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HomeNewsCause analysis and solutions of common failures of brushless DC motors

Cause analysis and solutions of common failures of brushless DC motors

2020-09-12
This article is mainly about the introduction of the Brushless DC Motor, and focuses on the reasons for the excessive vibration of the Brushless dc motor at no-load starting.

Brushless Dc motor
The brushless DC motor is composed of a motor body and a driver, and is a typical mechatronic product. Because the brushless DC motor works in a self-controlled manner, it will not add a starting winding on the rotor like a synchronous motor driven by a heavy load under variable frequency speed regulation, nor will it vibrate and lose step when the load changes suddenly. For the permanent magnets of small and medium-capacity brushless DC motors, the rare earth neodymium iron boron (Nd-Fe-B) materials with high magnetic energy levels are now mostly used. Therefore, the volume of the rare earth permanent magnet Brushless Motor is one size smaller than the three-phase asynchronous motor of the same capacity.

Principle of operation
Brushless DC motors use semiconductor switching equipment to complete electronic commutation, that is, electronic switching equipment is used to replace traditional touch commutators and brushes. It has the advantages of low mechanical noise, no reversing sparks, and high reliability. It is widely used in high-level automated office equipment, electronic instruments, video recorders, and recording stands.
The brushless DC motor consists of a permanent magnet rotor, a multi-pole winding stator, and an orientation sensor. The orientation sensor changes the current of the stator winding along a certain order according to the change of the rotor orientation (that is, detects the orientation of the rotor pole relative to the stator winding, and generates an orientation sensor signal at the determined orientation, which is processed by the signal conversion circuit To control the power switch circuit, switch the winding current according to a certain logical connection). The operating voltage of the stator winding is supplied by an electronic switch circuit controlled by the output of the orientation sensor.
There are three types of orientation sensors: photoelectric, electromagnetic and magnetic sensitive.
The brushless DC motor with magneto-sensitive azimuth sensor is selected, and its magneto-sensitive sensor materials (such as magneto-sensitive pole tube, magneto-sensitive diode, Hall element, magneto-sensitive resistor or special integrated circuit, etc.) are installed on the stator assembly. To detect the change of the magnetic field generated when the permanent magnet and the rotor rotate.
The brushless DC motor with photoelectric azimuth sensor is selected. The stator assembly is equipped with photoelectric sensor material in a certain direction, the rotor is equipped with a light shield, and the light source is a light-emitting diode or a small bulb. When the rotor rotates, due to the effect of the shading plate, the photosensitive element equipment on the stator will generate pulse signals intermittently at a certain frequency.
The brushless DC motor with electromagnetic orientation sensor is equipped with electromagnetic sensor components (such as coupling transformer, LC resonance circuit, proximity switch, etc.) on the stator assembly. When the permanent magnet rotor orientation changes, the electromagnetic effect will cause the electromagnetic sensor A high-frequency modulation signal occurs (its amplitude changes with the rotor position).

Structure analysis
In terms of structure, brushless DC motors and brushed DC motors have similarities. There are also rotors and stators, but they are opposite to the structure of brushed motors. The rotors of brushed motors are coil windings connected to the power output shaft, and the stator is permanent. Magnetic steel: The rotor of the brushless DC motor is a permanent magnet, which is connected to the output shaft together with the casing. The stator is a winding coil. The Brushed Dc Motor is removed to replace the commutation brush for changing the electromagnetic field, so it is called Brushless DC Motor (Brushless DC Motor), then there is a problem now. Without the change of the electromagnetic field, how to make the brushless DC motor roll?
In short, relying on changing the alternating frequency and waveform of the current wave input to the stator coil of the brushless DC motor, a magnetic field rotating around the motor axis is formed around the winding coil, and this magnetic field drives the permanent magnet on the rotor to roll. , The motor starts to rotate. The function of the motor is related to the number of magnets, the strength of the magnetic steel flux, the size of the motor input voltage and other factors. It is also closely related to the control function of the brushless DC motor, because the input is DC, current The electronic speed governor is required to turn it into a 3-phase communication circuit, and it is also required to receive the control signal from the remote control receiver to control the speed of the motor to satisfy the requirements of the model application. In general, the structure of the brushless DC motor is relatively simple. The actual function of the brushless DC motor is still the brushless electronic governor. A good electronic governor requires microcontroller control program design, circuit design, and messy processing technology. Wait for the overall control of the process, so the price is much higher than the brushed DC motor.

