Sizing Your Generator
Monday, March 29th, 2010Sizing Your Generator
In selecting an Alternating Current (A.C.) generating set consider the following points:
1. Total electrical load connected – including any motors i.e. Bore hole pump etc.
2. Voltage, frequency and phase
3. Special requirements
4. Whether automatically started or not
5. Acceptable sound emissions
CONNECTED LOAD or ASSESSED LOAD
Having assessed the total connected load, consider next the portion of the load that is likely to be switched on at any time. A Generator set with an output a little in excess of the figure should be selected to allow for contingencies. Always allow for future expansion.
VOLTAGE, FREQUENCY AND PHASE
A.C. current is usually supplied by Public Electricity undertakings in sub-Saharan Africa at 230volts and a frequency of 50 cycles (Hertz). The normal supply to the average house is single phase being a two wire systems. The three phase four wire system is usually taken to factories to give higher voltage (The voltage between phases, known as the line voltage has a direct relationship to the phase voltage. 1.73 to 1, i.e. – 230 volt phase voltage, 400 volt line voltage). It is advantageous to use a three – phase supply for electric motor loads both on consideration of price and starting characteristics. Single phase sets with outputs up to 12kW or 15kVA are normally sufficient for domestic installations.
Manual change over:
This is a manual switch normally with a pilot light wired so that when the mains is on the light shows. It would have a Mains/Neutral/Generator switch to ensure that it is either mains on or generator on but never both.
Automatic Standby to Mains:
Sets can normally be supplied which start automatically and take up the load when the mains fail. On restoration of the mains supply the load is automatically transferred to mains and the generator set shuts down. There will always be a delay between mains failure and the generator starting up and supplying full load so if sensitive equipment is being used or computer systems an Uninterrupted Power Supply (UPS) should be installed to ensure no loss of Data. Time delay on switch over can be between 3 seconds and 30 seconds dependant on the generator system installed.