The regenerative drive elevators: what, how and why

For several hundred years, we are dealing with different types of elevators. First it was the type of mechanical lifts crane and winch drum, which is driven manually, beasts of burden, water wheels or steam. Traveling system ropes allowed to lift even the heaviest loads.

For repeated use, the inventors learned rapidly equilibrated with the load lifting platform or cab stand to reduce the force required to move the payload up and down the shaft. By the end of the XIX century invented many improvements in terms of safety of lifts and the practical use of electric motors. In spite of the scientific debate about the comparative characteristics of energy use of AC and DC, to the 20th years of XX century, almost all the lifts have been rebuilt or newly constructed, based on the use of electric motors. The drum winch gave way to a system with a counterweight, pulleys forward thrust and steel suspension cables.

Electric elevators became a catalyst for the construction of a huge number of high-rise buildings in large cities. Despite the many improvements aimed at increasing the safety and ease of use, in the elevators of the day still used lifting mechanism with a counterweight and a drive pulley with hanging ropes. The reason is simple -uravnoveshivanie weight (mass) of the cab against the action of gravity reduces the amount of force required to move the shaft in the cabin up and down. Furthermore, when the added additional counterweight for balancing the system with an average payload requires the least effort for the intended load range of typical values.

Less effort to move the average load and the smaller is the average amount of energy consumed. In today’s world one of the main objectives of improving -energosberezhenie. Normally we pay for electricity by meter reading kilowatt-hours. Anything you can do to save energy or to minimize its consumption, of course, allows you to save money and preserve energy sources.

It is known that in many low-rise buildings using hydraulic lift mechanism, but this article will be considered only traction elevator.

Energy

What we call energy? From a scientific point of view, the energy — it is the action of performing work where basic definition is a force multiplied by the distance over which the object moves. The elevator operation involves two main types of mechanical energy: potential and kinetic.

Subject (weight) gains or loses potential energy as it is raised or lowered by the action of gravity. Elevator must invest energy in the payload when it rises (weight multiplied by distance). The same amount of potential energy is lost payload when it falls under the force of gravity.

Kinetic energy is the energy of motion, to accumulate in the mass as it moves. We often call it inertia or momentum. The magnitude of the kinetic energy of a moving object is equal to half the mass multiplied by the square of the speed. It is necessary to add to any object, to speed up its movement, or set it in motion. Similarly, the same value of the kinetic energy of an object must be removed to make him stop. The kinetic energy is of great importance when we consider the structure of an elevator or the speed of their movement.

All major components of the elevator should move to propel the payload. To ensure the required mechanical strength necessary to the mass of the frame cabin and the counterweight has a much greater mass than the nominal payload lift. As a result, the total kinetic energy that must be added to accelerate the movement of the elevator, or remove it to slow down, four to five times higher than the kinetic energy that would be when moving alone payload. Please note that we are talking about mechanical energy, and its bi-directional flow. That is, we need to make energy to mechanical components and payload to make them move with acceleration and climb. The energy is released again when we stop the movement, or down load. To lift the action is repeated several times throughout the day. The cyclical nature of energy flow provides an ideal opportunity to minimize total energy consumption.

The rate at which energy flows for the job is called power. It is equal to the force times the velocity in the mechanical units. In electrical terms, it is expressed in watts (or kilowatts) representing the voltage (power), multiplied by the current (the flow velocity of the electrons), which occur simultaneously. The power flow is bi-directional, too. When the demand for electrical power is measured and stored for some period of time, resulting in a total value of all the work carried out during this period. Electricity meters, standing in place liner power supply to the building, do exactly that. They determine the total energy flux at the end of the month. This is the true value of the electrical energy supplied to the general-purpose power station building where the measured amount of electricity.

The motors that move the elevators — are devices that convert the power. Electric power is supplied to the input (in volts x amps or watts) and is converted into mechanical power (torque x / min or horsepower). Motors and generators have a similar structure and are also similar. All electric motors can operate as electric generators. Each time under the influence of the motor shaft is mechanically forced to move more quickly than does the source of electrical power, mechanical power (torque x speed of rotation) is converted into electrical power (Amps x Volts). Consequently, the power can go through the motor in either of these directions. This is true DC motor (with or without brushes), AC induction motors and synchronous motors, permanent magnet. This principle allows you to get back to the energy temporarily stored in the mass of the moving elevator.