Reasons for excessive vibration of brushless DC motor at no-load starting
On the other hand, the water resistance between the moving plate and the case heats up due to the passing current, and under the effect of the negative temperature characteristics of the water resistance, the resistance will gradually decrease. Summarizing the effects of the above two aspects, the motor is started with a constant torque and a constant current.
What is the reason for the excessive vibration of the brushless DC motor at no-load starting? The brushless automatic control motor soft starter is to directly install the starting resistance on the shaft of the motor, and use the centrifugal force generated when the motor rotates as the power to control the size of the resistance to reduce the motor starting current, increase the starting torque, and make the winding asynchronous The motor completes the brushless automatic control equipment. It is mainly composed of a casing, electrolyte, moving plate, tension spring, terminal, safety valve, exhaust valve, etc. The reason for the excessive vibration of the brushless DC motor at no-load starting is about the above conditions.


Common motor faults and solutions
When the motor is working or malfunctioning, the four methods of watching, hearing, smelling and touching can be used to prevent and troubleshoot in time to ensure the safe operation of the motor.

one look
Investigate whether there are abnormalities in the working process of the motor, which are mainly manifested in the following situations.
1. If there are discoloration, burn marks and smoke trails at the touch points and joints in the motor, it means that there may be some overheating, poor touch at the conductor joints or burnt windings.
2. If the motor vibrates violently, it may be that the transmission equipment is jammed, the motor is not properly fixed, and the foot bolts are loose.
3. The motor repair net works normally, but when it suddenly stops, you will see sparks from the loose wiring; the fuse is blown or a part is stuck.
4. When the motor is severely overloaded or working under phase loss, the speed will slow down and there will be a heavier buzzing sound.
5. When the stator winding is short-circuited, you may see smoke from the motor.

Second, touch
Touching the temperature of some parts of the motor can also determine the cause of the fault. To ensure safety, use the back of your hand to touch the motor housing and the surrounding parts of the bearing when touching with your hands. If the temperature is abnormal, the reasons may be as follows.
1. Start or brake frequently.
2. If the temperature around the bearing is too high, it may be caused by damage to the bearing or lack of oil.
3. Short circuit between stator winding turns or unbalanced three-phase current.
4. Poor ventilation. Such as the falling of the electric fan and the obstruction of the ventilation duct.
5. Overload. The current is too large and the stator winding is overheated.

Three, smell
After smelling the smell of the motor, it can also be judged and prepared for failure. If there is a special paint smell, it indicates that the internal temperature of the motor is too high; if there is a heavy paste or burnt smell, it may be that the insulation repair net has been broken down or the winding has been burned.

