Load Test Capacity
The desired capacity of the load test is expressed in kilowatts (KW). KW corresponds directly to engine HP (.746 HP = KW) and is developed exclusively by the resistive load bank. Adding reactive load changes the total KVA of the load, but does not alter the KW or HP load. A purely resistive load bank provides a realistic and effective test of the prime mover and causes full load currents to be developed by the generator. The engine will develop full rated HP, operating pressures and temperatures, and the performance of the governor will be fully evaluated. Adding inductive load allows for the full KVA capacity of the generator to be developed for a full functional test of temperature rise, waveform quality, voltage regulator response and reactive load sharing.
Voltage
Low voltage AC systems are rated 120/240v, single-phase, 2 or 3 wire; 208-240/416-480/575-600vAC, 3 or 4 wire. Medium voltage systems are in the 5kV or 15kV class. DC systems can range from battery voltages of 12/24/32v, 125/250v, or 350/700v, typically. Keep in mind that the AC load banks in this catalog are generally 3-phase, 3-wire loads, meaning that a neutral is not required and that loads are applied in 3-phase balanced steps.
Current
Current, expressed in amperes, is found by the formula on the right.
Current must be known in order to size connection cables. Keep in mind that connection cables have a certain small resistance, and that a small voltage drop will be seen across the cable set and at the terminals of the load bank. Therefore, although the generator instrumentation may indicate 480v, the load bank instrumentation will measure the terminal voltage at the load bank, for example, 473v.
KW as a Function of Voltage
Connection cables have a certain small resistance, and a small voltage drop will be seen across the cable set and at the terminals of the load bank. Although the generator instrumentation may indicate 480v, the load bank instrumentation will measure the terminal voltage at the load bank, for example, 473v.
KW varies as the square of the voltage: therefore, in the preceding example, if the load bank terminal voltage is reduced to 473v, then the resultant KW will be 97% of the rated KW. Where did that power go? It is lost as heat in the connection cables.
Low Power Factor Testing
By adding an inductive load to the resistive load, lagging power factor can be obtained.
Commercial loads, as well as generator sets, are rated at the nominal power factor of 0.8 lagging. This number is the cosine of the angle made between the KW and the KVA in the diagram at right.
The magnitude of inductive load required to obtain 0.8 power factor is 0.75 X the KW (1000kw + 750kvar = 1250kva at 0.8 lagging power factor).
As a point of interest, 0.75 is the tangent of the aforementioned angle.
Keep in mind when sizing cables that the full load current at the rated KVA must be calculated.
Leading Power Factor Testing
See Low Power Factor Testing. The same rationale applies to leading power factor, except capacitive load is added.
Leading power factor testing is rare and specialized. Most engine generator sets cannot tolerate more than a very small degree of leading power factor.
KW Applied
KW Applied is used to determine actual kilowatt load being applied to load bank when line voltages and currents are known (at 1.0 power factor)
For addiitional calculators visit SimplexOnsite - Load Bank Interactive Electrical Calculators