How to Use a Multimeter Effectively

When the six terminals of a three-phase asynchronous motor are lost, unclear, or rewound, it is necessary to determine which two terminals belong to the same phase, which is the start of the coil, and which is the end. Below are several methods for operation.

Use the resistance setting on the multimeter to distinguish the two ends of each phase of the three-phase winding and assign hypothetical numbers. Connect the wires as shown in the diagram below.

Observe the direction of the multimeter (microamp setting) needle’s swing. When the switch is closed, if the needle swings to the side greater than 0, then the wire connected to the positive terminal of the battery and the wire connected to the negative terminal of the multimeter are both either the start or end. If the needle swings in the opposite direction, then the wire connected to the positive terminal of the battery and the wire connected to the positive terminal of the multimeter are both either the start or end.

Next, connect the battery and switch to the other two terminals of a different phase for testing, and you can accurately determine the start and end of each phase.

First, use the resistance setting on the multimeter to distinguish the two ends of each phase of the three-phase winding. Assign hypothetical numbers to each phase winding as U1, U2, V1, V2, and W1, W2. Connect the wires as shown in the diagram below to determine the start and end. Rotate the motor rotor by hand; if the needle on the multimeter (microamp setting) does not move, it proves that the assigned numbers are correct. If the needle deflects, it indicates that the assumed numbering of the start and end of one of the phases is incorrect, and adjustments should be made one phase at a time until correct.

First, use the resistance setting on the multimeter to distinguish the two ends of each phase of the three-phase winding and assign hypothetical numbers, then connect the wires according to the diagram below. Connect any two phase windings in series and then connect them to the AC voltage setting of the multimeter, while connecting the third phase winding to a 36V low-voltage AC power source.

After powering on, if the voltmeter shows no reading, it indicates that the two connected ends are either both the start or end. If the voltmeter shows a reading, one of the connected ends is the start, and the other is the end. By determining one end as the known start, the start and end of the third phase can also be established.

If the motor does not have a nameplate and there is no speed meter, the speed of the motor can be determined using a multimeter without disassembling the motor.

The method is: use the minimum milliamp setting on the multimeter to connect one end of the previously identified winding’s start and end, and slowly rotate the rotor at a uniform speed. Observe how many times the multimeter needle swings. If it swings once, it indicates a positive and negative current change for one cycle, meaning it is a 2-pole motor. Similarly, if it swings twice, it indicates a 4-pole motor; if it swings three times, it indicates a 6-pole motor, and so on.

Once the number of poles is determined, the approximate speed can also be calculated (slightly lower than the synchronous speed). The synchronous speed of the motor is related to the number of magnetic poles, and it can be roughly calculated as follows for a power frequency of 50Hz: 2 poles = 3000r/min, 4 poles = 1500r/min, 6 poles = 1000r/min.

During operation, ensure that the multimeter probes maintain good contact with the terminals. Otherwise, during the rotor’s rotation, the needle may swing, making it impossible to determine the result.

Many analog multimeters only have a DC current testing function and do not have an AC current testing function. To meet measurement needs, by adding rectification and filtering circuits, the DC current setting of the multimeter can be modified to measure AC current within a certain range. The improved additional circuit is shown in the diagram below.

The circuit in the diagram adds three components. Among them, VD1 is a rectifying diode, VD2 is an AC bypass diode, and C is a filtering capacitor. The current flowing through the DC ammeter is half-wave rectified current. If the negative half-wave bypass diode VD2 is not added, the test results will be slightly higher. Of course, if the testing data does not require high accuracy, VD2 can be omitted, simplifying the circuit. If the filter capacitor is not added, the multimeter’s needle will oscillate.

The capacity of capacitor C can be calculated; it must reach an appropriate capacity to eliminate the needle’s jitter during measurement.

For ease of operation, we will use the experimental circuit shown in the diagram below to introduce the basic principles of testing AC current using the DC current setting on the multimeter.

(1) Determine the load working current. According to the load power shown in the diagram, calculate the effective value of the AC current passing through the load to be 182mA.

(2) Determine the range of the multimeter’s DC current setting. Based on the relationship between the average value of half-wave rectification and the effective value, calculate the average current through the load EL to be 82mA.

According to the principle that the range of current must be greater than the measured value, the multimeter’s DC current range can be set to 100mA.

(3) Close the power switch S1, the load gets powered, the bulb EL lights up, and the multimeter shows 80.2mA, while the AC ammeter A reads about 181mA.

