Uneven air gap problem and adjustment method of direct hoist motor

The mine hoist is an important equipment in mines, responsible for lifting minerals, transporting personnel up and down, and transporting materials and equipment. With the expansion of mining production scale and the development of technological level, the capacity of mine lifting equipment has increased, the number of large-scale equipment has increased, and the demand for large lifting machines, especially large direct friction lifting machines, has increased significantly. The main shaft of the direct connection hoist is connected to the cone sleeve of the motor rotor through interference fit. The motor is the power mechanism of the hoist, and its normal operation directly affects the normal operation of the hoist. Once the motor malfunctions, it not only reduces work efficiency, but may also cause significant casualties and economic losses.

1. Structure and unbalanced magnetic pulling force of direct hoist

1.1 Structure of Large Direct Friction Hoist

The large direct friction hoist is mainly composed of main shaft, friction wheel, rolling bearing, bearing seat, bearing cover, electric motor and other components, as shown in Figure 1. Among them, the main shaft of the hoist is connected to the cone sleeve of the motor rotor through interference fit, and the main shaft and the motor rotor are integrated. The gap between the stator and rotor of an electric motor is the motor air gap, and adjusting the air gap is also an important part of motor installation.

Figure 1 Structure of a large direct coupled hoist

1. Bearings and bearing seats; 2. Friction wheel; 3. Spindle; 4. Electric motor.

1.2 Effects of Unbalanced Magnetic Tension

In theory, when the magnetic field of the hoist motor is absolutely uniform, the magnetic pulling force acting on the radial points of the rotor is uniform, and the resultant force is zero. However, due to production processes, on-site installation adjustments, and measurement errors, eccentricity can inevitably lead to uneven air gaps between the stator and rotor, resulting in unbalanced magnetic pulling force.

According to experience and installation and operation requirements, the uneven air gap should not exceed 10% of the air gap value of the motor. Once the air gap value does not meet this requirement, it will cause the hoist to generate unbalanced magnetic pulling force in the direction of smaller air gap, pulling the hoist spindle and motor rotor towards the side with greater magnetic pulling force. The unbalanced magnetic pulling force not only accelerates the damage of bearings, but also causes vibration and noise in the hoist, resulting in the hoist brake disc deviation exceeding the tolerance and an increase in motor heat generation. When this adverse effect develops to a certain extent, it can cause friction between the stator and rotor, leading to motor "sweeping" accidents and even burning out the motor.

Uneven air gap and unbalanced magnetic pulling force

2.1 Research Status

Liu Zhizhen et al. derived several formulas for calculating unbalanced magnetic pulling force of asynchronous motors and introduced an empirical coefficient β to propose a new formula for calculating unbalanced magnetic pulling force. By comparing with other methods through calculation examples, the correctness of the obtained formula was proved. Faiz et al. used finite element method to analyze the magnetic field characteristics of induction motors under dynamic and static eccentricity, and found that the main tooth harmonic frequency band can effectively predict rotor eccentricity and eccentricity type. Pillai et al. established an expression for unbalanced magnetic pulling force considering different rotor positions, proposed an analytical formula, and compared it with the two-dimensional finite element analysis method to verify the rationality of the method. Huang Biao calculated the unbalanced magnetic pulling force and used Maxwell software to solve and analyze it, and provided solutions to reduce the unbalanced magnetic pulling force. Dang Wenjuan studied the theoretical calculation of unbalanced magnetic pulling force and motor torque, and provided a nonlinear analytical expression. There are many reasons for the unbalanced magnetic pulling force caused by electric motors during operation, and the mechanism of generating unbalanced magnetic pulling force is also very complex. Research has found that there is a close nonlinear relationship between unbalanced magnetic pulling force and rotor eccentricity. Although many scholars have provided nonlinear analytical expressions for unbalanced magnetic pulling force at present, the magnetic pulling force expressions obtained are not the same due to different assumptions and simplification methods used, and each expression has different applicable environments.

