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The motor is the most common driving power device in our production and daily life. For example, common toys, fans, water pumps, etc., all have motors. Apart from those that are obviously burnt out, sometimes the motor’s exterior does not show visible damage. How can we determine if these non-functioning motors are simply due to a broken wire or if the internal winding is burnt? A simple check can be done using a multimeter that costs just a few hundred yuan. How to easily check the condition of a motor with a multimeter? Here are some methods; I can’t claim they are exhaustive, but knowing some is certainly better than nothing!
Single-Phase Motor
One probe is grounded, and the other is used to measure the motor’s starting, running, and common windings. The resistance should be infinite or close to infinite. No resistance or very low resistance indicates the motor is faulty. Then measure the resistance of the three windings separately; the starting winding should equal the sum of the running and common winding resistances.
Three-Phase Motor
The measurement method is the same as for single-phase motors, except that the resistance of the three windings in a three-phase motor should be the same.
When measuring with a multimeter:
1. The inter-phase resistance should be uniform; it doesn’t matter how large the resistance is, just that it is uniform. For motors greater than 15KW, you might measure 0 ohms no matter how you measure it, which is because the multimeter’s range is too large; you need to use a micro-ohmmeter.
2. The insulation between phase wires and the motor casing should be greater than 1M ohm.
How to Determine the Condition of a Three-Phase Asynchronous Motor?
1) Measure the current with a multimeter; the three-phase imbalance rate should not exceed 10%;
2) Use an insulation resistance tester; each phase to ground and inter-phase should be no less than 0.5 megohms;
3) Use a bridge to measure direct current resistance; the three-phase imbalance rate should not exceed 2%;
In addition to the above methods, to check if the windings are normal, connect a small current range (like 50 microamps) of the multimeter to any two lead wires, then forcefully rotate the motor. The multimeter’s needle should visibly swing (related to the speed of rotation). This is a simple method to check the windings using the motor’s remanence. If the windings are burnt, the needle will definitely not move.
A multimeter can determine inter-phase short circuits, grounding, and open circuits, but it is not good at determining inter-turn short circuits.
Measuring with just a multimeter may not be very accurate; it is best to use a megohmmeter to measure the insulation resistance of each phase, then use a DC bridge to measure the direct current resistance between the three windings to see if they are balanced and whether the values are too high or too low, for absolute accuracy. The resistance of the three windings should be equal, and the insulation resistance between phases and between phases and the casing should be greater than 1 megohm.
How to Determine the Condition of a Single-Phase Motor?
The direct current resistance of the starting winding of a single-phase motor is generally greater than that of the running winding. The simplest method to determine it is:
1). First, use a multimeter to measure the direct current resistance from the common terminal to the running winding terminal and the starting winding terminal.
2). Then use the multimeter to measure the direct current resistance from the running winding terminal to the starting winding terminal.
3). If the arithmetic sum of the two measurements in “1” does not equal the measurement in “2”, then the motor is definitely burnt out!
If they are equal, it is best to compare with a motor of the same model or find the factory parameters of the motor for comparison to determine the motor’s condition. When measuring a single-phase motor, the capacitor should be disconnected. The short circuit of a single-phase motor requires a reference value of resistance under normal conditions.
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