dg set

MaintenanceCircleTeam December 30, 2008

This document contains information for reference only. We assume no responsibility for its implication.

Maintenance circle

Word for the day: DG SET (DIESEL GENERATING SET) One of the most important supporting utility equipment in any industry / commercial establishment / complex or even apartments is a DG set, which gives essential power back-up when the state electricity fails. Diesel Generating set, as the name implies is a combination of an engine running on conventional diesel (technically called High Speed Diesel / HSD) fuel coupled to an AC (or DC sometimes) generator that will generate electricity at various voltage levels. For this article, let us limit our discussion to a standard 415V AC voltage at 50 Hertz. Of course, there are big generating sets producing 11kv, 33kv & higher voltages and running on various other fuels like furnace oil, petroleum gases. They are not generally used by end user but by “captive power stations,” which further supply the power to end users through a central distribution system and step down transformers. Let us study in general, few of the major components of a standard DG set and understand various indicators that give different readings. Although DG sets are very reliable and can run continuously, little basic attention and maintenance activities are essential for its reliable and trouble-free performance. There are three major performance and one static component in a standard DG set which influence the efficiency. They are: I) Diesel Engine (also called Prime Mover) II) Alternator or Generator III) Cooling System IV) Air Filter System. Let us understand a few basic details of them, one by one in the following paragraphs. Diesel Engine (Prime Mover): Engine is the most important part of the DG set and an average engine can have thousands of moving parts working in a timely sequence to deliver the required output. Each component must work perfectly for efficient running of the DG set. The diesel engine is identified by number of cylinders it has and the brake horse power it can generate under full load. There are Straight In-line and V-type cylinders available in the market and vary from each manufacturer. Generally, V-type engines are compact compared to In-line engines, but have more moving parts. And from application point of view, each type has its own merits & demerits. Three measurable parameters and one “sensible” parameter can give an indication of the condition of diesel engine in general.

1. Coolant Temperature: This should be ideally between 70 and 85 degree Celsius (158 and 185 degree Fahrenheit). Engine will shutdown if temperature exceeds 95 degree Celsius (203 degree Fahrenheit) since water mixed coolant can start boiling and will not cool the engine component temperature. Premixed coolants are available in the market which can be directly filled. If not, usually one liter of coolant is mixed to five liters of plain (not distilled) water.

2. Lube Oil Pressure: When engine is running, oil will be circulated to all sliding and moving parts like piston liners, rocker arms, valves and sprints to lubricate them for smoother operation. Certain minimum pressure is required for proper circulation of oil throughout the engine casing. The lube oil pressure should be ideally between 3 to 5 Kg / cm2 for satisfactory operation. Insufficient lube oil pressure damages the components and excessive lube oil pressure will result in oil entering into combustion area and some components may fracture. Thick white smoke in exhaust, is an indication of high lube oil pressure or some leakage in the system, resulting in oil mixing with the fuel during combustion. 15W50 and 20W40 are the most commonly used engine oils.

3. Engine Speed: For generating 415V AC at 50 Hz, engine must run at 1500 RPM (Revolution Per Minute) with an acceptable tolerance of +/- 30 RPM. The engine speed is controlled by varying fuel quantity to engine which is done either during using a standard mechanical governor or solid-state electronically controlled proportional governors. The response time of engine to respond to varying load is very critical in deciding which speed control method should be used. Overloading of engine causes speed reduction as well.

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4. Sound: One of the most important attributes of a good engine is its sound. Even though there are certain values that can decide the health of engine, it is always the human ear that can sense any deviation. The easiest method is to “memorize” the sound of an engine during “quite” period. Quite period is when there are no other sounds from surroundings, like compressor / vehicles and other machineries. It is ideal to “memorize” this sound during night time. Once it is recorded in our memory, any deviation can easily be identified and a major breakdown can be prevented.

Alternator (Generator): For this article, let us consider the standard AC generator used in typical DG set. AC brushless generator (they are termed Brushless since the voltage is directly generated from stator and not from conventional rotor thru carbon brushes) is the most reliable equipment with minimum moving parts. Usually the voltage generated by AC generator – 415 Volts at 50 Hz – is directly related to speed stability of the diesel engine. And, output wirings are generally connected in STAR mode. In order to compensate for speed variation of engine and to maintain a stable voltage within certain percentage, AVRs or Automatic Voltage Regulators are installed. The alternators can have bearings on single or both sides depending on the rating. Two common symptoms seen on Alternator are explained below:

1. Output voltage fluctuation – One of the most common symptoms of voltage fluctuation is either a faulty AVR or wrong setting of the excitation voltage. Also the AVR system will have 3 pairs of diodes fitted on the rotor shaft. They are called Rotating Diodes since they generally rotate. Regular check of the diode condition (forward and reverse bias voltage drop) using a conventional multi-meter is sufficient to detect any faults. If one of them fail, there will be some variation in the excitation voltage and hence in output voltage. The voltage level is usually 48V DC maximum at about 2 amperes. Second reason could be extreme speed fluctuation in engine. Third reason could be sudden instantaneous loads – like spot welding, punching – dropping the voltage.

