Power Generators

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A power generator is a portable substitute for mains power. Depending on power requirements a generator can vary from being very portable to large commercial applications.

Some reasons for requiring an alternative source of mains power are;

  1. Hospitals – typically large commercial type generators which are configured to self start in case of a loss of mains power.
  2. Offices, especially data dependant businesses such as ISP’s, phone companies and emergency call centres.
  3. Travellers who require mains power for appliances while on the road.
  4. Home user who have back up power in the event of black outs due to storms or during construction work.

There are different types of generators available, depending on the application.

These include single phase, three phase, petrol, diesel, manual start by pull cord or key, auto start, sine wave or square wave outputs.
They come in skid mounted form, enclosed, weatherproof and sound attenuated enclosures.

Skid Style with pull cord start Generator Yamaha EF3000iSE Inverter Generator electric start
Skid Style with pull cord start Generator Enclosed electric start sine wave Generator

Generators Are Rated In kVA

Generators are generally rated in kVA (kilovolt amps), why not in watts or kilowatts?

Well first let’s explain what kVA is.

240 volt AC mains power is a sinusoidal wave form as viewed on an oscilloscope, as seen below.

Sinusoidal wave

In a perfect world all appliances would consume power directly from the supply and this would be expressed in watts or kilowatts depending on the amount being consumed. An example would be a purely resistive load such as a 1kw bar radiating heater which would take 1kw of power from the mains supply.
kVA is a way of expressing the amount of electrical energy being handled by the circuit, even if it is not being used as real power. This is either the same or higher than the kW, and is important in calculating the capacity of wiring circuits. Electric motors are the main items you'll come across where the kVA is more important, as the electrical energy used is generally much higher than the real power output.

This is due to a phase difference between the voltage and the current, commonly known as power factor.

Normally we talk about Kilowatts which is the unit a household is charged for. However with AC power there is another factor which comes into play and this is the power factor.
To try and keep it simple, watts are generally a non inductive load like a electric fire; amps x volts = watts where the amps are a factor of ohms (purely resistive load), but if you use a motor in a washing machine, the load on this is inductive (it has a coil which induces magnetism which turns the shaft of the motor) a coil has a lagging current which can be a circulating current which is non useful current (as a matter of interest a capacitor has a leading current and these are used to correct a lagging current) the correction for this is normally referred to as power factor.
Large factories are billed by the electricity supply authority by KVA so they get billed for any non useful circulating currents, so factories normally have power factor correction units installed (capacitors) to correct this which reduces the amount of money they are billed.

You do not need to fully understand this principle and it is beyond the scope of this article to try and explain power factor, but just be aware that when looking for a generator the kVA rating does not equate to the same as kW.

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