Ebooks

This book has been written preliminary as a reference for electrical machine practical course. Practical’s are often conducted with a dull monotony, place components, hook up wires, connect wires and record results that are expected well before hand. To add misery, there is a great paucity of books and manuals that can motivate and guide students during experimentation.

Aim: To perform open and short circuit test on a single-phase transformer, to obtain equivalent circuit parameters. Open circuit test The core loss or iron loss i.e. the no-load loss in a transformer is determined by performing the no-load test at normal sine wave impressed voltage. Through the iron loss in a transformer is not governed by the supply being connected to primary or secondary side but when measuring this no-load loss. It is usually more convenient and safer to supply the voltage to the low voltage side. This makes the test apparatus cheaper and becomes safer for the low testing personal. To determine the shunt branch parameter of the equivalent circuit, the primary side (L.V.) is connected to a normal supply voltage and rated frequency and secondary side (H.V.) is left open with care so that there is no possibility of coming in contact with any part of (H.V.) terminals. The wattmeter connected in the primary (L.V.), will then read the total hysteresis.

Aim: Determination of percentage regulation and efficiency of a single phase transformer by direct loading method. Percentage regulation When the load on the transformer is increased, the voltage drop in its primary and secondary windings also increases causing variations in secondary terminal voltage. The term voltage regulation of transformer means the change in secondary terminal voltage when full load is thrown off keeping supply voltage and frequency constant. It is generally expressed as percentage of no load terminal voltage for a desired power factor.

Aim: To perform Sumpner’s test (back-to-back test) on a pair of similar transformer and thus calculating efficiency of operation. Sumpner’s test givens an effective way, to actually see the performance of transformers of large rating without loading them, at full load. This back to back test allows heat run test so that temperature rise and its rate can be analysed. The two identical transformers are connected back to back as shown in figure such that their polarities are in opposition. To do this, two terminals of the secondary winding are connected as shown and the voltages between the other two are measured by exciting primaries.

Aim: Perform the parallel operation of single-phase transformer and observe the load sharing of each transformer. A power transformer is one of the most vital and an equally expensive component in a power system. It may so happen that, over time, due to load growth in its service area, an existing transformer may not be able to withstand the demand during peak-hours without exceeding its long-term MVA rating. Operating a transformer in such a fashion would cause overheating and degrade its expected life. In most cases, instead of commissioning an entirely new higher capacity unit, a more viable alternative exists in adding a smaller unit in parallel to complement the existing one. In other words, a new smaller capacity transformer can now be connected in parallel to the existing one such that the two shares a large peak load in a specific proportion and the one operating near limits is relieved of the burden.

Aim: To verify the relationship between the input and output voltage for 3-phase transformer connection in various ways: A 3-phase transformer can be considered as a set of 3 single-phase transformer bank or as single unit of 3 phase windings. Since three winding are available in both primary and secondary side then there can be different types of connections and so far, different characteristics.

Aim: To obtain balanced two-phase supply from three-phase supply by Scott arrangement of two single-phase transformers. The phase conversion from three to two phase is needed in special cases, such as in supplying 2-phase electric arc furnaces. Scott connection of two-single phase transformers is employed for conversion of a three-phase system to two phase system or vice-versa. Rating of one transformer should be 15% greater than that of the other, but in practical two identical transformers are used for interchangeability and spares. The 50% tap of one transformer (Main transformer) is connected to 86.6% tap of the other transformer (Teaser transformer). The secondaries for balanced supply system have equal number of turns.

Aim: To determine the Load characteristics (External characteristics) of self excited D.C. shunt generator. If a dc shunt generator, after building up to its voltage, were loaded, its terminal voltage will drop. This drop increases if load increases. But this type of terminal drop is undesirable for a specified service. The relation between terminal voltage and load current is called external characteristics. Speed is to be kept constant. Reason for voltage drop:

Aim: To perform speed control of D.C. shunt motor by 1. Armature resistance control method. 2. Field (flux) control method. This method is based on the fact that by varying the voltage available across the armature, the back EMF and hence the speed of the motor can be changed. This is done by inserting a variable resistance RC (known as controller resistance) in series with the armature as shown in Fig. (a).

Aim: To determine the Efficiency Vs Load characteristics (External characteristics) of D.C. shunt motor Swinburne’s method. This method is an indirect method of testing a DC machine. It is named after Sir James Swinburne. Swinburne’s test is the most commonly used and simplest method of testing of shunt and compound wound DC machines which have constant flux. In this test the efficiency of the machine at any load is pre-determined. We can run the machine as a motor or as a generator. The iron and friction losses are determined by measuring the input to the DC machine at no-load. The machine is to be run as a motor at normal voltage and speed. The Swinburne’s method includes all the losses in the field circuit and winding which do not occur actually in the machine but the error introduced due to this is quite insignificant and still shows the accurate results. In this method of testing no-load losses are measured separately and eventually we can determine the efficiency.

Aim: Determination the equivalent circuit parameters of a single-phase induction motor. 1. No-load test. 2. Blocked rotor test. The performance of a single-phase induction motor is generally studied either by double revolving field theory or by cross field theory, the former being simple and gives a clear physical understanding.

Aim: Perform an experiment to determine the parameters of a three-phase induction machine. As the circuit modal of an induction motor is similar to that of a transformer, the parameter of the modal can be obtained by no-load test and blocked rotor test. No-load test In this test, the motor is to run at no-load at rated voltage and frequency. The applied voltage and current and power input is measured. Power input at no load (Po ) provides only the losses that are rotational losses.

Aim (a) Perform load test on 3-phase induction motor. (b) Compute Torque, Output power, Input power, Efficiency, Input power factor and Slip for every load setting and to determine how speed, efficiency, power factor, stator current torque, and slip of an induction motor vary with load. The load test on induction motor is performed to compute its complete performance i.e. torque, slip, efficiency, power factor etc. During this test, the motor is operated at rated voltage and frequency and normally loaded mechanically by brake and pulley arrangement from the observed data, the performance can be calculated, following the steps given below.

Aim: To study the effect of variation of field current upon the stator current and power factor with synchronous motor running at no-load, hence to draw V and inverted V curves of the motor. With constant mechanical load on the synchronous motor, the variation of field current changes the armature current drawn by the motor and also its operating power factor. As such, the behavior of the synchronous motor is described below under three different modes of field excitation.

Aim: Study of 3 point and 4-point starter. Theory A 3-point starter in simple words is a device that helps in the starting and running of a shunt field or compound excited dc motor. Now the question is why these types of dc motors requires the assistance of the starter in the f irst case. The only explanation to that is given by the presence of back emf Eb , which plays a critical role in governing the operation of the motor. The back emf, develops as the motor armature starts to rotate in presence of the magnetic field, by generating action and counters the supply voltage. This also essentially means that the back emf at the starting is zero, and develops gradually as the motor gathers speed.

Objective Questions and Answers on Electrical Machines  I. Transformers  1. Low voltage windings are placed nearer the core in the case of concentric windingsbecause  (a) It reduces leakage fluxes (b) It reduces insulation requirement  (c) It reduces eddy current loss  (d) It reduces hysteresis loss.  2. For voltage above 66 kV, condenser bushings are preferred than non-condenser bushings because  (a) The radial stresses can be made independent of the radial thickness of thedielectric  (b) Optimum utilization of the dielectric can be made due to uniform potential distribution which also reduces both the radial and the axial dimensions of the bushings