Requirements for electric drives of elevators

The elevator is a single electromechanical system, the dynamic characteristics of which depend both on the parameters of the mechanical part and on the structure and parameters of the electrical part. The kinematic diagram of the elevator has a significant impact on the requirements for the motor control system and the electric drive.

So, in the case of a fully balanced mechanical system (the weight of the car with the load is equal to the weight of the counterweight and the balancing rope compensates for the change in the load due to the change in the length of the towing rope when the car is moved) there is no active load moment on the traction shaft , and the engine must develop a torque that provides for overcoming the frictional moment in the mechanical transmission, and the dynamic moment that provides acceleration and braking of the cabin.

In the absence of a counterweight, the engine must additionally overcome the moment created by the weight of the loaded cabin, which requires an increase in engine power, weight and dimensions.At the same time, if in the process of acceleration and deceleration the engine develops the same torque, the acceleration values ​​in these modes will differ significantly and additional measures are needed to equalize them, which increases the requirements for the tuning characteristics of the electric drive and complicates the control system .

It is true that the presence of a counterweight cannot completely eliminate the unevenness of the load due to a change in the cabin load, but the absolute value of the load decreases significantly.

The presence of a counterweight also facilitates the operation of the electromechanical brake and allows to reduce its dimensions and weight, since this significantly reduces the amount of torque required to hold the cabin at a given level with the engine off (with a fully balanced system, this moment is zero) .

In turn, the choice of the type of electric drive and the parameters of the electric motor can affect the kinematic diagram of the elevator. So when using a high-speed asynchronous drive, the presence of a gearbox in a mechanical transmission is inevitable to match the speeds of the electric motor and traction harness.

When choosing a direct current electric drive, low-speed motors are often used, the speed of which matches the required speed of the traction beam, which eliminates the need for a reducer. This simplifies the mechanical transmission and reduces power loss in that transmission. The system turns out to be quite silent.

However, when comparing geared and gearless drive options, the designer must also consider the fact that a low-speed motor has significantly larger dimensions and weight and an increased armature moment of inertia.

The operating mode of the elevator drive is characterized by frequent switching on and off. In this case, the following movement stages can be distinguished:

• acceleration of the electric motor to the set speed,

• constant speed motion,

• speed reduction when approaching the destination floor (directly to zero or to low approach speed),

• stop and stop the elevator car at the destination floor with the required accuracy.

It should be taken into account that the stage of movement at constant speed may be absent if the sum of the paths of acceleration to constant speed and deceleration from constant speed is less than the distance between the departure and destination floors (with floor crossing).

One of the main requirements for the electric drive of elevators is to ensure the minimum time to move the car from the initial floor of the car position to the destination floor when calling or ordering. This naturally leads to the desire to increase the stationary speed of movement of the elevator in order to increase its productivity, but increasing this speed is far from always justified.

Elevators with a high speed of movement of the car in the event that the latter have to make stops on each floor are not actually used in terms of speed, since acceleration and deceleration restrictions are introduced on the section between floors, the car does not have time to reach the rated speed, since the acceleration path to this speed in this case is usually more than half the span.

Based on the above, depending on the operating conditions, it is advisable to use drives that provide different stationary speeds.

For example, depending on the purpose, it is recommended to use passenger elevators with the following rated speeds:

• in buildings: up to 9 floors — from 0.7 m/s to 1 m/s;

• from 9 to 16 floors — from 1 to 1.4 m / s;

• in buildings of 16 floors — 2 and 4 m / s.

It is recommended to have express zones when installing elevators in buildings with a speed of more than 2 m / s, i.e. elevators should not serve all floors in a row, but for example multiples of 4-5. In the areas between the expressways, the elevators must operate at lower speeds. At the same time, control circuits are used, which, with the help of speed switching, can set two modes of operation of the electric drive: with high speed for express zones and with reduced speed for floor coverings.

In practice, when installing, for example, two elevators in one entrance, a simple solution is often used, in which the control system ensures that one elevator stops only on odd floors and the other only on even floors. This increases the speed utilization of the drives and therefore increases the productivity of the elevators.

