out in the same way, but in coupling up at the onset, care must be taken to connect so that the series coils oppose the shunt and tend to demagnetize the magnets. Exactly similar tabulation of results and plotting of curves must be carried out with the inferences deducible. N.B.-The applied E.M.F. to the motor should be maintained constant. (91) Efficiency and B.H.P. of Small Direct or Alternating Current Electro-Motors. (Cradle-Balance Method.) to Introduction. In testing small motors, such as from of a H.P., difficulties present themselves in measuring the power developed by them or the work which they will do, owing to the relatively large amounts of extraneous friction introduced in applying the usual brake tests. In fact, in the case of the smaller power motor, this source of friction would entirely vitiate the results and make them worthless. The following method practically gets over this difficulty entirely, and may be carried out in one of two ways— (a) The motor to be tested is suspended freely with its armature spindle in centres, or on friction wheels, the field magnet system with its bed-plate, etc. being carefully balanced by counterpoise weights so as to bring the centre of gravity of the system in a line with the spindle. On the motor being supplied with electrical energy, and made to rotate and do work against the friction introduced at the face of its pulley by a stretched cord passing once round, the armature reacts on the field magnets tending to rotate them in the opposite direction with a certain force. If then this action is resisted by a weight or force W attached to the field magnet system at a leverage L, then the moment of this force resisting the tendency, i. e. the torque, = WL. Thus the arrangement is practically an electro-magnetic dynamometer in which the magnetic friction between armature and field magnets takes the place of mechanical friction in the ordinary dynamometer. The preceding arrangement of the method has the disadvantage that the weight of the heaviest portion of the machine is resting on the shaft, and consequently there will be a bearing friction assisting the magnetic pull of the armature on the field. a A better arrangement in this respect is one devised by Prof. C. F. Bracket, which is merely a slight modification of the preceding one. It consists in fixing the motor in "cradle" supported freely by knife edges resting on steel or agate planes, or on friction rollers, carried in a suitable fixed frame. The whole suspended system is very carefully balanced by means of counterpoise weights so that the centre of gravity lies in the axis of the motor shaft, this latter having been set in a line joining the knife edges of the cradle. A horizontal balanced lever controls the cradle, the end of it either supporting weights or being attached to a spring balance. Thus it will be seen that now the weight of the armature only is on the bearings, and it is being used under ordinary conditions. The balanced lever might be graduated and a sliding weight used to run along it to balance the torque, but the arrangement of the spring balance shown in Fig. 192 is the simplest and easiest to manipulate. This method has a further advantage that the friction at the journals of the motor does not affect or vitiate the measurement, but in the case of the application to a dynamo it should be remembered that it does. It should be borne in mind that a fruitful source of error may arise due to loss of power in driving the speed indicator. When small motors are being tested, care should be taken to choose an indicator that is very easily driven, and to drive it by means of a spiral or helical spring of, say, thin hard-drawn brass. Any eccentricity between the two shafts does not then matter so much as it would if they were direct coupled. Apparatus. That required for this test is precisely similar to what is detailed under one or other of the preceding methods of testing series, shunt, or compound wound direct current motors or alternating current single phase motor, according to which of these types of motors the one being tested belongs. In addition the cradle absorption dynamometer is needed, for a complete description of which see p. 417. Observations.—As, with the exception of the somewhat dif ferent type of brake herein to be manipulated, the whole test will be precisely similar to one of the foregoing motor tests, depending on which kind of electromotor is to be tested, the rationale of this present test will not be repeated here. The experimenter should refer to the proper corresponding test and carry out the present one in an exactly similar manner, tabulating and plotting the results in just the same way. = Though the following expression will be found in connection with the description of the cradle dynamometer on p. 417, we may repeat that if W weight or force applied at the end of the cradle lever in order to keep the same at zero when the motor is doing work, and if L distance between its point of application and the fulcrum of the cradle, then torque exerted by motor T, and the work it does per sec. WL = = = T Consequently if different tensions are applied on the cord wrapped round the motor pulley, causing it to do various amounts of work, thereby taking in different currents (A) amps. at different voltages V, the efficiency of the motor at each load is H.P. developed Efficiency = H.P. absorbed 2nT × 746 (92) Efficiency and B.H.P. of Direct Current Electro-Motors by Swinburne's Electrical Method. Introduction.-In the usual brake tests it is difficult and often impossible to obtain very accurate results, owing to variation of the co-efficient of friction between the rubbing surfaces and the resulting jerky behaviour of the brake. The advantage of any method, therefore, of measuring the input and output of a motor by solely electrical means will at once be apparent, as it is possible to obtain much more accurate results with such a method. The present method, which is purely an electrical one, is due to Mr. James Swinburne, and is sometimes termed the "Stray Power" method. The principle of it and all similar methods is based on the fact that Total Power given out Total Power put in = = Power lost internally, or in symbols, WoW, W; where the suffixes O, I, and I denote the output, input, and total losses in Watts (W) respectively. We thus at once obtain the commercial efficiency of the motor to be Wo WI W WL WI The input in Watts W given to the motor is at once obtained by the product of the volts and amperes of the supply. The total loss W in Watts we will consider now more in detail, and which in any machine is made up as follows: (a) the copper losses Lc in armature and exciting coils due to heating by the passage of current, and which can easily be calculated when the currents and resistances are known; (b) the friction losses Le due to air churning, journal and brush friction; (c) magnetic frictions or iron losses Lm due to eddy or Foucault currents and magnetic hysteresis. = Hence total internal loss W sum (Lp + L) Mr. Swinburne Lc + Lp + Lm, and to the has given the somewhat appropriate name of " Stray Power." The copper losses are calculable as follows = Let C total current flowing into the motor from the supply, and let Ra, Rse, and Rsh be the resistances of the armature series coils and shunt coils respectively of any motor of which Rsh can be measured by a Wheatstone Bridge, and Ra, Rse by the "Potential Difference" method (p. 82). for a Shunt motor Lc working voltage, Then we shall have The remaining losses, i.e. the stray power (LF+ Lm), can readily be obtained by running the motor at no load, i. e. with no other load than its own friction, eddy currents and hysteresis, at normal excitation of the field. Then we have where A now = = current flowing into the motor armature at voltage Va across the armature, and Leis the copper loss in the armature occurring for this current and voltage. Note. Only quite a small current at the normal voltage of the motor is required to be furnished by an auxiliary source of E.M.F., and if Ra is very small, Le can be neglected in comparison with AV in this last formula. Apparatus.-Motor M to be tested, which for purposes of discussion merely we will assume is shunt wound; voltmeter E M ; low reading long scale ammeter A; rheostat R; tachometer; complete Wheatstone Bridge set (W.B.); two-way voltmeter key K; switch S; source of E. M.F. (E) at least equal to that for which the motor was built; rheostat in the field coil circuit. Observations.-(1) Connect up as in Fig. 79, and adjust the instruments V and A to zero if necessary. Insert E, when the field should then be excited to the normal amount, which can be seen by closing K1 and noting whether the normal voltage is read off on V. (2) With Rat its maximum value (not less than about 10 ohms), close S2, adjusting R and if necessary the excitation by the rheostat r, so that the machine runs at its normal speed. Now note, by closing K 2, the volts Va across the armature terminals and the current A amps. flowing through it. (3) Repeat 2 at the same excitation for some ten different speeds in all, both below and above normal. (4) Open E, S, and K, and measure by means of W.B. the resistance R of the armature and Rsh of the shunt, re |