LOAD TESTS ON A SYNCHRONOUS GENERATOR

Product Code : EEL0025

Description: Objective: To start and synchronise the synchronous machine (SM) to the Bus Bars. To conduct load test on the SM as a motor To conduct load test on the SM as a generator Preview: A synchronous machine has a three-phase stator winding which carries three-phase balanced currents producing rotating field at synchronous speed (ns=120f/p). The rotor is dc excited and produces fixed poles when the rotor is made to run at a speed close to synchronous speed. The two fields get locked in each other and the rotor must run at a speed equal to synchronous speed. The torque developed is proportional to Sin A. The power angle A is the angle between the rotor field axis and resultant air-gap field. When the power is put into the machine by a prime mover coupled to it, then the rotor field leads (in the direction of the rotor) the resultant air gap field by an angle delta. The electrical power flows out of the machine to the load or to the bus bar, as the case may be, while the mechanical power flows into it throughout the shaft from the prime mover. This is generating operation of the machine. If instead a mechanical load is placed on the shaft of the machine the rotor field then begins to lag behind the resultant air gap field by an angle delta. The electrical power is now drawn from the mains while the mechanical power goes out of the shaft into the load. This is the motoring operation of the machine. If the electrical load on the generating machine is the maximum load of the motor the matching of the two fields is lost and the machine is said to lose synchronism or fall out of step. In such a condition the machine must be electrically disconnected from the bus and the input to the prime mover must be immediately brought down. This loss of synchronism can occur at values of delta much less than 900 in conditions of electrical or mechanical shock to the machine. So in practical operation of synchronous machine, value of delta is about 300. A per phase circuit model of the machine with dc field excitation is drawn in Fig.1. Xs is the synchronous reactance of the machine and Ef is the excitation emf due to the rotor alone. In generating operation Ef leads V by angle delta while in motoring operation it lags. In any given load condition if the dc field excitation is changed the excitation emf changes accordingly and thus results in change in power factor of the current being fed by the generator or drawn by the motor. While the machine is run at synchronous speed the excitation gives a simple control over the power factor of its operation. Of course if the field excitation is excessively reduced it reduces the maximum torque (power capability of the machine) and it may lose Synchronism (fall out of step). In order to connect synchronous machine to the bus bars, it has to be properly synchronized. i.e. the rotor is brought to a speed close to synchronous speed, adjusted to give rated voltage at its terminals and the machine is switched over to the bus bar at the exact moment when the machine voltage phasor and the bus bars voltage phasor are coincident. This process is known as synchronization. Practically all generating stations small or large use synchronous generators. Synchronous motor is used in special applications where constancy of speed is required and control over power factor helps in improving the power factor of the total plant. It is to be pointed out here that when the motor field is over –excited it draws a lagging power factor current (inducted behavior). At a certain load, excitation is called normal, when the power factor is unity. In order to conduct experimental test on a synchronous machine the machine is duly coupled to a dc machine. It can either act as a generator or as a motor. It also of course helps in synchronizing the machine to the bus bar. Such a set up in our laboratory will be used for synchronous machine tests. Equipment Required: All the measuring instruments associated with each experiment are industrial grade. These instruments are housed in an elegant cabinet as a package. Range of instruments and accessories (standard format) 1. Three Phase 400V 3KVA 1500rpm synchronous alternator coupled with 5HP DC shunt motor 220V 1500rpm on a common base plate 2. 3/4point DC starter 3. synchronizing panel consisting of 2frequency meters, 2voltmeters, 1synchronscope, 1set of bulbs for dark and bright method, complete with ON / OFF switch and a knife switch for synchronizing 4. Suitable Digital Ammeter AC 5. Suitable Digital Wattmeter 6. Suitable Digital Voltmeter DC 7. Suitable Digital Ammeter DC 8. Suitable Loading rheostat 3¢ 9. Suitable Single tube rheostat 10. Suitable Double tube rheostat 11. Suitable DC Rectifier unit 12. Phase sequence indicator