Elevator World
5/01/96
Lift Power Consumption

Author: Lutfi R. Al-Sharif

This article discusses several topics related to the energy consumption of lift systems. It reviews most of the work carried out by others in the field, while outlining some measurements of DC lifts. It compares the consumption of various types of drives , outlines the concept of regenerating power back into the supply, and reviews a method for calculating the energy consumption based on the lift system's various parameters.

Two points worth mentioning:

*The efficiency of the motor will have an impact on the overall efficiency of the system, usually depending on the motor's size. Hence, the motor is the major component through which all energy transfer processes will take place.

*An under-utilised lift system will run at a very low power factor, having an effect on the electricity cost (depending on the tariff structure). However, this problem is beyond the scope of this article and not addressed.

The main reason for needing to know the energy consumption of lifts (or escalators) is to calculate the cost of such consumption, to reflect that into the system's design. However, in some cases, the energy consumption of a lift or escalator can be used for passenger-surveying purposes.

Percentage of a Building's Energy Costs

There are different estimates for the percentage energy dissipation of lifts as a percentage of the energy consumption of the whole building (5-15% of the total costs, depending on what other services are running in the building). KONE estimates it at 5- 10%, whereas Schroeder estimates 15% of the building's total electric energy consumption (assuming no air conditioning or oil heating). Moreover, Schroeder also estimates it at 1% of the total rental cost of the office space.

Comparison of Different Types of Lift Drives

Different types of drives use different levels of energy. This section compares the energy losses of the hydraulic, single-speed, two-speed, variable voltage and variable voltage variable frequency (VVVF) drives against the rated speed of the lift system .

Doolaard compared the energy consumption of various types of drives, comparing the following four:

* Hydraulic lifts: As these systems are usually not counterweighted, they are the least efficient of the four systems, because energy is stored as potential energy when lifting the car, then dissipates as heat when the car is lowered.

*Two-speed lifts: These systems employ a pole-changing motor, with the low-speed winding producing 25% of rated speed. Where the two-speed system is installed, a large flywheel has to be used to smooth the sudden change in torque, thus reducing the jerk perception of passengers. However, the flywheel stores energy, which is dissipated later, contributing to the lower efficiency of these systems.

*Eddy current braking systems (DC-injection braking): This system uses three pairs of back-to-back thyristors to control the phase angle of the stator voltage and uses a controlled bridge rectifier to control the DC-injected current in the low-speed wind ings of the motor. These systems were very popular before the VVVF systems were introduced; however, they suffer from a high level of harmonic current injected into the motor during low-speed levelling drive, causing severe heating of the motor. Thus, the y cannot drive the motor at low speed for long periods. Moreover, they cannot produce rated torque at lower speeds; therefore, their accuracy in starting and stopping relies on feedback. Consequently, driving accuracy suffers at low speed, as the system t ends to oscillate. The two main problems with this type of drive are:

1.As the speed is reduced, the torque is reduced, causing the system to become unstable, and does not cope satisfactorily with starting and stopping performance.

2.As the firing angle is increased (in order to reduce voltage and thus speed), the harmonic content of the voltage waveform imposed on the motor windings rises dramatically. This harmonic content does not contribute to any useful mechanical output from the motor, only generating more heat in the motor.

*Variable frequency drives: All of the problems encountered in the eddy current braking systems have been resolved in the VVVF systems or variable frequency systems in general. These systems overcome the problems encountered by the DC-injection braking s ystem, by offering the motor a nearly sinusoidal waveform at any speed and virtually constant torque across the speed range. VVVF drives must be sized depending on the peak conditions and have to be dimensioned in accordance with starting power.

As Figure 1 shows, the hydraulic system is the least efficient system. The use of a flywheel on two-speed systems makes them less attractive when compared to solid-state systems.

The other interesting point is that energy consumption rises dramatically with the increase in rated speed.

Formula for Calculating Energy Losses

Schroeder has developed a generalised formula for calculating the daily energy consumption of a lift installation. He based the formula on the typical trip time, which he labelled as TP. This factor, TP, depends on the number of floors, the type of drive and, consequently, the rated speed. Table 1 shows the values calculated by Schroeder for various installations and drives.

Once the relevant installation has been found in the table, the mean value of TP in the table is used. Alternatively, for more accuracy, 'the lower end of the range is used for 1:1 roping with a relatively large motor, or the upper end of the range is us ed for 2:1 roping with a relatively small motor.'

This is then used in the following formula:

E = (R x ST x TP)/3600

. . . where

E = daily energy consumed in kWH/day

R = motor rating in kW

ST = number of starts per day

The difficulty with ST is that it has to be estimated or measured, thus affecting the final accuracy of the estimate.

The value found is then used to calculate the yearly energy consumed (e) per floor unit area (in square meters), as follows:

E(kWh/day) x days/1 year x 0.85

e = population x space/person

This gives the annual energy consumed in units of energy (kWh) per square meter.

