1. Common fault diagnosis and treatment of single-phase motor
The reference address of this article: http://www.eepw.com.cn/article/201808/385227.htm
1. The power supply voltage is normal, and the motor does not start after power-on
1) The power wiring is open circuit (the motor is completely silent). There should be no voltage across the measurement terminals.
2) The main winding or the auxiliary winding is disconnected. The open circuit can be determined by measuring the DC resistance.
3) The contact of the centrifugal switch is not closed, so that the auxiliary winding can not be energized to work. Disconnect the connection point between the main winding and the auxiliary winding, and then use the method of measuring the DC resistance to determine, or use the method of the second part to determine.
4) The starting capacitor wiring is open or internally disconnected. The search method is the same as the above item 3).
5) For the shaded-pole motor, the shaded-pole coil (short-circuit ring) is open or falls off. For the short-circuit ring that can be seen from the outside, it can often be found by observation, otherwise it can be determined by the method of the second part.
6) For series-excited motors, the brushes cannot be connected to the commutator without the brushes or because the brushes are too short or stuck, or the lead wires of the brushes are disconnected, or the armature windings and the magnetic field windings are open-circuited.
2. The power supply voltage is normal. After the power is turned on, the motor rotates at a low speed, there is a "humming" sound and a sense of vibration, and the current does not drop.
1) The load is too heavy.
2) The stator and rotor of the motor rub against each other. An unusual rubbing noise will be emitted.
3) The bearing is stuck due to poor assembly of the bearing, consolidation of grease in the bearing, damage to the bearing roller bracket or roller, etc.
4) For series-excited motors, short circuit between commutator segments or internal short circuit of armature winding, or too much deviation of brush from the center line (for motor with movable brush).
3. After the power is turned on, the power fuse will blow quickly
1) Serious short circuit between winding turns or to ground. Measure the DC resistance, if the value is much smaller than the normal value, it is a short circuit between turns of the winding; a serious short circuit to ground can be determined by measuring with an insulation resistance meter or a higher resistance range of a multimeter (such as R×1k range). The current will be greater than the rated value.
2) The motor lead-out phase line is grounded. The inspection method is the same as item 1).
3) The capacitor is short-circuited. Determine by measuring the DC resistance between the two ends of the starting winding circuit (including capacitor and starting winding, excluding centrifugal switch) with the lower resistance range of the multimeter (for example, R×1 range).
4) The centrifugal switch is short-circuited to ground. The inspection method is the same as item 1).
5) The load is too heavy. The sound will be abnormal and the current will be greater than the rated value.
4. After the motor starts, the speed is lower than the normal value
1) The main winding has a short-circuit fault between turns or to ground. The inspection method is the same as item 1) in 3.
2) There is a coil reverse connection fault in the main winding. The sound will be abnormal and the current will be greater than the rated value.
3) The centrifugal switch is not disconnected, so that the auxiliary winding cannot be disconnected from the power supply. The current will be greater than the rated value.
4) The load is heavy or the bearing is damaged. The sound will be abnormal and the current will be greater than the rated value.
5) For series excitation motors, short circuit between commutator segments or internal short circuit of armature winding, or poor contact between brush and commutator.
5. When the motor is running, it quickly heats up
1) The winding (including the main winding and the auxiliary winding) is short-circuited between turns or to ground. The inspection method is the same as item 1) in 3.
2) There is a short-circuit fault between the main winding and the auxiliary winding (outside the end connection point). The current will be greater than the rated value.
3) After starting, the centrifugal switch is not disconnected, so that the auxiliary winding cannot be disconnected from the power supply. The current will be greater than the rated value.
4) For motors that mainly or only rely on the main windings during operation (other single-phase split-phase motors except the single-value capacitor motors that start and run with the same capacitance of both windings), the main windings and auxiliary windings are connected wrongly. The current will be much greater than the rated value.
5) The working capacitor is damaged or the wrong capacity is used.
6) The stator and rotor cores are rubbing against each other or the bearing is damaged. The sound will be abnormal and the current will be greater than the rated value.
7) Heavy load. The current will be greater than the rated value.
8) For series excitation motors, short circuit between commutator segments or internal short circuit of armature winding, or poor contact between brush and commutator.
6. Motor running noise and vibration are large
Compared with three-phase asynchronous motors of the same capacity or the same frame size, the noise and vibration (especially vibration) of single-phase motors are relatively large. This is because its stator rotating magnetic field is not a regular circle, so the torque will not be equal at all times, that is to say, there will be fluctuations in size within a circle, resulting in radial vibration of the rotor.
Common causes of high noise and vibration are as follows:
1) Poor dipping paint, resulting in looseness between the core pieces, resulting in higher frequency electromagnetic noise.
2) The centrifugal switch is damaged.
3) The bearing is damaged or the axial movement is too large.
4) Uneven air gap or axial dislocation between stator and rotor.
5) There is a foreign body inside the motor.
6) For series excitation motor, short circuit between commutator segments or internal short circuit of armature winding, or poor contact between brush and commutator (mica between commutator segments is higher than commutator segment or commutator segment is rough, or brush is too hard , excessive pressure, etc.).
