There are many aspects to judging the quality of an electric motor. Here are a few simple points for your reference.
First, let's summarize how to determine the condition of a three-phase asynchronous motor coil and what instruments to use for inspection:
1. Multimeter: Used to check the continuity of motor coils.
2. Single-arm bridge: It can accurately measure the coil resistance and determine whether the resistance of each phase coil is close.
3. Megohmmeter: Used to measure the insulation resistance between phases and between phases and ground of a motor, not less than 0.5 megohms. A 500V megohmmeter reading greater than 0.5MΩ indicates the motor is good; a reading less than 0.5MΩ indicates the motor is faulty. However, this is only a visual assessment; for inter-turn short circuits or phase-to-phase breakdowns, disassembly and inspection are necessary.
Electric motor failures can be broadly categorized into two types: mechanical and electrical.
The electrical aspects mainly include:
1. To determine if the three-phase DC resistance is up to standard, use a double-arm bridge to measure it.
2. Whether the insulation resistance is up to standard.
3. Whether the rotor bars are broken, the DC resistance of the motor is an important basis for judging the motor.
The mechanical aspects mainly include:
1. Check if the end cap is "moving out of the outer sleeve" and the bearing is "moving out of the inner sleeve".
2. Check if the bearing is lacking oil or damaged.
How to test if a motor is good or bad?
1. Test with a megohmmeter: A 500V megohmmeter is sufficient. Test the insulation resistance between the wires on the three terminals and the motor casing. If the resistance is above 0.5M ohms, it indicates that there is no short circuit to ground.
2. Multimeter test: Measure the resistance between the three phases A, B, and C to see if they are equal. The larger the motor, the smaller the resistance! But all three phases cannot be 0 ohms, unless it is a particularly large motor.
3. Check the bearings and fan. If the motor is tangled, the entire motor needs to be replaced. Sometimes, a seized bearing can also burn out the motor.
4. The no-load current of a motor is generally 10% to 50% of the rated current, and sometimes the no-load current of the motor is still zero.
5. When the motor is running at its rated current, it is operating at full load, with its output power essentially at 100%. A lower operating current indicates that the motor's output power is reduced, meaning it is operating under a light load.
How to determine if a DC motor is good or bad?
First, check for broken wires and whether the resistance is normal. If it is a brushed DC motor, let the rotor rotate and use a multimeter to measure whether the output DC voltage is normal. If it is a brushless DC motor with three-phase leads, and the rotor can rotate, use a multimeter to measure whether the output AC voltage is normal. The output voltage is directly proportional to the speed.
How to test the condition of a single-phase AC motor?
1. The insulation resistance between the motor windings and the casing, measured with a 500V megohmmeter, should not be less than 0.5 megohms; the insulation resistance of each winding lead, measured with a multimeter, should be normal, with no broken wires. If all of the above requirements are met, the motor is performing well.
2. To test the quality of a capacitor, use an analog multimeter or a digital multimeter with a capacitance range; it can be measured directly.
3. Set the multimeter to the 1k or 10k resistance range and measure the two leads of the capacitor. If the pointer deflects quickly to the right and then slowly returns to the left, the capacitor is good; if it always deflects to the right, the capacitor is short-circuited; if the pointer does not move, the capacitor has an internal broken wire or no capacitance.
How to determine if an offline motor is good or bad?
1. Use a multimeter to measure the DC resistance of the three-phase windings of the motor. The closer the results of the three measurements are, the better. If the difference is more than 10%, the phase with the lower resistance may have an inter-turn short circuit.
2. Use a 500V insulation resistance megohmmeter to measure the phase-to-phase insulation resistance and the insulation resistance to ground (to the motor casing) of the three-phase windings. A motor with an insulation resistance of 0.5 megohms or higher after long-term storage is considered a good motor and can be used. A resistance between 0.1 megohms and 0.5 megohms indicates the motor has become damp and needs to be dried before use. Very low insulation resistance indicates damaged insulation.
