What do you mean by Alternators? | What are the main Parts Of Alternator? | What do you mean by Maintenance Of Engine Alternator?
What is difference between generator & alternator?
A/C Generators are also called ‘ALTERNATORS’. Here the mechanical energy is converted into electrical energy. Basically a generator produces electricity by the rotation of a group of conductors in a magnetic field. Hence the input of the generator is the mechanical energy to rotate the conductors. The output of a generator is the e.m.f. induced in the conductors as they move through the magnetic field. The principle used here is Law of Electromagnetic Induction.
Different Parts of Alternator
1.1. STATOR :
Slip ring series:– The Stator frame is made up of rolled steel plates with welded feet. It forms the yoke of the magnetic circuit. The laminated pole bricks are bolted to frame with impregnated field coils.
Brush less series: The stator frame is of solid cast / fabricated structure designed to ensure the correct distribution of airflow over the stator core and windings. The main core is built up of insulated silicon steel sheets, pressed together suitably. Armature winding is housed in a stator core pack. Stator core pack has skewed construction for a better wave from.
1.2 ROTOR :
Slipring series :- Rotor forms a built up construction of high grade electrical steel lamination on a steel shaft. Rotor lamination has a semi-closed slot construction for reduced losses, in which armature winding is housed. Rotor core is formed with skewed construction for better wave form.
Brush less series : Rotor core is made up of cruciform laminations having four projections , forming four poles . Laminations are stacked and held under pressure in place by core clamp plate and key . Pole windings are well supported by specially designed aluminum alloy support block and bright bars offering good mechanical support to prevent bulging of winding due to heavy centrifugal forces . The poles carry continuous damper windings to facilitate parallel operation .
1.3 ENDSHIELDS : End shields are of cast iron / fabricated steel construction , spigotted to the stator frame and are fixed by easily accessible screws .
1.4 BEARINGS : The bearings are of anti-friction type and are grease lubricated . Normally a ball bearing is provided on the non driving end (NDE) and a ball or roller bearing on the driving end (DE).
1.5 SHAFT: The shaft is of high grade carbon steel machined and ground on fitting surfaces . The shaft is designed for overload conditions .
1.6 Slip Ring And Brush Gear (For NB Series only ) :
The slip rings are of molded type , wherein rings are molded with high grade epoxy resin which is press fitted on shaft . Slip rings are made up of cuprous – nickel or phosphor bronze . Design provides sufficient space between the rings to facilitate cleaning . The brush gear assembly comprises radial type brush holders clamped to insulted spindles which are screwed to the NDE end shields.
1.7 ARMATURE WINDING : Armature coils are made from super enameled copper wire which are formed on special formers . The coils are held in slots by epoxy wedges . The overhangs are held in position by special varnished fibre glass tape banding . The insulating materials s used are of class F for all frames .
1.8 FIELD WIDING :
Slipring series :- Field oils are made up of super enameled copper wire , former wound pressed to proper shape , varnished and fitted on poles .
Brush less series :- Main field poles are directly wound with the strip enameled copper conductor on a special winding machine without any joints between poles and having good layer formation . These rotors are vacuum pressure impregnated with unsaturated polyester resin for good insulation strength .
- EXCITATION SYSTEM :-
Slip Ring series :- The open loop current compounding excitation unit comprising a Transformer ( 3phase or 1 phase ) a surge protected rectifier and a capacitor is mounted on a truncated NDE end shield just above the slip ring unit .The exciter transformer has a provision for adjusting air gap and tapping on shunt winding for terminal voltage adjustment .
Brush less Series :- A separate rotating exciter unit is used . Exciter rotor is mounted on the same shaft at NDE side and exciter stator is fitted with NDE and shield . Exciter is liberally designed for good motor starting capacity .
Slip ring series :- A molded open type construction 3 phase silicon bridge rectifiers , using high PIV and sufficient DC current rating silicon diodes , is placed in the terminal box . Intake air flow of machine, improves performance and reliability . It is protected from surge voltages by a selenium plate surge suppressor .
Brush less Series :- Three phase rotating bridge rectifier consist of liberally rated Avalanche diode ( three normal polarity and three reverse polarity ) mounted on specially designed aluminum bus bars . These bus bars mounted diagonally opposites on exciter rotor . Diodes are protected from surges with special windings on main rotor.
