NINTH LECTURE HUNTING OF SYNCHRONOUS MACHINES C"^ROSS currents can flow between alternators due to dif- ferences in voltage, that is, differences in excitation; ■—^ and due to differences in phase, that is, differences in position of their rotors. Cross currents due to differences in excitation are watt- less currents, magnetizing the under-excited and demagnetiz- ing the over-excited machine. Cross currents due to differences in position are energy currents, accelerating the lagging and retarding the leading machine. Their magnetic action is a distortion or a shift of the field, ithat is, they increase the magnetic density at the one and decrease it at the other pole corner. If two machines are thrown together out of phase, or brought out of the phase by some cause (as the beat of an en- gine, or the change of load of a synchronous motor) then the two machines pull each other in phase again, oscillate a few times against each other, which oscillation gradually decreases and dies out, and the machines run steadily. If the oscillations do not decrease, but continue, the machines are said to be hunting. If the oscillation is small it may do no harm ; if it is greater, it may cause fluctuation of voltage, resulting in flick- ering of lights, etc. ; if it gets very large, it may throw the ma- chines out of step. Some causes of hunting are: 1st. Magnetic lag. 2nd. Pulsation of engine speed. 3rd. Hunting of engine governors. 4th. Wrong speed characteristic of engine. ii6 GENERAL LECTURES I St. When the machines move apart from each other, magnetic attraction opposes their separation. When they pull together again, magnetic attraction pushes them together with the same force, so that they would move over the position of coincidence in phase and separate again in the opposite direc- tion just as much as before. Energy losses as friction, etc., retard the separation and so make them separate less than before, every time they do so, that is, cause them gradually to stop see-sawing. If, however, there is a lag in the magnetic attraction, then they come together with greater force than they separated, so separate more in the opposite direction, that is, the oscillation increases until the machines fall out of step, or the further increase of oscillation is stopped by the increasing energy losses. This kind of hunting is stopped by increasing the energy losses due to the oscillation, by copper bridges between the poles, by aluminum collars around the pole faces, or by a com- plete squirrel cage winding in the pole faces. The frequency of this hunting depends on the magnetic attraction, that is, on the field excitation, and on the weight of the rotating mass. The higher the field excitation the greater is the magnetic force, that is, quicker the motion of the ma- chine and therefore the higher the frequency. The greater the weight, the slower it is set in motion, that is, the lower the frequency. Characteristic of this hunting therefore is that its fre- quency is changed by changing the field excitation. 2nd. If the speed of the engine varies during the rota- tion, rising and falling with the steam impulses, then the alternator speed and the frequency also pulsate with a speed equal to, or a multiple of the engine speed. If now two HUNTING OF SYNCHRONOUS MACHINES 117 such alternators happen to be thrown together so that the moment of maximum frequency of one coincides with the moment of minimum frequency of the other, the two machines cannot run in perfect phase with each other, but pulsate, alter- natingly getting out of phase with each other, coming together, and getting out again in the opposite direction. If the deviation of the two engines from uniform rate of rotation is very little — the maximum displacement of the alternator from the position of uniform rotation not more than three electrical degrees — the pulsating cross currents, which flow between the alternators, are moderate, and the phenomenon harmless, as long as the oscillation is not cumulative. An increase of the weight of the flywheel of the engine decreases the speed pul- sation and thereby decreases this form of hunting, which is the most harmless, but increases the tendency to the hunting in No. I and No. 3, and therefore is not desirable; but steadiness of engine speed should be secured by the design of the engine, that is, by balancing the different forces in the engine, as the steam impulses and the momentum of the reciprocating masses, so as to give a uniform resultant. In such a case, when running from a single alternator, driven by a reciprocating engine with moderate speed pulsa- tion, (therefore receiving a slightly pulsating frequency) a synchronous motor without anti-hunting devices, but of high armature reaction, and therefore high stability, may run very steadily, with no appreciable current pulsation ; while the same synchronous motor, when supplied with a squirrel cage wind- ing in the field pole faces as the most powerful anti-hunting device, may show pulsation in the current supplied, which in a high speed motor, of high momentum, may be considerable. The cause is, that in the former case the synchronous motor does not follow the pulsation of frequency, but keeps constant ii8 GENERAL LECTURES speed, while in the latter case the squirrel cage winding forces the motor to follow the variation in frequency by accelerat- ing and decelerating, and the pulsation of the current therefore is not hunting, but energy current required to make the motor speed follow the engine pulsation. If the frequency of oscillation of the machine (as deter- mined by its field excitation and the weight of its moving part) is the same as the frequency of engine impulses, that is, the same as the number of engine revolutions or a multiple thereof, then successive engine impulses will always come at the same moment of the machine beat and so continuously increase it: that is, the machine oscillation increases, or the machine hunts. In this case of cumulative hunting caused by the engine impulses, the frequency of oscillation agrees with'the engine oscillation. 3rd. If one alternator is a little ahead, that is, takes a little more load, its engine governor regulates by reducing the steam, slowing down the alternator to its normal position. When slowing down, the flywheel is giving power, therefore the steam supply has been reduced more than it should be, that is, the alternator drops behind and takes less load until the governor has admitted steam again. In the meantime, while the first alternator was behind and took less load, the second alternator had to take the load, that is, the governor of the second alternator admitted more steam. When the first alternator has picked up again to its normal load, the second alternator gets too much steam and its governor must cut off, but then cuts off too much, the same way as the first alternator did before; so the two governors hunt against each other by alternatingly admitting too much and too little steam. HUNTING OF SYNCHRONOUS MACHINES 119 In this case the frequency of hunting does not depend on the engine speed and does not vary much with the field exci- tation, but the hunting is usually much less at heavy load than at light load. The reason is that at load, when the engfines take much steam, a little change in the steam supply does not make so much difference as at light load, where the engines take very little steam, and so a small change of the governor has a great effect. 