A variable frequency drive for an elevator: why it matters and what it brings in a modernization
A modern elevator is rarely designed without variable frequency control today. Twenty years ago the drive ran on a "two-speed" scheme: accelerate quickly, then switch hard to a creep speed just before the floor. The passenger felt the jolt, leveling accuracy drifted, and the motor drew peak currents on every start. A variable frequency drive removes these problems: it smoothly controls both the speed and the torque of the motor throughout the entire travel cycle of the car.
When customers come to us for the modernization of an old elevator, the question is usually framed like this: keep the proven mechanics and the hoisting machine, but replace the relay-contactor control with a modern drive. In most cases this is justified and cheaper than a full replacement. A variable frequency drive becomes the heart of the renewed motion control system.
Elevator classification and drive types
Passenger elevators are the most common class. Their speed usually lies between 45 and 105 metres per minute, and the rated load between 450 and 1350 kg, which corresponds to six to twenty passengers. Freight elevators have a similar speed but a higher rated load, from 600 to 2500 kg. Car lifts in multi-level parking garages and escalators in supermarkets, railway stations and the metro stand apart as separate cases.
By drive type, the most common option is the traction drive (a hoisting machine with ropes), where the car and the counterweight move on ropes over a traction sheave. The second type is hydraulic, with a plunger and a hydraulic cylinder. Historically, traction elevators used geared asynchronous motors. Today, gearless permanent magnet synchronous machines (PMSM, gearless) are applied more and more often: they are more compact, quieter and more precise in control. Each motor type is matched with its own frequency converter for an electric motor.
S-curve acceleration and passenger comfort
What the passenger perceives as a "smooth elevator" is described in engineering terms by the acceleration profile. A simple linear ramp produces an abrupt change of acceleration at the start and at the braking point - this is the jerk. A variable frequency drive shapes an S-curve: acceleration builds up and falls off gradually, without steps. The car sets off softly, reaches working speed, and then brakes just as smoothly before the floor.
In practice several segments of the profile are tuned: initial smoothing, the acceleration rate, constant speed, the deceleration rate and the final approach. A correctly chosen S-curve is a compromise between comfort and travel time: a profile that is too gentle makes the elevator feel subjectively "slow", while one that is too sharp feels uncomfortable.
Leveling accuracy and the closed-loop vector control
Stopping exactly level with the floor is a separate engineering task. If the car ends up a few centimetres above or below the threshold, it is both inconvenient and dangerous for people with trolleys and suitcases. The motor control type is what decides this.
A simple variable frequency drive (open-loop V/f scheme) holds the speed approximately, without feedback about the real shaft position. For elevators a closed-loop vector control is used: an encoder is fitted to the motor, the drive knows the speed and position in real time, holds torque even at zero speed and brings the car exactly to floor level. It is precisely this loop that gives stable final approach and repeatable stopping regardless of the car load. In such a scheme the encoder is an additional option for the drive, selected to suit the specific motor.
Energy regeneration and the rescue drive
An elevator is a mechanism that regularly works in generator mode: when a heavy car goes down or a light one goes up, the counterweight "outweighs" it and the motor turns as a generator. This energy has to go somewhere. There are two approaches. A braking resistor simply dissipates the surplus as heat - simpler and cheaper, suitable for low traffic. An active regenerative unit returns the energy back to the grid - more expensive in hardware, but it saves electricity on elevators with intensive traffic. The choice between a resistor and regeneration is made according to the actual traffic of the building and the tariff.
A separate question is behaviour during a power failure. Many modern elevators have a rescue / evacuation drive mode: from a backup source the drive carries the car at low speed to the nearest floor and opens the doors, so that people are not left trapped between floors. It is important to understand the boundary: the variable frequency drive controls the motion, but the mechanical safety - the machine brake, the safety gear, the overspeed governor - are separate elevator subsystems with their own standards (EN 81-20/50). The drive and these systems complement each other, they do not replace one another.
How to tell it is time, and where to start
The basic selection principle is simple: the power of the drive must match the rated power of the motor, and the supply type must match the grid of the site (most often three-phase 380 V, less often single-phase 220-240 V for small drives). Next you look at the required control range, the availability of vector mode with an encoder, and the way the braking energy is handled.
When to choose modernization and when a full replacement? If the hoisting machine, ropes and guide rails are in good condition and the problem is the old relay control, the jolts and the inaccurate stops, then replacing the drive with a variable frequency one delivers most of the effect for less money. But if the mechanics themselves are worn out or the elevator does not meet current standards, it makes more sense to replace the elevator as a whole. To avoid a mistake in selecting the drive for a specific hoisting machine and motor, it is better to calculate it together with an engineer - write to us in the Contacts section, and we will pick a solution for your site.
Frequently asked questions
Why does an elevator need a variable frequency drive?
It smoothly controls the speed and torque of the motor throughout the entire travel cycle. This removes the jolts at the start and during braking, gives a precise stop at floor level, reduces inrush currents and mechanical wear, and saves electricity compared with the old two-speed scheme.
What does energy regeneration give?
When the elevator works in generator mode (a heavy car going down or the counterweight going up), the motor gives energy back. An active regenerative unit returns it to the grid instead of burning it on a braking resistor. In buildings with intensive traffic this noticeably reduces the electricity bill; for infrequent trips a resistor is usually enough.
Is it safe during a power failure?
Modern systems have a rescue mode: from a backup source the drive carries the car at low speed to the nearest floor and opens the doors. At the same time, mechanical safety - the brake, the safety gear, the overspeed governor - is provided by separate elevator subsystems, not by the drive itself.
Why is a PMSM better than an asynchronous motor?
Gearless permanent magnet synchronous machines are more compact, quieter and have a higher efficiency at low speeds. Paired with a vector drive and an encoder, they deliver better stopping accuracy and comfort. The asynchronous drive is cheaper and still widespread, especially in geared elevators and in budget modernizations.
Should I modernize an old elevator or replace it entirely?
If the mechanics (the hoisting machine, ropes and guide rails) are in good condition and the problem is outdated control, modernizing the drive to a variable frequency one delivers a bigger effect for less money. If the mechanics themselves are worn out or the elevator does not meet standards, a full replacement is more appropriate. The decision is made after an inspection of the specific site.
