The output frequency of a generator is one of the important parameters that determine the generator’s rating. The electrical output of the generator must be maintained at a fixed frequency, 50 Hz or 60 Hz, to match the output of a standard electrical grid or the frequency rating of your appliances. The frequency is typically 60 Hz in the US and 50 Hz in Europe. You may also come across different isolated sections of the same grid operating at different frequencies. It then becomes essential to vary the output frequency of the diesel generator to match that of the appliances to be powered or of the grid to which your diesel generator is connected.
(1) Changing Engine Speed to Vary Output Frequency
Present-day generators consist of an engine directly connected to an alternator to produce electricity. One of the most common ways of changing the output frequency of a generator is to change the rotation speed of the engine.
The two factors are related as per the following formula –
Diesel Generator Frequency (f) = Number of revolutions per minute of the engine (N) * Number of magnetic poles (P) / 120
Conversely, P = 120*f/N
As per the above formulae, a 2-pole generator producing an output frequency of 60 Hz has an engine speed of 3,600 rpm. To change the output frequency to 50 Hz for the same generator configuration, the engine speed needs to be reduced to 3,000 rpm. Similarly, for a 4-pole diesel generator, an engine speed of 1,800 rpm produces output of 60 Hz. Reducing the engine speed to 1,500 rpm yields an output of 50 Hz.
In the case of small or residential generators, you can alter the engine rpm settings by making a few changes on the control panel of your unit. Follow the steps given below to alter your generator frequency from 60 Hz to 50 Hz:
(a) Run your generator engine and tune the frequency meter on your control panel to read 50 Hz.
(b) Check the AC voltmeter or potentiometer as the case may be and read the voltage output of the generator. The output voltage decreases as you reduce the frequency and may be lower than the desired value.
(c) Adjust the AC voltmeter or potentiometer on your control panel untill you obtain the desired output voltage at 50 Hz.
(d) By making similar modifications on the control panel, you can increase the frequency from 50 Hz to 60 Hz.
(e) If the control panel does not display the frequency, you will need to first connect a device that will measure the frequency while the generator is in operation and then change the engine rpm.
Diesel Generator controller units undertake real-time monitoring and control of your unit. Built-in protective functions automatically shut down your generator in the case of excess engine rpm or very low output frequency. For additional information on the functionality of a diesel generator please read the following article, How Generators Work.
(2) Frequency Convertors
If you use a fixed-speed generator, you could attach a frequency convertor to your unit. A frequency convertor is a combination of a rectifier and an inverter. The rectifier uses the generator’s alternating current (AC) output to produce direct current (DC). The inverter then converts this to produce AC output of the desired frequency. Any accompanying change in voltage is incidental to the purpose of the unit and also depends on the application for which the frequency convertor is being used.
Traditionally, frequency convertors, such as rotary convertors and motor-generator sets, were made of electromechanical components. With the advent of solid state electronics, these are now built as completely electronic units.
In addition to altering the output frequency, these units are also used to control the torque and speed of AC motors. Frequency convertors also find application in the aerospace industry for conversion of 50 Hz or 60 Hz to an output of 400 Hz that is used in the ground power unit of airplanes. These systems are also used to control the speed of fans and pumps and other variable torque loads operating on variable speed.
(3) Electronic Variable Speed Gensets
There is a special class of generators known as electronic variable speed gensets in which varying the speed of the engine changes the speed of the alternator to automatically produce an output of variable frequency. A frequency inverter is then used to rectify the varying generator output to match the required output frequency of 50 Hz or 60 Hz. Use of this device eliminates the need of variable speed drive and transformer. The drawback of this technology is that the electronic component, in addition to being expensive, is not suitable for use in rough environments in which a generator normally operates.
It is important to note that these are different from variable speed gensets that have continuously variable transmission (CVT), which allows the rpm of the engine to be changed but maintains a constant speed of the alternator. This does not alter the output frequency but allows the generator to change the output power of the generator to address variable load requirements.