1. Targeted Measurement with Multimeters
1. Correct Use of Multimeters
To correctly use a multimeter, one must be familiar with the dial. Adjust the two zero position regulators gently. Choose the correct terminals, ensuring the red and black probes are inserted into the right holes.
Rotate the switch to the correct range setting. Choose the appropriate measurement range for accurate readings. Ensure to look at the scale line directly above the surface for precise readings.
After measurement, switch the probes back and set the switch to the high voltage range. Regularly check the internal battery, as deterioration can lead to electrolyte leakage. Store the instrument in a good environment, free from vibration, humidity, and weak magnetic fields.
2. Correct Use of the Ohm Setting on Multimeters
To correctly use the ohm setting, one should know and be able to perform eight tasks. Ensure the battery voltage is sufficient and that the circuit being tested is free of voltage. Select the appropriate range, aiming for the needle to point to the middle of the scale.
Each time the range is changed, the resistance zero must be recalibrated. Ensure good contact at the probe tips, and do not touch the probe ends with your hands. For measuring continuity in wires, use a range above one thousand ohms.
When testing diodes, different ranges will yield different resistances. When measuring transformer windings, avoid touching them to prevent electric shock.
3. Precautions When Measuring Voltage with a Multimeter
When measuring voltage with a multimeter, there are eight precautions to consider. Be aware of the internal resistance of the meter, and ensure someone is monitoring the process. The multimeter should be connected in parallel to the live circuit, and the range should not be changed while powered.
When measuring DC voltage, clarify the polarity of the circuit. For measuring AC voltage in reactive circuits, do not disconnect the power supply during measurement. For testing high voltage (kilovolt), use specialized probes.
Induced voltage can vary significantly depending on the range used.
4. Methods for Measuring DC Current with a Multimeter
To measure current with a multimeter, set the switch to the milliamp range, confirm the polarity of the circuit, and connect the meter in series with the circuit. Choose a higher range to minimize the impact on the circuit.
5. DC Method for Identifying the Ends of a Three-Phase Motor Stator Winding
For three-phase motor windings, use the DC method to identify the ends. Set the multimeter to the milliamp range and use a DC power source like a battery. Connect one phase winding to the meter and the other to the battery.
When powered, if the needle moves, the positive and negative are identified. If it does not reverse, switch the connections and test the remaining phase winding.
6. Residual Magnetism Method for Identifying the Ends of a Three-Phase Motor Stator Winding
For motors that have been operated, use the residual magnetism method to identify the ends. After marking the three-phase winding leads, connect them in parallel. Set the multimeter to the milliamp range and connect to the common point.
Slowly rotate the motor shaft while observing the meter needle. If the needle does not move significantly, the three ends are connected. If the needle swings left and right, two ends are connected while one is not.
Adjust one phase winding and repeat the method until the needle stabilizes, indicating the ends are correctly identified.
7. Loop Method for Identifying the Ends of a Three-Phase Motor Stator Winding
For motors that have been operated, use the loop method to identify the ends. Connect the three-phase winding leads in a triangle. Set the multimeter to the milliamp range and connect in series with the three-phase windings.
Rotate the motor shaft evenly while observing the meter needle. If the needle does not move significantly, the ends are connected. If the needle swings widely, one phase winding is reversed.
Two connection points indicate the start and end of the windings.
8. Measuring the Speed of a Three-Phase Motor with a Multimeter
To measure the speed of a three-phase motor, use a multimeter. Open the motor junction box and disconnect the terminal connections. Set the multimeter to the milliamp range and connect to any phase winding.
Rotate the rotor one full turn and observe how many times the needle moves. A two-pole motor will show one swing, indicating a synchronous speed of 3000 RPM. A four-pole motor will show two swings, indicating a synchronous speed of 1500 RPM.
Continue this method to determine the speed, which will be slightly lower than the synchronous speed.
9. Testing the Ground Resistance of Household Grounding Protection Lines
To test the grounding resistance of household grounding lines, set the multimeter to the voltage range and connect a kilowatt electric stove to the phase and neutral. Measure the voltage at the stove terminals to calculate the working current value.
Reconnect the stove to the phase ground line and measure the voltage again. The difference in voltage readings divided by the working current value gives the grounding resistance, with an approximate error of five percent.
10. Identifying the Phase and Neutral Wires of Low Voltage AC Power Supply
In a low voltage three-phase four-wire system, identify the phase and neutral wires. Set the multimeter to the voltage range, with a setting of 250V AC. Connect one probe to the ground and touch the other probe to the power line.
