The DC resistance of a motor winding is a crucial parameter, and measuring this parameter is an important part of motor testing. Analysis of the measured resistance values can preliminarily determine whether the number of turns, wire diameter, number of parallel windings, wiring method, and wiring quality of the tested motor winding meet the design or expected requirements, and whether there are serious short circuits between the winding turns. The DC resistance of the winding is also a necessary parameter for loss calculation and temperature rise calculation. Therefore, the measurement of the winding DC resistance must be performed using high-precision instruments and with meticulous care to ensure high accuracy of the measured data.
When measuring the DC resistance of a motor winding during testing, a DC resistance measuring instrument is generally used to directly measure the value. Alternatively, a voltmeter and an ammeter can be used to simultaneously measure the DC current flowing through the winding and the voltage across the winding, and then Ohm's law can be used to calculate the DC resistance of the winding under test. This method is simply called the "voltage-current method." Ms. Can will today summarize and share some common problems that may arise during the testing process.
1. Common Types of DC Resistance Meters
● DC resistance bridge
The most commonly used instrument for measuring the DC resistance of motor windings is the DC resistance bridge. There are two main types of commonly used DC resistance bridges: one is called a single-arm bridge, also known as a Wheatstone bridge; the other is called a double-arm bridge, also known as a Kelvin bridge. Here, "arm" refers to the connection between the bridge and the resistor being measured; a single-arm bridge has one connection at each end, while a double-arm bridge has two connections at each end. A single-arm bridge is used to measure resistances greater than 1Ω; a double-arm bridge is used to measure resistances of 1Ω and below. Compared to a single-arm bridge, the advantage of a double-arm bridge is that it can largely eliminate errors caused by the resistance of the lead wires.
● Digital display electronic DC resistance meter
Digital display electronic DC resistance meters are also commonly used, and they come in many different sizes. The working principle of this type of meter is based on the "voltage-current method" mentioned above. Compared to a DC resistance bridge, it has the advantages of ease of use and quick reading, and within a certain measurement range, it also has the advantage of high accuracy; its disadvantage is that its stability is relatively poor. When using this type of meter, care should be taken to ensure that the current supplied to the winding does not exceed 10% of the winding's rated current, and the energizing time should be as short as possible to avoid the winding overheating and affecting the measurement accuracy.
2 Common mistakes made when using DC bridges
(1) When using a single-arm bridge to measure small resistances, the accuracy is insufficient, leading to inaccurate measurements and the inability to detect uneven resistance. This type of problem frequently occurs in motor repair shops.
(2) When the power supply battery is insufficient, the stability of the bridge is poor, the measurement data is sometimes large and sometimes small, and the measurement imbalance is likely to occur.
(3) Inappropriate gear selection can cause the pointer to swing to its limit, which can easily lead to the burnout of the bridge.
(4) The power was not turned off in time, and the battery was instantly depleted due to contact between the measuring terminals.
(5) When the start and end of a certain phase of the motor are reversed, it cannot be detected by the bridge circuit and can only be detected during later inspection and testing.
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