In addition to the basic speed of the car, which largely determines the operation of the elevator, the electric drive and the control system of the elevator with a nominal speed of more than 0.71 m / s must ensure the possibility of moving the car at a speed of no more than 0, 4 m / s, which is necessary for a control survey of the mine (revision mode).

One of the most important requirements, the fulfillment of which largely depends on the structure of the electric drive and its control system, is the need to limit the acceleration and deceleration of the cabin and their derivatives (kicks).

The maximum value of the acceleration (deceleration) of the car movement during normal operation should not exceed: for all elevators, except for the hospital, 2 m / s2, for the hospital elevator — 1 m / s2.

The derivative of acceleration and deceleration (kick) is not regulated by the rules, but the need for its limitation, as well as the limitation of acceleration, is determined by the need to limit dynamic loads in mechanical transmission during transient processes and the task of providing the necessary comfort for passengers. Limiting the values ​​of acceleration and sudden movement should ensure a high smoothness of the transient processes and thus exclude the negative impact on the well-being of the passengers.

The requirement to limit the accelerations and thrusts to permissible values ​​contradicts the above requirement to ensure the maximum performance of the elevator, since it follows that the duration of the acceleration and deceleration of the elevator car cannot be less than a certain value determined by this limitation. It follows that in order to ensure maximum performance of the elevator during transients, the electric drive must provide acceleration and deceleration of the car with the maximum allowable values ​​of acceleration and sudden movement.

An important requirement for the electric drive of the elevator is to ensure the precise stopping of the car at a given level. For passenger elevators, the poor stopping accuracy of the car reduces its performance, because the time of entering and exiting passengers increases, and the comfort of the elevator and the safety of using the elevator decrease.

In freight elevators, inaccurate braking makes it difficult, and in some cases impossible, to unload the car.

In some cases, the need to meet braking accuracy requirements has a decisive influence on the choice of an elevator drive system.

In accordance with the rules, the accuracy of stopping the car at the landing level must be maintained within limits that do not exceed: for freight elevators loaded with floor transport and for hospital — ± 15 mm, and for other elevators — ± 50 mm

In low-speed elevators, the braking distance is small, therefore the potential change in this distance causing inaccurate braking is small.Therefore, in such elevators, meeting the requirements for stopping accuracy is usually not difficult.

As the speed of the elevator increases, so does the eventual spread of the car's stopping points, which usually requires additional measures to meet the stopping accuracy requirements.

A natural requirement for the electric drive of the elevator is the possibility of its reversal to ensure the raising and lowering of the car.

The starting frequency per hour for passenger elevators should be 100-240, and for freight - 70-100 with a duration of 15-60%.

In addition, the rules provide for a number of additional requirements for the electric drive of the elevator, determined by the need to ensure the safety of its operation.

The voltage of power circuits in machine rooms should not exceed 660 V, which excludes the possibility of using motors with a high rated voltage.

Disengagement of the mechanical brake must be possible only after the creation (of an electric torque sufficient for the normal acceleration of the electric motor.

In asynchronous electric drives, commonly used in low-speed and high-speed elevators, this requirement is usually met by supplying the supply voltage to the electric motors at the same time as the voltage applied to the brake solenoid.In DC electric drives used in high-speed elevators, before the brake is released, the control circuit is usually signaled to set the motor torque and current sufficient to hold the car at platform level without a brake (initial current setting ).

Stopping the cab must be accompanied by the actuation of a mechanical brake. Shutting down the electric motor when stopping the cab must occur after applying the brake.

In the event of a failure in the mechanical brake when the car is at the landing level, the electric motor and the power converter must remain on and ensure that the car is kept at the landing level.

It is not allowed to include fuses, switches or other miscellaneous devices in the armature circuit between the motor and the power converter.

In the event of an overload of the electric motor, as well as a short circuit in the supply circuit or in the control circuits of the electric drive, it must be ensured that the voltage is removed from the elevator drive motor and the mechanical brake is applied.