Is it possible to measure the quality of the motor with a multimeter?
The multimeter can measure AC voltage, DC voltage, AC current, DC current, resistance and capacitance, buzzer, diode, etc., and also has intelligent anti-burning protection design, automatic shutdown, etc. It can be said that the function is quite powerful. For an ordinary electrician, it is completely enough. The specific range parameter table is available, so I won't go into details.
So can a multimeter be used to measure the quality of the motor? And how to measure it?
I think this has to be looked at in two ways. If it is a short circuit between coils, a short circuit between turns, or an open circuit, these can be measured with a multimeter. If it is a short circuit to the ground or leakage to the ground, the insulation resistance value in this regard should use a megohmmeter (insulation resistance meter, shaking meter).
Let's briefly talk about it, use a multimeter to measure the short circuit between the motor coil and the coil, as well as the short circuit and open circuit between the coil turns.
Motors come in big and small sizes, but all three-phase motors are made up of three windings. To judge whether the motor is faulty, it is essential to measure the resistance of the three-phase winding. The resistance value of the three-phase winding varies according to the power of the motor, but the difference between their resistance values will not be very large, generally between 2 and 5 ohms.
Usually three-phase motors have 6 terminals, but for some with smaller power, the manufacturer has formed a star or angle type inside the motor, and there are only three terminals in the junction box. The detection method of this kind of motor is the same as the detection method of the 6 terminals, except that the coil is short-circuited between turns, and ordinary electricians cannot detect it.
Before we measure, we must first disconnect the power supply and remove the connecting piece. Assume that the top three lines are the beginning and the bottom three lines are the end. The letters at the first and last ends indicate that typing is inconvenient, so I named the first ends 1, 3, 5, and the tail ends 2, 4, 6.
Because we don't know the resistance value between the three-phase windings, we first turn the multimeter to a certain gear of the electric barrier, and finally find a suitable gear, and then make an accurate comparison.
After finding the appropriate gear, measure the resistance value between the three-phase windings at the head end respectively. If the resistance value difference between the three-phase windings is not large and within a reasonable range, we consider the resistance value between the three-phase windings is normal. Don't worry, we can measure again at the end and make a comparison, so that nothing will go wrong.
The problems between the three-phase windings are relatively easy to solve. The inter-turn fault judgment of the coil is relatively troublesome.
Let's talk about inter-turn short circuit first. The phenomenon of inter-turn short circuit is local heating of the motor and unbalanced three-phase current. Measured with a clamp meter, the constant current of the short circuit is much larger. Use a multimeter to measure the resistance of the first and last ends of the three-phase coils, and the short-circuited one-phase is much smaller.
If there is an open circuit between turns, the resistance at the beginning and end of the coil winding should be infinite or very large.
If the coil is short-circuited to ground or leaks electricity, you need to use a megohmmeter. Because the voltage level of the multimeter is too low, there is no way to measure accurate data.
To measure low-voltage equipment or lines, we can choose a 500 megohmmeter.
The megohmmeter is also called a shaking meter because it is a hand-operated DC generator.
The megohmmeter is also called the insulation resistance meter, which is specially used to measure the insulation resistance above the megohm level.
The usual industry standard is that the insulation resistance to ground is normal if it reaches above 0.5 megohm level.
The operation is very simple. We can measure the insulation resistance meter by leveling it and adjusting it.
Connect the L terminal of the megohmmeter to the motor winding, connect the E terminal to the motor casing, and measure the resistance of the 6 wire ends of the motor to the motor casing. Europe is regarded as a leakage, and the resistance is small, it is a short circuit.
Some friends think that it is possible to use a megohmmeter to measure the insulation resistance of the motor coil or between turns. I personally think it is inappropriate, because the voltage level of the megohmmeter exceeds the withstand voltage level of the coil, and it may damage the insulation resistance of the coil. breakdown.
The measurement of single-phase motors also uses the above method. The purpose is also to judge whether there is a fault between the coils, whether there is an open circuit and a short circuit inside the coil, and whether there is a short circuit and leakage to the ground.
