1.Why does the motor generate shaft current?

Shaft current has always been a hot topic among major motor manufacturers. In fact, every motor has shaft current, and most of them will not endanger the normal operation of the motor.The distributed capacitance between the winding and the housing of a large motor is large, and the shaft current has a high probability of burning the bearing; the switching frequency of the power module of the variable frequency motor is high, and the impedance of the high-frequency pulse current passing through the distributed capacitance between the winding and the housing is small and the peak current is large. The bearing moving body and raceway are also easily corroded and damaged.

Under normal circumstances, a three-phase symmetrical current flows through the three-phase symmetrical windings of a three-phase AC motor, generating a circular rotating magnetic field. At this time, the magnetic fields at both ends of the motor are symmetrical, there is no alternating magnetic field interlinked with the motor shaft, there is no potential difference at both ends of the shaft, and no current flows through the bearings. The following situations may break the symmetry of the magnetic field, there is an alternating magnetic field interlinked with the motor shaft, and the shaft current is induced.

Causes of shaft current:

(1) Asymmetric three-phase current;

(2) Harmonics in the power supply current;

(3) Poor manufacturing and installation, uneven air gap due to rotor eccentricity;

(4) There is a gap between the two semicircles of the detachable stator core;

(5) The number of fan-shaped stator core pieces is not selected appropriately.

Hazards: The motor bearing surface or ball is corroded, forming micropores, which deteriorates the bearing operation performance, increases friction loss and heat generation, and eventually causes the bearing to burn out.

Prevention:

(1) Eliminate pulsating magnetic flux and power supply harmonics (such as installing an AC reactor on the output side of the inverter);

(2) Install a grounding soft carbon brush to ensure that the grounding carbon brush is reliably grounded and reliably contacts the shaft to ensure that the shaft potential is zero;

(3) When designing the motor, insulate the bearing seat and base of the sliding bearing, and insulate the outer ring and end cover of the rolling bearing.

2. Why can’t general motors be used in plateau areas?

Generally, the motor uses a self-cooling fan to dissipate heat to ensure that it can take away its own heat at a certain ambient temperature and achieve thermal balance. However, the air on the plateau is thin, and the same speed can take away less heat, which will cause the motor temperature to be too high. It should be noted that too high temperature will cause the insulation life to decrease exponentially, so the life will be shorter.

Reason 1: Creepage distance problem. Generally, the air pressure in plateau areas is low, so the insulation distance of the motor needs to be far. For example, the exposed parts such as the motor terminals are normal under normal pressure, but sparks will be generated under low pressure in the plateau.

Reason 2: Heat dissipation problem. The motor takes away heat through air flow. The air in the plateau is thin, and the heat dissipation effect of the motor is not good, so the temperature rise of the motor is high and the life is short.

Reason 3: Lubricating oil problem. There are mainly two types of motors: lubricating oil and grease. Lubricating oil evaporates under low pressure, and grease becomes liquid under low pressure, which affects the life of the motor.

Reason 4: Ambient temperature problem. Generally, the temperature difference between day and night in plateau areas is large, which will exceed the use range of the motor. High temperature weather plus motor temperature rise will damage the motor insulation, and low temperature will also cause insulation brittle damage.

Altitude has adverse effects on motor temperature rise, motor corona (high-voltage motor) and commutation of DC motor. The following three aspects should be noted:

(1) The higher the altitude, the greater the motor temperature rise and the smaller the output power. However, when the temperature decreases with the increase in altitude to compensate for the effect of altitude on temperature rise, the rated output power of the motor can remain unchanged;

(2) When high-voltage motors are used in plateaus, anti-corona measures should be taken;

(3) Altitude is not conducive to the commutation of DC motors, so pay attention to the selection of carbon brush materials.

3. Why is it not suitable for motors to run under light load?

The motor light load state means that the motor is running, but its load is small, the working current does not reach the rated current and the motor running state is stable.

The motor load is directly related to the mechanical load it runs. The greater its mechanical load, the greater its working current. Therefore, the reasons for the motor light load state may include the following:

1. Small load: When the load is small, the motor cannot reach the rated current level.

2. Mechanical load changes: During the operation of the motor, the size of the mechanical load may change, causing the motor to be lightly loaded.

