Use and maintenance of DC motors.
1. Preparation and inspection before use
(1) Clean the dust, brush powder and dirt inside the motor.
(2) Check the insulation resistance of the motor. Normally, it should not be less than 0.5MΩ. If it is lower than 0.5MΩ, it needs to be dried before use.
(3) Check whether the surface of the commutator is smooth and clean. If mechanical damage or spark burn marks are found, the commutator should be maintained.
(4) Check whether the brush is worn too short, whether the pressure of the brush holder is appropriate, and whether the position of the brush holder meets the regulations. If the brush needs to be replaced if it does not meet the requirements, it should be replaced according to the original size and model.
2. Maintenance of DC motor in operation
For the DC motor in operation, it is necessary to carry out frequent maintenance in order to detect abnormal conditions in time, eliminate equipment defects, and ensure the long-term safe operation of the motor.
(1) During the operation of the motor, the temperature, vibration, sound and commutation of each part should be checked, and attention should be paid to whether there is overheating discoloration and the smell of burnt insulation.
(2) If it is a pressure oil circulation system, it should also check whether the oil pressure and the temperature of the incoming and outgoing oil meet the specified requirements. Generally, the inlet oil temperature should be less than 40℃, and the oil outlet temperature should be less than or equal to 65℃.
(3) Check the sound of each part with a listening stick, and pay attention to whether there are other friction sounds between the rotor and stator except for electromagnetic sound and ventilation sound. Check the bearing shell or bearing for abnormal sound.
(4) For the connection points and insulators of the main circuit, pay attention to whether there is any abnormal smell such as overheating and discoloration, and whether there is any abnormal smell such as burning of the insulation.
(5) For the closed cooling system, pay attention to the water temperature and air temperature, and also check whether the cooler has water leakage and condensation, and whether the supplementary air network is blocked or not.
(6) Always pay attention to the current and voltage values of the motor, and be careful not to overload. For the DC system with insulation inspection device, its insulation to ground should be checked regularly.
(7) Check whether the color of the oxide film on the surface of the commutator is normal, whether there is spark between the brush and the commutator, whether there is accumulation of carbon powder and grease on the surface of the commutator, and whether there is dust on the brush holder and brush holder.
(8) Check whether the brush edge is chipped and worn to the minimum allowable length.
(9) Check whether the brush braid is complete, whether there is broken or broken strands, whether the connection with the brush holder is good, and whether there is grounding and short circuit.
(10) Check whether any brushes or brush braids are discolored due to overheating, and whether the brushes are stuck or wobbled in the brush holder.
(11) Check whether the brush pressure between the brushes is uniform, and whether the pressure finger is pressed well.
(12) Check whether the commutator is unevenly worn, whether the unevenness has exceeded the allowable value, and whether there is a bulge of mica between sheets that causes the brush to vibrate.
Common faults and maintenance methods of DC motors
DC motor does not start
Possible cause of failure
(1) The motor is not connected to a suitable voltage or the power supply has no voltage.
(2) The excitation winding circuit is open circuit.
(3) The brush has poor contact.
(4) The current passed to the DC motor is too small at startup.
(5) It is difficult to start if the load is too heavy.
Maintenance methods and skills
(1) First, use a multimeter to measure whether the power supply has voltage, and restore the power supply voltage when there is no voltage. Check whether the fuse is blown, and whether the line wiring is open circuit, if so, connect the connecting line. Also check the action of the overload protection device. If it has acted, it should be reset after identifying the cause, and then restart the motor.
(2) Use a multimeter to electrically block the on-off of the excitation winding. If the joint is burnt, it should be reconnected; if the winding is burnt and open, the winding should be replaced. Finally, use a multimeter to check whether the excitation voltage is normal, find the cause from the distribution line, and repair it.
(3) Check the contact condition of the DC motor brush, smooth the contact surface with fine sandpaper, and adjust the brush pressure to make it appropriate.
(4) Check whether the circuit has poor contact, and check whether the starting equipment is matched with the DC motor.
(5) Excessive load is one of the important reasons why the DC motor cannot be started. First turn the motor coupling or drive pulley by hand to see if it can rotate flexibly. If the motor is overloaded or stuck, the reason should be found from the mechanical transmission and the mechanical equipment should be repaired; if the bearing of the motor itself is damaged and stuck, it is necessary to open the motor and replace the bearing of the same type.
