Multimeter - Measuring Tips You Didn't Know
1. Use a multimeter to identify the beginning and end of the motor winding
When the labels of the six outlets of the three-phase asynchronous motor are lost or unclear or after the winding is rewound, it is necessary to find out which two outlets belong to the same phase, which is the beginning of the coil, and which is the end of the coil. Here are several methods of operation.
Check method one
Use a multimeter to electrically block and distinguish the two wire ends of each phase of the three-phase winding, and make an assumption number.
Observe the direction in which the pointer of the multimeter (microampere gear) swings. When the switch is turned on, if the pointer swings to the side greater than 0, the wire connected to the positive pole of the battery and the wire connected to the negative pole of the multimeter are the same end or tail. If the pointer swings in the opposite direction, the wire end connected to the positive pole of the battery and the wire end connected to the positive pole of the multimeter are the same as the beginning or end.
Then connect the battery and the switch to the two wires of the other phase, and test it, and you can correctly determine the beginning and end of each phase.
Check method two
First, use a multimeter to electrically block and distinguish the two wire ends of each phase of the three-phase winding. Assume that each phase winding is numbered as U1, U2, V1, V2 and W1, W2. Connect the wires as shown in the figure below to judge the beginning and end. Turn the rotor of the motor by hand, if the pointer of the multimeter (microampere) does not move, it proves that the assumed number is correct. If the pointer deflects, it means that the number of the beginning and end of one of the phases is wrong, and it should be re-tested one by one until it is correct. until.
Check method three
First, use a multimeter to electrically block and distinguish the two wire ends of each phase of the three-phase winding, and make assumptions, and connect them as shown in the figure below. Connect any two-phase windings in series and then connect them to the AC voltage block of the multimeter, and connect the third-phase winding to the 36V low-voltage AC power supply.
After power on, if there is no reading on the voltmeter, it means that the two wire ends connected together are both the beginning or the end. The voltmeter has readings, and one of the two wire ends connected together is the head end and the other is the tail end. One end is determined as the known head end, and the first and last ends of the third phase can be determined in the same way.
2. Use a multimeter to judge the speed and number of poles of the motor
If the motor has no nameplate and tachometer, you can use a multimeter to judge the speed of the motor without disassembling the motor.
The method is: use the minimum milliampere gear of the multimeter to connect the head end and tail end of a certain winding that has been judged above, rotate the rotor slowly and at a constant speed, and watch the pointer of the multimeter swing several times. If it swings once, it means that the current is normal. , Negative change for one cycle, it can be judged that it is a 2-pole motor. For the same reason, it is judged to be a 4-pole motor by swinging twice, and it is judged to be a 6-pole motor by swinging three times, and so on.
By judging the number of poles of the motor, you can know its approximate speed (slightly lower than the synchronous speed). The relationship between the synchronous speed of the motor and the number of magnetic poles can basically be calculated as follows when the power frequency is 50Hz: the two poles are 3000r/min, the four poles are 1500r/min, and the six poles are 1000r/min.
During operation, the multimeter pen and the terminal should be kept in good contact. Otherwise, the hands of the watch will swing during the process of turning the rotor, and the result cannot be judged.
