Friday, December 30, 2011

General Guide Lines to Choose Generator Set

Power and voltage

The main thing is to calculate the amount of power needed. Some devices require a large power at start (inrush current), such as some machines with electric motors (A/C, fan, refrigerator, water pump, air compressor, etc.), when normal operation those machines will consume power as per manufacture specification (running current). Inrush current can be as much as 7 times higher than running current.

If many tools are often used with on-off operation or intermittent, then we need to consider to increase the amount of power needed to about 1.5 to 2 times the power that is written on the manufacture specification.  This is often confusing because the generator that was chosen based on power calculation was not able to supply enough power, but in fact many equipments are working with on-off operation and consume a lot more power.

Another way is to reduce the intermittent operation and make it work more steady, for example: increase water tank volume for water pump, using water level switch to detect maximal and minimal water level in tank.

For the compressor, reducing inrush current can be done by adding the volume of the air tank, use soft starter to limit inrush current, increase motor rotation (rpm) by changing the ratio of pulleys so that the compressor motor will work lighter and easier to start.

For air conditioner, ensure building tightly sealed so that the temperature inside the building is stable, therefore the A/C is not working hard to stabilize the temperature. If the room temperature changing rapidly then the A/C compressor will work with on-off operation frequently in an attempt to stabilize the room temperature, this will lead to a wasteful use of electricity.




Ensure voltage requirement for each equipment, the most popular systems are using 220 Volt and 110 Volt systems. If you need two systems with different voltages, you can use step up / step-down transformer to raise or lower the voltage.

Nominal power and maximum power
There are two systems of power measurement at each generator, namely nominal power and maximum power. Maximum power is always greater than the nominal power. But the generator is only able to work for about 20-30 minutes at maximum power. If more than 30 minutes to work at maximum power then the temperature will rise, and if the generator is equipped with an automatic circuit breaker then the electric current will be cut-off, or the generator will stop working if it is too hot (overheating). Nominal power is sometimes referred as rated power or continuous power. Therefore, you should always be based on the nominal power.

Power units
There two units of measurement for power. First is voltamperes (va) or kva (kilo voltamperes), second unit is watt or kilo watt. In alternating current (AC), va unit is always greater than watt. Because of the alternating current system, voltamperes must be multiplied by power factor (pf) in order to get watts.

Power factor is usually considered to be 0.85, unless it is written other. So if written power of generator is 2kva, then power in watts = 0.85 x 2 kva = 1.7 kilo watts.

Similarly on the tools that will be driven by electricity, we need to ensure its power units. Usually the power units for equipment is watt, not voltamperes.

Is it the primary or standby power generation?
If it is the main power plant, the generator must be able to work 24 hours a day 7 days a week continuously, so it is better to use the generators of good quality and expensive. If only for standby, then we can use generator with lower quality and with more affordable price.

If the generator works more than 12 hours per day then it can be categorized as a primary or prime power source. If it only worked less than 6 hours per day then it can be categorized as a standby.

Continuous generator operation means about the same as prime, but with less fluctuative power and about 10% less power than prime.

Some manufacturers specification explain power for standby and power for prime, prime power is always smaller than standby power as the prime generator works longer.

Fuel
For small size generator, gasoline is more suitable, because less noisy, smooth running, and easy to start. But gasoline is flammable and explosive. It need a better safety consideration.

For large size, diesel-fueled generators are more suitable. Diesel driven generator is usually more efficient fuel consumption than gasoline. Diesel fuel is also non-flammable and not explosive, making it safe even if stored in huge quantities.

Phase
For domestic use, single phase is commonly used. 3 phase is better for the industry because some equipment require industrial 3 phase electrical supply. 3 phase has more complicated grid, but it has small equipment size then single phase with same power.

Housing
Generally, a small generator is designed without a lid or a housing, so it need a special building or placed in a warehouse or garage. Some generator are designed with housing, to resist weather and low noise (silent generator), and of course more expensive, but do not need to build a special building for this generator.

Connecting to the grid
If you intend to connect your generator to an existing power grid in your building, then you  must instal selector switch (transfer switch) to select the power source. If the generator is connected to the active grid, generator will most likely be damaged by electric power from that active grid. You can not claim warranty if this happened.

If your generator is connected to the city power grid when electricity shut off, the generator may overloaded as it supply power to city grid. If that city power grid is being repaired and the generator is connected to the grid, power of your generator could endanger workers who repair the grid.

Fuse
Always use the correct fuse. Fuses are simple and inexpensive component and has proven very helpful to prevent fatal accidents.

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