How to use a multimeter to quickly and easily detect whether a motor is good or bad
Using a multimeter to measure the quality of a motor is a commonly used method in our work. There are two main methods for measuring the quality of a motor: resistance measurement and current measurement. Below, I will talk about how to use a multimeter to quickly and easily detect the quality of a motor.
The most common method I use to check with a multimeter is to disconnect the connecting wires between the three-phase windings before checking, so that none of the windings of the three-phase motor are connected to each other. Then, set the multimeter to the RX10K ohm gear, connect one probe of the multimeter to one end of the winding, and connect the other probe to the motor casing. At this point, we can check the resistance value indicated by the multimeter. If the resistance value indicated by the multimeter is very small, even zero, it indicates that there is a grounding fault between the phase winding of the motor and the motor casing. If the measured resistance value is very large, it indicates that there is no grounding fault.
We can also use a multimeter to measure the resistance of three independent windings separately. First, we need to determine the normal resistance values of the three windings. If the resistance values of the three windings are the same during measurement, it means that the windings are normal. If there is a difference in the resistance values of the three independent windings, it means that the motor is not normal. Then, a megohmmeter can be used to measure the insulation resistance between the three windings of the motor. The resistance value between them is generally between 0.5 megohms and 1 megohm, which should be considered normal. Finally, use a shaking table to measure the insulation of the three windings to the casing (ground) separately. The normal resistance value should be around 0.5 megaohms to 1 megaohm, which is considered normal.
Another method of measuring with a multimeter is to use the current range of the multimeter, but it can only be used on small three-phase motors, such as three-phase asynchronous motors below 4KW, which have a rated current of 8.8 amperes during operation. And the maximum current measurement range of our general multimeter is 10A. If we want to measure motors with high power, we can use a dedicated clamp ammeter for measurement. We mainly use this method to measure whether the three-phase current of the motor is balanced. If the current is unbalanced, it indicates that the motor is also faulty.
In fact, in many cases, we do not need a multimeter to determine the basic condition of the motor. For example, by "looking", "smelling", and "touching", we can roughly judge the basic fault of the motor, because when the motor malfunctions, we can see it from its operation. For example, the motor's speed suddenly slows down, there is abnormal noise, the surface temperature of the motor is too high, and the motor's shell is charged, all of which are caused by different faults of the motor.