Variable speed drives are one of the types of converters of electric power, which is normally used to convert a fixed-voltage power supply in the regulated voltage, controls the speed of the elevator motor. On variable speed drives usually think only as drives and AC motors, but the speed of DC motors can also be easily adjusted with the voltage, and that for many years has been used in elevators. Aggregates generator-motor type systems Ward-Leonard represent a type DC drive, as well as SCR is a control device DC variable speed. In any of these cases, the adjustment voltage (and frequency for the AC drive) is the primary mechanism by which we control the amount of power going to the motor or from it.

When the power converter produces a voltage exceeding the magnitude of counteracting voltage (counter-electromotive force generated by rotation inside the engine), the electric power to the motor is to create an acceleration or lifting torque on the shaft and sheave of the elevator. When the output voltage of the power converter is less than EMF of the motor, the motor operates as a generator, converting the mechanical power (torque x speed) into electricity, which flows back to the power converter. From the design and type of electric power inverter used to control motor speed depends on what happens with the regenerated energy coming from the motor lift.

Elevator motors DC control type engine generator unit

Engine-generator unit is an AC induction motor connected to the power supply, which rotates the shaft of a DC generator with a relatively constant rate. Dc elevator motor connected to the output of the generator. Adjusting the force field generator directly modifies the output voltage to the motor lift DC. The voltage of the generator with respect to the elevator motor counter determines the direction and magnitude of the real power flow. When the elevator motor applies a braking force to the load, the power comes from the engine to the generator. Then, the generator begins to operate as a motor and trying to speed up the rotation of the shaft connected to the AC induction motor. Since the asynchronous motor is slightly faster than allow the frequency and voltage, it essentially becomes an asynchronous generator, producing electrical energy and sending it back to the supply network. Here, of course, plays a role of equipment efficiency, but the fact that the engine-generator unit regenerates energy often forgotten. Even though the AC motor generator unit engine idling current takes a considerable amount of low power factor, the current in the power supply is always free from harmonics.

DC motors of the elevator with the control of the drive of SCR

Thyristor drive converts the AC voltage from the power supply into a regulated DC voltage by selectively rectification process known as phase control. Thyristors act as switches, lead-in certain parts of the sine wave voltage from the power supply to the DC motor. Adjusting the timing phase of each thyristor switch relative instantaneous ac voltage regulates the average DC voltage applied to the motor. A double thyristor bridge allows the drive current to flow in either direction, to the power supply and from it. By adjusting the average DC voltage at the output of the power converter with respect to counteract engine manages the direction and magnitude of power. The process of power conversion using SCR is particularly effective when motoring or regeneration. However, the mechanism of the phase control variable voltage creates a significant amount of current harmonics in the power supply in all operating conditions at significantly changing the power factor.

Engines lift AC inverters

The speed and torque of AC motor — synchronous or asynchronous — governed by an electronic inverter power by changing the frequency and voltage applied to the motor leads. There are several types of inverters, and all used quickly adjustable pulsing with an electronic switch (pulse width modulation) to convert the temporary constant voltage DC bus voltage or three-phase variable AC voltage with a variable frequency. Although used for precise control of various technical schemes, the engine is driven, when the voltage and frequency supplied to the AC motor exceeds the EMF generated by the motor is proportional to the speed. Power comes from a DC source via an inverter and the motor to the mechanical load. When the applied voltage and the frequency is lower than the generated EMF, the motor acts as a generator. Mechanical power rotary shaft motor is converted into AC electric power, and the inverter communicate this power back to the DC bus.

