How to quickly and easily detect whether a motor is good or bad with a multimeter
Using a multimeter to simply measure the quality of a motor is a commonly used method in our daily work. There are two main methods for measuring the quality of a motor using a multimeter: resistance measurement and current measurement. Below, I will talk about how to use a multimeter to quickly and easily detect whether a motor is good or bad.
The most common method I use to check with a multimeter is to disconnect the connecting wires between the three-phase windings before the inspection, so that each winding of the three-phase motor is not connected to each other. Then, set the multimeter to the ohmic gear of RX10K, and 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 see the magnitude of the resistance indicated by the multimeter. If the resistance indicated by the multimeter is very small or even zero, it indicates that there is a grounding fault between this phase of the motor winding and the motor casing. If the measured resistance value is very large, it indicates that there is no ground fault.
We can also use a multimeter to measure three independent windings separately. Let's first figure out the normal resistance values of the three windings. If the resistance values of the three windings are the same during measurement, it indicates that the windings are normal. If there is a difference in the resistance values of the three independent windings, it indicates that the motor is abnormal. 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 megohm and 1 megohm, which should be a normal value. Finally, use a megger to measure the insulation of the three windings to the casing (ground) separately. The normal resistance value should be around 0.5 megohm to 1 megohm, which should be considered normal.
Another method of measuring with a multimeter is to use the current range of the multimeter, which can only be used on small three-phase motors, such as three-phase asynchronous motors below 4KW. Its rated current during operation is 8.8 amperes. However, the maximum current measurement value of our multimeter is 10A. If measuring high-power motors, we can use a dedicated clamp type ammeter to measure them. 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 can also use a multimeter to determine the basic condition of the motor. For example, we can roughly determine the basic fault of the motor by "looking", "smelling", and "touching", because when the motor malfunctions, we can see it from its operation, such as sudden slow down of the motor speed, abnormal noise, and high surface temperature of the motor The electrification of the motor casing is caused by different faults of the motor.