Four, listen
When the motor is working normally, it should declare a uniform and light hum, no noise and special sound. If the noise is declared to be too large, including electromagnetic noise, bearing noise, ventilation noise, mechanical conflict sound, etc., it may be a precursor to a failure or a failure phenomenon.
1. If the transmission organization and the transmission organization announce that the sound is continuous rather than fluctuating, it can be dealt with in the following situations.
(1) The uneven knocking sound is caused by the wind blade knocking against the fan cover.
(2) Periodic buzzing sound is caused by the loosening between the coupling or the pulley and the shaft and the wear of the key or keyway.
(3) Periodic snapping noise is caused by uneven belt joints.
2. Regarding bearing noise, you should always monitor the motor while working.
The monitoring method is: press one end of the screwdriver against the bearing equipment part and the other end close to the ear, you can hear the sound of the bearing. If the bearing is working normally, its sound will be successive and small rustling, and there will be no high and low changes and metal clashes. If the following sounds are present, it is abnormal.
(1) If there is a clicking sound or a creaking sound, it is the sound produced by the irregular movement of the balls in the bearing, which is caused by the damage of the balls in the bearing or the drying of the grease after the motor is not used for a long time.
(2) If there is a chirping sound, which is the sound announced when the ball is rolling, it is usually caused by the drying of the grease or lack of oil, and an appropriate amount of grease can be added.
(3) There is a squeaking sound when the bearing is working. This is the sound of metal conflict, which is generally caused by the lack of oil in the bearing. The bearing should be disassembled and added with an appropriate amount of grease.
3. Regarding electromagnetic noise, if the motor announces a high, low and heavy sound, the reasons may be as follows.
(1) The iron core is loose. When the motor is working, due to vibration, the fixing bolts of the iron core are loosened and the iron core silicon steel sheet is loosened, which generates noise.
(2) The three-phase current is unbalanced. This is because the three-phase windings are grounded incorrectly, short-circuited, or poorly touched. If the sound is very annoying, it means that the motor is severely overloaded or the motor is working without phase.
(3) The air gap between the stator and the rotor is not uniform. At this moment, the sound fluctuates and the interval between high and low frequencies does not change. This is caused by the wear of the bearing, which makes the stator and the rotor not concentric.


Motor fault detection and troubleshooting methods

1. The no-load current of the motor is large
The measurement method is as follows: Use the 20A gear of the multimeter to connect the red and black test leads of the multimeter in series to the power input of the controller. Turn on the power, record the maximum current value A1 of the multimeter at the moment when the motor is not rolling, and roll the handle to make the motor roll at high speed without load for more than 10 seconds, and the motor speed is stable. After that, start to investigate and record the maximum value of the multimeter at the moment A2. The no-load current of the motor = A2-A1.
Refer to the table below for the maximum limit no-load current without failure of various motors:
Motor mode extra voltage 24V extra voltage 36V
Side hanging motor 2.2A1.8A
High-speed brushed DC motor 1.7A1.0A
Low speed brushed DC motor 1.0A 0.6A
High speed brushless DC motor 1.7A1.0A
Low speed brushless DC motor 1.0A 0.6A
When the no-load current of the motor is greater than the limit data in the table, it indicates that the motor is malfunctioning. The reasons for the large no-load current of the motor are:
1) Demagnetization of magnetic steel. 2) The coil is partially short-circuited. 3) The internal mechanical conflict of the motor is large. We continue to do related tests and check items, and we can further identify the cause of the failure or the location of the failure.

2. Phase loss of brushless DC motor
The lack of phase of the Brushless motor is generally caused by the damage of the hall element of the brushless motor. We can measure the resistance of the output lead of the hall element relative to the hall ground wire and the relative hall power lead, and use the comparison method to determine which Hall element is. The ear element is malfunctioning.
To ensure the accuracy of the motor commutation orientation, it is generally recommended to replace all three Hall elements at the same time. Before replacing the Hall element, it is necessary to find out whether the phase algebraic angle of the motor is 120 degrees or 60 degrees. Generally, the three Hall elements of a 60-degree phase angle motor are placed in parallel. In the 120-degree phase angle motor, one hall element in the center of the three hall elements is placed in an orientation that is turned 180 degrees.

3. Motor heats up
Use a non-touch infrared thermometer, or the temperature measurement gear of a multimeter (multimeter with temperature measurement), when the temperature of the motor end cover exceeds the ambient temperature by more than 30℃, it indicates that the temperature rise of the motor has exceeded the normal range. The temperature rise of the motor is below 25°C.
The direct cause of the motor heating is caused by the large current. Motor current i, motor input electromotive force E1, motor rotation induced electromotive force (also known as back electromotive force) E2, and the relationship between the motor coil resistance R is:
i=(E1-E2)÷R
The increase in i indicates that R becomes smaller or E2 is reduced. The decrease of R is generally caused by a short circuit or an open circuit of the coil. The reduction of E2 is generally caused by the demagnetization of the magnetic steel or by the short circuit or open circuit of the coil.
In the practice of repairing the entire vehicle of an electric vehicle, to deal with the fault of the motor heating, it is generally to replace the motor.

