Generators

Let's talk about Generators

 

Generators are widely used in the Camping and Caravan community to provide a number of uses and benefits. However before purchasing such a generator it is wise to consider what use it will be put to and assess what capacity generator will provide the purchaser with his/her requirements.

 

Typical uses for a genset will include the following:

 

Run lights for evening illumination.                                                                                      Typically 150 watts

 

Run battery charger                                                                                                                     100 to 200 watts

Run a TV                                                                                                                                         150 to 300 watts

Run sound and radio equipment                                                                                                             50 watts

Electrical hot water heater                                                                                                                      500 watts

Air conditioner                                                                                                                          1000 to 1600 watts (depending on cap)                                  

Fan                                                                                                                                                                 50 watts 

Fridge (150 )                                                                                                                                                  80 watts

Electrical resistance air heater                                                                                                1000 to 1500 watts

Electric blanket                                                                                                                                            l50 watts

Microwave                                                                                                                                                   900 watts

 

Looking at the above list we can break it down roughly into 2 types of load that the generator has to service.

 

The first category is Resistive loads. This includes incandescent lights, electrical hot water heater, Electrical resistance air heater and an electric blanket.

 

These electrical loads are very straightforward to assess against a generators quoted continuous output capacity. For instance if you have a gen set rated for continuous output of say 1600 watts, a 1500 watt air heater will be run OK.

If we add all the Restive load items in the above list it would look like this:

Lights                                         150 watts

Hot water heater                     500 watts

Air heater                               1500 watts

Electric Blanket                        150 watts

 

This makes a grand total of 2300 watts and you would need a genset that could supply a minimum of 2300 watts

continuously,

 

The complication comes when considering the other items. They have a type of load that is referred to as an Inductive load or a combination of Inductive and Resistive load.

 

It is more difficult to estimate the gensets requirements to adequately drive these loads. because they generally have a momentarily high inrush current when first activated. This can be 3 to 4 times the appliance’s normal running current.

 

For the small ticket items such as the sound equipment, the TV or the Fridge , the extra genset capacity is usually not significant if you are using say a 1S00W continuously rated gen set, but when you add say a 1300 watts airconditioner to the mix, then we need Some understanding about what is going to occur.

 

Before we go further, let’s look at how gensets are rated.

 

Up to a few years ago, most generators were field excited units but now days the Inverter units are becoming most popular. They have the great advantage of being lighter and they are better at starting motors (inductive loads).

 

Gensets are usually advertised with the Maximum output to the fore, but the Continuous rating is usually amid the fine

print. Gensets will put out the maximum power for a short period of time to be able to start motors etc...

 

An airconditioner rated at 1300 watts input will for a small fraction of a second require perhaps 4000 watts to start and accelerate the compressor up to speed. This starting procedure is of very short duration but in the case of for example a genset rated at MAX. 2000WaNs then inertia of the petrol motor/genset is of great importance to get the compressor successfully started.

 

Gensets are driven with an internal combustion engine (either petrol or diesel) and therefore have some short comings regarding performance. The hotter the ambient air is (the air that is sucked into the engine for combustion) so the pawer output of the engine and generator will decrease. A good rule is to allow a 1.5% decrease in output for every 5 degree C above 25C. So on a 45 C day, a genset will be de-rated by 4 x 1.5 = 6%. So with a 1500 W continuously rated genset, at 45C will only produce 1500-90 = 1410Watts. Height above sea level will also cause then genset to be de-rated further.

 

Having some understanding of the above, it can be seen that it is very easy to underestimate your genset requirements. Let’s now conslder airconditioners that one may wish to run on a genset.

 

The rated power input of an airconditioner is that steady state power input at the rating point of the aircon. This is usually measured at an outside temperature of 35C and an inside temp. of 27C & 50% humidity. As the outside temperature increases, so does the power requirement of the airconditioner. Typically an aircon rated at say 1300W may well require 1400Watts when the outside temp reaches 40C.

 

So you can see that as the genset is losing power as the outside temp increases, so the aircon is requiring more! So we have the following:

We would all choose to have the lightest genset to manhandle around. More powerful gensets have to have larger engines and therefore the weight increases.

 

If you have a significant inductive load (an airconditioner or a microwave) then you need significantly more power to start them than what is printed on the data sheet for that appliance. Normal engineering requirements for starting capacitor motors (that is most single phase airconditioners) off a genset is, in fact, 4 x rated power input. So by normal engineering criteria, a 1300W aircon should be driven with a 4 x 1300W or a 5200W (max rated) genset.

 

These days with the advent of inverter type gensets, it is quite apparent that smaller gensets can be used. However, it must be borne in mind that the margin between what the genset can put out (particularly in hot conditions) and the power requirement of the aircon can be borderline, and occasionally insufficient. Purchase these remarkably well engineered small gensets by all means, but bear in mind they will often be working “on the edge” and the occasional failure will have to be looked at in light of the above explanation.

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