Inverters are not regenerating energy

In many inverters used to drive alternating current motors installed unidirectional rectifier capacity at the front end for converting an AC voltage from the power supply into a DC voltage, which is temporarily stored on the internal power bus capacitively filtration. It is an ideal low-cost way to supply a DC voltage to control the AC motor via the inverter. However rectifier may supply power only in one direction. When the elevator is working so that the motor and inverter regenerated power to control the rate controlled load moving under the influence of gravity (rising empty cubicle or descending cabin) or simply release the kinetic energy to slow moving mass, energy is pumped into the DC bus, thereby increasing the bus voltage in the capacitors that store energy.

The traditional method, solves the problem of the accumulation of excessive amounts of energy, is the installation of a circuit that temporarily closes the switch, discharge the excess energy in the block of resistors. Resistor braking is one of the ways of using the energy recovered from the elevator equipment. But it is a net loss measured in terms of energy consumption. There are other losses due to the efficiency of the equipment of the elevator, but 25-40% of the energy consumed by the elevator, it is usually lost on the block of resistors. The heat generated by the resistors should be removed from the premises management system. You have to pay extra for the energy used to run fans or air conditioners that may be required to remove the heat from the resistor block.

Regenerating consoles

For the majority of traditional inverter used to control the operation of the engine, you can add a module regeneration power to regulate the flow of power from the DC power supply into the mains supply. The action is similar to the second inverter operates synchronously frequency power supply. When the elevator motor and the inverter are such that the power is pumped back to the DC bus, causing a voltage rise above the peak mains (ascent or descent empty car cabin, as discussed above), the module sends a surplus regenerative power back to the network. It holds the key to the rise of the voltage DC bus, so that the DC braking resistor is not used. The energy that would be lost to the braking resistor in the form of heat, is returned to the power distribution system from the power plant. This gives a significant difference in the total energy consumption, especially for gearless elevator system permanent magnet having a low coefficient of friction.

Where is regenerated energy?

The distribution of electric power inside a building often takes a form shown in Fig. 1 or 2. The purpose of the building, for example, a corporate office or hotel, you can put one electricity meter, where the energy from the power supplied to the building. The mixed-use commercial buildings have electric public lighting hallways, heating, ventilation and air conditioning, also elevators, and, in addition, there are individual meters for different tenants such as restaurants, offices and residences. Step-down transformer that distributes power from the power plant can be located outside the building — in the basement of a nearby power pole or small buildings. In large buildings, often set down transformer substation, built in the basement of a building or other convenient location. In any case, there is a power distribution center equipped counters (panel switch), where multiple power line disperse to other parts of the building. To lift usually provides its own electricity grid, but it almost never put a single electricity meter.

Even under the condition that the aggregate nominal capacity of all the lifts can be a great work of elevators is random, and the actual electrical load is relatively small compared to the total load of other continuous supply lines suitable to the same distribution panel. The total power consumption of all the electrical loads of the building, measured electric meter and the rotation speed of the electric meter depends on the population of a building in the daytime and night-time, outdoor temperature, the use of other modes of machinery and on whether the hoist lifts. Note: when the equipment lift regenerate energy, it can be sent back directly to the distribution panel, and from it to any other equipment used in the building, bypassing the electric meter.

Within a few seconds, during which there is a regeneration equipment lift, the counter should not rotate in the opposite direction: just slowing its rotation at the time when the energy received from the elevator, helps to provide power to other elevator, the lighting in the lobby or fans air-conditioning systems . There are no other visible manifestations of this process, such as a surge voltage or the distortion of its momentum, which may cause flickering lights or other equipment malfunction. Just some of the energy, which is measured by the meter and used to start an elevator or lift the load, to get back and used again for other work within the building. An intelligent building owner noticed that the repeated use of the same energy reduces energy costs.

Keep in mind that the rated performance of the elevator motor shown on the nameplate (in volts, amps, horsepower or kilowatts), based on the value of the power required to raise the maximum rated load of the elevator rated speed. However, the elevator is not always filled completely up to the rated load, and it works only for short periods of time. Furthermore, the losses due to the efficiency dictate that the total amount of regenerated power is always less than the value necessary for operation under full load of the elevator and the elevator capacity can be regenerated for a time of less than half of its operating cycle. Finally, on a normal day it is regenerated only a small part of the nominal electric power of the elevator at full load, and it’s even smaller part of the total amount of energy consumed by the building.

