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How to use a multimeter to determine the speed of a motor and distinguish the beginning and end of t
Release Time：2023-02-22

At the debugging site, we often encounter motors without nameplates, but we really want to know the speed of this motor. There is no tachometer, and we don't want to bother dismantling the motor. At this point, you can use the multimeter in your hand to solve this problem.

We know that as long as we know the number of poles of the motor, we can know the approximate speed of the motor. The judgment method is as follows:

1. Firstly, disconnect the connecting wires and short connectors of the six heads of the motor, and use the ohm range of the multimeter to find any set of windings.

2. Set the multimeter to a smaller milliampere range and connect it to both ends of this winding.

3. Then, slowly and evenly rotate the rotor of the motor once, and observe how many times the pointer of the multimeter swings left and right. If it swings once, it indicates that the current changes positive and negative for one cycle, which is a bipolar motor. For the same reason, swinging twice is a four pole motor, and swinging three times is a six pole motor. By analogy, the milliampere gear of the multimeter can be used to rotate the motor once and the pointer can swing several times to determine which pole the motor is and determine its approximate speed (i.e. slightly lower than the synchronous speed).

We know the relationship between the synchronous speed of an electric motor and the number of magnetic poles. When the power frequency is 50HZ, the second pole is 3000 rpm, the fourth pole is 1500 rpm, and the sixth pole is 1000 rpm. The formula is: N=6000/P (at a power frequency of 50HZ, N is the synchronous speed, and P is the number of poles of the motor).

Distinguishing the winding ends of AC motors

You can use a multimeter to first distinguish six wire terminals, divide the two connected wire terminals into one phase, and then divide the six wire terminals into three phases. Then, determine the beginning and end of the winding. The specific method is to set the DC milliampere range of the multimeter to a smaller range, and connect the probe to one end of the three-phase winding group, while the positive and negative poles of the battery are connected to the two wire terminals of the other phase. When the switch is closed, if the needle swing is greater than zero, it indicates that the wire head connected to the negative electrode of the battery and the wire head connected to the multimeter probe have the same name (both can be considered heads). By analogy, the heads and tails of the other two phases can be measured.

You can use a multimeter inspection method or winding series inspection method; For example, using the winding series inspection method, connect any two phase winding in series with a light bulb, and then connect 220V AC voltage to the three-phase winding [36v for large and medium-sized motors]. If the light bulb is lit, it indicates that the two phase winding is correct. If the light bulb does not light, it indicates that the two phases are incorrect. You can reverse the beginning and end of one of the stages and try again. After determining the two ends, you can follow this method to find the di three-phase.

For example, using a multimeter to check, assuming the windings are D1-D2, D3-D4, D5-D6, D1-D2, place a dry battery in the winding, connect D1 to the negative pole, D2 to the positive pole, connect a switch between D2 and the positive pole of the battery, connect D5 to the negative pole of the multimeter, D6 to the positive pole of the multimeter, and set the multimeter to the milliampere position. When the switch is turned on, if the pointer of the multimeter swings to the side greater than 0, the positive electrode of the battery and the positive electrode of the multimeter are at the same head or tail; If the pointer is inverted, it indicates that the terminal connected to the positive terminal of the battery and the terminal connected to the negative terminal of the multimeter are of the same head or tail. Then connect the battery and multimeter to the two terminals of the other phase for testing to determine the beginning and end of each phase.

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