www.Vidyarthiplus.comDEPARTMENT OF ELECTRONICS AND INSTRUMENTATION ENGINEERING EE6402 ELECTRICAL MACHINESUNIT I- D.C. MACHINESPART A (2 MARKSWhy are electro-magnets referred over permanent magnets for use in large DC Machines? By switching the current on or off in a coil its magnetic field can be switched on or off or by switching the direction of the current in the coil the direction of the generated magnetic field can be switched 180 How can the direction of rotation of a DC shunt motor be reversed? Either the direction of the main field or the direction of current through the armature conductors is to be reserved. What is armature reaction? The interaction between the main flux and armature flux cause disturbance called as armature reaction. Write the induced emf equation when the machine act as s DC motor and DC generator. Machine act as motor: Back emf =PPMachine act as motor: Generated emf=State the conditions which determines if a DC machine is generating or Motoring Generators are normally placed in closed room and accessed by skilled operators only. Therefore on ventilation point of view they may be constructed with large opening in the frame. Motors have to be installed right in the place of use which may have dust, dampness, inflammable gases, chemicals.etc. to protect the motors against these elements, the motor frames are made either partially closed or totally closed or flame proof. Write the voltage equation of DC motor. P What is the function of commutator in a DC machine? The function of the commutator is to facilitate the collection of current from the armature conductors. It converts the alternating current induced in the armature conductors into unidirectional current in the external load circuit. Name the type of DC motor used in electric train and justify. D.C Series Motor In this case, as field windings also carry the armaturecurrent,proptional to the armature current upto the the magnetic saturation.,.,9.What is the significance of back emf?The back emf acts as a governor and self regulates the motor by taking the necessarycurrent from the supply.www.Vidyarthiplus.comwww.Vidyarthiplus.com10.List the different types of DC generators.Self-Excited Generators Seperately Excited Generators: a. Series Generators b. Shunt Generators c. Compound Generators: i. Cumulative Compound ii. Differential Compound Define armature reaction and commutation The effect of magnetic field set up by the armature current on the distribution of flux under main poles of a generator is known as armature reaction. The commutator changes the alternating current produced in the armature into direct current. State working principle of operation of a DC generator. A generator is a device that converts mechanical energy into electrical energy.Direct-current generators operate on the principle of magnetic induction.Conductors cut through magnetic flux lines when the shaft is mechanically turned emf induced in the conductor. What are the effects of armature reaction in DCmachines? It has two effectsit demagnetizes the main flux it cross magnetizes or distorts it. Sketch the external characteristics of a DC series generator. 15. What are the essential parts of DC generator?16. What are the performance characteristics of a DC motor?www.Vidyarthiplus.comwww.Vidyarthiplus.comSpeed Vs Armature Current Torque Vs Armature Current Write the speed equation and What are the various methods of speed in DC series motor. N-Speed V-Applied Voltage Write down down the applications of DC series and shunt motors These are used for series arc lighting, series incandescent lighting and as a series booster for increasing the voltage in D.C. transmission lines. Shunt wound generator with field regulations are used for light and power supply purposes. These are also used for charging of batteries on account of its constant terminal voltage. What is meant by excitation of a dc machine? Apply DC voltage to the field winging. What is the necessity of a starter for a dc motor? When the motor is at rest, there is no back EMF developed in the armature. If now full supply voltage is applied across the stationary armature, it will draw a very large current because armature resistance is very small (I = V/Ra) this excessive current will blow out the fuses and prior to that, it will damage the commutator, brushes etc. To prevent this high starting current starters are used. PART B (16 MARKS)Explain how AC voltage generated is converted to DC voltage in a generator.(6) Explain necessity of starter in a DC motor and describe three-oint starter with neat sketch(10) Mention the reason for compounding of DC generator. Neatly sketch the external characteristics of a DC compound generator (4) Derive the torque equation of DC motor.(4) Explain the seed control of a DC series motor by (i)field divertors method and (ii) variable resistance in series with the motor (8)Explain the construction and working principle of D.C. generator with neat diagram.(10) A 10KW, 220 V, DC 6 pole shunt motor runs at 1000 rpm. Delivering full load. The armature has 534 la connected conductors. Full load core loss is 0.64 KW. The total brush drop is 1 volt. Determine the flux per ole neglecting shunt current (6) Draw the characteristic curves of D.C. shunt, series and compound motors. Compare the DC shunt and series motor characteristics and applications.(16) Write the speed equation and explain how to control the speed of a shunt motor by flux control method. (16) Derive the EMF equations of DC generator. (16) Explain the procedure for obtaining internal and external characteristics of a separately excited DC generator. (16) Explain the seed-torque characteristics of a Dc series motor and why the DC series motor not be started without any load.(16) www.Vidyarthiplus.comwww.Vidyarthiplus.comA shunt generator delivers 50 KW at 250 V and 400 rpm. The armature and field resistance are 0.2 and 50 ohms respectively. Find the seed machine running as a shunt motor and taking 50 KW input at 250 V.(16) Explain with neat diagram, the working of 3 point starter.(16) UNIT II- TRANSFORMERSPART A (2 MARKS)1. Draw the no-load phasor diagram of a transformerDefine transformer ratio Transformation ratio is defined as the ratio of number of turns in the secondary winding to number of turns in primary winding. Why is transformer rating in KVA? Copper loss of a transformer depends on current & iron loss on voltage. Hence total losses depend on Volt-Ampere and not on PF. That is why the rating of transformers is in kVA and not in kW. Write the condition for maximum efficiency of a transformer Condition for maximum efficiency: Iron loss = copperWhy an open circuit test is generally performed at rated voltage on LV side for a transformer Since the secondary of the transformer is open, the primary draws only no-load current, which will have some copper loss. This no-load current is very small and because the copper loss in the primary is proportional to the square of this current, it is negligible. There is no copper loss in the secondary because there is no secondary current Draw a single phase shell type transformer and name the parts www.Vidyarthiplus.comwww.Vidyarthiplus.comWhat is regulation? Name the test to be conducted to find it in the lab. The change in secondary terminal voltage from no load to full load expressed as a percentage of no load or full loadvoltage is termed as regulation. Load test O.C test, S.C test Define voltage regulation of a transformer? When a transformer is loaded with a constant primary voltage, the secondary voltage decreases for lagging PF load, and increases for leading PF load because of its internal resistance and leakage reactance. The change in secondary terminal voltage from no load to full load expressed as a percentage of no load or full loadvoltage is termed as regulation. %regulation =E2-V2/E2 *100 V2>E2 for leading p.f load V2