Regeneration compared cogeneration

While these terms may sound the same, there is a significant difference between the regenerative elevator system and cogeneration. Cogeneration systems (wind turbines, fuel cells, photovoltaic cells and generator operating on the exhaust gases) are designed for nearly continuous operation in the region of the nominal power. The aim is to ensure the CHP the main part of the energy required for the local buildings, or even for sale of electricity power plants for distribution to other paying customers. The power regenerated equipment lift is sporadic and relatively small. It is designed to conserve energy by reusing it to supply other loads within a building. Use of getting back energy to useful purposes, instead of losing it as heat — is an effective way of energy saving.

When the elevator drive regenerates power obtained new energy really flowing back into the supply network. In most cases, a local electric load takes this energy before it reaches the electric meter. If the local load is not large enough to absorb all the regenerated power, it still goes back to the supply network. However, in many electric meter is a ratchet mechanism that does not allow them to rotate in the opposite direction. However, power will still pass through them to the distribution network. The energy will be saved, but the owner of the building does not necessarily receive full compensation.

In case of emergency generator power to actuate lift when no power supply from the mains to the generator should be connected as the load, absorbing energy. It is necessary to analyze the consumption of energy in emergencies, to determine the minimum size of a generator used for these purposes. Very often, lighting equipment and heating, ventilation and air conditioning, are connected to the same generator will absorb all of the energy regenerated lifts. However, the connection of only one elevator to the emergency power generator of the appropriate size will not give the desired effect due to the random nature of the work and the inability to lift the generator to absorb any significant amount of regenerated energy.

You can reduce the value of the recovered energy by reducing the speed of the elevator in an emergency. The maximum number of regenerated power and energy will decrease in direct proportion to a decrease in the operating speed of the elevator. Internal losses in most systems, the elevator will run at 10-15% of nominal power regeneration without any substantial power to the supply network or power line generator.

In the case of an elevator drive with additional regenerating device may be able to use the control logic to turn off and turn on the regeneration of the traditional heat dissipation in the braking resistor under emergency operation. It is more difficult (and expensive) to provide backup heat dissipation in the braking resistor in the elevator drive, designed for automatic regeneration, while controlling the harmonics in the power supply and power factor. Most of the equipment elevators, produced at the moment, is not designed to enable and disable the regenerative function.

Other concerns about security in the use of regenerative drive elevators are totally unfounded. The design of drives, control the operation of the engine, as the regenerative energy, and without this feature provides the ability of perception of the state of the energy from the power supply. This is a prerequisite in the development of the design, to provide a reliable product in the elevator construction industry. If the power from the power supply suddenly weakens, or not served at all during the drive, he should feel bad conditions of power supply shut down in a controlled manner and to remain intact. If the power is interrupted during the trip of the elevator, the drive quickly perceives the situation on and off. According to the safety standards related to the control circuit also include a mechanical brake to safely stop the lift at a time when the drive stops working. Standard Elevator requires overspeed protection in the form of a speed controller with an electrical switch that opens the control circuit, disconnecting the motor and controls including mechanical brake. The high-speed elevators can be provided more dynamic braking circuit of the engine, helping to stop the elevator. The combination of these functions means that the elevator will be stopped quickly and safely, even if it regenerates energy into power is turned off the network.

The mechanism of perceiving off energy, regenerative drives, as mentioned above, will turn off the drive for a split second in the event of a power failure from the mains. The energy of a moving elevator will be given when stopping the elevator mechanical braking system. It will not continue to be transmitted to the supply network.

Regeneration is naturally in thousands of buildings around the world, while the system power supply from the network remains healthy and strong. The huge capacity of the power supply allows instant energy required for operation of the elevator, walk in any direction and be relatively invisible in large buildings. True regeneration captures most of the mechanical power operating elevators and sends it back to the electrical system, where it can be reused for other elevator or other types of electrical loads. Production of renewable energies can contribute to energy efficiency and minimize energy consumption that will definitely help to save money and conserve energy sources.

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