3. Working power supply voltage changes: If the working power supply voltage of the motor changes, it may also cause the light load state.

When the motor is running under light load, it will cause:

1. Energy consumption problem

Although the motor consumes less energy when it is under light load, its energy consumption problem also needs to be considered in long-term operation. Because the power factor of the motor is low under light load, the energy consumption of the motor will change with the load.

2. Overheating problem

When the motor is under light load, it may cause the motor to overheat and damage the motor windings and insulation materials.

3. Life problem

Light load may shorten the life of the motor, because the internal components of the motor are prone to shear stress when the motor works under low load for a long time, which affects the service life of the motor.

4.What are the causes of motor overheating?

1. Excessive load

If the mechanical transmission belt is too tight and the shaft is not flexible, the motor may be overloaded for a long time. At this time, the load should be adjusted to keep the motor running under rated load.

2. Harsh working environment

If the motor is exposed to the sun, the ambient temperature exceeds 40℃, or it is running under poor ventilation, the motor temperature will rise. You can build a simple shed for shade or use a blower or fan to blow air. You should pay more attention to removing oil and dust from the motor’s ventilation duct to improve cooling conditions.

3. The power supply voltage is too high or too low

When the motor runs within the range of -5%-+10% of the power supply voltage, the rated power can be kept unchanged. If the power supply voltage exceeds 10% of the rated voltage, the core magnetic flux density will increase sharply, the iron loss will increase, and the motor will overheat.

The specific inspection method is to use an AC voltmeter to measure the bus voltage or the terminal voltage of the motor. If it is caused by the grid voltage, it should be reported to the power supply department for resolution; if the circuit voltage drop is too large, the wire with a larger cross-sectional area should be replaced and the distance between the motor and the power supply should be shortened.

4. Power phase failure

If the power phase is broken, the motor will run in single phase, which will cause the motor winding to heat up rapidly and burn out in a short time. Therefore, you should first check the fuse and switch of the motor, and then use a multimeter to measure the front circuit.

5.What needs to be done before a motor that has been unused for a long time is put into use?

(1) Measure the insulation resistance between the stator and winding phases and between the winding and the ground.

The insulation resistance R should satisfy the following formula:

R＞Un/(1000+P/1000)(MΩ)

Un: rated voltage of motor winding (V)

P: motor power (KW)

For motors with Un＝380V, R＞0.38MΩ.

If the insulation resistance is low, you can:

a: run the motor at no load for 2 to 3 hours to dry it;

b: pass low-voltage AC power of 10% of the rated voltage through the winding or connect the three-phase winding in series and then use DC power to dry it, keeping the current at 50% of the rated current;

c: use a fan to send hot air or heating element to heat it.

(2) Clean the motor.

(3) Replace the bearing grease.

6. Why can’t you start the motor in a cold environment at will?

If the motor is kept in a low temperature environment for too long, the following may occur:

(1) The motor insulation will crack;

(2) The bearing grease will freeze;

(3) The solder on the wire joint will turn into powder.

Therefore, the motor should be heated when stored in a cold environment, and the windings and bearings should be checked before operation.

7. What are the reasons for the unbalanced three-phase current of the motor?

(1) Unbalanced three-phase voltage: If the three-phase voltage is unbalanced, reverse current and reverse magnetic field will be generated in the motor, resulting in uneven distribution of three-phase current, causing the current of one phase winding to increase

(2) Overload: The motor is in an overloaded operating state, especially when starting. The current of the motor stator and rotor increases and generates heat. If the time is slightly longer, the winding current is very likely to be unbalanced

(3) Faults in the stator and rotor windings of the motor: Turn-to-turn short circuits, local grounding, and open circuits in the stator windings will cause excessive current in one or two phases of the stator winding, causing serious imbalance in the three-phase current

(4) Improper operation and maintenance: Failure of operators to regularly inspect and maintain electrical equipment may cause the motor to leak electricity, run in a phase-missing state, and generate unbalanced current.

