During the manufacturing process of electric motors, due to the limitations and uncertainties inherent in multimeter testing accuracy, most motor manufacturers are equipped with specialized testing equipment. However, multimeters are still used in motor repair, installation, and initial inspection. In the use of multimeters, especially by non-professionals, the requirements for their use are often unclear, leading to occasional damage due to improper use. Ms. Can, based on her own experience and the instruction manuals of relevant instruments, has compiled some information to share with everyone.
Based on the display method of measured values, there are two main categories: pointer-based and digital-based. Relatively speaking, digital-based displays offer more functions and are simpler to use, but in some situations, they cannot replace pointer-based displays. Pointer-based displays are more complex to use, so some necessary explanation is required.
Main functions and accuracy of a multimeter
●Main Functions
(1) Measuring the DC resistance of a conductor. The minimum scale division is 0.20, and the maximum range (readable value) is within 5MΩ. (2) Measuring AC voltage. The general range is 500V, and some can reach 1500V or 2000V, etc. (3) Measuring DC voltage has the same range as AC voltage. (4) Measuring DC current generally has a maximum range of less than 2.5A.
● Measurement accuracy
Analog multimeters generally use rectifier systems. When measuring DC, they are rated 1.5 or 2.5; when measuring AC, they are rated 2.5 or 5.0.
How to use a multimeter and precautions
●Pre-measurement checks. (1) Check if the pointer is at 0. If not, turn the zero-adjustment screw to set it to 0. (2) Check the integrity of the two probes and leads. This is especially important when measuring higher AC voltages to prevent electric shock. (3) Check the lead plugs; if they are loose, tighten them. Sometimes loosening is caused by loose nuts on the sockets; in this case, the meter case needs to be disassembled for repair.
●Measuring DC resistance. (1) Select the appropriate range according to the estimated value of the measured value. When selecting the range, it is best to select a range where the measured value is as close as possible to the middle of the scale. (2) Short-circuit the two probes. The meter needle will then turn to 0. If it is not at 0, it should be zeroed. If it cannot be zeroed, it means that the battery voltage of the meter is insufficient. (3) Connect the two probes separately to the two terminals of the resistor being measured. The meter needle will then indicate a reading. Multiply the reading by the range setting of the meter to get the actual measured value. (4) After the measurement is completed, prevent the two probes from touching each other and short-circuiting. For safety, turn the indicator knob to the highest AC voltage setting and adjust it again when using the meter.
● Measuring AC voltage. (1) Select the AC voltage range according to the estimated voltage value to be measured. (2) Connect each of the two probes to one end of the voltage to be measured. Take precautions to prevent electric shock. To do so, wear insulated shoes or step on an object insulated from the ground, or wear insulated gloves. (3) Select a scale line that is as close as possible to the value of the selected range as possible, in order to facilitate reading and obtaining results.
●Measuring DC voltage. (1) Select the DC voltage range according to the estimated value to be measured; (2) Determine the positive and negative terminals of the voltage to be measured; (3) Connect the test lead marked "+" (usually red) to the positive terminal of the voltage to be measured, and the test lead marked "-" (black) to the negative terminal of the voltage to be measured. Note that the positive and negative terminals must not be reversed, otherwise the meter will be burned out; (4) Calculate the voltage value according to the selected range and the reading of the meter needle; (5) After use, turn the knob to the highest AC voltage range.
●Measuring DC current. (1) Set the knob position according to the estimated value of the current to be measured. (2) Disconnect the circuit to be measured and determine the positive and negative terminals of the two disconnections. Connect the "+" probe to the positive terminal of the circuit and the "-" probe to the negative terminal of the circuit, that is, connect the multimeter in series with the circuit to be measured. (3) Calculate the value of the current to be measured according to the selected range and the reading on the scale indicated by the pointer. (4) Turn the knob to the highest AC voltage range.
●Special Note: The maximum range of a multimeter for measuring DC current is generally small; a clamp meter should be used for measuring larger currents.
Comparison of analog and digital multimeters
An analog multimeter is an average-sampling instrument. It provides intuitive and visual readings. A digital multimeter, on the other hand, is a sampling instrument. It uses a sampling interval of 0.3 seconds to display the measurement result. Sometimes, each sampling result is only very close, not exactly the same, which makes reading the results less convenient than with an analog multimeter.
Analog multimeters generally do not have internal amplifiers, resulting in lower internal resistance. Digital multimeters, on the other hand, utilize operational amplifier circuits internally, allowing for significantly higher internal resistance. This minimizes the impact on the circuit being measured and results in higher measurement accuracy.
Analog multimeters have lower internal resistance and often use discrete components to construct shunt and voltage divider circuits. Therefore, their frequency response is non-uniform (compared to digital multimeters). Digital multimeters, on the other hand, have relatively better frequency response.
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