Ans. Hunting.
Ques. What is the cause of this?
Ans. It is due to a variation in frequency.
The inertia of the converter armature tends to maintain a constant speed; variations in the frequency of the supply circuit will cause a displacement of phase between the current in the armature and that in the line wires, which displacement, however, the synchronizing current strives to decrease. The synchronizing current, although beneficial in remedying the trouble after it occurs, exerts but little effort in preventing it, and many attempts have been made to devise a plan to eliminate this trouble.
NOTE.—Three phase motor test; polyphase wattmeter method. This is identical with the test of [fig. 2,882], except that the wattmeter itself combines the movement of the two wattmeters. Otherwise the method of making the measurements is identical. If the power factor be known to be less than 50 per cent., connect one movement so as to give a positive deflection; then disconnect movement one and connect movement two so as to give a positive deflection. Then reverse either the pressure or current leads of the movement, giving the smaller deflection, leaving the remaining movement with the original connections. The readings now obtained will be the correct total watts delivered to the motor. If the power factor be known to be over 50 per cent., the same methods should be employed, except that both movements should be independently connected to give positive readings. An unloaded induction motor has a power factor of less than 50 per cent., and may, therefore, be used as above for determining the correct connections. For a better understanding of the reasons for the above method of procedure, the explanation of the two wattmeter method, [fig. 2,882], should be read. The power factor may be calculated as explained under [fig. 2,882]. Connect as shown in [fig 2,882]. The following check on connection may be made. Let the polyphase induction motor run idle, that is, with no load. The motor will then operate with a power factor less than 50 per cent. The polyphase meter should give a positive indication, but if each movement be tried separately one will be found to give a negative reading, the other movement will give a positive reading. This can be done by disconnecting one of the pressure leads from the binding post of one movement. When the power factor is above 50 per cent. then both movements will give positive deflection.
Ques. What are the methods employed to prevent hunting?
Ans. 1, the employment of a strongly magnetized field relative to that developed by the armature; 2, a heavy flywheel effect in the converter; 3, the increasing of the inductance of the armature by sinking the windings thereon in deep slots in the core, the slots being provided with extended heads; and 4, the employment of damping devices or amortisseur winding on the pole pieces of the converter.
Fig. 2,883.—Three phase motor test; one wattmeter method. This method is equivalent to the two wattmeter method with the following difference. A single voltmeter (as shown above) with a switch, A, can be used to connect the voltmeter across either one of the two phases. Three switches, B, C and D, are employed for changing the connection of the ammeter and wattmeter in either one of the two lines. With the switches B and D in the position shown, the ammeter and wattmeter series coils are connected in the left hand line. The switch C must be closed under these conditions in order to have the middle line closed. Another reading should then be taken before any change of load has occurred, with switch A thrown to the right, switch B closed, switch D thrown to the right and switch C opened. The ammeter and the current coil of the wattmeter will then be connected to the middle line of the motor. In order to prevent any interruption of the circuit, the switches B, D and C should be operated in the order given above. With very light load on the motor the wattmeter will probably give a negative deflection in one phase or the other, and it will be necessary to reverse its connections before taking the readings. For this purpose a double pole, double throw switch is sometimes inserted in the circuit of the pressure coil of the wattmeter so that the indications can be reversed without disturbing any of the connections. It is suggested, before undertaking this test, that the instructions for test by the two wattmeter and by the polyphase wattmeter methods be read.