Patent GB-754865-A (GB)
Title: Improvements relating to apparatus for testing windings
IPC: G01R31/06
Filed: 1954-04-15
Abstract: 754,865. Testing armature windings. HOOVER, Ltd. (Hoover Co.). April 15, 1954, No. 11216/54. Class 37. Armature testing apparatus includes (i) a generator for generating an A.C. magnetic field; (ii) driving means for rotating in the field the armature to be tested; (iii) a pick-up for making contact with the armature commutator bars two at a time so as to pick up from one armature coil at a time a signal which is a function of the voltage induced in that coil when it is in a predetermined position in the field; and (iv) an indicator responsive to the said signal and timing means for selecting an operative portion between the ends of each bar period, that is to say the period during which a single armature coil is connected to the pick-up, and causing the indicator to be operative only during the said operative portion of each bar period, the indicator being arranged to give an indication as the armature rotates only when the signal is beyond predetermined limits. The apparatus may include pulse shaping and timing circuits so that transient-free' voltages trigger thyratrons to actuate fault indicators. General description, Fig. 1.-A motor 9 drives test armature 1 (at 1 revolution/second) in audio-frequency field (2400 cycles/second) of coil L with test prods 27, 28 engaging adjacent commutator bars 2. As each coil of armature 1 moves into position in which it links maximum field lines of coil L a prod signal is transmitted to the pulse shaper unit 31. The prod signals may vary in magnitude as shown in Fig. 4: N (normal); O-C (open circuit); D-T (deficient turns) ; S-T (surplus turns); I-C (reversed coil connections). In Fig. 4, t1-t4 is the duration of the prod signal or bar period and t2-t3 is the mid-bar or sampling period considered to be transient-free and truly representative of coil conditions. The pulse shaper unit 31 in conjunction with a timer unit 33 operates to convert the mid-bar signals into the family of voltage waveforms 125 shown at the extreme right of Fig. 5. The S-T and I-C waveforms are applied directly to S-T and I-C alarms 35, 36 but as the O-C and D-T voltage peaks are below the normal (N) peak O-C and D-T detectors 37, 38 are inserted and these energize the O-C and D-T alarms 39, 40. If any alarm is energized the associated motor control switch 20, 21, 22 or 23 is opened automatically, whereby motor 9 is de-energized and solenoid 13 also to apply brake 11. The motor and brake controls act very quickly so that armature 1 stops with test prods 27, 28 resting on the commutator bars 2 connected to the faulty armature coil. If the armature 1 is free from faults a cam 25 driven by gear 26 opens cam switch 15 to de-energize motor 9 and solenoid 13 and light lamp 19 after one and a half complete revolutions of motor shaft 8. This means that if a fault is indicated cam 25 will not be engaging switch 15 so that after marking the commutator bars 2 and lifting handle 29 to remove prods 27, 28 the operator actuates push 42 to energize motor 9 and solenoid 13 until the cam 25 opens switch 15. The alarms 35, 36 and 39, 40 fed from a common source 300R are reset when cam 45 driven by gears 46 from shaft 8 opens cam switch 44. When the handle 29 is lifted a cam 41 opens a switch 43 to isolate audio-frequency generator 17 feeding coil L. When the faulty armature 1 has been replaced by a fresh one to be tested a push 18 is actuated until cam 25 clears the cam switch 15. Shaper and timer circuit details, Fig 2.