Generator Basics

When selecting a generator, there are certain steps to take in order to make sure that you buy the correct product for the application. Mistakes could cost huge amounts of money to rectify at a later stage.

Source: Various generator providers and installers. 

Following, is a step by step guide, which will help you with generator selection:

Tips, Hints and general comments.

1. 1. Refueling – NEVER refuel a petrol generator when the engine is running. A spark from the spark plug can ignite the fuel and start a fire. I have received a number of burnt out sets back over the years. The problem could easily be recognized as the fuel cap is normally burnt on the inside. This means the cap was not fitted to the machine at the point of catching fire. The reason is obvious: refueling while the engine is running.
   2. Starting up the generator – ALWAYS make sure that the appliances or power tools are switched off before starting up the generator. When the revs have settled to a constant level it is then the time to switch on the appliances.
   3. Extensions – Do not use an extension which is longer than 15 m without increasing the diameter of the cable so as to avoid voltage drops which destroy appliances and tools.
   4. Fuel – NEVER run out of fuel – Always refuel before the tank is empty. (remember to switch off) Just before cutting out, when running out of fuel, the alternator does not produce the correct voltage and can burn out your appliances or tools.
   5. Misconception – generator power is not as good as Eskom power. The power from a generator is as good if not more constant as that of the locally supplied power. Most reputable alternator suppliers offer alternators with less than 5% harmonic distortion. The problem could come in during start up or switch off. This is the time when your generator puts out an unstable current. An AVR (automatic voltage regulator) could eliminate this problem.

Determine the size of the generator needed for the required application.

From experience we know that your customer would like to use a vast range of appliances on his generator. After receiving the quote, he would come back to you, and ask you to also quote on a smaller unit. He might mention that it is not absolutely important to have his stove (or other appliances) to be working from the generator. Therefore make sure from the start that your customer knows that a stove, geyser, air conditioner or any appliance with an element adds huge amounts of money to his generators price. Try to quote him on a generator with and without these appliances.

Calculate the required size:

It is advisable to always ask the customer to specify the size of his requirement. This will free you up from making a mistake and carrying the consequences at a later stage. It is however important that you know what you are doing. People don’t want to purchase from someone that is unsure about his product.

Following is an appliance chart. The values on this list are given as an average, and are subject to change from case to case. So when you want to quote, you need to get the correct information from your customer. If your customer doesn’t know, you have to explain to him where it could be found, or you will have to read this from the appliance spec-plate yourselves.

Voltage Requirements

Most appliances operate on 240 volts. Most electrical services for homes and small businesses are made up of 240 Volt or 380 Volt feeds. This is also commonly known as Single – or Three Phase. If you ever expect to have to power any 240 volt load, you should select a generator with 240 volt output.

Running Load Requirements

Running load requirements are simply a total of all the loads to be operated simultaneously. This can be based on actual measurements if a clamp-on probe type AC Current Meter is available. Any electrician should be able to assist your customer with this exercise. Another way of determining the actual required size is to refer to the Main Circuit Breaker /s. There could be more than one. By simply adding up the total value of the circuit breakers you will be able to calculate the total usage. This method can obviously not be used if the application doesn’t include all the breakers on the board.

Otherwise, an estimate of the total running load can be made by adding up the running wattages of all items to be powered at one time. Some appliances may not list the wattage on the nameplate, but may show the rated current in amps and voltage. The wattage can estimated from these two figures by multiplying them together to get the watts. See formula below:

Watts = volts x amps
OR
Amps = watts / amps

Starting Load Requirements

Determining the starting requirements can be a bit more complicated. Certain electrical devices require additional power and current when initially turned on. This is true for motors because the rotor of the motor and the shaft driven load (fan, pump, compressor, saw, etc.) is initially at a standstill. It requires more energy to accelerate these rotating parts to operating speed than it does to keep them rotating. Therefore, during the period of acceleration, the demand on the power supply is greater. To precisely evaluate the motor starting capability the detailed motor characteristics need to be known. However, a rule of thumb is usually sufficient. Most engine driven generators will start a motor with up to 1/5th the horsepower of the engine, if it is the first load connected. For example, a 2500 watt generator driven by a 5 horsepower engine will usually start up to a 1 horsepower electric motor.

Typical Running and Starting Loads

The graph below shows typical values for the start up of a refrigerator. It is clearly indicated that a refrigerator, or freezer uses much more current to start up than to run. In a case like this you can’t just read the spec-plate and mach the generator to that.