After the DC motor starts, the speed is abnormal, the speed is too high or too low, and it is accompanied by violent sparks
Possible cause of failure
(1) The position of the brush is offset and not in the normal position.
(2) The circuit resistance of the magnetic field winding is too large.
(3) The armature and field windings have short-circuit or open-circuit points.
(4) The load of the series excitation motor is too light.
(5) The series excitation field winding is reversed.
Maintenance methods and skills
(1) Re-distribute the brush according to the original marked position or adjust the position of the brush by induction method.
(2) Check whether all the joints of the magnetic field winding circuit have an oxide layer to prevent poor contact. If there is an oxide layer, it should be smoothed with sandpaper. Check the magnetic field winding with a multimeter. When the resistance is too large, the motor should be turned on to find the cause, and the voltage value of the actual magnetic field should be measured. If it is not correct, the cause should be found from the distribution line.
(3) Turn on the DC motor, separate the wiring connectors, measure the resistance of the magnetic field windings respectively, find the short-circuit point of each group of coils, and replace or repair the coils locally.
(4) When the speed of the series-excited motor is abnormal under light load, and the motor circuit is no problem, the load of the motor should be appropriately increased.
(5) Check the wiring of the series excitation field winding, and rewire it according to the correct method.
DC motor oscillates during operation
Possible cause of failure
(1) The voltage of the DC motor power supply fluctuates.
(2) The motor brush is not on the neutral wire.
(3) The excitation current is too small or the excitation circuit has a short-circuit point.
(4) The series excitation winding or the commutation pole winding is reversed.
(5) The mechanical load fluctuates too much during rotation.
Maintenance methods and skills
(1) First disconnect the motor armature voltage, connect the load, and find the cause of the armature voltage fluctuation. Find the cause from the power distribution equipment and deal with it.
(2) Reassemble the brush according to the original marked position, or adjust the position of the brush by induction method.
(3) Appropriately increase the excitation current or disconnect the power supply, and check whether there is a short-circuit point or open circuit in the excitation circuit. When the circuit is short-circuited seriously, the coil should be replaced.
(4) Reconnect the commutating pole windings according to the wiring diagram on the nameplate of the DC motor.
(5) Check the mechanical wear and tear, whether there is instantaneous overload, and replace the parts with serious mechanical wear.
The brush spark is too large during the operation of the DC motor
Possible cause of failure
(1) There is poor contact between the brush and the commutator.
(2) The brush and the brush holder are too tight and get stuck inside.
(3) The brushes are worn out excessively or the size and performance of the newly replaced brushes are inconsistent with the original ones.
(4) uneven distribution of brushes and uneven current distribution between brushes.
(5) The commutation pole winding is reversely connected or there is a short circuit point.
(6) Desoldering of armature winding and commutator.
(7) The brush position is not on the neutral line or the brush holder moves loosely.
(8) Inappropriate or uneven brush pressure.
(9) The commutator is dirty, the contact resistance is too large, or the insulator between the commutator segments protrudes.
(10) The mechanical load is too heavy.
Maintenance methods and skills
(1) Grind the contact surface of the brush with fine sandpaper, and wipe the contact between the brush and the commutator with a clean cloth, install the brush, and apply the voltage to make the DC motor idle for about 1h at low speed and light load.
(2) Use fine sandpaper to slightly grind the small brush, so that the brush can move up and down in the brush holder freely.
(3) Replace the new brushes with qualified quality according to the original model and size of the DC motor.
(4) Readjust the distribution position of the brushes.
(5) Reconnect the wiring of the commutating pole winding in the correct direction, and shake its insulation with a 500V megohmmeter. The coil should be replaced when the coil is short-circuited seriously.
(6) Turn on the DC motor, carefully find the de-soldering point between the armature winding and the commutator, and re-weld.
(7) Adjust the brush holder to the original position or find out the neutral line position of the brush by induction method. For example, when the motor is powered off, connect the millivolt meter to the brushes of the adjacent two groups, connect a switch in series with the excitation winding, and connect the 3V battery. When the switch is turned on and off, the pointer of the millivolt meter swings left and right, and this is repeated continuously. Turn the switch on and off, and move the brush holder position back and forth until the pointer swings to a minimum, and the brush position is just on the neutral line, see Figure 9-68.
