Patent GB-1327285-A (GB)
Title: Brushless synchronous electrical machine provided with  apparatus for measuring the insulation resistance of the  rotor
IPC: G01R31/34
Filed: 1971-07-05
Abstract: 1327285 Measuring insulation resistance GOSUDARSTVENNY NAUCHNO-ISSLEDOVATELSKY ENERGETICHESKY INSTITUT IMENI G M KRZHIZHANOVSKOGO and NAUCHNOISSLEDOVATELSKY PROEKTNO-KONSTRUKTORSKY I TEKHNOLOGICHESKY INSTITUT TYAZHELOGO ELEKTROMASHINOSTROENIA ZAVODA "ELEKTROTYAZHMASH" 5 July 1971 31377/71 Heading G1U [Also in Division H2] Apparatus associated with a brushless synchronous electrical machine 2 to measure the resistance of the insulation of the rotor circuit 1 whilst the machine is running comprises, coupled to the rotor, a source 10, 9 of unidirectional voltage connected in series with a current sensing winding 11 between earth 14 and a point 13 in the rotor circuit nominally at earth potential, the current sensing winding 11 being electromagnetically coupled to a stationary winding 19 to supply to a meter 18 a signal proportional to the current flowing from the source 10, 9. This current is in turn indicative of the insulation resistance so long as this  resistance remains distributed uniformly. The. source 10, 9 comprises an auxiliary exciter machine, connectible to a supply 4, and rotary rectifier, and the current sensing arrangement 8 comprises an auxiliary synchronous generator. The nominal earth point 13 may be a star point, as in Fig. 1, or a centre tap on a winding 22 connected between the + and - terminals of the rotor circuit and forming part of a rotor voltage sensing arrangement 7. When a fault occurs in the insulation, so that the resistance is no longer distributed evenly, the voltage in the rotor circuit superimposes a further insulation resistance current on that flowing due to the source 10, 9 alone and to allow for its effect two further measurements are made: (i) the supply 4 is disconnected (move switch 6 to position I 2 ) so that the magnitude of the further current alone is measured; (ii) with the supply 4 still disconnected, the meter 18 is connected to a winding 20 (switch position 0) which senses the voltage across the amature 12 of the auxiliary exciter 10, which voltage will, because the rectifiers 9 shunt the winding 12 in one direction, be zero or non- zero according to the direction of the further current. Since the direction depends on whether the fault is on the positive or negative side of the rotor circuit 1, this test also serves to locate the fault. The apparatus may also be used (switch position C) to monitor the rotor circuit 1 during normal running of the machine and provide an indication when an insulation fault occurs by allowing meter 18, protected by diodes 21, to detect when current flows through winding 11. The location of a fault is this time determined from the magnitude of the current, since resistors 16 limit the current if it is flowing in one direction but not if it is flowing in the other. In modification Fig. 2 (not shown) the winding 11 is shunted by a diode (26) opposing the rectifiers 9 and in a single alternative switch position replacing positions I 2 , O, C of Fig. 1 signals proportional to the voltages across windings 11 and 12 are applied in series opposition to a centre zero meter (27). The meter reading will indicate the magnitude and direction of any insulation resistance current flowing due to the voltage in the rotor circuit 1, since this current will bypass one or other of the windings 11, 12 depending on its direction.