Traction elevator balance factor and detection - Solutions - Huaqiang Electronic Network

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The balance factor is a familiar and unfamiliar parameter for elevator professionals. It is said that it is familiar because everyone knows that the traction elevator has a "balance factor" for the weight configuration. It is known that the national standard has a "balance factor should be in the range of 40% to 50%", saying that it is unfamiliar Because what role does the balance factor play in the elevator? What will its value affect? How should the value be most appropriate? And how exactly is it accurate? Many elevator installations and inspection personnel are not clear.

At present, the inspection and inspection institutions of special equipment in various places are the most time-consuming and labor-intensive, and the most important factor is to check the balance coefficient of the elevator. According to the inspection regulations, the load-current curve of the elevator operation must be measured under the rated load of 0, 25%, 40%, 50%, 75%, 100% and 110% of the car, and the intersection of the upper and lower curves should be taken. The load factor of the point is the balance factor of the ladder, and the junction point is qualified within the range of 40% to 50%. In order to determine this parameter, in addition to the two inspectors, more than one worker carrying the code back and forth is required. Because there are too many factors affecting the test, is the result credible and not to mention, even if the test result is within the range of 40%~50%, is it qualified? If it is beyond this range, why is it not qualified? What is the meaning of "balance factor"? What is the impact on the elevator? I don't know why, the "balance factor" is lost.

1. The essence of the balance factor

To explore the essence of the balance factor, we must start with the principle of the traction elevator. The vertical elevator is a lifting device that moves the heavy objects vertically up and down. From a mechanical point of view, in order for a heavy object to remain stationary in the air, a tensile force T must be balanced with the gravity Q of the object, that is, T = Q, at which time the object is in a stationary or uniform motion state, called the balance of forces. This system is called a balancing system. In order to make the object move upward and the speed changes, this tension T, in addition to overcoming the gravity Q of the object, also provides a force F for generating acceleration, ie T = Q + F = Q + ma ( m -- for the object Quality; a—for acceleration).

If the gravity Q of the object is balanced by another balancing force W, W = Q, which constitutes a balancing system, then the pulling force T does not have to overcome the gravity Q, but only the force required to generate acceleration of the object, ie T = F = ma This greatly reduces the pulling force T. This is the "balance principle" used on the elevator. This balance is provided by the counterweight. Therefore, we require the counterweight gravity W to be equal to the gravity of the car and the load (P+Q).

But to really do this, it is very difficult in the practical application of the elevator, or that there is not yet a way to achieve this. Since the car's load Q is randomly variable, it may be any value within the range of 0 (no load) or 100% QH (full load), so we can only choose an appropriate counterweight.

That is, take W = P + K QH ---------------(1)

This coefficient K is the "balance factor". Therefore, the essence of the balance factor is the quality of the design configuration. It will affect the quality of the counterweight and the unbalanced load of the elevator. When the car and the load are P + Q, (where P - is the car's own weight; Q - is the actual load of the car; QH - the rated load of the car), the car side and the counterweight side The unbalanced load is:

△T = (P+Q) –( P+KQH ) = Q – KQH --------(2)

2. The value of the balance coefficient K

It can be seen from the above formula (2) that the system is only balanced when Q = KQH, so regardless of the value of K, the balance is only relative, and the imbalance is absolute. We can only hope that the system is as close as possible to the balance. A simple way is to take the average of the car load changes. Because the change in car load is: 0 ~ 100%, it is reasonable to take K = 50%, it is difficult to say how much better. When the elevator is shipped from the factory, it does not fully understand the load during actual operation. To truly achieve the ideal balance, the actual running load should be measured during the actual operation of the elevator. For example, the actual load change of a large number of residential elevators is basically 0 ~ 60%, and there is very little full load. Therefore, it is more appropriate to take K = 30% ~ 40%. Nowadays, the general passenger elevator enters the direct driving state when the load exceeds 80%, so the actual full load is also less, so it is appropriate to take the balance coefficient K = 40% ~ 50%. On the contrary, some cargo elevators, due to the super-area of ​​the car, will have a load change of 0 to 105%, so it should be more appropriate to take a balance factor of K ≥ 50%. It must be pointed out that the value of K here refers to the value of the balance coefficient K when the elevator is designed. It is called the design value, and it is not the K value that is randomly configured when the elevator is installed or after use.

