Understanding Elevator Regenerative Drives: A Guide to Energy-Efficient Vertical Transportation

Introduction

Elevators are essential in modern buildings, but they are also significant energy consumers, accounting for up to 10% of a building's total energy use. In high-performance buildings where other energy uses have been minimized, elevator consumption becomes even more critical. As building codes push for greater energy efficiency, the elevator industry has undergone a major transformation in control algorithms, electric drives, and motor technology.

One of the most significant advancements is the regenerative drive, which plays a pivotal role in making elevator technology more energy efficient. Rather than wasting the recovery energy from the machine as heat—as traditional systems do—regenerative drives recover it as usable "green" energy that can be fed back into the building’s electrical network.

How Regenerative Drives Work

A motor is fundamentally an energy conversion device. Most commonly, we think of inputting electrical energy to a motor, which converts it to mechanical torque to perform work. This is called "motoring mode." However, depending on the nature of the connected load, a motor can also operate in reverse—converting mechanical torque back into electrical energy. This is called "generating mode" or regeneration.

Elevators are ideally suited for regenerative technology because the motor acts as a generator for approximately half of its operational use.

In most lifts, the counterweight is sized so that if a half load is placed in the cabin and the mechanical brake is released, the lift remains stationary. The counterweight is heavier than the empty cabin, so:

• With an empty lift cabin travelling up, the motor overhauls and pushes energy backwards
• With a fully loaded cabin travelling down, the cabin is heavier than the counterweight—again, overhauling the motor

When the motor overhauls, it requires negative torque and creates energy that would historically have been exported to resistors and lost as heat. Not only was the energy lost, but further energy was required from building systems such as HVAC to manage the resulting heat.

The Technology Behind Regenerative Drives

A typical 6-pulse variable frequency drive (VFD) has three main parts:

1. The Rectifier Stage – Converts incoming AC power to DC power. Importantly, the rectifier is a one-way street, allowing energy to flow into the inverter but not back onto the utility.
1. The DC Bus – Consists of a capacitor bank that smooths the rectified power and acts as an energy buffer.
1. The IGBT Output Stage – Uses pulse width modulation (PWM) to switch the DC bus voltage to the motor, producing an AC current. Energy flow here is bi-directional.

Before regenerative drives were available, excess DC bus voltage from the drive unit was dumped across a braking resistor. While this method is simple, all excess energy is turned to heat, which adds to machine-room cooling costs.

Regenerative drives enable the excess energy from the overhauling motor to be returned to the building's power grid. These regenerative systems are typically wired in parallel with the adjustable frequency drive, either as part of a new control system or as a retrofit to an existing AC drive.

Key Benefits

Energy Savings and ROI

Regenerative systems can recover up to 30% of an elevator's total energy consumption and are an improved technology over non-regenerative drives. Depending on the application, line regenerative systems can pay for themselves in as little as two years.

ABB's regenerative drive technology demonstrates this potential:

• Daily Energy Savings: 640 kWh
• Annual Energy Savings: 233,600 kWh
• Annual Cost Savings: ~$23,360 (at $0.10/kWh)
• Payback Period: Approximately 2.1 years

Space and Cooling Benefits

By eliminating the need for large braking resistors and the associated heat dissipation, regenerative drives reduce machine-room cooling costs and free up valuable space. Modern component costs have dramatically reduced regenerative drive costs to non-regenerative prices.

Sustainability and Building Certifications

High-efficiency elevators with regenerative drives contribute to green building rating systems such as BREEAM. Efficient elevators with speeds >0.15 m/s can contribute up to three BREEAM points by meeting the criteria published in the Energy section of the BREEAM assessment.

The use of regeneration on elevators alone can contribute to one of those points. Buildings that receive a high BREEAM rating often benefit from lower running costs because they are naturally more energy efficient.

Design Considerations and Challenges

Operation with UPS and Generators

A key design consideration is the operation of regenerative drives during emergency power scenarios. Buildings such as hospitals, nursing homes and apartment complexes may require emergency power operation during an outage.

• Generator Backup: Regenerative drives can regenerate back to a generator while in emergency power operation, assuming there is sufficient load to consume the energy.
• UPS Backup: Regenerating AC power back to an active UPS is not an option, because the UPS is essentially a large DC battery. Regenerating back to this DC supply could cause damage.

For UPS applications, the regenerative drive must be disabled while the UPS is active. This can be achieved with a dual-level redundancy setup where relay contacts open the enable signal and activate an external fault condition to prevent modulation by the regenerative drive. The drive still acts as the rectifier, supplying power to the drive unit to continue to drive the motor.

During UPS operation, any regenerated energy can be dissipated through a braking resistor on the VFD unit. Since the regenerative drive generally begins modulation at a lower DC bus trigger level than the braking transistor activation level, no additional contactor is needed to deactivate the braking resistor under normal operation.

Modern Motor Technology

The introduction of Permanent Magnet Synchronous Motors (PMSM) has accelerated the elevator revolution in terms of power quality, ride quality, and green energy. Compared with different types of vertical transportation machines, PMSMs have better powerful execution, compact size, and higher system-level efficiency. With the introduction of regenerative PMSM, electrical drives coupled to system-integrated frameworks for recovery energy have enhanced savings in power consumptions.

The Future of Elevator Efficiency

As building heights continue to increase and energy efficiency plays a larger role in new construction and modernization projects, line regenerative systems will continue to play an important role. Modernisation of ageing lifts to newer systems featuring advanced intelligent control technology can help reduce operating costs, improve ride quality, and extend the lift’s life cycle.

By working with experts that understand how the whole lift system works, building owners can understand the cost benefits of different technologies and how they can help them achieve efficiency accreditations.

Disclaimer: This article provides a summary of technical information on elevator regenerative drives from various industry publications and manufacturer resources. While efforts have been made to accurately represent the content, please refer to official manufacturer documentation and relevant standards for complete technical details. Final design, specification, and implementation decisions should be made by qualified professionals in accordance with all applicable regulations and standards.