Fig 32: Lap winding test dataFig 32 shows a lap winding, a style where the windings are connected to bars laying next to each other. To make a test, the current probe should be placed at the end of the commutator bar and the potential probe should be placed at the connection to the winding (the riser on the commutator bar). The user measures the resistance of the windings between each set of bars under test (1 - 2, 2 - 3, 3 - 4, etc.). In this example, there is a possible weak solder joint between bars 4 and 5, and a break in the coil between bars 12 and 13 (the instrument will show this as an open). Fig 33: Commutator with 24 coils in seriesIn Fig 33 (lap winding, 24 coils), all the coils are connected in series. The resistance of each coil will be measured with the resistance of all of the other coils connected in parallel. The primary question for the user is what constitutes an acceptable reading for a specific coil (Rm) since the remaining 23 coils in parallel will lower the resistance of the coil being tested. For this example, we will assume that the resistance of the coil before insertion into the motor (Rc) was 1 A. The expected resistance can be calculated by the equation:Expected Rm = (Rc)(# of coils being tested)(# of coils in parallel)/(# of coils being tested + # of coils in parallel). In this example:Expected Rm = (1 A)(1)(23)/(1 + 23)Expected Rm = 0.958 AFig 34 shows a wave winding, another manufacturing technique for putting high resistance coils in a motor. In this example, the coil runs from commutator bar 1 to 6 to 11 to 16 and then loops back around the armature to commutator bar 2 (connected in series). When the user measures between bars 1 and 2, he / she is checking the resistance of the wave wound coil (the complete loop). In this example, there is a break in the coil between bars 12 and 17. This problem will appear when measuring bars 2 and 3, since they are the start and end bars of the loop. Fig 34: Wave winding test dataFig 35 on the following page shows wave winding commutator connections to the internal coils and test probe connections to individual commutator bars. This is a simplified layout, as the heavy ring shows the series connections for all the coils in the armature. A d.c. motor will have a different number of coils depending on the horse power and the voltage rating. In this example (tests from bar #1 to bar #2), two coils are in series and nineteen are in parallel. If one coil is open in the ring, the measurement from bar #1 to bar #2 will be the series value of the two coils. If the test probes are across the open coil, the total resistance of the other nineteen coils will be shown. Fig 35: Wave winding coil arrangementBattery strap testsWhen battery straps are tested, the user should have baseline values or targets to compare against the actual results. The following are examples of how these target levels are determined:Example 1: In Fig 36, the user is measuring the resistance (R0) across a single battery strap (both sides of the terminal). The straps on each side of the terminal have a resistance of 20 microhm and the connections to the terminals each have a resistance of 5 microhm. Under these conditions, the target resistance that the user wants to see is 15 microhm. A significant variance from this resistance in the actual reading would show a loose connection.Example 2: Fig 37 shows terminals connected in parallel by carrier strips with a resistance of 100 microhm. In this case, the target resistance that the user wants to see is 14 microhm.If there was an open strap between terminal 'a' and terminal 'b', the resistance reading would be significantly higher than the target, as follows:Ra-b = Ra-c + Rc-d + Rb-dRa-b = 100 + 15 + 100Ra-b = 215 μΩAdditional tests can be done between the same polarity terminals on a cell. Such a test will help determine the quality of the terminal-to-bar welds and major problems with the internal bar to which the plates are welded, as all are series connected. In this example, the measured resistance between like terminals on the same cell should be in the 100 microhm range. Fig 36: Single strap resistance targetFig 37: Parallel strap resistance targetRamp testingA ramp test delivers a controlled 'ramp' of the output current from 0 up to the required output. This ability is particularly beneficial where there are protection relays in place, typically in the form of differential relays. When the contact resistance of a circuit breaker is tested, a differential relay monitors the line for any sudden rise in current that may be an a.c. signal. If the rise in current is too fast the differential relay detects this as a fault and trips the circuit breaker, as it would do under normal operating conditions.By the application of the current at a slower rate, which is variable and configurable, it allows low resistance test equipment to be used with a multitude of protection relays, each with different sensitivities. This means that the protection relays can stay in place and removes the undesirable need to disconnect the protection relay in a test.www.megger.com 3130 A guide to low resistance testing www.megger.com
