Electric railways

by American School of Correspondence; James Raley Cravath

However the controller may be arranged, the principle involved is that when the motors are revolving by the motion of the car, and the armature connections are reversed as they would be to reverse the direction of motion of the car, the motors begin to generate current as series-wound dynamos. The amount of current generated and the retarding effect will depend on two things—namely, the speed of the car, with the consequent electromotive force in the motors, and the amount of resistance in the circuit. The amount of resistance is regulated by the motorman by means of his electric brake controller. The function of the electric brake controller is to reverse the motors and to insert enough resistance in the circuit to make a comfortable stop. This current in the motors acting as dynamos, in itself acts as a powerful brake to retard the motion of the car. In the General Electric type of electric brake, the current generated in the motors, in addition to having this retarding effect in the motors themselves, is conducted to brake discs that act as magnetic clutches against one of the car wheels on each axle. The car wheel has a disc cast upon it, and against this the magnetic disc acts. The magnetic disc contains a coil which is in series in the brake circuit.

In applying an electric brake of this kind the motorman first puts the controller on a point that inserts considerable resistance in the circuit. When the motors have slowed down, the electromotive force, of course, drops, so that to maintain the same braking current there must be a reduction of the amount of resistance, until, when the car is almost at a standstill, the resistance is nearly all cut out. It might seem at first that the current would die down before the car came to a stop, but it is found that there is enough induction in the motor fields to cause current to flow for a short time after the car has stopped. The residual magnetism in the steel in the fields of the motor is sufficient to cause the motors to begin to generate current when the electric-brake controller is first turned on.

The greatest advantage of an electric brake using motors as generators is in the fact that the braking current instantly falls in value as soon as the wheels begin to slide, and releases the brake until the wheels again revolve. In fact, it is almost impossible to skid the wheels as they are sometimes skidded by being locked by brake shoes. This not only prevents flat wheels but insures a quick stop, because when the wheels are locked and sliding, the braking or retarding power is only about one-third what it was before the wheels began to slide. The electric brake requires extra large motors because of the heating caused by the current generated while braking.

Fig. 56. Magnetic Brake Shoe.

Westinghouse Electromagnetic Brake. The Westinghouse magnetic brake is in principle similar to the General Electric brake as far as the use of motors as generators is concerned; but, instead of assisting the motors by means of a magnetic brake disc acting against the car wheel, a magnetic brake shoe is used (see [Fig. 56]), which acts against both car wheel and track. This not only retards through the medium of the wheels but acts directly on the track. It is not dependent upon the coefficient of friction between the wheels and track; and it should, therefore, be possible to stop much more quickly than with any form of brake depending upon the coefficient of friction between the wheels and track.

MAGNETIC BRAKE SHOWING METHOD OF ATTACHING TO CAR FRAME AND TRUCKS.
Westinghouse Air Brake Co.

Track Brakes. Track brakes have been used to some extent on very hilly electric roads. These have a shoe fastened to the truck frame, which acts directly on the track.

Motors as Emergency Brakes. The motors can of course be used to brake the car by simply reversing them if current is applied to them from the line. But in case the trolley flies off or if the circuit breaker or the fuse opens the circuit, or the supply of current is interrupted for any other reason, they may be used as brakes by throwing the reverse lever and moving the controller handle to the multiple position of a two-motor equipment or by simply throwing the reverse lever of a four-motor equipment. These movements throw the motors in multiple and connect the fields and armatures of the motor in such relation that they can generate current. One of the motors then acts similarly to a generator in a power house, deriving its power from the momentum of the moving car instead of from an engine, and sends current through the other motor of the pair which acts like any auxiliary motor trying to revolve its wheels in the opposite direction from that in which they are revolving. The motors of a four-motor equipment are permanently connected in two multiple groups as long as the reverse is not in the central position. In the two motor equipment such connections are not made until the controller handle is turned to the multiple position. As the external resistance is beyond the junction of the two motor circuits, the braking effect is not increased by cutting out the resistance.