Regenerated Energy

In any lifting system, potential energy can be gained or lost by the system. Thus, in some cases, it draws energy from the supply, and in other cases, it returns energy to the main supply. Lifts always need to dissipate excess energy from the system. In some cases, this is returned to the main supply naturally (if a direct connection to the mains is used, as would be the case for a single-speed or two-speed AC motor, which will attempt to regenerate energy back into the mains when acting as an induction generator) or through a special power electronic setup (as in the case of inverter-driven systems with regenerative capabilities). In other cases, the energy is dissipated through resistors or as heat in the motor. This section discusses some of the aspec ts of regeneration and gives a practical example.

If the lift system had been an ideal system, with no friction and 100% efficiency, the regenerated energy would be equal to the consumed energy. However, this regenerated power is not equal to the consumed power for the following reasons:

1.The system needs to overcome friction in cases of both motoring and regeneration, i.e., friction in guide rails, air resistance, etc.

2.The motor (when acting as a generator) still needs to overcome its internal losses (i.e., the copper losses, the iron losses and the friction and windage losses).

3.If the gearbox is a worm type, its efficiency in the reverse direction will be significantly lower than in the forward direction.

Example On Motoring and Regenerating Modes

The following is a practical example to demonstrate the ideas discussed in regards to regeneration. Although the example is unique (employing a DC prime mover in a motor-generator setup), the principle of energy flow could be applied in any other system, provided it has the capability to return energy back to the main supply.

Motoring Mode

Figure 2 shows a general overview of the setup. It assumes that the hoist motor is motoring, thus drawing current from the generator. The measurements have been carried out on the terminals of the motor within the motor-generator set, i.e., the DC motor on the left of the figure.

When the lift is travelling down empty or up full, the hoist motor will be acting as a motor, thus drawing power from the generator. The generator will be driven by the prime mover (the DC motor in this case of the motor-generator set). The DC motor of t he motor-generator set will draw power from the DC supply.

Figure 3 shows a current trace which corresponds to the power (and hence the energy drawn) during a cycle, as the DC voltage is virtually constant during the trip. It can be seen from the figure that the highest power is drawn during the starting phase ( 70 A). It drops down to a constant value during running (45 A), then to a lower value during levelling (10 A).

Regenerating Mode

However, when the lift is moving up empty or down full, the hoist motor will act as a generator (Figure 4).

This mode is referred to as regenerating (or generating) mode. In this mode, the system will send power back into the mains, as shown by the reverse direction of the current from the prime mover.

When the lift is run up empty, it regenerates around 15 A back into the main supply. This is the current needed to stop it from overspeeding when it is driven as a generator.

Factors Affecting Energy Consumed

As seen from the previous sections, several factors Ð among many others Ð affect the energy consumption of the lift system:

*Type of drive: hydraulic, two-speed, etc.;

*Mechanical design aspects Ð discussed in much detail by Stawinoga;

*Efficiency of various components, especially the motor and the gear-box for a geared system;

*Reduction of inertia: the use of flywheels (and all other moving masses) reduce the system's efficiency;

*Type of gearing (if applicable): worm-wheel gears generally have lower efficiency than helical gears;

*Possibility of regeneration back into the mains, depending on whether the system can return energy and whether the metering system can cope with reverse energy;

*Running power factor, especially on Ward-Leonard systems; and

*Loading level: level of usage, number of passengers and the number of journeys.

Obviously, the tariff structure will affect the final cost of electricity consumption.

Conclusions

The energy consumption of lifts has been estimated between 5-10% of a typical building's total energy costs. The choice of drive system and rated speed affect the energy consumption and where the hydraulic system is least efficient and the VVVF system mo st efficient.

A general equation has been presented, dependent upon the type of drive, number of floors, number of starts per day and the rating of the motor to calculate the daily energy consumed. This can then be used to calculate the yearly energy consumed per unit floor area.

Lifts can and sometimes do regenerate power back into the main supply. A practical example was used to illustrate the general concepts involved.

References

1. Al-Sharif, L.R. 'The Use of Power Measurement to Calculate the Numbers of Passengers Travelling on an Escalator.' 1995.

2. Doolaard, D.A. 'Energy Consumption of Different Types of Lift Drive System.' Elevator Technology 4: Proceedings of Elevcon '92. Editor Dr. G. Barney, The International Association of Elevator Engineers, 1992.

3. Greenwood, P.B. Energy Efficiency Report. Brook Crompton Parkinson Motors, H1311, CP/E/283, 1983.

4. Kolmeder, W. & K. Hofbauer. 'Energy Losses and Energy Saving.' Elevator Electric Drives, Concepts and Principles, Control and Practice. Eds. Dr. G.C. Barney and A.G. Loher. Ellis Horwood, 1990.

5. KONE. 'V3F: The Green Power.'

6. Schroeder, J. 'Elevator Traction Drives.' Lift Report, May/June 1987.

7. Schroeder, J. 'The Energy Consumption of Elevators.' Elevator Technology. Editor Dr. G. Barney. Ellis Horwood, 1986.

8. Stawinoga, R. Private Communication (slides accompanying Designing for Reduced Energy Costs. May 1995).

9. Stawinoga, R. 'Designing for Reduced Energy Elevator Costs.' Lift Technology 1, Proceedings of LiftTech 94. Editor Dr. G.C. Barney. The International Association of Elevator Engineers, 1994.