When a motor fails to start, it should first be ascertained if a supply of electrical energy be available in the mains. This may readily be discovered by means of a voltmeter, or if low tension service, by means of the fingers bridging across the main terminals. If the supply of energy be present, the contact arm of the starter should be moved into such position that all resistance is inserted into circuit with the motor. This is important, as the motor may start suddenly while trying to ascertain the cause of the stoppage.
Fig. 743.--Starting panel. In installing any kind of motor starting rheostat, it is necessary to provide main line knife switch and fuses in addition to the starting box. The appearance of the installation can be much improved by mounting all of these upon one panel.
Having closed the switch, if the motor fail to start, it will be advisable to remove the load if possible, as the failure may arise from an overload of the machine. This being effected and the motor not starting, the terminals of the latter should be tested by the means already described for voltage. If no voltage be generated, a broken circuit or a defective contact may be looked for in the main fuse, switch, or starting box. The resistance coils of the latter, through the heat developed, frequently break in positions out of sight. If a defective contact of this nature cannot readily be seen, the contact arm should be moved slowly over the contacts, as it is possible the broken coil may be cut out of circuit by this means.
Figs. 744 to 746.--Cutler-Hammer motor starting rheostats with no voltage and overload release. [Fig. 744], starter with fixed button contact, [fig. 745], with renewable button contact, and [fig. 746], with contact segments. In construction the resistance is enclosed in a pressed steel box on which is mounted a marbleized slate panel carrying the starting lever, contacts and protective devices. By means of a calibrated scale, the overload release (shown in the lower left hand corner, figs. 744 and 745, and in the lower right hand corner [fig. 746]) can be set to break the circuit on any overload not exceeding 50 per cent. of the rated capacity of the motor. This calibrated scale can also be used for determining, with a fair degree of accuracy, the amount of current being consumed by the motor.
If a difference of pressure exist between the motor terminals, the field magnets will, if shunt or compound wound and in good order, be excited, which may be ascertained by means of a bar of iron. If no magnetism be present, it will of course, indicate a broken or bad connection, either between the terminals of the field coils, or one or more of the coils themselves. If the bar pull strongly, the position of the brushes upon the commutator in regard to the neutral points should be ascertained, and the rocker adjusted, if necessary, to bring them into their correct positions. If this fail to start the motor, the connecting leads from the motor terminals to the brushes and the brushes themselves should be carefully examined for broken or bad connections, and defective contact of the brushes with the commutator. In the latter case, it may arise from a dirty state of the commutator, or from the brushes not being fed properly. If due to these causes, pressing the brushes down upon the commutator with the fingers will probably start the motor. If the failure to start arise from none of these causes, it is probably due to the field coils acting in opposition, or to a short circuited armature. This latter remark applies more especially to motors provided with drum armatures.
Fig. 747.--Allen-Bradley compression type resistance units. The contact resistance between the discs composing the resistance column is subject to variations of pressure thereby producing proportionate resistance changes in the column as a whole. In the complete resistance unit, the resistances column is encased in a drawn steel tube, which is lined with a highly refractory cement, for purpose of insulation, affording the column both mechanical and electrical protection and excluding the air which effectually prevents any combustion should the column become red hot due to overload. The ends of the tube are closed by means of caps through which pass electrodes for making connections between the discs and exterior conductors. The steel tube, when necessary, is provided with ribs or fins for the dissipation of acquired heat.