4. The no-load/load speed ratio of the motor is greater than 1.5
The measurement method is as follows: turn on the power, roll the handle, make the motor roll at high speed and no-load for more than 10 seconds, and the motor speed is stable. In the future, use a hand-held speed/speed meter to measure the no-load maximum speed N1 of the motor at the moment. Under the standard load, Travel more than 200m intervals, start to measure the maximum load speed N2 of the motor.
No-load/load speed ratio=N2÷N1
When the no-load/load speed ratio of the motor is greater than 1.5, it indicates that the magnet demagnetization of the motor is now adequately strong, and the entire set of magnets in the motor should be replaced. Generally, the entire motor is replaced in the actual repair process of electric vehicles.

5. There are mechanical bumps or mechanical noise inside the motor when it is working
Regardless of whether the high-speed motor is still a low-speed motor, there should be no mechanical bumps or irregular mechanical noise when the load is working.


Analysis of common faults of motors and countermeasures
Low-voltage three-phase communication motors (hereinafter referred to as motors) have the advantages of simple structure, low price, and convenient repair. Therefore, it is widely used in the driving equipment of industrial and agricultural machinery and equipment, and it is the most widely used kind of all motors. Nevertheless, the motor will inevitably have various failures in actual work. Below, analyze the causes of common motor faults and the countermeasures adopted for the reference of colleagues.

1. The motor speed becomes lower
1. The connection method of the stator winding is wrong, please check and correct the connection method.
2. If the rotor guide bar is cracked or desoldered, check and repair the broken bar.
3. The power supply voltage is too low. Check and find out the reason.

2. When the motor is working, the ammeter pointer swings back and forth
The cause of this failure is likely to be an open circuit of the rotor guide bar.

3. Three-phase current imbalance is formed during work
1. One parallel branch or several parallel branches of a group of three-phase windings is open, resulting in unequal three-phase impedance.
2. One-phase open circuit or one-phase fuse blown in the three-phase winding causes phase loss work.
3. The beginning and end of a phase in the three-phase stator winding are wrong, but the fuse is not blown.
4. The three-phase power supply voltage is severely unbalanced (that is, not equal). Such as poor touch of switch contacts.
5. There is a partial short circuit between winding turns or phases, and the fuse is not broken.

4. Excessive vibration
1. The fan is unbalanced;
2. The transmission pulley is unbalanced, so the static balance should be checked.
3. The driven machine loses its dynamic balance.
4. The iron core is deformed or loose.
5. The rotor bar is cracked, causing the load current to vibrate from time to time.
6. The coupling device is incorrect or loose.
7. The shaft is twisted. The shaft should be replaced, or the car straight insert sleeve (hot sleeve).
8. Part of the stator winding is faulty, and the rotating magnetic field is unbalanced, causing vibration.
9. The bearing clearance is too large due to wear;
10. The bottom of the equipment is uneven or unstable. The equipment should be re-installed and fixed smoothly.
11. If the rotor of the motor is unbalanced (such as the weight screw on the rotor is dropped), the dynamic balance should be checked.

5. Abnormal noise
1. The first and last ends of the stator winding are wired incorrectly, there is a low roar, and the speed drops.
2. When the wind blade hits the shell or has debris, it will announce the impact sound. The wind blades should be calibrated and the debris around the wind blades should be removed.
3. When the bearing is severely short of oil, a "sizzling" sound can be heard from the bearing chamber. The bearings should be cleaned and new oil should be added.
4. The motor is working under phase loss and the roar is very loud. It can be switched off and on again to see if it can start normally again. If it cannot be started, there may be a phase open circuit.

Shenzhen Maintex Intelligent Control Co., Ltd.
Business Type:Manufacturer
Product Range:DC Motor , AC Motor , Stepper Motor
Products/Service:Stepper Motor , Brushless motor , Dc motor , treadmill used motor , Brushed Dc Motor , Fuji motor
Certificate:ISO14001 , ISO/TS16949 , ISO9001 , RoHS
Company Address:Zhengde Science Park, No. 8, Pingdi Street, Longgang District, Shenzhen, Guangdong, China

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