Patent GB-781067-A (GB)
Title: A load measuring and discriminating system for a.c. electrical machines
IPC: G01R31/34
Filed: 1954-01-06
Abstract: 781,067. Power measurements. LAURENCE, SCOTT & ELECTROMOTORS, Ltd., and SCHWARZ, K. K. Jan. 6, 1955 [Jan. 6, 1954], No. 411/54. Class 37. [Also in Group XXXVII] A system for deriving a signal voltage for measurement of the load of a 3-phase electrical machine comprises a current transformer in one phase wire whose secondary develops across a load resistance a voltage representing total current which is series connected with the secondary outputs of two voltage transformers, respectively connected between the phase wires not containing the current transformer, and between the current transformer phase wire and neutral; such outputs representing the wattless and wattful components due to machine loss to be eliminated from the total current measurement, and the vectorial sum of the three voltages representing either motor shaft torque or the true power component of a generator output, for application to an indicating or control device. In Fig. 1, a pole-changing squirrel cage induction hoist motor M is energized from a 3-phase and neutral supply A, B, C, N, and a current transformer 1 connected in phase wire C develops across load resistance 2 a voltage V2 representing the total current in magnitude and phase. A voltage transformer 3 energized between phase wire C and neutral develops at its variably tapped secondary a voltage which is subtractively combined with V2 and is adjustable to represent the in-phase frictional losses of the motor, while a voltage transformer 4 energized between phase wires A and B develops at its variably tapped secondary a voltage which is vectorially and subtractively combined with the resultant and is adjustable to represent the reactive magnetizing components of the motor load current. The vectorial resultant measurable voltage Vo is proportional to the motor output torque with a small error due to the variable reactive voltage drop of the machine and is rectified and smoothed to operate D.C. relay 5 over variable ballast resistance 9 and non-linear resistance 6 at the voltage corresponding to half the maximum torque value, and the relay is connected to preclude changing from high to low poleage of the motor (i.e. from low speed high load to high speed low load operation) unless the torque load is less than half the maximum. The non-linear resistance limits the current through the relay up to a fixed level of applied voltage; and thereafter allows the current to increase rapidly for a small increase of applied voltage, so that the relay operates and releases at the same voltage value irrespective of its delay characteristics and the sense of the voltage variation. The variable tappings of resistances 2, 9 and the secondaries of transformers 3, 4 are adjustable for correct relay operation and torque indication for each pole condition. In a modification (Fig. 3), for a three-phase supply A, B, C without neutral, the current transformer load resistance 2 is connected in series with the secondaries of voltage transformers 10, 11 respectively energized between phase wires B, C; A, B; to develop voltages V 10, V11, and is provided with a tapping 2T operated by a switch X to develop a fractional voltage V2T or the full voltage V2 respectively for the high and low poleage (low speed high load and high speed low load) conditions of the motor; the switch being operated by the pole-changing gear. The vectorially combined voltages are applied to a bridge rectifier 7 having voltage response characteristics which in conjunction with the saturation characteristics of transformer 1 approximate to those of the non-linear resistance used in the device shown in Fig. 1. The rectifier output voltage V7 operates control relay 5 over a ballast resistance 12 which is short-circuited by switch Y, operated by the pole-changing gear, in the high poleage lowspeed condition and opened in the low poleage high-speed condition. Closure of switch Z enables relay 5 to be held operated for all motor conditions, while relay operation is disabled during pole changing by over-riding time switches (not shown). Circuit operating curves for the motoring, no load, and generating conditions are given (Figs. 4, 5, 6, not shown). The system is applicable to slip ring and commutator A.C. motors and also to generators.