The above graph shows a maximum inrush current of about 13 amps which lasts only about one-half second. Also, the running current is significantly less than the nameplate value. It should be noted that most refrigerators (including this one) are “frost free.” This means that on a regular basis a timer shuts off the compressor and turns on resistance heaters to clear frost build-up in the freezer section. This defrost current was not measured and is probably greater than the compressor running current. This may explain the large difference between the nameplate current value and the measured value.

It is clear that your generator needs about 5 times the potential compared your freezer / fridge in order to start the appliance without putting your other appliances as well as your fridge / freezer at risk.

Power Quality & Distortion

Perfectly pure AC power is a sine wave for both the voltage and current. Resistive loads such as incandescent lights and heaters are linear loads since the current is always proportional to the voltage applied. Some types of generators and non-linear loads can alter this perfect sine wave. A non-linear electrical load does not have a linear relationship between the voltage applied and the current that flows into the load. Certain types of electronics, lighting ballasts, arc welders and other devices are non-linear. Welding generators due to their design and poorly designed generators may also produce a distorted AC wave. When a significant portion of the load on a generator (or any power source for that matter) is non-linear, all the loads fed by the source will see this distortion. A measure of this distortion is called THD, or total harmonic distortion. If the distortion is severe enough, motors and transformers will operate hotter. Over a long period of time this can cause a reduction in life. And some other sensitive electronic equipment may not operate. An specific example is uninterruptible power supply (UPS) systems powering computers or communications equipment. These types of devices cause some distortion of the AC wave and at the same time can be negatively affected by it. A UPS system powered by an inadequately sized backup generator may continue draining the internal battery rather than switching over to generator power and charging the battery.

To reduce chances for THD problems, the rule of thumb is to select a backup generator kW size at least three times the kW of non-linear loads to be powered. For example, if you have 2000 watts of computers fed by UPS systems and 1000 watts of incandescent lighting to be fed by a generator first total the power:

2000 W + 1000 W = 3000 W

Then compare the total with three times the non-linear load portion:

3 x 2000 W = 6000 W

The generator needs to be at least 6000 watts in accordance with this rule of thumb.

Calculation rules:

1. 1. • 1000 Watt = 1 KW = 1 Kva. This is true for all smaller sets under 10 Kva. These sets operate at a power factor of 1.0. Your larger generators (10 Kva and more) will have a power factor of 0,8. A 20 Kva generator will give you only 16000 Watt = 16 KW.
      • Should you want to work out the amount of Amps you will get from a generator you take the Wattage and divide that by the voltage. E.g.: 2500 Watt / 220 V = 11 Amp refer to the formula given above.
      • On any 3 Phase unit ( 380 V) you will have to take the output E.g. 15 KVa. 15000 Watt x 0,8 (Power Factor) = 12000 Watt ( 12 KW) and divide this by 380 V = 32 Amps. You now take 32 Amps and divide this by 1.732 ( Root to the 3rd) = 18.23 Amps. This will be the Amps you will get from a 15 Kva unit per phase.
      • A generator is de-rated to get its actual performance at the point of use. This is because any engine will loose power if used at higher altitude. This is normally taken at 1% for every 100M above sea level. The first 300M is free. E.g.: a 5000 Watt generator will only give you 4250 Watt in Johannesburg ( 1800M above sea level). You will further loose 1% for every 3 deg above 30 deg Celsius. At 36 deg. Celsius you will loose another 2%. The de-rating process is the same for all machines except engines with turbo chargers. These engines compensate for altitude.
      • Method of Change over from locally supplied electricity to your generator

The following installation methods are available when you want to connect the generator to the buildings electrical system.

1. 1. • Automatic transfer swtch (ATS) with UPS (Uninterrupted Power Supply) has no delay before the power is restored. It switches your normal power supply off and your generator on, without you leaving your chair as you watch TV.
      • ATS Panel only: this has approximately 15 Sec delay before the power is restored. It switches your normal power supply off and your generator on without you leaving your chair as you watch TV.
      • MMF Panel (Manuel Mains Failure) will switch your power supply off and requires that you leave your chair and go to the generator to start it up.
      • Manual change over switch – This requires that you switch the sth electric supply from your main supply to the generator and start the generator yourself. Electric start is optional in this case.
      • Direct method. – this is the simplest method. The appliance is plugged into the generator directly via and extension. (Beware not to use a too long or too thin extension cord). In this case just the cost of the generator is what you need to out lay.
      • Always make sure that the generator is started first and that the power is switched on when the generator is running. Also switch the power off before your generator is restarted. This will eliminate spikes and will protect your appliances.

