The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 39 has no effect on the action of the machine, at any rate when the connections are made as in Fig. 14. If the connections are made as in Fig. 15 the effect is again that the series coil is short-circuited, and that the machine works as a single shunt machine (Fig. 19). Wrong Connection of Compound-Wound Ma- chines.—The wrong relative connection of the series and shunt windings of both dynamos and motors may, as is well known, lead to trouble. If a new or repaired compound-wound machine with the Held regulator kept in one place drops in its voltage, although the speed has not varied, then if no short-circuit has occurred on the mains to account for the drop, the first thing to do is to see whether the series winding is rightly connected. If it has been reversed, it will of course tend to lower the voltage at full load, instead of raising it. The simple remedy is to reverse its con- nections at the terminal board. While with dynamos such a wrong connection can hardly cause a break- down, yet with motors it is almost dangerous. In a correctly connected compound motor, that is to say, in a shunt motor with a series winding to increase the torque at full load, the current in the series winding must flow in the same way as in the shunt winding. This increases the magnetisation at full load. The result is, that with increased load the speed drops more than with a shunt motor ; it will usually drop about 25 per cent. This is no disadvantage for the purpose for which compound motors are usually used,