AUTOMATIC VOLTAGE REGULATORS (AVR) FOR NRF SERIES:-
Electronic AVR is used for controlling the terminal voltage within ± 1% . It is fitted inside or side of the terminal box .
Special features of AVR
- Filter circuit is incorporated inside AVR . No external filter is required for thyristor load application .
- AVR components are sufficiently over rated to suit Indian atmospheric conditions.
- Less number of components to increase reliability .
- Good motor starting capacity .
- No need of QDC kit , as auxiliary CT for parallel operation is provided along with AVR .
- Fuse box is provided inside the AVR which has also a Spare fuse .
- Over excitation protection is incorporated inside the AVR .
1.12 TERMINATIONS :- Three phase winding terminals and neutral are brought into spacious terminal box and terminated on special terminal block . Terminals are marked U,V,W, N .
2.0 ENCLOSURES:- Screen protected drip proof enclosures is provided with a degree of protection IP 2 IS for NB series and NRF series as standard . IP 23 S protection can also be offered .
- PRINCIPLE OF OPERATION :-
Slip ring series :- When the AC generators is being run up to rated speed , the residual flux in the pole bricks develops some voltage in the armature which in turn drives through rectifiers assembly some DC current in the field windings , which in turn enhances the terminal voltage . The increasing terminal voltage increases the field current and this process continues till rated voltage is reached .When the AC generator is loaded part of the load current passing through load winding L gets reflected in C winding which feeds the field . This increase in field current take care of additional requirement of field current due to armature reaction and the voltage is maintained within ± 5% 0f set voltage for any load from no load to full load at unity to 0.8 lagging P.F. and also for engine speed drop of 4% .
Brush less series :- NRF type A.C generator is a two machine system comprising of four pole main field winding inside a stationary armature and 12 pole exciter . Exciter rotor output is converted into DC by rotating 3 phase bridge rectifier and given to main field winding . AVR input and sensing is taken from main armature winding and DC output is given to the exciter field winding . When AC generator is rotated by a prime mover , because of the residual magnetism in the exciter field , voltage is induced in the exciter rotor which is converted to DC and given to main field winding which drives some current in the main field windings which in turn produces flux.
This flux links with the armature conductor in the stator and due to electromagnetic induction voltage is induced in the armature winding. This induced voltage is given to AVR which converts it into DC and supplies to exciter field windings as the speed goes up the terminal voltage also increases to rated voltage , than the AVR starts regulating this voltage by controlling the exciter field current supply and will maintain the output voltage within ± 1% of set voltage for any load from no load to full load , for any power factor from unity to 0.8 lag and engine speed droop of 4% .
Rating A-C Generators
The power rating indicates the maximum power that can constantly be supplied by a generator. The power of a-c generators can not be rated in KW because the power consumed in an a-c circuit depends on the circuit power factor . If power factor is low, even though the true power is less, the apparent power actually delivered by the generator will be large and that situation results in the generator burning out.
For this reason, a-c generators are rated on the basis of the maximum apparent power they can deliver. Thus the capacity of a-c generator is generally expressed in terms of volt-amperes (VA) or kilovolt-amperes (KVA). For example if an a-c generator with a rating of 100 KVA has an output of 50 kilovolts, the maximum current that it can safely deliver is 2 amps only.
Maintenance of Engine Alternator
Engine alternators are being installed in our department mainly to give standby power to our Telecom network . These are essential ingredients of providing reliable power to the system and thus play vital role in satisfactory operation of Telecom services . Due to the importance as stated earlier , it is very essential that E/A are maintained properly so that their services is ensured at any time during failure of main supply .
In this lecture / talk , we will be focusing mainly on various maintenance aspects of electrical parts of E/A set . Diesel engine acts as prime mover to alternator which generates A/C power . Various aspects of mechanical parts have been already dealt in previous lecture .
Following components / arrangements required special attention from the maintenance point of view :-
- Control panel
Connecting circuit between alternator and control panel
- Terminal box and regulator
- Earthing and
- Starting arrangement and battery .
We will discuss the maintenance aspects of various electrical components related to E/A sets.
