as the ratio of output power to the input power. Commercial efficiency = InputPower OutputPower There are certain types of transformers whose efficiency cannot be judged by this efficiency. For instance, distribution transformers used for supplying lighting loads have their primaries energized all the 24 hours in a day but the secondaries supply little or no load during the major portion of the day. It means that the constant loss (iron loss) occurs during the whole day but copper loss occurs only when transformer is loaded and would depend upon the magnitude of load. Consequently, the copper loss varies considerably during the day and commercial efficiency of such transformers will vary from a low value to a high value when the load is high. Performance of such transformers is judged on the basis of energy consumption during the whole day. This is known as All-day efficiency. KTUQBANK.COM
The ratio of output KWh to the input KWh of a transformer over 24hr period is known as all-day efficiency. KWh input in hours KWh output in hours all day 24 24    All-day efficiency is a of special importance for those transformers whose primaries are never open-circuited and secondaries carry little or no-load much of the time during a day. In the design of such transformers, efforts to reduce the iron losses which continuously occurs during the whole day. 6. KTUQBANK.COM
7. 8. In some high rating transformers, a third winding is constructed in addition to the primary and secondary windings. This third winding is called Tertiary Winding and the transformer is called Three Winding Transformer. The voltage ratings of all three windings of transformer are usually unequal. The primary winding has the highest voltage rating; the tertiary has the lowest voltage rating, and the secondary has the intermediate voltage rating. The most significant advantage of the third winding is that the harmonic generated by the primary and secondary winding extinguish by the third winding. The third winding is connected in delta.  It reduces the unbalancing in the primary due to three phase unbalanced load.  It can supply additional load providing isolation from the secondary windings.  It can act as a voltage source to supply substation auxiliaries at a voltage different from those of primary and secondary winding. KTUQBANK.COM