2. The method of determining that the motor does not start due to the open circuit of the auxiliary winding or the damage of the capacitor
The single-phase capacitor starts and runs. After the motor is connected to the power supply, it does not start and there is almost no sound. If measured with an ammeter, there is a certain current. At this time, use the resistance (R×1) file of the multimeter to check whether the auxiliary winding circuit is blocked. The reason for the failure is that the winding or wiring is disconnected, or the capacitor is broken and damaged.
In the field without a multimeter, the following simple method can be used to check whether there is an open circuit fault in the auxiliary winding or capacitor.
In the case of power failure, use a wire or other conductive tools (such as screwdrivers) to short-circuit the two electrodes of the capacitor to discharge, so as to prevent the stored charge from being stored in the capacitor without damage, so that the human body will get an electric shock (if there is any damage at this time). Strong discharge phenomenon can rule out the problem of capacitor damage). After that, disconnect the wire between the capacitor and the motor and wrap it with insulating material.
Remove the load of the motor (for example, remove the drive belt. For the load that requires a small starting torque, if it is difficult to remove the load, it may not be removed), then energize the motor (pay attention to the insulation work), use your hand (or tool) to twist the shaft in order to make it rotate in one direction, as shown in the figure below. If the rotor of the motor rotates at this time, it will automatically accelerate until it reaches the normal speed. After the power is turned off and stopped, rotate the motor shaft extension in the opposite direction. If the motor rotor also rotates with the same trend, it can basically be determined that the auxiliary winding or the capacitor does not start due to the open circuit. Then further check whether the capacitor or the winding (including the wiring) has an open circuit fault.
Third, the simple method of judging the quality of capacitors
When checking the used capacitor, the two poles of the capacitor should be connected and discharged with a wire (or other metal), so as to avoid electric shock damage to the test personnel due to the electric charge stored in it.
1. Use a multimeter to check the quality of the capacitor
When it is suspected whether a capacitor is damaged or has quality problems, an analog multimeter can be used to make a rough judgment. Please refer to the image below.
Set the multimeter to the R×1k (or R×100) block in the resistance column. Touch the two electrodes of the capacitor under test with two test leads respectively. Watch the reaction of the hands and determine the quality status of the capacitor according to the reaction.
1) The pointer quickly swings to zero (0Ω) or close to zero, then slowly goes back (to the ∞Ω side), and stops when it reaches a certain place. This shows that the capacitor is basically intact. The closer the return stop position is to the ∞Ω point, the better the quality of the capacitor. The farther it is, the more leakage it is.
This is because the principle of measuring resistance by the multimeter is actually to add a fixed value of DC voltage (provided by the battery installed in the meter) to the conductor under test. At this time, there will be a corresponding current. Using the relationship of Ohm's law, the This current is converted into a resistance value on a scale on the dial. For example, when the voltage is 9V, the current is 0.03A, the resistance of the conductor is 9V/0.03A=300Ω, and the scale at the 0.03A position on the dial is 300Ω.
For a good capacitor, when a DC voltage is just applied to its two ends, it starts to charge, and the current will reach the maximum value instantaneously. For the resistance of the multimeter resistance gear, it is close to 0Ω. As the charging process progresses, the current will also will gradually decrease. In theory, the two plates of the capacitor should be completely insulated, so the final result of the above charging process should be that the current reaches zero, reflected on the resistance, and finally should return to the ∞Ω point (that is, where the current equals zero). But in fact, all the capacitor plates are not completely insulated, so there will be a small current under the applied voltage, which is called the "leakage current" of the capacitor, which means that the pointer cannot completely return to the ∞Ω point. reason. How much the multimeter needle returns indicates the magnitude of the leakage current. If the needle returns more, the leakage current is small, and if it returns less, the leakage current is large. The leakage current should not be too large, otherwise it will cause some abnormal phenomena in the circuit, and it will not work normally in severe cases. When the leakage current is large, the capacitor will be much hotter than normal.
2) The pointer quickly swings to the zero position (0Ω) or close to the zero position and then does not move, indicating that a short-circuit fault has occurred between the two plates of the capacitor, and the capacitor can no longer be used.
3) When the two electrodes of the test lead and the capacitor start to be connected, the pointer does not move at all, indicating that the internal connection of the capacitor has been disconnected (generally occurs at the connection between the electrode and the plate), and naturally cannot be used again.
2. Use the charging and discharging method to judge the quality of the capacitor
When you do not have a multimeter at hand, you can roughly check the quality of the capacitor by charging and discharging. The power supply used is generally direct current (especially electrolytic capacitors and other polar capacitors, must use direct current power supply), the voltage should not exceed the withstand voltage value of the tested capacitor (marked on the capacitor), commonly used 3 ~ 6V dry battery Or 24V, 48V batteries for electric bicycles and automobiles. For capacitors connected to the AC circuit during operation, AC power can also be used, but when the voltage is high, safety should be paid during operation, and insulating gloves or insulating tools should be worn.
After the DC power supply is connected to both ends of the capacitor, wait a short time before disconnecting the power supply. Then, use a piece of wire, one end is connected to one pole of the capacitor, and the other end is connected to the other electrode of the capacitor, and at the same time observe whether there is a discharge spark between the electrode and the wire. As shown below.
If there is a larger discharge spark and a crackling discharge sound, it means that it is good, and the larger the spark has a larger capacitance (for the capacitor of the same specification, when using the same power supply to charge); the discharge spark and the discharge sound are small , indicating that the quality is not very good; if there is no discharge spark, it means that it is bad.
