The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 49 starting point, so that the total starting resistance is in series with armature, and so that when the current is turned on again, the armature is not short-circuited. When a breakdown occurs in the current supply, the starters of every motor connected to the system must be brought back to the first step. Short-Circuit in the Supply Leads.—If a short- Circuit in the supply cables occurs while the dynamo is stopped, the dynamo will not generate any current when started up, since the shunt field cannot be pro- duced owing to the short circuit. This cause may be found out in the following manner. Disconnect the leads from the machine, and run up the machine with a voltmeter across its terminals. If a short-circuit is in the leads, the only remedy is naturally its removal. Short-Cireuit in the Leads due to Motors not being Switched Off, or to a number of arc lamps being connected in parallel. If, when the plant is shut down, the switching out of the motors is forgotten, then this may, under certain circumstances, have the same effect as a short-circuit in the mains. It follows, hence, that under all circumstances, when shutting down, all motors and arc lamps must be switched off. By using starters with automatic release this may be ensured, as far as motors are concerned, since these starters, when the current is cut off automatically, return to the starting point. Determination of the Temperature.—Dynamos and electric motors naturally become heated when