How does generator alternator (induction generator) work?principle

How does diesel generator(induction generator) work?

When induction motor running as generator it takes mechanical power and supply electrical power from stator. To run the induction motor as generator, it's slip must be less than zero. i.e negative
Negative slip show that the rotor is running at a speed above the synchronous speed. Thus when slip of induction motor is negative means induction motor runs faster than synchronous speed, the induction motor runs as a generator called induction generator.

Explanation about How does an alternator work?

 

simple electric generator diagram given below:

fig:generator working principle

If a simple loop of wire is rotated between the poles of a permanent magnet, as shown in Fig the loop of wire will cut the lines of magnetic flux between the north and south poles. This flux cutting will induce an emf in the wire by Faraday’s law which states that when a conductor cuts or is cut by a magnetic field, an emf is induced in that conductor. If the generated emf is collected by carbon brushes at the slip rings and displayed on the screen of a cathode ray oscilloscope, the wave form will be seen to be approximately sinusoidal.Alternately changing, first positive and then negative,then positive again, giving an alternating output

Main parts of a generator
The main components of an electric generator can be broadly classified as follows:

(1) Engine
(2) Alternator
(3) Fuel tank
(4) Voltage Regulator
(5) Cooling Systems
(6) Lubrication System
(7) Battery Charger
(8) Control Panel
(9) Main Assembly

Explanation of the main part of a induction generator is given below.

1) Engine
The engine is the source of the input mechanical energy to the generator. The size of the engine is directly proportional to the maximum power output the generator can supply. There are several factors that you need to keep in mind while assessing the engine of your generator. The manufacturer of the engine should be consulted to obtain full engine operation specifications and maintenance schedules.Generator engines operate on a variety of fuels such as diesel, gasoline, propane, or natural gas. Smaller engines usually operate on gasoline while larger engines run on diesel, liquid propane, propane gas, or natural gas .

(2) Alternator
The alternator, also known as the ‘generator ’, is the part of the generator that produces the electrical output from the mechanical input supplied by the engine. It contains an assembly of stationary and moving parts encased in a housing. The components work together to cause relative movement between the magnetic and electric fields, which in turn generates electricity.

(a) Stator – This is the stationary part of generator. It contains a starter winding.

(b) Rotor or armature – This is rotating part of generator which produces a rotating magnetic field (RMF) in any one of the following three ways:

(i) By induction – These are known as brushless alternators and are usually used in large   generators.
(ii) By permanent magnets – This is common in small alternator units.
(iii) By using an exciter – An exciter is a small source of direct current (DC) that energizes the rotor through an assembly of conducting slip rings and brushes.

The rotor generates a moving magnetic field around the stator, which induces a voltage difference between the windings of the stator. This produces the alternating current (AC) output of the generator.

The following are the factors that you need to keep in mind while assessing the alternator of a generator:

(a) Metal versus Plastic Housing – An all-metal design ensures durability of the alternator. Plastic housings get deformed with time and cause the moving parts of the alternator to be exposed. This increases wear and tear and more importantly, is hazardous to the user.

(b) Ball Bearings versus Needle Bearings – Ball bearings are preferred and last longer.

(c) Brushless Design – An alternator that does not use brushes requires less maintenance and also produces cleaner power.

(3) Fuel tank
The fuel tank usually has sufficient capacity to keep the generator operational for long hours on an average. In the case of small generator units, the fuel tank is a part of the generator’s skid base or is mounted on top of the generator frame. For commercial applications, it may be necessary to erect and install an external fuel tank. All such installations are subject to the approval of the City Planning Division.  Click the following link for further details regarding fuel tanks for generators.

Common features of the fuel system include the following:

(a) Pipe connection from fuel tank to engine – The supply line directs fuel from the tank to the engine and the return line directs fuel from the engine to the tank.

(b) Ventilation pipe for fuel tank – The fuel tank has a ventilation pipe to prevent the build-up of pressure or vacuum during refilling and drainage of the tank. When you refill the fuel tank, ensure metal-to-metal contact between the filler nozzle and the fuel tank to avoid sparks.

(c) Overflow connection from fuel tank to the drain pipe – This is required so that any overflow during refilling of the tank does not cause spillage of the liquid on the generator set.

(d) Fuel pump – This transfers fuel from the main storage tank to the day tank. The fuel pump is typically electrically operated.

(e) Fuel Water Separator / Fuel Filter – This separates water and foreign matter from the liquid fuel to protect other components of the generator from corrosion and contamination.