The unilateral magnetic pulling force of an electric motor refers to the force generated by the uneven distribution of the motor's magnetic field. In traditional domestic design, the calculation of unilateral magnetic pulling force has always followed the formulas in references [9] and [14]. Unilateral magnetic pulling force

In the formula: F0 is the unilateral magnetic pulling force, N; D is the outer diameter of the rotor, cm; L is the length of the magnetic field, which refers to the length range of the magnetic field, cm; g is the air gap value of the motor, cm; Bg is the magnetic induction intensity, which is the magnitude of the magnetic flux per unit area, Gs; e0 is the eccentricity, cm.

Nowadays, when calculating the initial unilateral magnetic pulling force of low-speed motors in electric motor factories, it is generally calculated based on experience, with an air gap deviation of 10% between the stator and rotor of the motor. In the above equation

Equation (1) can be simplified as

2.2 Installation and adjustment of air gap

When installing a hoist on site, the hoist spindle device is usually installed first, followed by the installation of the electric motor. When installing the electric motor, first install the motor rotor onto the main shaft of the hoist, and then install the stator in place. When installing the stator, the clearance between the rotor and stator should be adjusted according to the requirements specified in the drawings provided by the manufacturer.

Because the mechanical weight of the hoist spindle and the weight of the motor rotor will cause certain deflection on the hoist spindle, but the deflection caused by the weight of these two parts already exists during the installation of the motor; The stator air gap has been adjusted based on the rotor position at that time, and will not have any additional impact on the air gap between the stator and rotor.

Generally, the steel wire rope is only suspended after the installation and adjustment of the electric motor and before the formal operation of the hoist. The three parts of the load, namely the weight of the steel wire rope, the weight of the container, and the load weight, are all exerted on the friction wheel of the hoist through the steel wire rope, collectively referred to as the tension of the steel wire rope. The tension of the steel wire rope will cause deflection at the motor end of the hoist spindle, and the tension will affect the air gap between the stator and rotor of the hoist motor; Therefore, when installing and adjusting the air gap of the motor, it is necessary to reserve an air gap in the direction where the tension of the steel wire rope causes deflection, that is, in the opposite direction to the direction of the steel wire rope coming out. The tension of the steel wire rope acting on the hoist of the well tower hoist and the floor hoist is different, and the direction of the reserved air gap is also different, as shown in Figures 2 and 3. Measure the air gap of the motor again after hanging the hoist rope. If the air gap value does not meet the requirements, it needs to be adjusted again until the air gap is qualified.

Figure 2 Changes in air gap caused by tension of steel wire rope

Figure 3 Reserved air gap for two types of elevators

Case analysis of uneven air gap in 3 elevators

Taking a certain site as an example, the equipment is a JKMD-4 × 4Z III multi rope friction hoist with a steel wire rope diameter of 42 mm and a lifting speed of 8.8 m/s; The electric motor is a DC motor, model ZKTD250/75, with a power of 1800 kW and a speed of 42 r/min.

3.1 Overview of Uneven Air Gap Issues

During maintenance, the user discovered an uneven air gap between the rotor and stator of the electric motor, which has been a problem for over a year. Users are concerned that the continued development of this issue may affect the normal operation of the hoist. After our company's technical personnel arrived at the site, they observed that there were significant air gap changes in the rotor of the elevator during excitation power on and excitation power off, with a visual change of 2-3 mm, as shown in Figure 4. The air gap of the motor does not show significant changes during smooth operation after starting. Technicians initially suspect that the looseness or displacement of the motor stator and rotor may cause changes in the air gap, and it is also possible that the air gap was not adjusted evenly during the initial installation.

Figure 4 Changes in air gap before and after excitation of magnetic flux

3.2 Measuring Air Gap and Finding Problems

3.2.1 Measuring Air Gap and Investigating Causes

Remove the outer protective cover of the motor and use a feeler gauge to check the air gap between the stator and rotor of the motor. Use a long feeler gauge to measure the air gap evenly on the circumference of the stator and rotor, and it is required to use a feeler gauge to measure and record on both the inner and outer sides. The results indicate that there is unevenness in the air gap, with a maximum gap of 10.3 mm and a minimum gap of 4.4 mm; The direction of change in the air gap between the stator and rotor is roughly consistent with the direction of excitation movement caused by the decrease in air gap.