-The voltage picked up by prod 27 (see Fig. 1), is amplified by triode 50 via resistors 51, 52 and the output of triode 50 goes through oppositely connected diodes 60, 61 into matched integrating filters 54, 62 so that the timer circuit 33 receives the negative portion from filter 54 (see voltage line 76 at the extreme left of Fig. 6), and the positive component 63 from filter 62 (see voltage line 63 at the extreme left of Fig. 5), is compared with a reference voltage taken from the transformer 16 feeding coil L whereby variations in the output of generator 17 will not appear as coil faults. This reference voltage is derived from transformer 16 through a diode 64 so that the instantaneous voltage on the cathode of diode 64 is always equal and opposite to the input to coil L. The diode 64 co-operates with an integrating filter 65/65a matched to filter 62 and series resistors 68, 69 are connected to filters 62 and 65/65a and when the circuit is calibrated the voltage across resistor 69 is equal and opposite to voltage 63 (see line 67 below 63 in Fig. 5) so that during bar period t1-t4 the voltage at the junction of the resistors 68, 69 is zero for a faultless armature coil and occupies the level shown at 70 when the test reveals I-C &c. fault conditions. The time constants of filters 54, 62 ensure voltage decline to zero (see 70 in Fig. 5) whilst the prods 27, 28 are engaging insulating segments on the commutator 2 during time period t4-t1. The voltage 76 (see Fig. 6) representing the negative half of the prod signal feeds timer 33 and is therein squared and amplitude limited (to remove transients) in stages prior to triode 90 so that the output of triode 90 is a near-perfect square wave 93 going via differentiator 94, 95 into pulse generator 96, 98 which generates voltage pulses when it is triggered by positive peaks at t1 (see 97) and delivers an output 103 which shows that triode 98 is cut-off between t1 and t2 (as determined by time-constant combination 101a/ 102). A differentiator 106/113 triggers triode pair 104, 105 by generating a sharp negative peak as at 103a and the normally saturated triode 105 is so controlled by resistor-condenser 111/112 as to be cut-off for the required pulse sampling period or mid-bar period t2-t3 as shown by breaks in the voltage level 115. The keyed pentode 73 shown in the shaper circuit 31 has its control grid fed from the junction of resistors 68, 69 and its suppressor grid taken to the anode of triode 105 in the timer circuit 33 so that the suppressor grid is driven to earth potential during sampling period t2-t3 when triode 105 is cut-off. Between t2-t3 the pentode 73 is conductive and controlled by the I-C &c. voltage levels 70 (Fig. 5). During cut-off of the valve 73 the condenser 120 coupled to clamping diode 122 and resistor 121 charges via resistor 116 so that the voltage across 120 at each instant t3 is determined by the magnitude of the voltage 70 during the sampling period and the voltage across the resistor 121 is represented by the waveforms 125 and these are applied to a cathode follower 126 whose output is taken via line 130 to the alarm and detecting circuit, Fig. 3. In the diagram 125 the line O-C represents the boundary between voltages caused by armatures which are classed as O-C and those classed as D-T. The lines D-T and S-T are boundaries of voltages caused by armatures acceptable as normal. The line N represents the voltages caused by an ideal standard armature and the lines S-T and I-C are the lower limits of S-T and I-C defective armatures. S-T and I-C alarms, Fig. 3.-If the output from the shaper unit 31 is an I-C voltage thyratron 132 in alarm unit 36 is fired to operate a relay 136 to open motor switch 20 and change the position of switch 138 to isolate the other alarm units and light a neon I-C indicator 139. In the case of an. S-T condition thyratron 132<SP>1</SP> in alarm unit 35 is triggered and relay 1361 operates. O-C and D-T detectors and alarms, Fig. 3.- If the output from the shaper unit 31 is an O-C voltage triode gate 141 in O-C detector unit 37 is triggered and thyratron 154 fails to fire and earths the grid of thyratron 163 in alarm unit 39 so that thyratron 163 conducts to operate relay 162 whereby motor switch 23 and alarm unit 40 isolator switch 164 both open and O-C neon 165 lights up. The D-T arrangements are similar to the O-C and comprise' detector unit 38 and alarm thyratron 167. The Specification describes the detector units in considerable detail with the aid of waveform diagrams, Figs. 7 and 8 (not shown). Calibration.-Full details and instructions concerning this are given in the Specification. Specification 750,995 is referred to.