Ans. In starting a shunt motor, no trouble is likely to occur in connecting the field coils to the circuit. Since the resistance of the armature is very low, it is necessary on constant voltage circuits to use a starting rheostat in series with the armature.
The necessary connections are shown in [fig. 756]. The switch is first closed thus sending current through the field coils, before any passes through the armature. The rheostat lever P is then moved to the first contact to allow a moderate amount of current to pass through the armature. The resistance of the rheostat is gradually cut out by further movement of the lever P, thus bringing the motor up to speed.
Figs. 739 and 740.--Multiple switch starters. [Fig. 739], starter with no voltage release; [fig. 740], starter with no voltage release and circuit breaker. The multiple switch type of starter is designed to overcome the arcing on sliding contacts which, in the case of large motors would be very severe. The cutting out of each step of resistance is accomplished in the multiple switch starter by a separate carbon contact switch which breaks the circuit with a quick snappy action.
Ques. How does the reverse voltage affect the starting of a motor?
Ans. When a motor is standing still, there is no reverse voltage, and the current taken at first is governed principally by the resistance of the circuit. If the motor be series wound, there is a momentary reverse voltage, due to self-induction while the field is building up. If the motor be shunt wound, self-induction delays the current through the field coils, but that through the armature is not impeded by such cause. When the armature begins to revolve, reverse voltage is developed which increases with the speed. The resistance of the starting box may be gradually cut out as the armature comes to speed. Thus the reverse voltage gradually replaces ohmic drop in limiting the current as the motor comes to speed.
Fig. 741.--Starting rheostat with no voltage and overload release. The no voltage release permits the starting lever to fly to the "off position" should the voltage fail momentarily, thus protecting the motor against damage should the voltage suddenly return to the line. The movement of the lever is due to a spring. The overload device causes the lever to back to the off position should the current exceed a predetermined maximum for which the release is adjusted.
Fig. 742.--Compound starter. Rheostats designed for the double duty of starting a motor and regulating its speed are commonly known as compound starters, the resistance provided being a combination of armature resistance for starting duty and shunt field resistance for speed regulation.
Failure to Start.--This fault, which is liable to occur in a motor of any description, is similar to failure to excite in a dynamo, and is liable to be produced by any of the causes mentioned in connection with the latter fault, excluding insufficient speed, and insufficient residual magnetism.