Alternator when driven through prime mover i.e. Engine, generates A.C. power . Thus, it is energy conversion device which converts mechanical energy into electrical energy.
Now a days Brush less type A.C. Generators are normally used . Brush less Alternator , eliminates use of slip ring , commutators and brush gears besides offering other advantages over conventional alternators . Its main components are :-
Automatic voltage regulators :-
Now a days solid state automatic voltage regulators are being used in Brush less Alternators , for maintaining the terminal voltage constant within close limits over wide operating conditions . The input to the AVR is derived from the generator output itself and output is connected to an exciter field .
In addition to voltage regulating function , the AVR incorporates an under frequency protection circuit . This feature automatically reduces the generator output terminal voltage, in proportion to the speed-of prime mover , below a certain cut off frequency. Reduces in voltage safe guards the following components against heating:
- Exciter (Main and pilot)
- The voltage regulating circuit
- The control circuit .
Above all , this feature also protects the prime mover from getting over loaded under such slow speed condition .
Voltage regulation up to ± 0.5 % can be obtained under following conditions .
- Variation of load from no load to full load .
- Variation of power factor between 0.8 and unity .
- Hot and cold condition of generator and
- For prime mover speed variation of ± 4 %
Filter circuit can be provided for unbalanced and thyristor load application .
1.2 MAINTENANCE OF ALTERNATOR
Depending on the site condition frequent maintenance should be carried out . Before carrying out maintenance alternator should be stocked and isolated electrically from all external loads or supplies .
A) General Cleaning
B) Both exterior and interior of machine particularly .
- Ventilation openings
Should be cleared from oil and dust by blowing out frequently at least once in three month .
All the external and internal connections should be checked for tightness at joints and continuity . For links should be made good fit in the holders .
D) Loss of residual magnetism
If the residual magnetism is completely loss due to any mal-operation , it will be necessary to disconnect the water field terminals and connect a 12 V battery directly across the field (With correct polarity for the seconds to re-establish the residual )
The bearings are to be lubricated with carried quantity of grease.
1.3 Common Faults , Causes and action
|Sr no||Fault||Probable causes||Action|
|1||Voltage Builds up but staying low||Objective potentiometer||Change potentiometer of problem possible charger AVR|
|2||Voltage reduces beyond CS % of nominal on load||single / parallel switch in parallel portion||Put switch in single position|
|3||Voltage oscillator on loose no load or put on load|
|4||Out put voltage not being maintained correctly on load||i. Incorrect prime minor speed||Adjust speed|
|5||Overheating of machine||i. Overload||Adjust load|
|ii low load power factor||Adjust power factor|
|iii. Low operating voltage||Adjust voltage|
|iv. Low speed on load||Adjust speed|
|v. Ventilation blocked||Clean the ventilation|
|6||Excessive vibration and notice||i. Poor alignment||Align properly|
|ii. Coupling and foundation hold loose||Tighten the bolts and bearing|
|iii. Damage bearing|
|7||Over heating of bearings||i. Incorrect assembly of bearing|
|ii. Incorrect quality .||use specified quality .|
2.0 Control Panel
Electric supply generated by alternator is fed to the control panel from where it is taken to feed the load . Control panel shall be checked periodically for various items. Some of them are :
(i) Meters like ammeter, voltmeter , frequency meter etc. if any of them found faulty it should be replaced or rectified as the case may be .
(ii) Switch or circuit breakers shall be checked for circuits proper operation . Contacts should be suitably greased .
(iii) All fuses should be checked .
(iv) Relays should be checked for proper operation
(v) All connection and terminations in control panel should be checked .Any loose connection should be tightened .
(vi) Megger value of various components shall be checked periodically with 500 V megger .
3.0 Connecting Circuit
Cables of sufficient capacity to carry rated current must be selected from switch gear and generates to minimize heating , voltage drop. cables are to be kept as short as possible .
If cables are getting heated , it shall be replaced by suitable size or check terminates as it may be due to loose connections .
The alternator frame shall be earthed thru the conductor heavy enough to carry momentary short circuit without burning out . Earthing screws are provided on alternator at two distinct point . It should be checked that earthing is provided effectively.
It should be checked that all ventilation guide , glance plate and corners are fruity and correctly fitted and generator inlet and outlet ventilation ducts are not obstructed in any way.