3. The influence of the value of the balance coefficient K on the elevator

It has been explained above that no matter how the balance coefficient K is taken, it is impossible to change the load Q with a constant K value, so the imbalance between the car and the counterweight system is absolute, from the design point of view. The value of K first affects the magnitude of the unbalanced moment acting on both sides of the traction sheave. If the maximum load is 110% QH of the overload load and the value of K is 40%~50%, the unbalanced load at no load is :(0.4~0.5)QH, the unbalanced load at overload is: (1.1-K)QH = (0.6~0.7) QH. If the most severe load is 125% QH according to the elevator acceptance test, the unbalanced load is 1.25- K = (0.75 ~ 0.85) QH, which is the maximum unbalanced load (referred to as static load) that the elevator may have, that is, the minimum static traction that the elevator must provide.

This first affects the power P of the selected host motor. The power P of the main motor is determined by the following equation: P∝(1-K) QH VH . If the motor power used by the elevator is large enough, the choice of K will affect the energy consumption of the elevator. If the power margin of the motor is small, the value of the balance factor is not suitable, which may cause the elevator to pull up after starting. There is an accident of rolling or topping.

The value of the balance coefficient affects the size of the unbalanced load, and also affects the tension of the wire ropes on both sides of the traction sheave. The magnitude of this tension will affect the specific pressure of the traction wire rope in the rope groove. The larger the drag, the stronger the traction capacity provided by the traction wire. Therefore, the value of the balance coefficient determines both the unbalanced load and the traction capacity of the elevator. When the maximum unbalanced load is greater than the maximum traction force of the elevator, the traction wire rope will slip in the rope groove and a rolling accident will occur. In the design of the elevator, the choice of the balance factor K must take into account the power of the main motor and the impact on the traction capacity. #p#分页头#e#

The value of the balance factor K also affects the total mass of the car and counterweight system:

M = P + Q + W + Y = ( P + Q ) + ( P + K QH ) + Y (Y - the mass of the wire rope and other devices), which is easily overlooked. The total mass of the car and counterweight system will affect the safety factor of the elevator and affect the selection of components such as the traction wire rope and the traction rope groove. At the same time, the total mass also affects the acceleration and deceleration of the starting and braking of the elevator. The selection of safety components such as safety tongs and bumpers that affect the use of the elevator. When installing the elevator, in order to cope with the acceptance inspection and reduce the unbalanced load during acceptance, the installer often takes the balance factor K to a larger value, and is configured to approach 50%. Increasing the balance coefficient means increasing the weight of the counterweight, which will bring the elevator. The acceleration of the start and brake is reduced, and the braking is difficult. Therefore, the value of the balance factor K is only a ratio on the surface. In fact, it is closely related to the quality of the car and the counterweight. It is one of the important parameters in the overall design of the elevator. It is meaningless to set off the parameters such as the rated load and the weight of the car. The pure balance factor is not meaningful.

Therefore, the determination of the balance factor K must be considered in the design of the elevator, combined with the traction sheave, the shape of the rope groove, the traction wire rope, the weight of the car and the supporting traction machine motor, brake, safety gear, buffer and so on. The relationship between them has been described in my own paper "Elevator Parameters and Their Relationships" and will not be described here. This is why the balance factor should be configured according to the design value in the range of 40% to 50% when the elevator is installed. It is worth mentioning that some of the cars have been reinstalled in the elevators recently, so that the weight of the car is increased. In order to keep the balance factor K constant, the method of increasing the weight is adopted to greatly increase the overall quality of the system. This is extremely wrong. At this time, the balance factor has lost its original meaning. The safety factor of the elevator is reduced, and the deceleration of the starting and braking is reduced, which will cause serious safety hazards to the elevator.