Choosing: Petrol VS Diesel fuelled machines.

For certain applications a diesel set is a much better option:

a) Longevity – Think of all the18 wheeler trucks capable of 1,000,000 Kilometres of operation before major repairs. Most trucks are diesel powered.

b) Lower fuel costs – Diesel uses less fuel consumption per kilowatt (kW) produced).

c) Lower maintenance costs – diesel have fewer parts. no spark system, more rugged and more reliable engine.

Today’s modern diesels are quiet and normally require less maintenance than comparably sized gas (natural gas or propane) units. Overall operating costs are typically thirty to fifty percent less than gasoline units.

Diesel engines running at 1500 RPM and water cooled operate on average for 12,000 to 30,000 hours before major maintenance is required.

Petrol engines running at 3000 rpm and air cooled are normally replaced – not overhauled at 500 to 1500 hours.

Petrol units run hotter due to the higher BTU rating of the fuel, you will therefore see significantly shorter gasoline engine life. Diesels run cooler and last longer.

To Summarize:

Use a petrol unit when:

1. 1. • • Price is important
        • The unit will be used for standby purpose only
        • The unit will be running for 6 hours or less a day
        • Portability is important

Use a Diesel Generator when:

1. 1. • • The generator is used for prime power supply
        • The unit is used for any mining or similar application
        • The unit needs to be built into a silent enclosure
        • The unit will be used daily for more than 6 hours per day
        • You need a true 15 Kva or larger

Standby/emergency/maximum and prime/continuous duty applications?

Most products have a “maximum” rating, also called standby and emergency rating. This value is the most that a generator can produce under normal conditions although it may still have some peak reserve for surges. The prime or continuous rating is the rating that the equipment should not exceed under normal running on a continuous basis. Even though small portables have both ratings that do not mean that they are designed for continuous use. A prime power generator must be 1500 RPM and liquid cooled.

Any air cooled or 3000 RPM generator is a standby generator no matter what our opposition may tell you. Most residential generators are standby rated. You should use a prime rated, 1500 RPM, liquid cooled generator when the number of hours per year will exceed 500 and the use is on a regular basis. If you start the generator many times a year for short periods you may also need a prime power rated generator. You must use a prime power rated generator when the generator is used 24 hours per day and 7-days per week. If you use a standby generator in a prime power situation the generators life expectancy can be shortened considerably and the potential for premature failure is increased dramatically. Buying a Gen Set rated for standby duty and using it for prime or prime continuous application is just dumb and will normally void your factory warranty as well. Your generator system should be considered a long-term investment and should be sized and selected properly.

Be careful:
Don’t supply a diesel generator when the unit will not use at least 75% of its potential load. The unit will only last for a day or two. Diesel engines don’t like to run without any load. They will glaze, and a major engine repair will be due within the warranty period. Your customer will expect a warranty and we will refuse. Avoid this unpleasant situation at cost.

Option: If your customer wants to purchase a larger unit than needed now, maybe because there will be a demand for a larger unit at a later stage, you can offer him the option of installing a dummy load onto the generator. This will ensure that no glazing takes place.

Installing the set.

1. 1. • 1. Use a local electrical contractor to do your installation. Ask for a wiring certificate.
        2. Try a number of contractors for quotes and advice to confirm that your price is reasonable.
        3. Check that the contractor is registered and qualified to work on the City Council in your areas network.
        4. Remember that if the contractor does not do the job properly the generator can burn out and no company will warrantee the product.
        5. It is recommended that you refer the customer to the contractor. Getting involved with the installation could tie you up right in between the installation problem and your customer. This could be very time consuming and unproductive.

To Summarize:

Follow the following steps in order to select the correct generator:

1. • 1. What is the application? (Camping, Hire, Prime Power Supply, and Standby Power)
     2. Single or Three Phase
     3. Silent or Open Frame
     4. Your customers budget
     5. Petrol or Diesel
     6. AVR or Normal
     7. Portable or stationary
     8. Generator Size
     9. Starting Method: Electric, Recoil or rope?
     10. AMF, MMF, Direct connection?