(4) Close switch S2, disconnect the AC ammeter from the test circuit, and observe that the reading on the multimeter’s DC current setting remains unchanged. The test is successfully completed.

(5) Data processing for the test. The reading obtained from the DC ammeter in step four divided by the half-wave rectification conversion coefficient gives the effective value of the AC current passing through the load.

If the DC current setting on the multimeter is frequently used to test AC current, it is also possible to calculate the meter constant based on the range of the multimeter’s DC current settings, making usage very convenient. For instance, using the MF30 multimeter, the half-wave rectification conversion coefficient can be taken as 0.45, as shown in the table below.

For example, if using the MF30 multimeter’s DC current 500mA setting, and employing half-wave rectification, if the multimeter needle indicates 38 scales when testing the current consumption of a certain AC load, the effective value of the AC current can be calculated.

From the table, the AC range meter constant for the 500mA DC current setting is 22mA/scale, so the effective value of the AC current passing through the load is 836mA.

Tests have proven that using the DC current setting on the multimeter to test AC current is feasible and the data is reliable. However, the reading on the multimeter’s DC current setting is not the effective value of the AC current, but the average value of the AC current, and the reading from the multimeter’s DC current setting must be calculated to obtain the effective value of the AC current in the circuit.

When there is a broken wire fault in a cable or its internal components, the exact location of the break is difficult to determine due to the insulation covering the outside. The traditional method is to use the resistance setting on the multimeter to search for the cable break segment by segment, which not only wastes time but also greatly damages the cable insulation. A digital multimeter can easily solve this problem.

The specific method: connect one end of the wire (cable) with a breakpoint to the live line of 220V AC power, leaving the other end open. Set the digital multimeter to AC 2V, starting from the live line connection point of the wire (cable), hold the black probe tip with one hand while slowly moving the red probe along the wire’s insulation. At this point, the display shows a voltage value of about 0.445V (as measured by the DT890D model). When the red probe moves to a certain point, the display suddenly drops to 0.0V (about one-tenth of the original voltage), and the breakpoint of the wire (cable) is approximately 15cm in front of that position (towards the live line connection point).

This method is ineffective when checking shielded wires if only the core wire is broken and the shielding layer remains intact.

This method can also be used to locate the break points in faulty electric blankets and other resistance wires.

During line maintenance, if the amplifier input level is lower than the specified loss value for the cable length, the resistance (R×1 or R×100) of the cable can be measured using an analog multimeter. If the needle rises slowly like a capacitor charging, this phenomenon indicates that the cable is severely damp or has a lot of accumulated water.

Generally, an intact cable should have an infinite resistance value; if the cable is severely damp and has a lot of water, the resistance value is usually around several hundred ohms; if the cable is filled with water, its resistance value is essentially zero, equivalent to a short circuit, and the needle will also rise slowly like a capacitor charging. This clearly indicates that the cable is damp and waterlogged, leading to the fault; at this point, the television receiver’s TV signal quality is very poor, or even unwatchable.

Remove the cable with an open circuit or short circuit fault from both ends, separating the internal and external conductors. If the cable has a short circuit fault, the needle will definitely point to zero; when the cable is open circuit, the other end of the cable should be shorted, and then checked with the multimeter. If the needle still does not move, it proves that there is a break in the cable.

Damage to the outer plastic sheath of coaxial cables due to construction or poor production quality can cause the plastic sheath to crack and allow water ingress after exposure to sunlight and rain, leading to corrosion of the external conductor metal layer (or metal mesh). This fault can be checked with the multimeter’s resistance setting; if the measured loop resistance is significantly higher than the original cable loop value, it indicates severe corrosion of the external conductor. If the measured loop value is infinite, it indicates that the external conductor has broken after corrosion. In summary, there are many methods for testing coaxial cables, as long as we analyze and judge carefully, mastering the correct testing methods, any fault can be detected.

1. Contact Measurement Method

Set the multimeter to the AC 250V or 500V setting, connect the first probe to one end of the power supply, and the second probe to the ground (such as a water pipe, radiator, or damp ground). If the ground connection is good, when the multimeter reading is around 220V, the first probe is connected to the live wire of the power supply, as shown in the diagram (a). If the needle does not move, it indicates that the first probe is connected to the neutral wire. Even if the second probe has a high resistance to ground, when the first probe is connected to the live wire, the needle will show a significant deflection.

