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 allows the engine to work at the rated capacity. An electrical unbalance can result in an electrical overload and overheating if one phase current exceeds the nameplate amperage.
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 | |
When an electrical load is applied to a generator set, block loading occurs. 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
- the amount of load that is already present
If a block load derating is required, refer to ISO 8268 Standards or SAE J1349 Standards. Also, reference Engine Data Sheet, LEKX4066, "Loading Transient Response" and Engine Data Sheet, LEKX4067, "Block and Transient Response".
The power factor represents the efficiency of the load. The power factor is the ratio of apparent power to total power. This ratio is expressed as a decimal. The power factor represents the 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. This current is called the reactive load. Engine power is not required to maintain the reactive load.
In most applications, electric motors, controls, and transformers determine the power factor of the system. Induction motors usually have a power factor that is no larger than 0.8. Incandescent lighting is a resistive load of about 1.0 power factor, or unity. Controls can operate at any power factor. Variable frequency drivers and variable speed drivers can operate at any power factor. An uninterruptible power supply can operate at any power factor. In this case, the power factor can be between 0.4 and 1.0.
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. With equal loads, a lower power factor will draw more current. A high power factor will result in full engine load that is less than the generator's rated amperage. A lower power factor increases the possibility of overloading the generator.
Note: Normally, Caterpillar generators are designed for a power factor of 0.8 lagging. Please consult your Caterpillar dealer in order to check the generator rating if operation at less than 0.7 lagging power factor is desired.
Self-excited generators receive excitation power and regulator-voltage sensing from the generator's main armature output. The voltage regulator senses the generator output voltage. A regulated output is provided to the generator exciter. This regulated output is provided by the voltage regulator. The exciter then provides power to the main rotating field. As the main field rotates, a voltage is induced into the main armature. This voltage is a generator output voltage.
Note: The main armature is also called the stator. The main rotating field is also called the rotor.
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 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 the generator's output voltage are not fed back into the excitation system. Such irregularities can be caused by load conditions. The independent operation also allows the generator to better sustain an overload for a short duration.
Typically, the low idle on 3500B generator sets is 900 rpm. On 60 Hz units, low idle will be approximately 66 percent of the full load speed. On 50 Hz units, low idle will be approximately 80 percent of full load speed.
There is no low idle stop on generator sets with electronic governors. The low idle is set at the factory on generator sets that have mechanical governors. The low idle should only be adjusted by your Caterpillar dealer if adjustment is required.
Note: Operating the electric set at low idle speed for an extended time will cause some voltage regulators to shut off. The electric set must be completely shut down. Then, the electric set must be restarted. This will allow the voltage regulator to again produce an output.
Most standby units are automatic. Without an operator in attendance, standby units will perform the following functions: start, pick up the load, run and stop.
Standby units will not change the governor speed control or voltage level settings automatically. The governor speed and voltage level must be preset for the proper operation of that unit. Whenever the set is operated manually, ensure that the governor speed and the voltage level settings are set correctly for automatic operation. Check all switches for the proper setting. The Start Selector Switch should be in the AUTOMATIC position. Emergency Stop Switches should be in RUN position.
The SR4B generator is capable of operating in high humidity conditions without problems. However, problems can occur when the generator is idle and the surrounding air is warmer than the generator. Moisture can form on the windings that will result in poor performance from the windings. Moisture can also result in damage to the windings. Whenever the generator is not active, ensure that the space heaters are in operation.
Whenever the generator is operating, ensure that the space heaters are disconnected.
An external source of either 115 VAC or 230 VAC is required to operate the space heaters.
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| Illustration 2 | g00556903 |
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Space heater connection to external source (H1), (H2), (H3) and (H4) terminals. | |
If a 115 VAC source is available, connect both heaters in parallel across the source. If a 230 VAC source is available, connect both heaters in series across the source. Refer to Illustration 2.
Embedded Temperature Detectors
SR4B generators are available with embedded temperature detectors. The detectors are installed in the slots of the main armature. The main armature is also called a stator. The detectors are used with the equipment that is provided by the customer. Thus, the temperature of the main armature winding can be measured or monitored. Two types of temperature detectors are available: RTD and thermocouple. Contact your Caterpillar dealer for more information.
Temperature detectors for the bearings are available on large frame generators. These temperature detectors measure the main bearing temperature. Thus, the temperature of the bearing can be measured or monitored. Bearing temperature measurements may help to prevent premature bearing failure. Two types of temperature detectors are available. Temperature detectors are used with equipment that is provided by the customer. Contact your Caterpillar dealer for more information.