Therefore, the value of the balance factor K is not determined to be in compliance with the requirements as long as it is within 40% to 50% of the time of installation or acceptance inspection. If the value deviates from the design value, it does not meet the requirements, or although the value meets the design value, but the car's own weight P or the rated load QH changes, the same is not met. In the traction check of Appendix D of GB7588-2003, it is stated as follows: "It should be checked whether the balance factor is as stated by the installer." The "installer said" here means the design value, which is not the result of the installer's arbitrary desire. This requires the elevator manufacturer to inform the installation personnel of the balance factor value of the elevator design. The installation and construction personnel must configure the counterweight device according to the design value, and must not arbitrarily change the car's own weight. When the inspection organization conducts the acceptance inspection, it must determine whether the actual value is consistent with the design value, and check whether it changes the parameters such as the weight of the car. This should be brought to the attention of the industry.

The method of measuring the current of the traction motor recommended by the national standard belongs to this category. The basic principle is: when the elevator is running at a constant speed, the torque T2 output on the traction motor shaft is:
T2 = T0 ±△T ---------(3)
T0 ---- converted to the motor shaft, elevator mechanical transmission resistance resistance torque (referred to as resistance torque)
△T --- converted to the motor shaft, unbalanced load torque. ± represents the direction of the unbalanced load torque as the load changes. (referred to as unbalanced load)
When the gravity of the car and the load (P+Q) is equal to the weight of the counterweight (P+KQH) (ie, in equilibrium), then △T = (P+Q)—( P+KQH ) = 0
Then Q = KQH balance factor: K = Q / QH

The key to the current method is to use the measured current to determine whether the balance is balanced or not:

â–³T = 0, assuming that the resistance torque T0 of the car up and down is the same, the output torque T2 of the motor is the same when the vehicle is up and down, T2 = T0, and the current measured by the motor should be equal. The above and the downstream currents are equal to determine the balance, (note: not the current is the smallest), which is the principle of the current method.

Measuring the current to determine the torque is an indirect measurement method. The relationship between current and torque is indirectly derived from the power balance relationship on the motor.

The mechanical power of the motor output P2 = T2 Ω (Ω---motor angular velocity), and the electromagnetic power PM of the motor is: PM = PCU + P2 (PCU-----motor rotor copper loss), such as ignoring the rotor For copper loss, there are: PM = P2 For AC asynchronous motor, electromagnetic power PM = ( mp /2πf1) · ( I22 r / s) ---- (4) When the motor speed and frequency are constant, the electromagnetic power PM and The rotor current I22 is proportional. The rotor current of an AC asynchronous motor cannot be measured. Only the stator current I1 can be measured.
I1 = I0 + (-I2,) (I0---- is the excitation current of the motor). If the excitation current I0 is ignored, then I1 = (-I2,)
For DC motors, the electromagnetic power PM = CT φIS When the air gap flux φ is constant, the electromagnetic power PM is proportional to the armature current IS. The air gap flux φ is related to the voltage.

Therefore, when measuring with the current method, the speed and frequency must be kept constant for the AC motor. For DC motors, keep the voltage constant.

From the above analysis, the current method determines the unbalanced load ΔT = 0 on the motor shaft by measuring the stator current I1 of the motor, after a series of conversion relationships:

I1 → I2 → PM → P2 → T2 → △T

Each step must have certain conditions, in order: I0 is unchanged; f1 is unchanged; voltage U1 is unchanged; PCU is unchanged; speed n is constant; upper and lower T0 are equal, and the factors affecting these quantities are complex, such as When the elevator is going up and down, the air resistance of the car is different, and it is difficult to establish the resistance torque T0 of the up and down, which is especially true at high speed. There is also the influence of the human factors in the measurement process, such as: how to determine the current measured in the car to the same level as the counterweight; how to ensure the position of the upper and lower speed is the same; there are instruments for measuring current use; Measuring the position of the current, etc., will cause a large error. There is also a curve drawing. Since there is no measuring point in the range of 40%~50% load, the drawing of the curve contains a lot of human factors. These all affect the accuracy of the current method to determine the K value of the balance coefficient.

In summary, for the balance factor of the elevator, the meaning of the value should be correctly understood, and the influence of the change of the value on the elevator should be understood. The second is to take the correct and tangible measurement method.