The method for finding the position of the neutral line of the DC motor brush
(8) Find a spring balance and correct the brush pressure so that it is better at 150-250gf/cm (1gf/cm2=98.0665MPa).
(9) Scrub the dirty part of the commutator and remove some mica insulation properly.
(10) Check the reason for the overload of the load. It is caused by the motor bearing to replace the bearing. It is caused by the mechanical load and the mechanical fault point should be repaired. When the motor capacity is not suitable, a DC motor with large capacity should be replaced.
DC motor power distribution cabinet emits smoke
Possible cause of failure
(1) The motor overload time is long.
(2) The commutator or armature of the motor is short-circuited.
(3) The motor starts directly at high speed or runs frequently in forward and reverse directions.
(4) The motor terminal voltage is too low.
(5) The motor bearing is damaged.
(6) There is rust and debris in the stator and rotor rubbing against each other.
Maintenance methods and skills
(1) Immediately disconnect the motor power supply, find the cause of the failure of the mechanical load, and start the motor when the load is normal.
(2) Carefully observe whether there are foreign objects or metal chips falling into the commutator, and clean it if any. Then use a millivolt meter to check the short circuit of the armature winding. If the short circuit is slightly repairable, it should be repaired, and finally add insulating paint; if it cannot be repaired, the winding should be rewound.
(3) Use the matching power distribution cabinet, and adjust the start-up operation method to prolong the forward and reverse conversion time.
(4) Check the power distribution equipment, and use a multimeter to measure whether the motor armature voltage is too low. If it is too low, it should be adjusted to the normal value.
(5) Turn on the DC motor and check the damage of the motor bearing. If it is damaged, replace the bearing of the same type with better quality.
(6) Open the motor and pull out the rotor, remove rust and other debris, and check whether the air gap of the motor is uniform. If the air gap is not uniform, the bearing should be replaced.
Every other piece of the DC motor commutator is charred and blackened. After cleaning, the motor is still black.
Possible cause of failure
(1) There are de-soldering and open circuit faults between the scorched and blackened commutator segment voltage equalizing line and the armature winding connection line.
(2) The armature windings connected to these blackened commutator segments have short-circuit points.
Maintenance methods and skills
(1) Re-solder the open-circuit equalizing line, check whether there is a de-soldering between the armature winding and the armature winding, and re-solder if so.
(2) Turn on the motor, check the short circuit of the armature winding and repair it. If the short circuit is serious, replace the armature winding.
When the DC motor is started, the shunt motor rotates reversely, and then turns forward after starting.
Possible cause of failure
The series winding is reversed.
Maintenance methods and skills
Swap the connectors of the two lead wires of the series excitation winding.
DC motor casing leakage
Possible cause of failure
(1) Water enters the motor or other conductive impurities enter the interior of the motor.
(2) The lead wire of the motor is in contact with the motor casing somewhere.
(3) The motor grounding wire is open circuit or the insulation resistance of the motor winding to the casing is too small.
(4) The insulation of the motor winding is damaged and connected to the outer casing.
Maintenance methods and skills
(1) Turn on the motor, directly observe whether there is water in the interior, and check whether there is any damp place. If there is, it needs to be cleaned and dried. Then use a 500V megohmmeter to shake the winding of each phase of the motor and the insulation to the ground to meet the requirements. The motor can be used again.
(2) Open the motor, and replace the lead wire that contacts the casing or the wire with poor insulation of the casing.
(3) Reconnect the motor protective grounding wire with multi-strand copper wires. The motor with poor insulation performance should be brushed with insulating paint and dried.
(4) Replace the winding with damaged insulation of the motor.
DC motor has abnormal vibration
Possible cause of failure
(1) The foundation of the DC motor is not strong.
(2) The fixing motor bolts are not fastened or slipped.
(3) The motor bearing is damaged.
(4) The DC motor and the mechanical wheel are not concentric.
Maintenance methods and skills
(1) Reinstall the motor infrastructure.
(2) Tighten the motor anchor bolts or re-tap the anchor holes.
(3) Replace the motor bearing of the same type.
(4) Loosen the motor anchor bolts, and recalibrate the motor wheel and the mechanical load wheel to make them concentric.