2. Non-contact Measurement Method

Set the multimeter to the AC 250V or 500V setting, connect the first probe to either end of the power supply, and place the second probe on the table, holding the insulated part of the second probe with your hand (be careful not to touch the conductive part). If the first probe is connected to the live wire, the needle will generally deflect by 2-10 scales, as shown in the diagram (b). The higher the sensitivity of the multimeter, the more pronounced the deflection. If the first probe is connected to the neutral wire, the needle will not deflect.

This method is suitable for any model of multimeter. This method does not pose a danger to the human body. The internal resistance of the analog multimeter is 20K/V; when measuring 220V voltage, the internal resistance is 20K×220V=4400, which is relatively safe.

3. Precautions

It is best to use a test pencil to distinguish between live and neutral wires, as it is a fast and safe method.

When using an analog multimeter to distinguish between live and neutral wires, always pay attention to safety. Specifically, the insulation of the probe wires should be good, and the operator’s hands and any part of the body should not come into direct contact with the metal parts of the probes to avoid electric shock hazards.

As we know, the sensitivity of the AC voltage setting on a digital multimeter is very high; even weak voltage signals can be displayed on the LCD screen. Using the ACV setting on a digital multimeter to find the live wire of AC power has the characteristics of being intuitive, quick, accurate, and safe.

1. Contact Measurement Method

Set the digital multimeter to the AC 20V setting (or AC 2V setting), remove the black probe, insert the black probe into the V/Ω socket, hold the insulated handle of the red probe with one hand, and touch the two sockets with the probe tip in turn; the one with the larger displayed value is the live wire, as shown in the diagram.

2. Non-contact Measurement Method

Sometimes, it is necessary to find the live wire from outdoor lighting lines; in this case, there is no need to strip the insulation from the wires. Use the red probe to touch the insulated outer skin of the two wires in turn; the one with the larger reading is the live wire; since this is induced voltage, it is best to select the AC 2V range.

When there is a short circuit in the lighting circuit, the current in the circuit is very large, and the fuse quickly blows, cutting off the circuit. If the fuse is too thick, it may burn the wire and even cause a fire.

Possible causes of short circuits in lighting circuits include: wiring errors, contact between live and neutral wires; damage to the wire insulation, causing contact or grounding at the damaged point; internal damage to electrical appliances; loose internal contacts in the lamp holder causing metal pieces to touch and short-circuit, water ingress in lamp holders, etc.

The fault phenomena of short circuits in lighting circuits are quite obvious, but determining the location of the fault can be quite complex. We can use the resistance method to check for faults.

The resistance method involves using the resistance setting on the multimeter to measure the resistance values between wires or electrical appliances to determine the location of the short circuit. After a short circuit occurs, disconnect the knife switch (or circuit breaker) on the distribution board and unplug all electrical appliance plugs to completely cut off the power. Set the multimeter to R×100 and measure the resistance between the live and neutral wires. If the needle approaches zero (or deflects), it indicates a short circuit (or leakage) in the circuit. Check the main line and each branch line segment by segment; if necessary, disconnect a certain line and measure the resistance between the two wires to determine the fault location.

During maintenance, first locate the short circuit point; you can use the resistance setting on the multimeter to check the circuit segmentally and regionally while the power is off. Start from the load end and step by step check forward to see whether the problem is due to the wiring or the components, which can help determine the cause. After eliminating the short circuit fault point, connect a qualified fuse and restore power.

When there is an open circuit in the lighting circuit, the circuit has no voltage, the lighting does not turn on, and electrical appliances cannot work. The causes include: blown fuses, broken wires, loose wire ends, and damaged switches.

When using a multimeter to detect open circuit faults in lighting circuits, you can measure the continuity of the circuit using the resistance setting while the power is off; you can also measure the voltage of the circuit using the AC voltage setting while the power is on to determine the fault point.

Open circuit faults in lighting circuits can be divided into complete open circuits, partial open circuits, and individual open circuits.

(1) Complete Open Circuit

This type of fault mainly occurs in the main line, within the distribution and metering devices, and in the range of the incoming device. Typically, the first step is to check each connection point (including fuse connection points) in the above parts sequentially; generally, the most common fault is the disconnection of the wire ends. Secondly, check the on/off status of the contacts in the various switches.

(2) Partial Open Circuit

This type of fault mainly occurs within the branch circuit. Generally, first check each connection point of the wire ends, and then check the branch switch. If the cross-section of the branch wire is too small, consider that the core wire may have broken within the insulation, causing a partial open circuit.