Check the following items and identify the problem: Check if the connecting bolts between the motor and the foundation base are loose, if the connection between the motor poles and the motor stator housing is loose, and if the connecting bolts and wedges in various parts of the hoist are loose. After checking each of the above issues one by one, no problems were found, ruling out the possibility of air gap changes caused by displacement due to loose stator during the operation of the hoist.

3.2.2 Using a dial gauge to measure displacement

When the hoist motor is excited and energized, there is visible rotor displacement, and a dial gauge is needed to measure the rotor displacement. At the same time, install a dial gauge on the main shaft between the electric motor and the hoist to observe whether there is displacement of the main shaft and the amount of displacement; Install a dial gauge at the position where the maximum displacement has been observed at the tail of the motor rotor, as well as between the main shaft bearing and the motor, with the same orientation as the dial gauge at the tail of the motor rotor; Install a dial gauge on the stator of the electric motor. After installing and adjusting the dial gauge, perform excitation and power on. When the motor is excited and powered on, record the data on each dial gauge as follows: there is a displacement of 0.12 mm between the main shaft bearing and the motor, and a displacement of 4 mm at the tail of the rotor, and the two displacement directions are consistent; The stator of the electric motor has not undergone displacement.

4 Reasons Analysis and Adjustment Methods

4.1 Root cause analysis

The preliminary judgment is that the reason for the uneven air gap is that the air gap was not adjusted as required during the installation of the hoist. The unbalanced magnetic pulling force generated by the severe uneven air gap of the electric motor is too large, causing the main shaft of the hoist to "eat" the bearing clearance, resulting in significant displacement of the main shaft and rotor ends amplified by the lever principle; Therefore, it is necessary to adjust the air gap between the stator and rotor. After consulting the information, the air gap value of the motor is 7 mm, and the air gap deviation is required to be no more than 10% of the air gap value, which is 0.7 mm. After adjustment, it should be ensured that the air gap deviation does not exceed 0.7 mm.

4.2 Adjustment Methods

Due to the fact that the rotor of the electric motor is coaxial with the main shaft of the hoist, after inspection, the installation of the hoist meets the requirements. Therefore, it is not possible to adjust the hoist to adjust the air gap. The stator of the electric motor is connected to the motor base through 6 bolts, and the stator position can be adjusted to achieve the purpose of adjusting the air gap. The process of adjusting the stator position is as follows:

(1) Mark the current position of the motor stator with a line and record it as the original position;

(2) Move the stator towards the direction with a smaller air gap (rope exit direction) by about 3 mm;

(3) Remove the positioning pin between the stator and the base, and loosen the bolt between the stator and the base;

(4) Use a crane to slightly lift the stator of the motor, and use a jack to push the motor towards the side with the smaller air gap by about 3mm;

(5) Measure the air gap with a feeler gauge and adjust it again based on the measured value until the air gap meets the requirements;

(6) After adjusting the air gap evenly, tighten the bolts to the required torque;

(7) Air gap verification. After adjustment, the final minimum air gap is 6.6 mm and the maximum air gap is 7.4 mm, meeting the requirements of the standard air gap. Simultaneously install a dial gauge between the main shaft bearing of the hoist and the motor, at the tail of the hoist rotor motor, and at the rotor of the motor to excite and energize. Record the dial gauge data as follows: the main shaft drive bearing end experiences a displacement of 0 mm, the rotor tail experiences a displacement of 0.16 mm, and the motor stator remains unchanged. This indicates that the adjustment method is correct and effective.

5 Conclusion

(1) The uneven air gap of the motor directly affects the unbalanced magnetic pulling force of the motor, so the air gap deviation should be ensured not to exceed 10%. When installing the hoist, the impact of the hoist rope on the air gap should be considered, and an air gap needs to be reserved during installation. Before and during the operation of the hoist, check the air gap of the hoist motor. If the air gap is uneven, it should be adjusted in a timely manner until it meets the requirements.

(2) In this case, the maximum position of uneven air gap is in the horizontal direction, and the stator position can be adjusted horizontally to meet the air gap requirements. If the maximum position of uneven air gap is in a non horizontal position, the center height of the stator should also be adjusted when adjusting the horizontal position of the motor stator to ensure that the adjusted air gap fully meets the requirements.