If the needle deflects significantly, the probe is touching the phase wire. If the needle does not move or only slightly deflects, the probe is touching the neutral wire.
11. Testing the Polarity and Condition of Diodes
To test the polarity of a diode, set the multimeter to the kilo-ohm range. A reading of less than one kilo-ohm indicates forward resistance. Connect the black probe to the anode and the red probe to the cathode.
A reading of several mega-ohms indicates reverse resistance. Connect the red probe to the anode and the black probe to the cathode.
To determine if a diode is good or bad, set the multimeter to the kilo-ohm range. A significant difference between forward and reverse resistance indicates a good diode. If the values are close, the diode is likely faulty.
A reading of zero in both directions indicates a short circuit, while infinite resistance indicates an open circuit.
12. Testing the Condition of High Voltage Silicon Rectifiers
To check the condition of silicon rectifiers, set the multimeter to the voltage range. Connect the multimeter in series with the rectifier and apply 220V AC. Set the range to 250V DC and connect the rectifier in the forward direction.
A reading greater than 30V indicates a good rectifier; if the needle does not move, there is a fault. A reading of 250V AC indicates a short circuit, while a reading of zero indicates an open circuit.
13. Testing the Condition of Capacitors
To test the condition of microfarad capacitors, set the multimeter to the kilo-ohm range and connect the probes to the terminals. The needle should swing left and right; the greater the swing, the better the capacitor.
If the needle does not move, the capacitor is open. If the needle drops to zero and does not return, the capacitor has shorted.
14. Using a Digital Multimeter’s Buzzer Function to Test Electrolytic Capacitor Quality
To test the quality of electrolytic capacitors, use a digital multimeter. Set the switch to the buzzer function and connect the probes to the positive and negative terminals. A short beep indicates a good capacitor, while a continuous beep indicates a short circuit. No sound indicates an open circuit.
15. Safety Regulations When Using Clamp Meters
When using clamp meters, remember the safety regulations. Testing high voltage circuits must be done by two people. Ensure the potential of the wire being tested does not exceed the clamp meter’s voltage rating.
Always wear gloves and stand on an insulated platform. Maintain a safe distance from any live parts. When measuring low voltage bus currents, ensure proper insulation and protection.
Avoid using clamp meters on poorly insulated or bare wires.
16. Correct Use of Clamp Meters
When using clamp meters, select the appropriate model and specifications. Start with the maximum range for rough measurements, and choose the range wisely. Position the wire in the center of the clamp jaws for accurate readings.
Once the clamp is closed around the wire, do not change the range while powered. Clamp meters for current and voltage should be used separately. Do not insert both wires into the clamp at the same time.
After each test, set the range back to the maximum.
17. Techniques for Measuring Three-Phase Three-Wire Current with Clamp Meters
When using clamp meters to measure three-phase three-wire current, apply Kirchhoff’s law to derive measurement techniques. Insert one wire into the clamp to read the current of that phase.
Insert two wires to read the current of the third phase. If all three wires are inserted, the reading should be zero, indicating balanced load.
18. Techniques for Measuring Small AC Currents with Clamp Meters
When using clamp meters to measure small AC currents, wrap the insulated wire around the clamp core. Divide the reading by the number of turns plus one to obtain the actual current value.
19. Detecting Phase Loss in Three-Phase Resistance Furnaces
To detect phase loss in three-phase resistance furnaces, use a clamp meter. If the current values of two phase wires are both less than the rated current and one phase wire shows zero current, that phase’s resistance wire is burnt out.
20. Locating Short Circuit Ground Faults in Low Voltage Distribution Lines
In long low voltage distribution lines, locating short circuit ground faults can be challenging. If the faulty phase wire is connected to an electric stove, use a single-pole switch to connect the power supply. Use a clamp meter to measure the current in segments of the line.
Where there is no current, that is the location of the short circuit ground fault.
21. Testing Thyristor Rectifier Devices
To test thyristor rectifier devices, use a clamp meter. Connect the clamp to the anode lead and observe the current reading. If the reading is zero, the device is not functioning.
For three-phase devices, the current values should be balanced. Significant imbalance indicates phase shifting issues. If there are faults in the AC section, the rectifier transformer may be missing a phase.
22. Identifying User Phase Theft
If a single-phase energy meter shows low or no readings, check the meter before and after with a clamp meter. If the clamp is around the phase and neutral wires, and the reading is not zero, there may be phase theft.
Measure the current in both the phase and neutral wires; if there is a significant difference, it indicates phase theft.