2. Uncontrolled output voltage – Whenever AVR detects some fault, it stops regulating the output voltage. One of the most common faults is UFRO or Under Frequency Run Off. This fault occurs whenever the output voltage’s frequency reduces below a preset value, usually 47 Hertz. Generally, the frequency of output voltage is directly proportional to engine’s speed. Normally, the engine speed is adjusted in no-load conditions to generate 51 hertz of output frequency. Also faulty setting of AVR or a faulty component itself can result in voltage fluctuation.

Alternators are generally designed to run under extreme weather conditions. However, it is essential to maintain sufficient ventilation around the unit and not to block forced air vent holes. Do not use water to clean the alternator surface since it may be sucked into the winding when engine starts running and can damage the windings. Engine generates power by “internal combustion of fuel” process and produced lot of heat. In fact, engine is the most inefficient mechanical device with almost 40% of energy getting dissipated as heat. A good heat removal system is essential to keep diesel engine working at its optimum level. Alternator is cooled by forced air cooling – technically known as convection – and may not require special cooling process. Impeller fans are mounted for DG sets which are installed inside acoustic enclosures. A radiator is the most common and widely used “heat exchanging” device on a DG set. It has hundreds of aluminum or brass shells connected together in series. The coolant after “collecting” as much heat as possible from engine surface moves to the top of radiator. Due to smaller hole in the shell, the coolant starts coming down slowly and reaches the bottom. A large fan – generally coupled to engine – blows air thru the shells and exchanges heat with atmosphere and cools the coolant. Like a human body, engine and radiator need sufficient “breathing space” – ventilation – for efficient heat exchange. The radiator must never be blocked by any objects and its surfaces should be kept as clean as possible, without dust, grease, scale, flying insects and any other materials. To keep a DG set running efficiently, maximum attention should be given to radiator and coolant than any other components. A small piece of litmus paper is generally available with the coolant. Dipping it in the coolant opening radiator cap and checking the color gives the pH (French word: puissance de Hydrogen which indicate whether the coolant mixture need top up to avoid scale formation) level of the coolant. Little precaution with regular check up every week goes a long way in preventing in scale formation of cooling section on engine, which will be very difficult to “de-scale” later and consumes a lot of time. Lot of money could also be spent on replacing certain irreparable parts.



Air filter is the most important component for preventing entry of dust and other foreign particles into the engine. A clean and well maintained air filter is essential for getting maximum utilization from engine. Choked filter reduced air volume and engine burns more fuel and cannot take more load. Regular cleaning of air filter (use moderate compressed air at about 3 to 4 kg/cm2 from inside of filter) helps remove all the dust. The frequency of cleaning depends on the location and application of DG set. A DG set in construction site need more frequent cleaning than the one located in a clean acoustic enclosure. A simple method of checking filter paper for any rupture or blockage is using a 40 or 60 watt tungsten bulb from inside the filter. Engine should “breathe” clean air like human beings for efficient and reliable operation. A small list of some of the common points that can be considered when selecting a DG set, assessing the performance of an engine or for regular preventive maintenance checks.

The engine’s capacity is always defined in BHP (Brake Horse Power) and it will be usually higher than the electrical rating of AC Generator, which is rated in KVA. For example, a 500 KVA AC generator will be generally coupled to a 605 BHP engine.

If the variation in load during peak hours and lean hours is more than 50%, it is advisable to consider two DG sets of half the rating with proper change over and load distribution system.

15W40 and 20W40 are the most commonly used engine oils. 15W40 is meant for engines running in colder conditions compared to 20W40 which is generally used in warn weather. Do not mix engine oils of different grade and manufacturers to avoid damage to engine components.

3.5 units (KWh) per liter is the most commonly acceptable efficiency measure of engine, although 4 units per liter is also possible. Common reasons for lesser values are mentioned below:

o Engine running at less than 60% of its rating o Engine components – air filter, cooling system, governor, lubrication system – have some problem and

need to be immediately attended to.

80% is the minimum required load for efficient running of engines and to obtain better fuel consumption efficiency and overloading beyond 90% should be avoided.

Alternator frequency must be set at 51 Hz with no load condition. This will allow engine to stabilize around 50Hz with load.

No faults on DG set must be bypassed for any reasons.

Water separator unit must be installed as close to o fuel filter as possible and should be drained every day. An auto drain separator is recommended to avoid manual errors.

Generally, the cost of electricity generated from a DG set is almost 50% higher than state electricity and hence all steps should be taken to obtain the best.

Exhaust smoke, engine noise, its body temperature and vibrations are very good indicators of engine conditions. Observe them very closely – without any measuring devices, of course – to “feel” and “understand” the DG set.

Electronic governors are most common today which provide precise speed control and they are also essential in long term if more than one engine needs to be synchronized1.

We encourage our readers to share all the information they can for improvements they have bought in a typical DG room. A good and well maintained DG system can minimize machine downtime and save lot of money to the organization.

1 Connecting two or more generators in parallel to share the load and switch over automatically to avoid shutdowns.

dg set

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