The most important thing to consider when sizing a generator is the high inrush currents associated with starting electric motors and transformers, which are typically six times the full load current.
However, inrush currents for the type of high efficiency motors being specified today can be almost double that amount.
As a result, it has been common practice to take motor and transformer starting kVA requirements as a yardstick to determine the size of a generator.
This approach often results in generators being oversized for the motor running load and not based on the actual needs of the application. Moreover, it disregards other key factors that play a key role in sizing generators. For instance, harmonics caused by variable frequency drives and sequential starting of motors.
When starting motors or transformers, large voltage and frequency dips can also occur if the generator set is not sized properly. Furthermore, other loads connected to the generator output may be more sensitive to voltage and frequency dips than the motor or motor starter, which can cause problems.
Thankfully help is at hand. Many generators can now be equipped with solutions to overcome the extra excitation systems required in the alternator.
Typically, two options are offered: permanent magnet or auxiliary winding. Both provide the generator with three times their nominal current to cover inrush peaks from the electrical motor, for a minimum duration of ten seconds, via a residuary excitation current.
In certain cases even more advanced options are available. For instance, some generators feature a digital automatic voltage regulator (D-AVR) that is specifically designed to handle the high inrush currents associated with starting motors and transformers. In specific applications, this type of voltage controller allows operators to downsize the generator requirement because the transient behaviour of the power is better managed.
Another option could be to use a “Close Before Excitation” system that closes the breaker just when engine starts to run. This enables the excitation to increase gradually as the speed of the engine does, allowing for very soft start of loads connected to the generator.
This is especially useful for magnetising step up transformers in installations where medium voltage is required.
As a result, it is no longer necessary to buy larger generators than needed just to cope with the initial electrical surge upon starting. What’s more, with smart control of the generator’s voltage, it is possible to achieve lower fuel consumption, reduced maintenance cost and longer lifetimes.