Backup Power for an Elevator: What to Decide Before Choosing Equipment
Author: Svitlo-Lux editorial team. Published , updated . Review (engineering): approved (Project owner).
The elevator is the hardest load to back up: it is a motor with a sharp start, often three-phase, with safety requirements set by the manufacturer. So the first question should not be “what inverter power do we need” but “what exactly do we want from the elevator during an outage”.
Two different tasks: releasing people and carrying people
Elevator backup power is often described in a single phrase, but behind it are two different tasks with different requirements. The first: if power is lost, the cabin must reach the nearest floor and open its doors. The second: the elevator operates normally throughout the outage. The second task needs far more energy and power than the first.
| Mode | Essence | What affects the choice | Typical question for the engineer |
|---|---|---|---|
| Passenger release only | The cabin reaches a floor, the doors open, the elevator does not run further | Short duration, one trip, the start matters | Whether the control panel has a built-in emergency evacuation mode |
| Limited operation | The elevator runs for certain hours or for selected sections | Energy depends on the number of trips per hour | How many trips per hour are assumed and who counts them |
| Full operation | The elevator works as on a normal day | Power, phases, capacity, thermal conditions of the batteries | Whether the system is sized for simultaneous starting of other motors |
The board must clearly write down the target mode before talking to suppliers. Otherwise offers will compare systems sized for different tasks.
Why the motor’s rated power is not enough
An elevator motor draws more at start than in steady motion. The inrush current multiple depends on the drive, and a range from one and a half to three times the running value is only a guideline: it depends on the equipment, see the datasheet. Elevators with a frequency converter start more smoothly; older relay-based schemes start more sharply. That is why a typical mistake about the elevator sounds like this: “the motor is a few kilowatts, so we take an inverter of the same power”.
The platform’s calculator works with formulas from the methodology: the motor’s electrical power is derived from shaft power and efficiency, apparent power from cosφ, and the peak load is taken from a scenario where one motor starts while the others are already running. A starting safety factor is applied on top. Simultaneous starting of several motors is not covered by this model, so for elevators there is always a note “an engineering calculation is required”.
Illustrative example (hypothetical inputs, not the characteristics of a real elevator)
Suppose the motor shaft power is 7 kW, motor efficiency is 0.85, cosφ is 0.8, and the full starting power is twice the running power. Then the electrical power is about 8.2 kW, the running apparent power about 10.3 kVA, the starting power about 20.6 kVA, and with a starting safety factor of 1.25 the calculated peak requirement approaches 25.7 kVA. We can see that a backup source rated “7 kW” is clearly insufficient here, although from the datasheet that figure looks exhaustive.
Phases, UPS and inverter: what to check
Most elevator motors are three-phase. This means the backup source must either produce three-phase voltage or work with equipment that does. A single-phase inverter is not suitable for a three-phase motor without special arrangement, and three single-phase modules require careful balancing of the load across phases.
- Drive type: frequency converter, relay scheme or something else. Starting and sensitivity to voltage quality depend on it.
- Whether the control panel has an emergency travel mode or a separate input for backup power. A question for the elevator manufacturer.
- Whether the elevator will lose its warranty or certification when connected to third-party sources. Also a question for the manufacturer and the maintenance company.
- Requirements for the voltage waveform: some drives behave badly with non-sinusoidal sources.
- The method of switching from the grid to the backup: the device must prevent voltage from being fed back into the grid and must not cause control faults.
A separate guide on uninterruptible power sources for elevators and pumps describes how a UPS works and when it is used instead of full backup.
Energy: how much the elevator “eats” during an outage
Unlike starting peaks, an elevator’s energy is easier to estimate from the number of trips. Each trip needs a travel time and an average power under load, which depends on the weight of the cabin, the counterweight and the direction of travel. That is why data on typical daily consumption should be taken from a meter or from the maintenance company, not from a reference book.
In the calculator the elevator is entered as a motor with hours of operation per day and a duty factor. If this data is missing, the system uses conservative values and marks the unknown fields in the result. For the “release people only” mode the energy is so small compared with the power that the decision is driven by the starting power, not by capacity.
Typical elevator mistakes
- Adding up rated power without the start and finding a shortfall at the very first test.
- Not asking the manufacturer how the elevator behaves during a voltage sag.
- Addressing only battery capacity and forgetting inverter power.
- Choosing a single-phase solution for a three-phase drive.
- Not planning who promptly switches the mode on and who teaches residents not to use the elevator during an outage.
What to ask the engineer for
Documents and answers for the board
- The datasheet of the motor and control panel with starting data.
- A peak calculation stating the starting multiple and margin.
- A diagram of the backup input and a description of how the supply is switched.
- Written confirmation from the elevator manufacturer or service company that such a supply is acceptable.
- A description of the operating scenario: how many trips and what load are assumed.
- Instructions for residents and the dispatcher in case the elevator stops.
Frequently asked questions
- Can I request an inverter based only on the elevator motor’s rated power?
- No, the motor’s start exceeds its rating, and depending on the drive the difference is significant. The inverter is chosen for peak power, and the result is approved by an engineer after studying the datasheet.
- Is rescue mode enough for an elevator in an OSBB (condominium association)?
- For people’s safety, yes; it is the cheapest option. But if people with limited mobility live in the building, the board may choose an extended mode, which requires much more power.
Next steps
Inverter power calculatorPreliminary backup power calculation
This material is for general information only and is not a power supply design, legal or financial advice. Calculations are indicative; installation and protection selection must be done by qualified specialists only. The platform does not accept or hold funds.