Choosing a Generator

Choosing a generator can be a difficult and time consuming process. There are so many generators to choose from! How do you make up your mind up? Do you chose it on Price, Warranty, Back up/Support?

In this age of consumer freedoms the choice for many items, including generators, seems endless, buy in confidence from us, in the knowledge that all our generators are from top World renowned companies, we do not sell the inferior quality generators as sold on some auction websites, etc! Before you choose/buy a generator you may wish to consider the following:

What size of generator do I require?

All electrical products have a data plate on them with information regarding their power requirement/usage. For most power tools (not all) (drills, grinders, saws, etc.), their listed wattage is all the power they require to run. Household items such as lights, TV's, DVD player, etc. fall in the same category (these are known in the trade as ‘resitive’ loads). Items such as freezers, fridges, washing machines, lawn mowers are normally different, whilst they can state a wattage rating, due to the type of motor they use can require a much greater requirement on star-up, when started they then require a smaller usage.

If you are unsure as to what power usage a product has, it is always advisable to contact the manufacturers sales or technical department, quoting model number and requesting the ‘starting’ wattage/current & also the running wattage/current of your item. With this information you will be able to determine accurately the size of the generator you require (if that fails then contact us @ Elliotts).

Do I require a generator that has a long run fuel tank?

Some, not all generators are available in either standard run or long run versions. A standard tank will last for about 2-4 hours, where most long run generators can run for 8-10 hours (the running times are totally dependent on the connected load!). If you are looking to purchase a generator for back up (when you experience a power cut) then we would recommend a long run tank machine.

Do I want to run a computer off my generator?

Running home computers from a standard open generator is not advisable. You will need a generator that utilises an AVR (Automatic Voltage Regulator) or an Inverter generator such as the Honda EU Range. These generators will give you a more stable electricity supply than the mains. Please note: as a generator runs out of fuel, the engine is likely to surge/fluctuate. To avoid this affecting electronic equipment an Uninterruptible Power Supply (UPS) can be used. These are typically sold for use with computers so that data is not lost in the event of a power cut.

I have bought a generator, and I am concerned about safety. Can I use a personal power breaker?

Generators are configured differently to the mains supply. The generator has a ‘floating earth’, whilst the mains has an earthed neutral. Whereas it is definitely recommended to use a personal power breaker (RCD) from the mains, for the majority of cases, it is not necessary to use one with a generator. Personal power breakers are designed to operate from the mains. If one is to be used with a generator, then it is necessary to modify the generator so that it is configured in the same way as the mains. This is a relatively simple modification for a qualified electrician involving adding a link wire from the neutral terminal to the earth terminal. However, once the generator has been modified, it is necessary to then always use a personal power breaker and to also always use an earth spike, which connects between the generator frame and the ground. Since this is difficult to ensure, it is generally recommended not to modify the generator. There are also some issues with warranty!

I have bought a generator and would like to connect it to my house, in case of mains failure/power cuts. What do I need to do?

When using a generator as an alternative supply to the mains, there are several precautions that you must observe. It is vital that the generator is completely isolated from the mains supply. This ensures that the generator is not attempting to power up the whole neighborhood, but, also ensures that it does not electrocute a utility worker trying to restore the mains supply. To achieve this, a double-pole, break-before-make, changeover switch must be installed by a qualified electrician.

This should be fitted between the electricity meter and the building consumer unit. The switch (known as a ‘changeover switch’) connects the building to either the mains supply or to a lead which can be plugged into the generator.

Most buildings now have RCD’s (Residual Current Devices) built into the consumer unit. This is configured to operate from the mains supply with an earthed neutral, and not from a generator which has a floating earth. To utilize this protection device, it is necessary to modify the generator so that it is configured in the same way as the mains supply. This is a simple modification for a qualified electrician, involving adding a link wire from the neutral terminal to the earth terminal. It is recommended to make this connection in the plug that is to be used to connect to the generator. This ensures that the generator is unmodified when it is disconnected from the house, and therefore remains safe. The plug should be labeled “Do not connect to mains: Neutral-Earth link fitted”. The lead between the generator and the transfer switch is not protected by the RCD, it is therefore recommended to use a steel armored cable for this connection. Finally a local low-impedance earth spike needs to be installed.

What type of sockets do generators use?

Generators sold for use within the UK use the following types of 230/110V sockets.

13A 3 pin (similar to your house, usually in black plastic/rubber)

Blue round in either 16A or 32A

Yellow round in either 16A or 32A

Please note the white 230V 15A Schuko sockets are for European use only.

What is the difference between kW and kVA? What is power factor?

There are 3 types of electrical power:

  1. Real Power, measured in Watts (W). This is the power drawn by a resistive load, e.g. a heater element in a kettle, and has a power factor of 1. (unity power factor, cos F=1, 1.0pf or pf=1)

  2. Reactive Power, measured in Volt Amperes reactive (VAr’s). This is the power drawn by a reactive load (a load with a winding around a core), e.g. an electro-magnet, and has a power factor of 0. (zero power factor, cos F =0, 0pf or pf=0)

  3. Apparent Power, measured in Volt Amperes (VA). Many loads have a combination of resistive and reactive elements. (in fact it is not possible to produce a purely inductive load, since the wire used to form the windings has a resistance). This combination of elements means that both real power (W) and reactive power (VAr) are drawn together.