(f) Fuel Injector – This atomizes the liquid fuel and sprays the required amount of fuel into the combustion chamber of the engine.

(4) Voltage Regulator Exciter Windings

(1) Voltage Regulator: Conversion of AC Voltage to DC Current – The voltage regulator takes up a small portion of the generator’s output of AC voltage and converts it into DC current. The voltage regulator then feeds this DC current to a set of secondary windings in the stator, known as exciter windings.

(2) Exciter Windings: Conversion of DC Current to AC Current – The exciter windings now function similar to the primary stator windings and generate a small AC current. The exciter windings are connected to units known as rotating rectifiers.

(5) Cooling Systems in generator
(a) Cooling System
Continuous usage of the generator causes its various components to get heated up. It is essential to have a cooling and ventilation system to withdraw heat produced in the process.

Raw/fresh water is sometimes  used as a coolant for generators, but these are mostly limited to specific situations like small generators in city applications or very large units over 2250 kW and above. Hydrogen is sometimes used as a coolant for the stator windings of large generator units since it is more efficient at absorbing heat than other coolants. Hydrogen removes heat from the generator and transfers it through a heat exchanger into a secondary cooling circuit that contains de-mineralized water as a coolant. This is why very large generators and small power plants often have large cooling towers next to them.  For all other common applications, both residential and industrial, a standard radiator and fan is mounted on the generator and works as the primary cooling system.

(b) Exhaust System
Exhaust fumes emitted by a generator are just like exhaust from any other diesel or gasonline engine and contain highly toxic chemicals that need to be properly managed. Hence, it is essential to install an adequate exhaust system to dispose of the exhaust gases.  This point can not be emphasized enough as carbon monoxide poisoning remains one of the most common causes for death in post hurricane affected areas because people tend to not even think about it until it’s too late. 

Exhaust pipes are usually made of cast iron, wrought iron, or steel. These need to be freestanding and should not be supported by the engine of the generator. Exhaust pipes are usually attached to the engine using flexible connectors to minimize vibrations and prevent damage to the generator’s exhaust system. The exhaust pipe terminates outdoors and leads away from doors, windows and other openings to the house or building. You must ensure that the exhaust system of your generator is not connected to that of any other equipment. You should also consult the local city ordinances to determine whether your generator operation will need to obtain an approval from the local authorities to ensure you are conforming to local laws a protect against fines and other penalties.

(6) Lubricating System

An internal combustion engine would not last long if the moving parts of engine allowed to run metal-to-metal contact. The heat generated due to the tremendous amounts of friction would melt the metals, leading to the destruction of the engine. To prevent this, all moving parts ride on a thin film of oil that is pumped between all the moving parts of the engine.

Once between the moving parts, the oil serves two purposes. One purpose is to lubricate the bearing surfaces. The other purpose is to cool the bearings by absorbing the friction generated heat. The flow of oil to the moving parts is accomplished by the engine's internal lubricating system

(7) Battery Charger
The start function of a generator is battery-operated. The battery charger keeps the generator battery charged by supplying it with a precise ‘float’ voltage. If the float voltage is very low, the battery will remain undercharged. If the float voltage is very high, it will shorten the life of the battery. Battery chargers are usually made of stainless steel to prevent corrosion. They are also fully automatic and do not require any adjustments to be made or any settings to be changed. The DC output voltage of the battery charger is set at 2.33 Volts per cell, which is the precise float voltage for lead acid batteries. The battery charger has an isolated DC voltage output that does interfere with the normal functioning of the generator.

(8) Control Panel
A control panel is a set of displays that indicate the measurement of various parameters like voltage, current and frequency, through gauges and meters. These meters and gauges are set in a metallic body, usually corrosion proof, to protect from the effect of rain or snow. The panel may be set up on the body of the generator itself

In auto start control panels automatically start your generator during a power outage, monitor the generator while in operation, and automatically shut down the unit when no longer required.

(a) Engine gauges are different gauges indicate important parameters such as oil pressure, temperature of coolant, battery voltage, engine rotation speed, and duration of operation. Constant measurement and monitoring of these parameters enables built-in shut down of the generator when any of these cross their respective threshold levels.

(b) Generator gauges – The control panel also has meters for the measurement of output current and voltage, and operating frequency.

(d) Other controls – Phase selector switch, frequency switch, and engine control switch (manual mode, auto mode) among others.

(9) Frame
All generators, portable or stationary, have customized housings that provide a structural base support. The frame also allows for the generated to be earthed for safety.

generator working video tutorial