(3) Individual Open Circuit

This type of fault is generally limited to the range of junction boxes, lamp holders, light switches, and the connecting wires between them. Usually, check each connection point and the contact condition of components such as lamp holders, light switches, and sockets (for fluorescent lamps, check the connection of each component).

Once leakage occurs in the lighting circuit, it not only wastes electrical energy but also poses a risk of electric shock. Leakage and short circuits are fundamentally the same, differing only in the level of incident development; severe leakage can lead to short circuits. Therefore, leakage in lighting circuits should not be taken lightly; regular checks of the insulation condition of the circuit are necessary, especially when leakage is detected, to promptly identify the cause, locate the fault point, and eliminate it.

The main causes of leakage in lighting circuits are: damage to the insulation of wires or electrical equipment due to external forces; aging and deterioration of insulation due to long-term operation; and moisture or contamination affecting the insulation.

First, determine whether there is indeed leakage. You can use the R×10k setting on an analog multimeter to measure the insulation resistance of the circuit, or set a digital multimeter to the AC current setting (which acts as a current meter) in series with the main switch, turning on all switches and removing all loads (including light bulbs). If there is current, it indicates leakage.

Determine whether the leakage is between the live and neutral wires, between the live wire and the ground, or both. The method is to disconnect the neutral wire; if the current meter reading does not change, it indicates leakage between the live wire and the ground; if the current meter reading is zero, it indicates leakage between the live and neutral wires; if the current meter reading decreases but is not zero, it indicates leakage between the live and neutral wires and between the live wire and the ground.

Determine the scope of the leakage. Remove the branch fuse or open the circuit breaker; if the current meter reading does not change, it indicates leakage in the main line; if the current meter reading is zero, it indicates leakage in the branch line; if the current meter reading decreases but is not zero, it indicates leakage in both the main and branch lines.

Find the leakage point. After the above checks, sequentially disconnect the switches of the circuit lamps; when a certain switch is disconnected, if the current meter reading returns to zero, it indicates that the branch line is leaking; if it decreases, it indicates that this branch line has leakage and there is leakage elsewhere; if the current meter reading remains unchanged after all lamp switches are disconnected, it indicates that the main line is leaking. By narrowing down the scope of the incident, further check the connections of that segment of the line and any points where the wires pass through walls for leakage. Once the leakage point is found, promptly eliminate the leakage fault.

Short circuits, open circuits, and leakage are the most common faults in lighting circuits. Only through specific measurements and analysis can we accurately locate fault points, determine the nature of the faults, and take effective measures to eliminate them as soon as possible.

1. Measuring the DC Resistance Values of Components

In power drive control circuits, commonly used components include AC contactors, various relays, fuses, circuit breakers, control buttons, etc. We need to measure and record the DC resistance of the AC contactor coils and various relay coils used in the circuit (the specific values vary significantly among different models; for example, the DC resistance of commonly used AC contactor coils is about 2000Ω, while that of newer models may only be a few hundred ohms), for reference during repairs, as shown in the diagram below.

2. Measuring the Continuity of the Circuit

Before using the resistance method to locate faults, first disconnect the control circuit from the power source, then set the multimeter to R´10W or R´100W to measure. Generally, a segmented measurement method is used to check whether there are faults in the circuit, as shown in the diagram below.

(a) Segmented Measurement One

(b) Segmented Measurement Two

(c) Segmented Measurement Three

The long segment measurement method is used to measure the continuity of a certain branch; the short segment measurement method is used to measure the continuity of one or several components in the branch. For example, when the button SB21 is pressed, measure the resistance between points 2 and 3 in the circuit; under normal conditions, the resistance should be 0Ω. If the measured resistance is infinite, it indicates that SB1 has an open circuit fault, as shown in diagram (b).

3. Checking the Control Circuit

Generally, the multimeter’s two probes are connected to the starting point of the control circuit at FU2’s U11, V11 points (or FU2’s output points 0, 1), pressing buttons, contactor position switches, and other components to simulate the operation of control elements. Based on the continuity of each branch, the controlled contactor coils and relay coils will form parallel or open circuits, allowing us to judge the normality of the circuit based on the resistance values indicated by the multimeter.

We can measure the functions of buttons, the self-locking function of contactors, the interlocking function of contactors, and the main circuit to do so, connecting the multimeter’s two probes to the starting point of the control circuit at FU2’s U11, V11 points. If the multimeter reading indicates ∞ (if the resistance is 0Ω, the circuit has a short circuit; if the resistance is 2000Ω or 1000Ω, there may be a misconnection of the self-locking contacts or start button).

