Patent GB-1461725-A (GB)
Title: Method and apparatus for testing dc motors
IPC: H02K23/00
Filed: 1975-05-23
Abstract: 1461725 Testing dynamo-electric machines; measuring voltage FORD MOTOR CO Ltd 23 May 1975 [5 Aug 1974] 22628/75 Heading G1U To test a D.C. motor e.g. the permanent, magnet fan motor for a vehicle ventilation system, a constant voltage is applied to the motor for a first time period and a constant current is applied for a second time period, and if motor current falls below a predetermined level during the first period and/or the motor terminal voltage lies outside a predetermined range at the end of the second period, the motor is rejected. A timing circuit 22, Fig. 1, controls the apparatus via a chain of time delay circuits, Fig.3 (not shown), operating relays, the initial input being by a manually operated switch. The switch 18 connects the constant voltage supply 12 to the motor for an interval of 150msec. while the motor current stabilizes. At this point the 'first' time period (50msec.) is initiated and the contant voltage comparator circuit 24 monitors the motor current by means of a sensing shunt in its supply leads, Fig.2 (not shown). The circuit 24 compares the current with a reference level representing the minimum acceptable current and the test result is stored. At the end of this period, the motor is isolated from the supply and allowed time to come to rest (500 msec.). At the same time, the constant current supply 14 is switched on and allowed to stabilize, before being connected to the motor for the succeeding interval. After connection, the comparator circuit 28 first senses when the terminal voltage has exceeded a first low threshold and then enables further monitoring circuitry and initiates the 'second period' (250 msec.). The further circuitry establishes whether at the end of the second period, the terminal voltage lies within predetermined limits. Under the constant current supply, the terminal voltage increases throughout the period and at the end, a short sensing pulse (10 msec.) is provided to gate the test result to a store. At the end of this pulse, the accept/reject display 26 is enabled to provide the test results on four lamps, Fig.6 (not shown).  Voltage and current comparison circuits - Fig.5 The two comparator circuits 24, 28 utilise combinations of individual comparator stages to provide their monitoring function. For example in Fig.5, the motor terminals are connected to inputs 72, 74. The positive input is attenuated 200-208, and applied to the inverting input of a differential amplifier 212 via resistor 210. A reference level is set by the potentiometer 216 in the chain 216-218 between earth and -15V. Thus a negative current is supplied to the inverting input via resistor 214 which is equal to resistor 210. Thus if the input voltage is of greater magnitude than the reference level, the amplifier input is positive and its output negative and vice versa. This stage detects the low threshold at the motor terminals and when this is exceeded relay CR2 is energized to enable the following comparator stages and to initiate the 'second period'. The following stages comprising differential amplifiers 240, 300 sense whether the motor terminal voltage is between the respective reference levels set by potentiometers 312, 256. At the end of the 'second' period, a 10msec pulse gates the stage outputs 278, 336 to the gate electrode of SCR 292. If the motor is good, SCR 92 fires and stores the result as a low voltage on lead 364. The constant voltage comparator 24, comprises a similar comparator stage, but with preamplification, and a similar SCR store.