4th. To run in parallel, the speed of the engines driving the alternators must decrease with the load so that the alter- nators divide the load. If the speed did not change with the load, then there v/ould be no division of the load; the one engine could take all the load, the other nothing. If the speed curve of the engine is such that the speed does not fall off much between no load and moderate load, then the alternators will not well divide the load at light loads, and hunt while running in parallel at light load, but steady down at heavier loads. To distinguish between different kinds of hunting: 1st. Change of frequency with change of field excita- tion points to magnetic hunting, especially if very marked. 2nd. Equality of frequency with the generator speed points to engine hunting. 3rd. If the synchronous motor or converter steadies down when only one engine is running, it points to engine governor hunting. 4th. Steadiness of operation at load, and unsteadiness at light load points to governor hunting, but may also be due to engine and magnetic hunting. 5th. If by disconnecting one governor and governing one engine only, the hunting disappears, then it is due to 120 GENERAL LECTURES governor hunting. If it does not disappear, then both gover- nors may be diconnected and the engines run carefully without governors, by throttle. If the hunting then disappears, it is due to the governors; if it does not disappear, it is probably magnetic hunting. If by making the field excitation of the two alternators or two converters that hunt, unequal — ^by increasing the one and decreasing the other — the hunting disappears or decreases, it is magnetic hunting. In a case of hunting, the following points should be in- vestigated : A. HUNTING OF SYNCHRONOUS MOTORS OR CONVERTERS 1st. Count the number of beats to get the frequency of hunting. If the beats periodically increase and decrease, it shows two frequencies of hunting superimposed upon each other. Then count the total number of beats per minute (counting during intermissions) and count the number of intermissions per minute. The two frequencies are the number of beats per minute, plus and minus half the number of intermissions or nodes per minute. Instance : 80 beats per minute, 10 intermissions per minute. Frequencies 80 + 5 and 80 — 5 or 85 and 75 beats. If one of the two frequencies approximately coincides with the engine speed, it can be assumed as the engine speed. The number of revolutions of the engine obviously should be counted also. 2nd. See whether any machine in the system runs at a speed equal to the observed frequency of hunting. For instance, a generator may make 75 revolutions per minute, which accounts for this frequency. HUNTING OF SYNCHRONOUS MACHINES 121 3rd. With several converters in the same station see whether the station ammeter also hunts. If the station ammeter is very steady and the converter ammeters hunt, the converters hunt against each other. In this case lowering the one and raising the other con- verter field and, if necessary, readjusting the potential regula- tors, may stop the hunting by giving the two machines differ- ent frequencies of hunting which interfere with each other. If all three meters are unsteady, the converters may hunt against each other or hunt together against another station or against the generator. Then find out whether the ammeter needles of both converters go up and down together or one goes up when the other goes down. 4th. Change the field excitation and see whether the change of field excitation changes the frequency. See whether a decrease of field excitation steadies it. Occasionally hunt- ing can be stopped by lowering the field excitation, that is, running with lagging current. 5th. If several converters of a substation feed into the same direct current system, as the converters of other sub- stations, disconnect the direct current sides of the converters and see if they still hunt. If two or more converters run in the same station, run only one and see whether it hunts. CURE 1st. If the hunting is magnetic hunting between con- verters or synchronous motors, it is frequently reduced by mak- ing the field excitation unequal, or putting a flywheel on one converter, or belting some other machine to it, or running an induction motor in the same station or in any other way break- ing up the resonance. 122 GENERAL LECTURES 2nd. Several converters hunting against each other in the same substation are frequently steadied by connecting the collector rings with each other, that is, by equalizer connec- tions between converter and transformer or regulator. In this case the commutator brushes have to be carefully adjusted to avoid sparking. 3rd. The most effective way is to put copper bridges on the converters or synchronous motors, or better still a squirrel cage winding in the field pole faces. Not so good are short circuiting rings around the field poles. B. HUNTING OF GENERATORS 1st. Count the frequency in the same way as before. 2nd. See whether the frequency agrees with the genera- tor speed or with the speed of some large motor on the system. 3rd. See whether the frequency changes with the exci- tation. 4th. See whether the hunting changes with the load, that is, gets worse at light load. 5th. Disconnect governors and see whether this stops hunting. CURE 1st. If the hunting stops when disconnecting the gover- nors, it is hunting of the governors and can be cured by putting a stiff dashpot on the governors. 2nd. If the hunting does not stop by disconnecting the governors, copper bridges on the alternators will cure it. 3rd. If the hunting has the speed of the engine, it may be reduced by increasing the flywheel or decreasing it, by running an induction motor in the station, or in any other way breaking up the resonance. HUNTING OF vSYNCHRONOUS MACHINES 123 In general, systems having all kinds of loads, different sizes of generators, motors and converters, induction motors and synchronous motors mixed, etc., are very little liable to hunting. Hunting is most liable to occur when all the genera- tors are of the same kind and all the synchronous motors or converters are of the same kind. Resistance between the machines increases the tendency to hunting so that if the resistance drop is more than 10% to 15%, special precautions have to be taken, such as squirrel cage pole face windings, or synchronous machines must be alto- gether avoided and induction motor generator sets used. Reactance in general reduces the tendency to hunting except when very large. The tendency to hunting is very severe at the end of a long distance transmission line and induction machines as a rule are preferable in such a place. Machines with high armature reaction are much less liable to hunt than machines with low armature reaction, that is, close regulation, because with high armature reaction the current varies much less with a change of position of the machine. Therefore, 60 cycle converters are more liable to hunt than 25 cycle converters, because in 60 cycle converters there is not enough space on the armature to get high arma- ture reaction.