The proportion of Real Power to Reactive Power is defined as the power factor. [Nearly all resistive load (e.g. Universal motor used in hand tools) then power factor 0.95 to 1.0, nearly all inductive load then power factor ~ 0.3]

The vast majority of single-phase loads have power factors approaching 1. Therefore, single-phase generator power ratings are taken at power factor =1, and are consequently in Watts (W) or kilo Watts (kW), where 1 kW = 1000 W.

Three-phase loads tend to have lower power factors, approaching 0.8, therefore, three phase generator power ratings are taken at power factor =0.8 and are in VA or kVA.

There is obviously a relationship between real power, reactive power, apparent power and power factor.

a) Apparent Power (VA) = Ö [(real power (W))2 + (reactive power (VAr))2]

And

b) Power factor = Real Power (W)
Apparent Power (VA)

Therefore…

Apparent Power (VA) x Power factor = Real power (W)
If the Power factor =1, then all the Power is real, and
Apparent Power (VA) = Real Power (W)
(W = VA @ 1.0 pf)

For a single-phase generator, the rating should be at 1.0 pf, in which case
Watts = Volt Amperes.

But, for a three-phase generator the rating is at 0.8pf.

This is where confusion can arise!

Example.
A three-phase generator has a continuous rating of 5 kVA at 0.8 pf.

Now, at this rated load, the Real power (kW) will be …

Real Power (kW) = Apparent Power (kVA) x Power factor

Real Power = 5 x 0.8 = 4kW

This means that a generator producing 5kVA at 0.8pf is actually producing 4 kW of Real power, but it is also producing some reactive power.

From i)…
5000 VA = Ö [( 4000 W)2 + ( Reactive Power)2]
Reactive Power = 3000 VAr’s

It is this combination of 4kW of real power and 3kVAr’s of reactive power that has defined the limit for the generator rating.

If the same generator was loaded with a resistive load only, then it may be capable of more than 4kW, however, there is no formula that can be used to find this limit from the 0.8pf rating. It can only be found through testing of each machine.

Similarly, a single-phase generator rated at 4kW, cannot be expected to produce 5kVA at 0.8pf!

Question: Can I operate sensitive equipment from a generator?

The output from a portable generator might not be as stable as the supply from the mains! The speed of the engine driving the alternator is controlled by a simple mechanical governor; consequently, the speed drops as the load is increased. The frequency of the output voltage is directly dependent on the engine speed; therefore, the frequency of the output varies with load. In addition, the output voltage will vary with load, and with temperature. The output voltage of most standard generators will remain within 230V +/- 10% from no load up to the rated load current quoted on the data plate. This is the guaranteed range of voltage supplied from the mains utilities.

The frequency of the output voltage will vary typically from 53Hz at no load to 49Hz at rated load current, whereas the mains supply is unlikely to vary by no more than 0.1 Hz.
Most electronic equipment is designed to cope with these fluctuations and will operate normally. However, it is always recommended to ask the equipment supplier whether their equipment is suitable to be operated from a portable generator.
As a generator runs out of fuel, the engine is likely to surge/fluctuate. To avoid this affecting electronic equipment an Uninterruptible Power Supply (UPS) can be used. These are typically sold for use with computers so that data is not lost in the event of a power cut.

What is an AVR?

An AVR (Automatic Voltage Regulator) fitted to a generator basically gives the same output as an inverter machine. This feature limits the variation of the voltage of your generator to + or - 2%. With a generator equipped with an AVR you can run sensitive equipment such as computers.

What size generator do I need to start a motor?

This is a tricky one, since there are many different designs of motor, each with different characteristics. Some motors, e.g. Induction type motors (capacitor start/capacitor run) require additional current to start them, therefore requiring a larger generator. Motors fitted to hand tools generally do not require any additional start-up current. Consequently, it is recommended to ask the supplier of the equipment that you wish to run whether it requires additional start-up current. As a rough guide only, allow for a generator that has a continuous rating of 2½ to 3 times the motor rating.

Motors can be rated in kW or HP. To convert HP to kW, multiply by 0.746.

E.g. What generator power do I need to run a 3HP motor?

3HP = 3 x 0.746 = 2.238kW.
This motor would require a generator of between;
(2½ x 2.238) = 5.595kW
&

(3 x 2.238) = 6.714kW

What size generator do I need for my welder?

A welder is rated by its output current. To estimate its input power, divide the output rating by 30.
E.g. A 130A welder will have an input requirement of approximately 130/30=4.3kW. A 200A welder will have an input requirement of approximately 200/30=6.7kW.
This is only an estimate; therefore, it is recommended to choose a generator of the next size up. However, bear in mind, that many users will not actually require the full capacity of their welder, a smaller generator would still operate the welder, but, it would limit the welding current.

If you still find that you are unsure as to what particular generator you require, then please either telephone us on 01954 20 20 20 or email us your question on sales@elliotts.co.uk.



Please be aware that any modifications to a generator might affect it’s warranty, so as always if in doubt please contact the supplier!