Electric railways

by American School of Correspondence; James Raley Cravath

TESTING FOR FAULTS.

Fig. 94. Bond Testing.

Bond Testing. It is important to test the conductivity of rail bonds from time to time in order to determine if they have deteriorated so as to reduce their conductivity and introduce an unnecessary amount of resistance into the return circuits. One way of doing this is to measure the drop in potential over a bonded joint as compared with the drop in potential of an equal length of unbroken rail. To do this, an apparatus is employed whereby simultaneous contact will be made bridging three or more feet of rail and an equal length of rail including the bonded joint, as shown in [Fig. 94], which illustrates the connections of a common form of apparatus where two milli-voltmeters are employed that measure the drop in voltage of the bonded and unbonded rail simultaneously. If the current flowing through the rail due to the operation of the cars were constant, of course one milli-voltmeter might be used, being connected first to one circuit and then to the other. The current in the rail, however, fluctuates rapidly, so that two instruments are necessary for rapid work. The resistance of the bonded joint is usually considerably more than that of the unbroken rail, and the milli-voltmeter used to bridge the joint consequently need not be so sensitive as that bridging the unbroken rail.

In another form of apparatus, a telephone receiver is used instead of the milli-voltmeter, the resistance of a long unbroken rail being balanced against that of the bonded joint, as in a Wheatstone bridge, until, upon closing the circuit, these two resistances when balanced give no sound in the telephone receiver.

Bond tests of this kind can be made with satisfaction only when a considerable volume of current is flowing through the rails at the time of the test, because the drop in voltage is dependent on the current flowing, and in any event is small. It has sometimes been found necessary or advisable to fit up a testing car equipped with a rheostat which will itself use a considerable volume of current, so as to give a current in the rail which will give an appreciable drop of potential across a bonded joint. Some of the latest forms of testing cars carry motor generators which will pass a large current of known value through a bonded joint, and so cause a drop of potential across the joint large enough to be easily measured.

Motor-Coil Testing. Testing for faults in the motor armature and field coils is done in a great variety of ways. The resistance of these coils can be measured by means of a Wheatstone bridge employing a telephone receiver in place of the galvanometer used in such bridges in laboratory practice; but other less delicate tests are also in use.

Another method is to pass a known current through the coil to be tested and to measure the drop in the voltage between the terminals of the coil, the voltage divided by the current equaling the resistance.

A simple method, and one which involves no delicate instruments, has lately been introduced into railway shop practice very successfully. This is known as the transformer test for short-circuited coils. It requires an alternating current which can easily be supplied either by a regular motor generator or by putting collecting rings onto an ordinary direct-current motor and connecting these rings to bars of opposite polarity on the commutator.

The method of testing for short-circuited armature coils employed in the shops of the St. Louis Transit Company is indicated in diagram in [Fig. 95]. A core built up of soft laminated iron is wound with 28 turns of No. 6 copper wire. This coil is supplied with alternating current from a 110-volt circuit. The core has pole pieces made to fit the surface of the armature. When one side of a short-circuited coil in the armature is brought between the pole pieces of this testing transformer, as in [Fig. 95], the short-circuited armature coil becomes like the short-circuited secondary of a transformer, and a large current will flow in it. This current will in time manifest itself by heating the coil; but it is not necessary to wait for this, as a piece of iron held over that side of the coil not enclosed between the pole pieces, as indicated in. [Fig. 95], will be attracted to the face of the armature if held directly over the coil, but will be attracted at no other point.