4. Checking the Main Circuit

This is generally done after checking the control circuit, primarily to check for short circuits in the main circuit. When checking the main circuit, since the DC resistance of each phase winding of the motor is small, usually below 10Ω, the resistance setting should be set to ×1Ω. After connecting to the motor, press the contactor’s working order to simulate the contactor’s operation while measuring the resistance between the output points U11, V11, W11 of the main switch. The resistance should be equal and correspond to the resistance between any two power supply leads of the motor. If a resistance of zero is detected, it indicates a short circuit in the main circuit; if a large resistance or ∞ is detected, it indicates poor contact or an open circuit in the main circuit.

After measurement, if the resistance values conform to the above rules, the circuit wiring is basically correct with no serious faults (short circuits), and the success rate of powering on is very high. Additionally, during the measurement process, the level of analysis and judgment of the circuit is also improved.

Select the appropriate range for the multimeter; if the range is too large, the reading will be too small, misjudging as a short circuit; if the range is too small, the reading will be very large, misjudging as an open circuit, which can severely affect measurement accuracy. Generally, select ×10Ω or ×100Ω.

Before powering on for a trial run, the voltage measurement method should be used to check whether the output voltage of each fuse is normal; if not, find the cause. When the control circuit can function normally, it is essential to measure whether the voltage at the output terminals connected to the motor is normal to avoid phase loss when powering the motor.

1. Basic Principle of Voltage Measurement Method

The voltage measurement method involves using the multimeter to detect the working voltage in the circuit and comparing the measurement results with normal values to determine whether the circuit is functioning correctly.

When the circuit is functioning normally, the working voltage at various points in the circuit has a relatively stable normal value or a dynamic range of variation. If a short circuit fault, open circuit fault, or change in the performance parameters of components occurs in the circuit, the working voltage in that circuit will also change. Therefore, the voltage measurement method can detect whether the working voltage at certain key points in the circuit is present or absent, too high or too low, and whether the dynamic changes are normal. Based on different fault phenomena and the working principles of the circuit, we can analyze and identify the cause of the fault.

2. Basic Methods of Voltage Measurement

The power supply is a necessary condition for the normal operation of the circuit. Therefore, when a fault occurs in the circuit, the power supply section should be checked first. If the power supply voltage is abnormal, focus on checking whether there are open circuit or short circuit faults in the power supply circuit and load circuit. Generally, if there is an open circuit fault in the power supply section, such as a blown fuse, the power supply will have no voltage output; if the load has a short circuit fault, the power supply voltage will decrease.

When checking the power drive control circuit, set the multimeter to the 500V AC voltage range. The voltage measurement method can be divided into phased measurement and segmented measurement, as shown in the diagram below.

In the phased measurement method, first measure the voltage between points 1 and 0 with the multimeter; if the voltage is normal, it should be 380V. Then, while holding the start button SB1, connect the black probe to point 0, and sequentially connect the red probe to points 2, 3, 4, and 5, measuring the voltage between each stage. In a normal circuit, the voltage at each stage should be 380V.

In the segmented measurement method, the red and black probes are used to measure the voltage between adjacent labeled points (for example, 1-2, 2-3, 3-4, 4-5, 5-0). If the circuit is normal, except for the voltage between points 5-6 being equal to 380V, the voltage between all other adjacent points should be zero. If pressing SB3 causes the contactor KM not to engage, it indicates a fault in the circuit. At this time, you can use the voltage setting on the multimeter to sequentially test the voltage between adjacent points. If a voltage of 380V is measured between certain adjacent points, it indicates that the contacts and connecting wires between those points are either poorly connected or open. For example, if a voltage of 380V or a certain voltage value is measured between points 4 and 5, it indicates that the contacts of contactor KM are poorly connected or not conducting.

Examples of applying the voltage measurement method are shown in the table below:

(1) When using the voltage measurement method to test the circuit, it is essential to understand the conditions of the circuit being tested and the voltage range, then appropriately select the multimeter’s range according to the actual situation to avoid damaging the multimeter.

(2) Before measurement, distinguish whether the measured voltage is AC or DC, ensuring that the multimeter’s red probe is connected to the higher potential test point and the black probe to the lower potential test point to prevent damage to the multimeter due to reverse polarity.

(3) When using the voltage measurement method, be careful to prevent electric shock. During the measurement process, the body should not touch the metal parts of the probes. In general, it is advisable to fix the black probe and use one hand to hold the red probe for measurement.