Can a UPS Run a VFD Elevator During an Outage?
A request comes in: can a 22 kW elevator keep running for 20 minutes after a power cut? A controller nameplate in the same enquiry says 18.5 kW. Which number should go on the UPS quotation?
Neither, at least not yet. One may describe the motor or controller rating; neither tells us the elevator's complete electrical demand at the point where the UPS would connect. The elevator also has a variable frequency drive (VFD), which changes the starting behaviour and may handle braking energy in different ways. Those details decide whether a proposed UPS is suitable.
The short answer is yes, a UPS can support continued elevator operation if the elevator manufacturer approves the emergency-power arrangement and the entire UPS and battery system is designed and tested for it. A UPS that can move the car to a landing and open the doors has met a different requirement. It has not necessarily been approved to keep answering calls for 20 minutes.
Emergency rescue, reduced service and continued normal service should be written as separate requirements. In a rescue sequence, the car reaches a landing, opens its doors and may be taken out of service. With standby power, some elevator systems return cars one at a time and then leave a selected car in service. The controller's emergency-power settings and building controls decide what actually happens. Supplying AC power alone does not override that logic.
For this project, “20 minutes” means continued operation with passengers, door cycles and repeated trips. Ask the elevator supplier to confirm that this mode is permitted on a UPS supply and to provide the wiring and control requirements. The elevator contractor should also confirm what local requirements permit for passenger service on emergency power. Include the emergency-power signal, brake control, door operator, lighting and communications. The exact list depends on the elevator design; do not assume the traction motor is the whole protected load.
An 18.5 kW label on a controller may identify its rated capacity. A customer's 22 kW figure might be a motor rating, an installed-system estimate or a measured input. Until the source of each figure is clear, it is unsafe to treat either as the power the UPS will continuously deliver.
Get the elevator maker's supply data for the complete controller, then measure at the proposed UPS connection point if the installation exists. Record three-phase voltage, running kW and kVA, RMS current, and the highest input demand during acceleration, travel, braking and door operation. Capture more than one trip and the relevant car loads and travel directions. A VFD usually avoids the direct-on-line motor starting surge, but energising the drive and accelerating the car can still produce input peaks. Use the drive's actual input data and the UPS model's overload curve rather than a universal motor multiplier. Our guide to UPS sizing for inductive loads explains the broader motor-load checks.
Information checklist for a VFD elevator UPS enquiry; confirm the values with the elevator manufacturer.
| Information to request | Why it changes the UPS decision |
|---|---|
| Elevator and VFD model, input voltage and phase | Confirms the required supply and the manufacturer's approved backup arrangement |
| Measured input kW, kVA and current over representative trips | Establishes the load seen by the UPS, rather than motor nameplate power |
| Highest input current and how long it lasts | Allows a check against inverter current and overload limits on battery |
| Brake resistor or regenerative front end | Shows where generated energy goes during braking or an overhauling trip |
| Control, door, brake and auxiliary circuits | Identifies every load that must stay powered for continued service |
| Required traffic pattern and minutes | Defines the battery discharge duty and the site acceptance test |
An elevator drive can receive energy back from the motor on some parts of a trip. One design routes it through a brake chopper and resistor, where it becomes heat. Another may feed energy back toward the AC supply through a regenerative front end. These are different interfaces for the UPS.
If the installation can export power toward the UPS output, the elevator and UPS suppliers must agree how that energy will be absorbed or controlled when the UPS is on battery. Do not assume a standard UPS inverter can accept it, and do not count it as extra battery runtime without a verified system design. Check the actual VFD and braking circuit diagram; the word “VFD” alone does not identify the regeneration path.
An online double-conversion UPS can keep its inverter supplying the load when utility input disappears, provided the UPS is operating normally and within its ratings. That makes it a candidate for a critical elevator load; it does not make every online model suitable. Confirm the UPS's three-phase output, continuous kW and kVA ratings, battery-mode peak current, overload duration, output voltage response and bypass behaviour against the elevator maker's data. An overload that sends the UPS to bypass may leave no usable bypass source during an outage.
People often ask whether the UPS should be low-frequency or high-frequency. That choice may affect the models available and their isolation, short-term current and installation characteristics, but the label cannot replace the model's data. For example, TAFENG's 40 kVA / 32 kW low-frequency online UPS shows why both output ratings must be checked. Its catalogue rating alone does not establish that it can run this elevator or handle its regenerative behaviour. The same approval and load checks apply to a high-frequency proposal.
Twenty minutes at a stated load is a battery requirement; it is not a feature conferred by choosing a 40 kVA UPS. For continued operation, define the expected number of trips, passenger loading, travel directions, door cycles and any idle periods during those 20 minutes. The highest short-duration demand is important for UPS capacity; the load schedule and battery discharge data determine the bank required for the full interval. Allow for the battery's specified end-of-life condition and site temperature. Our UPS battery runtime sizing guide shows the calculation path from measured load to battery discharge data.
If a generator later takes over, measure the interval until the UPS accepts its supply. That is a separate bridge requirement from running the elevator on batteries for the full 20 minutes; our UPS and generator handover guide covers that sequence. If there is no generator, the battery must cover the agreed elevator operating duty itself.
For a VFD elevator UPS proposal, start with the controller wiring diagram, emergency-power mode and VFD/braking details from the elevator supplier. Give the UPS supplier the measured input trace, required operating time and any simultaneous building loads. Then test the approved arrangement with the car operating, not just with the controller idle: cut the normal supply under controlled conditions, run the agreed trips and door cycles, and record UPS current, voltage, alarms, battery discharge and any VFD faults. The elevator contractor should oversee the sequence and confirm that the safety circuits and controls behave as intended.
That test answers the real question. An 18.5 kW or 22 kW label may start the conversation, but it cannot promise 20 minutes of continued elevator service.