8. Why can’t a 50Hz motor be connected to a 60Hz power supply?

When designing a motor, the silicon steel sheets are generally made to operate in the saturation region of the magnetization curve. When the power supply voltage is constant, reducing the frequency will increase the magnetic flux and the excitation current, which will lead to increased motor current and copper loss, and ultimately increase the motor temperature rise. In severe cases, the motor may be burned due to coil overheating.

9.What are the reasons for motor phase loss?

Power supply:

(1) Poor switch contact; resulting in unstable power supply

(2) Transformer or line disconnection; resulting in power transmission interruption

(3) Fuse blown. Improper selection or incorrect installation of the fuse may cause the fuse to break during use

Motor:

(1) The screws of the motor terminal box are loose and in poor contact; or the hardware of the motor is damaged, such as broken lead wires

(2) Poor internal wiring welding;

(3) The motor winding is broken.

10. What are the causes of abnormal vibration and noise in the motor?

Mechanical aspects:

(1) The fan blades of the motor are damaged or the screws that fasten the fan blades are loose, causing the fan blades to collide with the fan blade cover. The sound it produces varies in volume depending on the severity of the collision.

(2) Due to bearing wear or misalignment of the shaft, the motor rotor will rub against each other when it is seriously eccentric, causing the motor to vibrate violently and produce uneven friction sounds.

(3) The motor’s anchor bolts are loose or the foundation is not firm due to long-term use, so the motor produces abnormal vibration under the action of electromagnetic torque.

(4) The motor that has been used for a long time has dry grinding due to lack of lubricating oil in the bearing or damage to the steel balls in the bearing, which causes abnormal hissing or gurgling sounds in the motor bearing chamber.

Electromagnetic aspects:

(1) Unbalanced three-phase current; abnormal noise suddenly appears when the motor is running normally, and the speed drops significantly when running under load, making a low roar. This may be due to unbalanced three-phase current, excessive load or single-phase operation.

(2) Short circuit fault in stator or rotor winding; if the stator or rotor winding of a motor is running normally, short circuit fault or cage rotor is broken, the motor will make a high and low humming sound, and the body will vibrate.

(3) Motor overload operation;

(4) Phase loss;

(5) The cage rotor welding part is open and causes broken bars.

11. What needs to be done before starting the motor?

(1) For newly installed motors or motors that have been out of service for more than three months, the insulation resistance should be measured using a 500-volt megohmmeter. Generally, the insulation resistance of motors with a voltage below 1 kV and a capacity of 1,000 kW or less should not be less than 0.5 megohms.

(2) Check whether the motor lead wires are connected correctly, whether the phase sequence and rotation direction meet the requirements, whether the grounding or zero connection is good, and whether the wire cross-section meets the requirements.

(3) Check whether the motor fastening bolts are loose, whether the bearings are lacking oil, whether the gap between the stator and the rotor is reasonable, and whether the gap is clean and free of debris.

(4) According to the nameplate data of the motor, check whether the connected power supply voltage is consistent, whether the power supply voltage is stable (usually the allowable power supply voltage fluctuation range is ±5%), and whether the winding connection is correct. If it is a step-down starter, also check whether the wiring of the starting equipment is correct.

(5) Check whether the brush is in good contact with the commutator or slip ring, and whether the brush pressure meets the manufacturer’s regulations.

(6) Use your hands to turn the motor rotor and the shaft of the driven machine to check whether the rotation is flexible, whether there is any jamming, friction or bore sweeping.

(7) Check whether the transmission device has any defects, such as whether the tape is too tight or too loose and whether it is broken, and whether the coupling connection is intact.

(8) Check whether the capacity of the control device is appropriate, whether the melt capacity meets the requirements and whether the installation is firm.

(9) Check whether the wiring of the starting device is correct, whether the moving and static contacts are in good contact, and whether the oil-immersed starting device is short of oil or the oil quality is deteriorated.

(10) Check whether the ventilation system, cooling system and lubrication system of the motor are normal.

(11) Check whether there are any debris around the unit that hinders the operation, and whether the foundation of the motor and the driven machine is firm.

12. What are the causes of motor bearing overheating?

(1) The rolling bearing is not installed correctly, and the fit tolerance is too tight or too loose.

(2) The axial clearance between the motor outer bearing cover and the outer circle of the rolling bearing is too small.

