SECTION 3

ELECTRICAL EQUIPMENT

300 General The electrical equipment of a diesel locomotive is generally more complicated and confusing to the engineman than are the mechanical items. Thus, this section is being included to discuss ingeneral, those electrical items that may need more explanation than has been given in Sections 1 and 2. it is hoped that the information in this section, while not being extensive, will enable the engineman to obtain a clearer picture of the operation of the locomotive.

301 General Electrical Scheme In the full throttle position the rated horsepower of the diesel engine is delivered to the direct coupled main generator (Model D15C). At the main generator the mechanical power of the engine is transformed into electrical power. The electrical power is then conducted to the four traction motors (Model D27), two motors being located in each truck.

The electrical system of the locomotive can be thought of as being divided into two separate systems: the high voltage system and the low voltage system.

The high voltage system is entirely and directly concerned with the function" of moving the locomotive. The main components of the high voltage system are the main generator, traction motors, reverser drum, wheel slip relay, forward transition relay, shunt field contactor, ground relay, series and parallel power contactors.

The low voltage system contains control circuits which control the flow of power in the high voltage system, and those auxiliary circuits conducting power fuel pump and the to the locomotive lights, heater fan, main generator battery field. A 64 volt storage battery is included in the low voltage system and it is from this source that power is taken to start the diesel engine. Once the engine is started, the auxiliary generator which is driven by the engine through a flexible coupling, takes over the job of supplying Power to the low voltage system. As in an automobile electrical system, the auxiliary generator charges the battery.

302 Main Generator The main generator (Model is directly coupled to the engine through a flexible coupling, and furnishes direct current at a nominal 600 volts to the traction motors. The main generator voltage varies with the speed of the engine and the condition of operation of the locomotive.

303 Traction Motors The traction motors (Model D27) are direct current, series-wound motors geared to the wheel axles. The motors are reversed by changing the direction of flow of current in the field windings the direction of current flow in the armature always being the same. A reverser drum operated by electro-pneumatic control from the cab reverses the current flow in the traction motor field windings.

The traction motors are cooled by blowers, one for each truck. The blowers are belt-driven and turn at approximately twice the speed of the diesel engine.

For low speed and heavy traction, the traction motors are connected in series with the main generator. When connected in series the current passes consecutively through each motor before returning to the main generator. For higher speed operation the traction motors are connected to the main generator in what is called a series-parallel connection. In series-parallel, the current from the main generator divides and follows two paths. Each branch consists of two traction motors I return in series. The current paths.then reunite and to the main generator.

304 Reversing Locomotive Movement of the reverse lever from full forward to full reverse energizes either the forward or reverse magnet valves electric on the reverser switch located in the electrical control cabinet. When either of the magnet valves is energized it allows control air to pass through the valve moving the reverser switch to the desired direction.

305 Electrical Cabinet An electrical cabinet at the forward end of the cab contains the high and low voltage equipment (contactors, relays, fuses and instruments (Fig. 3-1).

306 Wheel Slip Control In the event of a pair of wheels slipping the wheel slip relay initiates the action that:

• 1. Lights the wheel slip light.
• 2. Opens the shunt and battery field circuits.
• 3. Causes the load regulator to move to the minimum field position.

When there is a wheel slip indication, it will probably be only one set of wheels that are slipping, unless the rail conditions are extremely bad. During slipping, the wheel slip relay, which contains two normally open interlocks, will become energized thereby closing the intnterlocks. In closing, one of the interlocks will light the wheel slip indicator light; the second interlock establishes a circuit that by-passes, or short circuits, the operating coil of the shunt field contactor. This opens the shunt field contactor, and,consequently, the battery field contactor. When the battery field contactor opens, its normally closed interlock (AB) closes, energizing the overriding solenoid (ORS) in the governor. When the overriding solenoid is energized it causes the load regulator to move toward the minimum field position.

The power output of the main generator to the traction motors, being almost completely reduced, will result in stopping the slipping. As soon as the slipping stops the wheel slip relay drops out and restores delivery of power to the traction motors. See Arts. 109 and. 206.

Electrical Cabinet -- SW8 and SW9
Fig. 3.1

307 Ground Relay (GR) The ground relay, located in the electrical cabinet, is an electrical protective device connected to the high voltage system. The function of this relay is to automatically unload the main generator in case of a ground in the high voltage system (a ground can be defined as current passing through the frame, or carbody, of the locomotive).

If a ground in the high voltage system should occur, the ground relay will trip. When tripped, the ground relay drops out the shunt and battery field contactors, unloading the main generator. The ground relay must be reset before the unit can again deliver power. The relay is reset by pressing in on the reset button on the relay, or on the extension button in the door of the electrical cabinet. Should the relay repeatedly trip when power is applied, the power plant MUST be isolated.

CAUTION: Only reset the ground relay with the engine speed reduced to idle.

If the ground relay trips, the white needle in the relay will point to a red dot. In the normal position the needle points to a yellow dot, Fig. 6-2.

With the ground relay tripped, an air throttle equipped locomotive will allow the engine speed to increase, when the throttle is opened, but no power will be delivered. Under the same circumstances the speed of an engine controlled by an electro-hydraultc governor will be reduced to idle and will remain at that speed unless the throttle is placed in the 5th or 6th position, in which case the engine will stop.

Although a high voltage ground will normally be the only reason for the ground relay tripping, a low gr voltage ground can trip the relay when the engine is started; since at, that time the high and low voltage systems are temporarily connected. Ground relay action is not necessarily an indication of serious trouble but should be reported to the maintenance authorities.

The ground relay knife switch, when open, eliminates the protection of the ground relay. This switch must NOT be opened in normal operation unless definite instructions are issued by an official of the railroad.

308 Current Limit Relay (CLR) A current limit relay is supplied on 1200 HP locomotive units. This is a through cable type relay which closes if the current from the main generator becomes excessive. When closed this relay causes the wheel slip indicator light to burn steadily, signaling that backward transition should be made. This relay will not close when in series transition, since wheel slip action will normally take place before the main generator current becomes excessive.

309 Low Voltage Regulator The low voltage regulator regulates the auxiliary generator output voltage, keeping it at an approximate 74 volts, regardless of the speed of the engine..

310 Reverse Current Relay (RCR) This relay, located in the electrical cabinet controls the opening and closing of the battery charging contactor. The reverse current relay causes the battery charging contactor to open when the voltage of the auxiliary generator falls to a point below the battery voltage. This prevents a reverse flow of current from the battery attempting to "motor" the auxiliary generator.

311 Battery Charging Contactor (BC) The battery charging contactor is an electrically operated switch connecting the auxiliary generator to the low voltage system. When the auxuiary generator voltage drops for, any reason (such as stopping the engine), the battery charging contactor opens, due to the operation of the reverse current relay. The battery charging contactor and reverse current relay prevent current from the battery flowing back through the auxiliary generator when the engine is stopped.

312 Battery Charging Fuse (150-amp.) This fuse protects the auxiliary generator against any possible overload. A blown battery charging (auxiliary generator output) fuse will cut off the auxiliary generator from the low voltage system.

313 Auxiliary Generator Field Fuse (30-Amp.) A 30-ampere fuse protects the auxiliary generator field windings against excessive current. Blowing of this fuse will also prevent the auxiliary generator from supplying current to the low voltage system.

314 Battery Ammeter This ammeter is located on the side of the electrical cabinet and shows whether the battery is charging or discharging. The ammeter should show zero or a slight "charge" whenever the engine is running. If the battery ammeter shows a continual discharge an investigation should be made to prevent the battery from eventually running down (Art. 605).