How to use a multimeter to measure motor speed and number of poles
If the motor does not have a nameplate, there is no tachometer, in the case of not disassembling the motor, the multimeter can be used to determine the speed of the motor.
With the multimeter of the smallest milliamp file were connected to the above has been judged out of a winding of the first end and the end, the rotor will slowly rotate a circle at uniform speed, look at the multimeter pointer swing a few times, if swinging once, indicating that the current is positive, negative changes in a cycle, it can be judged that it is a 2-pole motor. For the same reason, it is a 4-pole motor by swinging 2 times, and a 6-pole motor by swinging 3 times, and so on.
Judging the number of poles of the motor, you can know its approximate speed (slightly lower than the synchronous speed). Synchronous speed of the motor and the number of poles of the relationship between the power supply frequency of 50Hz can be projected in this way: two extremely 3000r/min, four extremely 1500r/min, six extremely 1000r/min.
In operation, the multimeter pen and the terminal to maintain good contact. Otherwise, the process of rotating the rotor needle will swing, judgment is not the result.
How to use the multimeter to measure the cable break point
When the cable or the cable's internal broken line fault, due to the external insulation skin package, so that the exact location of the broken line is not easy to determine. With a digital multimeter can be easily fixed this problem.
Specific method: connect one end of the wire (cable) with a break point to the 220V mains power line, the other end is suspended. The digital multimeter pulled to AC2V block, from the wire (cable) of the firewire access to the end of the beginning, with a hand pinch the tip of the black pen, the other hand will be the red pen along the insulating skin of the wire to move slowly, this time the display shows the value of the voltage is about 0.445V or so. When the red pen moved to a certain place, the display shows the voltage suddenly dropped to 0.0 volts (about one-tenth of the original voltage), from the position forward (firewire access) of about 15cm is the wire (cable) where the breakpoint.
With this method of checking the shielded wire, if only the core wire broken and the shield is not broken, then this method is powerless.
This method can also be used to find faulty electric blankets and other resistance wire break point.