(3) The balls, rollers, inner and outer rings, and ball cages are severely worn or the metal is peeling off.

(4) The end covers or bearing covers on both sides of the motor are not installed correctly.

(5) The connection with the loader is poor.

(6) The selection or use and maintenance of grease is improper, the grease is of poor quality or deteriorated, or it is mixed with dust and impurities, which will cause the bearing to heat up.

Installation and inspection methods

Before checking the bearings, first remove the old lubricating oil from the small covers inside and outside the bearings, then clean the small covers inside and outside the bearings with a brush and gasoline. After cleaning, clean the bristles or cotton threads and do not leave any in the bearings.

(1) Carefully inspect the bearings after cleaning. The bearings should be clean and intact, without overheating, cracks, peeling, groove impurities, etc. The inner and outer raceways should be smooth and the clearances should be acceptable. If the support frame is loose and causes friction between the support frame and the bearing sleeve, a new bearing should be replaced.

(2) The bearings should rotate flexibly without jamming after inspection.

(3) Check that the inner and outer covers of the bearings are free of wear. If there is wear, find out the cause and deal with it.

(4) The inner sleeve of the bearing should fit tightly with the shaft, otherwise it should be dealt with.

(5) When assembling new bearings, use oil heating or eddy current method to heat the bearings. The heating temperature should be 90-100℃. Put the bearing sleeve on the motor shaft at high temperature and ensure that the bearing is assembled in place. It is strictly forbidden to install the bearing in a cold state to avoid damaging the bearing.

13. What are the reasons for low motor insulation resistance?

If the insulation resistance value of a motor that has been running, stored or in standby mode for a long time does not meet the requirements of the regulations, or the insulation resistance is zero, it indicates that the insulation of the motor is poor. The reasons are generally as follows:
(1) The motor is damp. Due to the humid environment, water drops fall into the motor, or cold air from the outdoor ventilation duct invades the motor, causing the insulation to become damp and the insulation resistance to decrease.

(2) The motor winding is aging. This mainly occurs in motors that have been running for a long time. The aging winding needs to be returned to the factory in time for re-varnishing or rewinding, and a new motor should be replaced if necessary.

(3) There is too much dust on the winding, or the bearing is seriously leaking oil, and the winding is stained with oil and dust, resulting in reduced insulation resistance.

(4) The insulation of the lead wire and the junction box is poor. Rewrap and reconnect the wires.

(5) The conductive powder dropped by the slip ring or brush falls into the winding, causing the rotor insulation resistance to decrease.

(6) The insulation is mechanically damaged or chemically corroded, resulting in the winding being grounded.
Treatment
(1) After the motor is shut down, the heater needs to be started in a humid environment. When the motor is shut down, in order to prevent moisture condensation, the anti-cold heater needs to be started in time to heat the air around the motor to a temperature slightly higher than the ambient temperature to drive out the moisture in the machine.

(2) Strengthen the temperature monitoring of the motor, and take cooling measures for the motor with high temperature in time to prevent the winding from aging faster due to high temperature.

(3) Keep a good motor maintenance record and clean the motor winding within a reasonable maintenance cycle.

(4) Strengthen the maintenance process training for maintenance personnel. Strictly implement the maintenance document package acceptance system.

In short, for motors with poor insulation, we should first clean them, and then check whether the insulation is damaged. If there is no damage, dry them. After drying, test the insulation voltage. If it is still low, use the test method to find the fault point for maintenance.

Anhui Mingteng Permanent-Magnetic Machinery & Electrical Equipment Co., Ltd.(https://www.mingtengmotor.com/) is a professional manufacturer of permanent magnet synchronous motors. Our technical center has more than 40 R&D personnel, divided into three departments: design, process, and testing, specializing in the research and development, design, and process innovation of permanent magnet synchronous motors. Using professional design software and self-developed permanent magnet motor special design programs, during the motor design and manufacturing process, we will ensure the performance and stability of the motor and improve the energy efficiency of the motor according to the actual needs and specific working conditions of the user.

Copyright: This article is a reprint of the original link:

https://mp.weixin.qq.com/s/M14T3G9HyQ1Fgav75kbrYQ

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