STRUCTURE OF INDUCTION MOTORS.

112. The stators of all induction motors are built of soft-iron stampings varying from 12 to 20 mils. in thickness. The running coils of monophase induction motors are wound in half-closed slots, which are placed close to the inside periphery, as shown in Fig. 64, while the starting coils are wound in completely closed slots. The stator stampings are built up inside a cast-iron case,

FIG. 64.

to which end shields are bolted. The slots are insulated with micanite or other insulating material. In monophase motors there are two distinct windings, one being the running winding—always in circuit, the other, the starting winding, in circuit only at starting, and whose function is to provide a cross-magnetization, the action of which on the rotor currents produces the starting-torque. In di-phase motors there are two similar distinct windings, both of which remain always in circuit.

In tri-phase motors there are three similar distinct windings, which always remain in circuit.

Rotors. The rotors consist of a slotted and laminated core,

the stampings of which need not be so thin as those of the stator, since the frequency of the flux is there much smaller than in the stator. A rotor stamping is depicted in Fig. 65. The rotor winding sometimes consist simply of bars of copper, short-circuited at

FIG. 65.

each end by heavy copper rings. This is called a "squirrelcage" winding. When a squirrel-cage winding is not used, a tri-phase star winding is almost universally employed, in which case three ends are jointed together, while the remaining three are

connected to three collector-rings mounted on the shaft. Three brushes rub on these rings, and are held by holders connected to the end shields of the case.

Fig. 66 shows the several parts of a Heyland monophase induction motor, manufactured by Messrs. Witting Bros., London.

Fig. 67 shows a starting resistance and switch, for use with a tri-phase star-wound rotor.

Fig. 68 shows a rotor of a British Thomson-Houston polyphase

induction motor, with a starting resistance inside the spider, and so arranged that it is automatically cut out when speed is got up.

PERFORMANCES OF INDUCTION MOTORS.

113. The following table gives the results of tests made with monophase motors of the Heyland type:

Fig. 69 gives the curves showing the results of a test of a 5 B.H.P. Heyland monophase motor; Fig. 70 gives similar curves for a tri-phase motor manufactured by Messrs. Witting Bros.; while Fig. 71 gives curves relating to a 1.5 B.H.P. monophase motor of the Fuller-Wenström Electrical Manufacturing Company.

From these details we see that induction motors compare favourably with the best shunt-wound direct-current motors,

FIG. 69.-Characteristic curve of a brake test for a 5 B.H.P. single-phase motor. 100 cycles. 200 volts. 2000 revs. per min.

both as regards efficiency and constancy of speed under variable loads.

FIG. 70.-Brake test of a 10 B.H.P. three-phase motor. 190 volts. 50 cycles. 1000 revs. per min.

The chief objection to such motors is that no easy means has been devised for varying their speed. It is possible, as has already

been done, to halve the speed by doubling the number of statorpoles, or, generally, to reduce the speed by increasing the number of stator-poles; but such variation is not continuous, and involves complicated windings. The only way of producing a continuous variation of speed is by supplying the motor with current at a

varying frequency, for which purpose a frequency transformer is required. As, however, no frequency transformer of wide range has hitherto been constructed, a satisfactory solution of the variation of speed of induction motors has not been arrived at. The method of changing the speed by altering the P.D. applied between the terminals of the stator windings is not satisfactory. The P.D. is varied by means of variable resistances, placed in series with the stator windings, entailing a serious waste of energy. An additional objection is that the motor cannot give its full rated output for any length of time when the P.D. is lowered, on account of the heavy currents it requires to do so.

114. The noticeable feature of induction motors is their simplicity of structure combined with great mechanical strength. The absence of commutators is an immense advantage, since there. is no sparking limit to the output as in direct-current motors;