23. Safety Regulations When Using Insulation Resistance Meters
When using insulation resistance meters, adhere to safety regulations. Testing high voltage equipment must be done by two people. Ensure the equipment is fully powered down and adequately discharged.
When measuring insulation on lines, obtain permission from the other party. Ensure both circuits are powered down, and avoid testing during thunderstorms. When measuring near live equipment, position yourself safely.
Maintain a safe distance and ensure monitoring to prevent electric shock.
24. Correct Use of Insulation Resistance Meters
When using insulation resistance meters, select the appropriate voltage level. Ensure the equipment is powered down and adequately discharged before testing. Clean the surface of the equipment to remove any dirt.
Position the meter away from electric and magnetic fields. Place it horizontally without tilting, and conduct both open and short circuit tests. Use insulated single-core leads that do not tangle.
Ensure the connection terminals are clearly identified and correctly connected. Rotate the handle clockwise to gradually reach a constant speed. There is no fixed time for testing; record the reading when the needle stabilizes.
25. Important Considerations When Using Insulation Resistance Meters
When using insulation resistance meters, remember eight key considerations. Do not touch the connection terminals during testing. Avoid cleaning the meter’s glass during the measurement process.
When measuring insulation to ground, connect the ground terminal to the casing. For large capacitive devices, disconnect at rated speed. When testing electrolytic capacitors, connect the ground terminal to the positive lead.
For multiple devices tested on the same platform, it is best to use the same meter. Record the temperature during insulation testing. Avoid measuring resistances below one hundred kilo-ohms, and do not use the meter for general purposes.
26. Connecting Diodes in Series to Prevent Discharge of the Device Under Test
To prevent the device under test from discharging the insulation resistance meter, connect a diode in series. This will help stabilize the reading and ensure accurate measurements.
After testing, stop rotating the handle to avoid damaging the meter.
27. Methods to Increase the Voltage of Insulation Resistance Meters
For low voltage insulation resistance meters, connect them in series to measure insulation. The voltage levels will add up, and the insulation resistance reading will be the sum.
28. Insulation Absorption Ratio of Power Transformers
To assess the quality of transformer insulation, use an insulation resistance meter. At room temperature (around 20 degrees Celsius), start timing from the moment of measurement: check the reading at fifteen seconds and stabilize the value after a few seconds.
The ratio of two insulation resistance values is called the insulation absorption ratio. A value greater than 1.3 is good, while less than 1.3 indicates moisture.
29. Quickly Assessing the Condition of Low Voltage Motors
To assess the condition of low voltage motors, open the junction box for testing. Use an insulation resistance meter to measure the minimum insulation resistance value, using thirty-five degrees Celsius as a baseline, reducing by half for every ten degrees increase.
If the reading exceeds the threshold, the motor is good. Set the multimeter to the milliamp range and use the star connection method. Connect the probes to any two phase leads and slowly rotate the shaft.
If the needle swings significantly, repeat the test three times. If the results are consistent, the motor is good; otherwise, it is faulty.
30. Testing the Condition of High Voltage Silicon Rectifiers with Insulation Resistance Meters
To test the condition of high voltage silicon rectifiers, use an insulation resistance meter. Connect the two leads to the rectifier terminals. Measure the resistance in both forward and reverse directions; a significant difference indicates a good rectifier.
If the readings are close, the rectifier is likely faulty. If both readings are infinite, the rectifier is open. If both readings are close to zero, the rectifier has shorted.
31. Testing the Condition of Self-Ballasted High Voltage Mercury Lamps
To test high voltage mercury lamps, use a kilovolt insulation resistance meter. Connect the leads to the lamp terminals. Test in a dim environment, gradually increasing the speed of the meter.
A reading of less than half a mega-ohm indicates a good lamp. If the lamp does not light, the reading will be zero, indicating a short circuit. If the needle indicates infinite resistance, the lamp has an open circuit fault.
32. Testing the Quality of Fluorescent Tubes with Insulation Resistance Meters
To test fluorescent tubes, use a kilovolt insulation resistance meter. Set the multimeter to the voltage range, with a setting of 500V DC. Connect the meter in parallel, ensuring the polarity is correct.
Connect the leads to the ends of the tube. If the lamp lights at rated speed, a reading below 300V is normal. If the tube glows slightly, it indicates aging above 300V.
If the tube does not light at all, it indicates that the tube is damaged.
33. Testing the Quality of Starters for Fluorescent Lamps with Insulation Resistance Meters
To test the starters for fluorescent lamps, use an insulation resistance meter. Connect the leads to the starter terminals. Slowly rotate the handle; if the neon bulb flashes red, the starter is good; otherwise, it is faulty.
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