Generator Set Inspection

After setting up a portable generator, your crew must do some preliminary work before placing the generator in operation. First, they should make a visual overall inspection of the generator. Have them look for broken or loose electrical connections, bolts, and cap screws, and see that the ground plate is properly grounded. Check the wiring diagrams in the instruction manual furnished with the generator to see if any wire connection is suspected of being improperly connected. If you find any faults, you should correct them immediately.

PRIME MOVER INSPECTION.\Servicing the prime mover is the next step in the process of placing the generator in operation. Be sure that the crankcase is filled with the proper grade of lubricant. A lubrication chart in the instruction manual furnished with each generator will show the proper grade of oil to use according to the operating temperature. If the plant is to be operated in freezing temperatures, be sure to use an antifreeze solution in the proportions recommended in the instruction manual for the generator. The fuel tank should be filled with clean fuel oil, strained if necessary.

Some of the prime movers of advanced base electrical power generators are started by starting units that obtain their power from batteries. If the prime mover that you are installing is equipped with a battery (or batteries), your crew has another servicing job to do. Batteries are usually shipped without the electrolyte but with the plates in a dry-charged condition. Thus, it is necessary to fill the battery with electrolyte. Usually, the electrolyte is shipped with the generator and is of the correct specific gravity. Because specific gravity of the electrolyte depends on the type of battery furnished with the generator, use the specific gravity value recommended by the manufacturer's instruction manual.

On large generators, you should check the area ventilation; the fan cover has to be opened and latched in that position. Be sure there is no cover or obstruction over the radiator section. The bypass shutters or doors may be closed to shorten the warming-up period, and roof hatches and side louvers may be opened for additional ventilation, if required.

ALTERNATOR INSPECTION.\Just as important as the preparation of the prime mover are the inspection and servicing of the alternator. Generally, you should take the following steps:

1. Check all the electrical connections by referring to the connection diagrams of the generator.

2. Megger the generator stator windings, generator rotor windings, exciter field windings, and exciter armature.

CAUTION

Do not exceed 500 volts dc for lowvoltage generating sets.

3. Check all electrical connections for tightness.

4. Check that the collector rings are clean and have a polished surface.

5. Check collector brushes to make sure they have no tendency to stick in the brush holders, that they are properly located, and that the pigtails will not interfere with the brush rigging.

6. Check the collector brush pressure to see if it agrees with the figure recommended in the manufacturer's instruction manual. If technical manuals are not available, a pressure of 2 to 2 1/2 psi of brush contact surface is recommended for integral horsepower and integral kilowatt machines, and about twice that pressure for fractional horsepower and fractional kilowatt machines.

Generator Connections

When you install a power plant that has a dual-voltage alternator unit, make certain that the stator coil leads are properly connected to produce the voltage required by the equipment.

Proper grounding is also a necessity for personnel safety and for prevention of unstable, fluctuating generator output.

INTERNAL LEADS.\Generators previously used for advanced base power requirements required you to make reconnections in the ammeter and voltmeter circuits when the generator output voltage was changed. The majority of manufacturers have made provisions that simplify the changeover from one voltage output to another. A newer type of changeover board is shown in figure 5-3.

The voltage changeover board permits reconnection of the generator phase windings to give all specified output voltages. One end of each coil of each phase winding runs from the generator through an instrumentation and a static exciter current transformer to the reconnection panel. This assures current sensing in each phase regardless of voltage connection at the reconnection board assembly. The changeover board assembly is equipped with a voltage change board to facilitate conversion to 120/208 or 240/416 generator output voltage. Positioning of the voltage change board connects two coils of each phase in series or in parallel. In parallel, the output is 120/208; in series, the output is 240/416 volts ac. The terminals on the changeover board assembly for connection to the generator loads are numbered according to the particular coil end of each phase of the generator to ensure proper connections.

Remember that you are responsible for the proper operation of the generating unit.

Figure 5-3.\Changeover panel.

Therefore, proceed with caution on any reconnection job. Study the wiring diagrams of the plant and follow the manufacturer's instructions to the letter. Before you start the plant up and close the circuit breaker, double-check all connections.

GROUNDING.\It is imperative that you solidly ground all electrical generators operating at 600 volts or less. The ground can be, in order of preference, an underground metallic water piping system, a driven metal rod, or a buried metal plate. A ground rod has to have a minimum diameter of 5/8 inch if solid and 3/4 inch if pipe, and it has to be driven to a minimum of 8 feet. A ground plate has to have a minimum of 2 square feet and be buried at a minimum depth of 2 1/2 feet. For the ground lead, use No. 6 AWG copper wire and bolt or clamp it to the rod, plate, or piping system. Connect the other end of the ground lead to the generator set ground stud.

The National Electric Code^R states that a single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by additional electrodes. Where multiple rod, pipe, or plate electrodes are installed to meet the requirements, they are required to be not less than 6 feet apart.

It is recommended that you perform an earth resistance test with a VibrogroundR Test Set before you connect the generator to ground. This test will determine the number of ground rods required to meet the requirements, or it may be necessary to construct a ground grid.