THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CXV. \ FRIDAY, FEBRUARY 15, 1918 No. 2981. Starters for Squirrebcage Induction Motors, and their Application, By L. It would be difficult to conceive of a more simple machine than the squirrel-cage induction motor. How- ever, although this type has often been advocated, and rightly so, as the ideal motor for underground work, such motors have the inherent weakness of being unable to start against load—a fact which seriously restricts their sphere of usefulness in and about a colliery. Moreover, certain forms of switchgear largely used at the present time are liable to be unsafe in operation, especially underground, because no pro- tection is provided for the motor during the starting periods. Whereas, in the case of a slip-ring motor, it is pos- sible to start the rotor by introducing a resistance FOKES. Fig. 1. Fig. 2. cannot be done with a across the windings, this squirrel-cage rotor, which forms a permanently closed circuit of extremely low resistance, and any means that may be employed for starting such a machine must necessarily be introduced into the stator circuit. To start a squirrel-cage induction motor by throwing it straight across the supply mains, would cause it to take an abnormal current, which, if the motor be a voltage of the system after allowing a certain current for magnetising the field. This field links with the secondary or rotor winding, and induces an electro-motive force which produces no further result so long as the rotor circuit is open. However, if the rotor circuit is closed through a resistance, a current will flow in the circuit in propor- tion to the resistance and self-induction of the wind- ing and to the electro-motive force induced. This current, which circulates in the rotor winding, pro- duces a magnetic field in the iron circuit reverse to that of the primary. (This ♦ is shown dotted in fig. 1.) The result of this reaction is to reduce the back electro- motive force of the stator, and to allow a larger cur- rent to flow in that winding. Returning to the squirrel-cage motor, if we imagine the secondary or rotor to be a closed circuit of extremely low resistance, and that the stator is switched straight across the supply, it will be under- stood that the enormous currents induced in the rotor absolutely wipe out the stator magnetism (which limits the current through the stator winding), and causes an abnormal current to flow, which is, practi- cally, limited by little more than the resistance of the stator winding. In the case of induction motors up to about 40 horse- power, this current, which is known as the short- circuit current, may reach a value of four to five times full-load current, whilst the torque varies from 1| to 2| times full-load torque. In larger motors the cur- rent may reach seven times full load, while the torque may be considerably less than full-load torque. The abnormal current produced on short-circuit does not cause the motor to exert a torque in proportion— a circumstance which is accounted for by the fact that the field produced by the large rotor currents neutralises the stator field, and so limits the torque of the motor. However, the torque is much higher than is necessary to start a motor, seeing that squirrel - cage motors are always started up light, and placed on load after full speed is attained. There are numerous ways in which squirrel-cage motors may As soon as the motor reaches full speed, the switch is thrown over to the running position, thus including the fuses in the circuit. The serious disadvantage, of course, lies in the motor having no protection while starting, but this point will be dealt with later. Variable Resistance in Series with Stator Windings. In connection with starting motors under these con- ditions, it should be noted that the fuses are in circuit the whole of the time, which is an improvement on the previous method. However, a motor must be adapted to this form of starting, as it will be apparent, from fig. 3, that the windings and resistances together form a star connec- tion, the latter being connected to form the neutral point through the starting switch. The contact arms of the switch are all connected together, and when moved to the first contact on each phase, the connected switch arms form the neutral point, which is gradually advanced, as resistance is cut out, until the ends of the windings are reached, and no resistance is in circuit. The windings of the stator must be wound and kept separate, the ends being brought to six terminals. There is nothing, however, to prevent such a motor being started in other ways, when the windings are so brought out; but, as both this method of starting and the previous one are only used for very small motors, either one or the other usually meets the case. Star Delta Switch. A motor must be provided with six terminals, as already described, to enable it to be started by a star delta switch. The normal operating connection of the stator wind- ings is a mesh connection, but the starting switch is so arranged that when it is placed in the “ starting ” position, the windings are connected in star, and thus the voltage impressed on the windings is equal to the normal operating voltage per winding divided by a^3, or about starting 58 per cent.; so that the current taken at is considerably reduced, although reaching O 4-00 V.---> Fig. 5. C 4? o z h. < h tf) ■Ol i ■ c.xo— ___9 h AXO"^ z ________9 z o-2 -Ola -pa./. ■ a ■ AXO- ■ 9 1 1 o OL.3. 1 1 >A.O- -pc • J! 9 “O Li 1 -O A.I. | I O~ ■ Ufr J 9 1 o- -Ou | | A 2.O- 1 * 9 1 o- -Ou | 1 BaO- -pc./ I L_9 J Ct Fig. 6. Z 0 z d Z t- Fig. 3. Fig. 4. large one, would interfere seriously with the voltage regulation of the system. The reason for this can easily be explained by refer- ence to fig. 1, which represents a simple single-phase transformer. Now, the only difference between a transformer and an induction motor is that, whereas the magnetic cir- cuit in the former is continuous, in the latter it is interrupted by the air gap, which, of course, is neces- sary to allow the rotor to turn. The action between the primary and secondary of the transformer is iden- tical with that which takes place between the rotor and stator of a motor; hence fig. 1 is applicable to either. The primary winding represents the stator of the motor, whilst the secondary the rotor circuit. It will be assumed that the rotor circuit is not a closed one, and that slip rings are provided in order to introduce a resistance across the windings. Now, when the primary winding or stator is switched on to the supply, the action of the magnetism induced in the iron circuit, the direction of which is shown by the full-line arrow, induces in the primary winding a back electro-motive force sufficient to balance the be started, and the most important of these will be discussed, with a view to pointing out their advantages and disadvantages, specially in regard to their application to motors in and about collieries. Broadly, these methods of starting may be placed under four headings, i.e.— (1) Switching the motor straight on to the line. (2) Variable resistance in series with the stator winding. (3) Starting by means of star delta switch. (4) Auto-transformers or compensators. Switching Motor Straight on to Line. This method of starting is only applicable to very small motors, such as might be used to work a lamp- cleaning machine in the lamp-room, or similar light work. The usual practice is to arrange for a three- pole change-over switch, as shown in fig. 2. When the switch is closed in the top contacts, the motor starts without any fuses in the circuit, as they would not stand against the comparatively heavy current, unless they were of such a size as to afford no protection to the machine when on load. 1J to 2 full-load current; whilst the torque exerted is about one-half that at full load. The mesh and star connections of the windings are shown in fig. 4, where it is assumed that the pressure of supply is 400 volts. The voltage impressed across the motor windings is there seen to be approximately 231 volts at starting, and the method commends itself at once by its simplicity. Fig. 5 shows a suitable switching diagram for start- ing a motor in this way. With the switch on the start- ing position, the windings of the motor are star-con- nected, the connection being changed to i( mesh ” by throwing the switch to the “ running ” side after the motor has attained sufficient speed. There is, however, a serious disadvantage with this starting arrangement, as no protection is afforded, in the present standard forms of apparatus, when the switch is in the starting position. This has already been referred to under method (1); but whereas with very small motors it is not a very serious matter, yet when dealing with larger motors it becomes objec- tionable. It will be obvious that, as a motor takes about twice