When a generator is installed or reconnected, be sure that the total current in one phase does not exceed the nameplate rating. Each phase should carry the same load. This load allows the engine to work at the rated capacity. If one phase current exceeds the nameplate amperage, an electrical unbalance can result in an electrical overload and overheating.
Allowable combinations of unbalanced loads are shown in Illustration 1. When you operate with significant single-phase loads, the combinations of single-phase load and three-phase load may be used. Such combinations should be located below the line on the graph.
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| Illustration 1 | g00627416 |
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Allowable Combinations of Unbalanced Loads | |
Block loading is the instantaneous application of an electrical load to a generator set. This load may be anywhere from a moderate percentage of the rated load up to the rated load.
The block loading capability of a generator set depends on the following factors.
- Engine transient response
- Voltage regulator response
- Type of the voltage regulator
- Altitude of operation of the generator set
- Type of load (power factor for the load)
- The percentage of load before the application of the block load
If derating for the block load is required, refer to ISO 3046 Standards or SAE J1349 Standards. Also, reference Engine Data Sheet, LEKX4066, "Loading Transient Response" and Engine Data Sheet, LEKX4067, "Block and Transient Response".
Note: ISO stands for International Standards Organization.
The power factor represents the efficiency of the load. Power factor is the ratio of apparent power to total power. The power factor is expressed as a decimal. The power factor represents that portion of the current which is doing useful work. The portion of current which is not doing useful work is absorbed in maintaining the magnetic field in motors or other devices.
In most applications, electric motors and transformers determine the power factor of the system. Induction motors usually have a 0.8 or smaller power factor. Incandescent lighting is a resistive load of about 1.0 power factor, or unity.
The power factor of a system may be determined by a power factor meter or by calculations. Determine the power requirement in kW by multiplying the power factor by the kVA that is supplied to the system. As the power factor increases, the total current that is supplied to a constant power demand will decrease. For example, a 100 kW load at a 0.8 power factor will draw more current than a 100 kW load at 0.9 power factor. High-power factor will result in full engine load at less than the rated amperage of the generator. A lower power factor increases the possibility of overloading the generator.
Note: If no other power factor is specified, Caterpillar generators are designed for a 0.8 lagging power factor.
Permanent Magnet Pilot Excited Generators
Permanent Magnet Pilot Excited (PMPE) generators receive power for the voltage regulator from a pilot exciter, rather than the main armature. The pilot exciter consists of a permanent magnet rotor and a permanent magnet stator. The pilot exciter operates independently from the generator output voltage. Constant excitation during a large load application is possible because the irregularities that occur in generator output voltage are not fed back into the exciter. Such irregularities can be caused by load conditions. The independent operation also allows the generator to sustain an overload for a short duration. The pilot exciter also ensures that the generator will start properly even if the rotating field becomes demagnetized.
Internal Excitation Generators
The "Internal Excitation" generator consists of two special sets of coils that are wound to fit in carefully selected slots of the main stator. The wire coils provide total separation from the main winding of the stator. The wire coils provide isolation from the main winding of the stator. The coils can only be fit to the main stator while the main stator is being wound.
The two auxiliary windings are designed in order to provide power to the voltage regulator. The two coils are connected in series and the two coils are connected to the three-phase power input of the voltage regulator.
One auxiliary winding produces a voltage which is proportional to the output voltage of the generator. The other auxiliary winding acts as a current transformer. The other auxiliary winding produces a voltage that is proportional to the output current of the generator.
The outputs from the two coils are combined on the inside of the voltage regulator. the outputs from the two coils provide a constant power source.
When the engine starts turning the "Rotating Field Assembly" (RFA), the residual magnetism that is in the exciter stator (L1) and the embedded permanent magnets that are in the exciter causes a small amount of AC voltage to be generated in exciter rotor (L2). Induced voltage causes current to flow. The current that is caused by the induced voltage is present in the exciter armature.
The AC is then rectified by a three-phase full-wave bridge rectifier circuit. Flashing of the field is not required in order to start the generator.
Generator sets normally have a higher low idle setting than industrial engines. Low idle will be approximately 66% of the full load speed of 60 Hz units. Low idle will be approximately 80% of the full load speed of 50 Hz units.
There is no low idle stop on generator sets with electronic governors. On generator sets with mechanical governors and natural gas generator sets, the low idle is set at the factory. The low idle should only be adjusted by your Cat dealer if adjustment is required.
Note: Operating the generator set at low idle speed for an extended time will cause some voltage regulators to shut off. The generator set must be completely shut down. Then, the generator set must be restarted. This restart will allow the voltage regulator to resume output.
Most standby units are installed with controls that will start the unit automatically. Standby units perform the following tasks without an operator in attendance: start, pick up the load, run and stop.
Standby units cannot change the governor control automatically. Standby units cannot change the voltage settings automatically. The governor speed and voltage level must be preset for the proper operation of the standby unit. Whenever the set is operated manually, be sure that the governor speed and voltage levels are correct for automatic operation. Check all switches for the proper setting. The start select switch should be in the AUTOMATIC position. Emergency Stop Switches should be in RUN position.
A customer may choose to have a generator mounted space heater installed. These space heaters are installed for operation in high humidity conditions. For more information on generator mounted space heaters, refer to Maintenance Section, "Space Heater - Check".
A customer may choose to have an enclosure mounted space heater installed. Refer to illustration 2. The enclosure mounted space heater is attached to the ceiling inside the enclosure. The operation of the space heater is automatic. The space heater thermostat is preset to operate with an upper temperature limit of
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| Illustration 2 | g03342296 |
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Enclosure mounted space heater (1) Enclosure ceiling (2) Space heater | |
Bearing temperature detectors are available as an option on the generators. Bearing temperature detectors measure main bearing temperature. Bearing temperature detectors are used with the equipment that is provided by the customer. Thus, the temperature of the bearing can be measured or monitored. Bearing temperature measurements may help to prevent premature bearing failure.