Advanced Elevator Drive Topologies

Regenerative Drive 

During a typical elevator cycle, energy is put into the system, stored and returned. During motoring mode, energy flows from the utility to the motor and mechanical work is done when the elevator cab moves. Additionally, both potential and kinetic energy are stored in the elevator system and returned to the drive during generating mode. Since the current cannot flow across the input rectifiers, the regenerative energy is stored in the DC bus capacitors. 

elevator interior
(photo credit:

Braking Resistors

Traditionally, braking resistors have been used to rid the capacitors of the excess regenerated energy. As current flows into the capacitors, the voltage across the capacitors increases. When the DC bus voltage reaches a threshold, a resistor circuit is closed across the capacitors, and current flows through the braking resistor, dissipating the excess electrical energy as heat. The heat represents energy loss in the system, since it cannot be recovered. Braking resistors are twice as inefficient when additional energy is needed to power an air-conditioning unit to cool the control room.

Regenerative Unit

A regenerative unit can be used in lieu of a braking resistor, and the regenerated power can be returned to the grid, where it is consumed by other building loads. Regen units are becoming increasingly popular, as they can dramatically increase system efficiency and lower the total cost of elevator ownership. The components of regen drives are similar to those of a six-pulse VFD, except there is no input diode stage. The regen drive has DC bus capacitors and an IGBT stage used to output current into the mainline. Additionally, an inductor is used to smooth current flow and synchronize the current to the voltage and frequency of the mainline. 

When the voltage of the coupled DC bus reaches a threshold, the regen unit’s IGBTs cycle at the same frequency of the line and current flows back onto the line. It should be noted that since the regen unit’s IGBT’s only cycle at 60 Hz, there are significantly less switching losses during operation than the PWM operation of a VFD. Since the regen’s IGBTs switch the current into the line in a similar manner to the diodes in a six-pulse rectifier, a regen unit will have similar harmonic content during regen mode as a VFD
Basic wiring topology of a DC-bus-connected regen drive and VFD
(photo credit: Elevator Drives, Power Quality and Energy Savings)

Cost/Benefit Analysis

Regen Drive Applications using a regen drive should be selected carefully to ensure reasonable payback. Due to their high efficiency, all applications using gearless permanent magnet motors should be considered. 

High-duty installations where elevators run 24/7 are great candidates for regen units 

Potential installations include 

  • Hospitals, 
  • Hotels 
  • Airports. 

High speed (>500 fpm) and high capacity (>3,000 lbs.) should also be strongly considered, as the large moving mass at high speeds provides significant energy returns, these installations include freight cars, office buildings, etc. 

Low-speed, low-capacity and inefficient worm geared applications will not regenerate a lot of energy, so the regen unit provides little value in these cases. 

Typically, the addition of a regen unit offers the quickest ROI and provides the greatest increase to the operating efficiency of the elevator system. Even existing elevators operating with a VFD can be easily upgraded with a standalone regen unit (e.g., a small control box that mounts outside the main panel). 

Most utilities will give offsetting rebates or credits based on the upgrade to or installation of a regen unit. Since a regen unit does contribute harmonics during regen operation, some harmonic mitigation through filters might be considered in certain applications. However, energy audits have shown that elevators account for a relatively low amount of a building’s energy consumption ,around 3-5% of the building total.This is significant, because it means there are other building loads, like the HVAC or lighting systems, which potentially contribute far more harmonics to the building’s electric system than the elevator could

This article has been extracted from "Elevator Drives, Power Quality and Energy Savings" by Jonathan Bullick and Brad Wilkinson

The full article can be accessed below. 

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