09 May 2011
Kerala Electrical & Allied Engineering Company Ltd (KEL) is a Public Sector Undertaken company fully owned by government of Kerala. Their main products are Alternators and Transformers. All alternators manufactured by KEL (in collaboration with Leroy Somer, France) are of Brush Less Excitation type. In lower rantings (<100kVA) no electronic regulator (AVR) is required to regulate the output voltage. In this type of alternator the voltage is build up and regulated with compound excitation system. In higher rating compound excitation & electronic voltage regulators are used to regulate the output voltage. This dual regulation gives high reliability. In this type of alternator one advantage is that we can operate the machine even after an AVR failure. With AVR we will get a 2% voltage regulation and in the second mode it will be 5%. The alternator manufacturing division of KEL is situated in Kasaragod district of Kerala.
The above picture shows the regulating part of a KEL alternator. The stator winding also have a 3phase star connected auxiliary winding. This winding generates a 40V output at rated speed. This output is fed to the secondary winding of compounding transformer. The secondary of compounding transformer has three independent winding. The second and third set have 15% and 5% turns of first set. By changing the way we connect theses windings we can change the effective number of turns of secondary winding from 80% to 120%. Any increase in the number of turns in the secondary winding will decrease the output voltage. (Increase in the number of turns will increase the impedance and thus decrease the output). Primary winding (outer winding) of the compounding transformer have only a few turns wound in opposite direction of secondary winding. This winding carries a portion of output current (load current). As this is wound in opposite direction flux generated by this winding will oppose flux generated by secondary winding. Thus as the load increases output voltage of each transformer will increase and this will help to regulate the output voltage of alternator. Both primary and secondary winding of each compounding transformer must be in phase. Output from the compounding transformer goes to a three phase bridge rectifier unit (4KD4). The output of this rectifier goes to the compounding field coils of exciter field. (in low rating there is only compounding system)
In higher rating (>100kVA) there is an electronic voltage regulator. Input to this regulator is from the the auxiliary winding. It monitor the output of the alternator (two phases). DC output of the AVR goes to regulating field coils on exciter field. The regulating field and exciter field are wound in opposite direction. The output generated by compounding field will be 10% higher than the rated output voltage. Flux created by regulating field opposes that created by compounding field and reduces the output voltage of alternator to normal value. Winding resistance of compounding field is less than that of regulating field. If there is an AVR failure (no output from AVR) then the output of the alternator will increase. Then disconnect the AVR and connect the regulating field to the full wave bridge rectifier unit (fixed above 4KD4, output are available at 6+ and 7- on the connector tray, just change link on connector tray). Then adjust the output voltage by adjusting the POT for manual voltage adjustment. (on picture just above AVR).
Cost of KEL AVR is very high (mostly around Rs. 25,000/-) comparing to that of other make. Now KEL is owned by Bharat Heavy Electricals Ltd (BHEL). Rotating Rectifier Assembly (RRA) have 6 diodes. 3 forward (violet tail colour) and 3 reverse (red tail colour). Getting genuine spares in time is always a problem with KEL. All orders are to be send to Kasaragod and some times they do not supply spares directly to the customer. In some models they are using imported diodes and there is always delay in getting these spares.