Lesson 5 of 8How lifts work
Lift controllers and drives.
The controller decides what the lift does next, and the drive makes the motor do it. Here is how the two work together with the buttons, doors, sensors and safety circuit around them.
What a controller does.
Every lift needs something to decide what it does next. That is the controller. It takes in information, makes decisions and sends out commands, many times a second, for as long as the lift is in service.
| Takes in | Decides | Sends out |
|---|---|---|
| Landing and car calls | Which floor to serve next, and in what order | Run, slow and stop commands to the drive |
| Car position and load | When to slow down, and whether the car can take more passengers | Open and close commands to the doors |
| Door and safety circuit status | Whether it is safe to start, and whether to take the lift out of service | Signals to indicators, gongs and announcements |
| Special mode switches and fire signals | Whether to change how the lift behaves | Fault records and status to monitoring |
Explore the control system.
The block diagram shows how the controller connects to the rest of the lift. Select a number or a part to see what it does.
Landing buttons
The buttons at each landing send landing calls to the controller. Most floors have an up and a down button, so the controller knows which way each passenger wants to go. In buildings with destination control, the buttons are replaced by a keypad or screen where you enter your floor.
Car buttons
The car operating panel sends car calls: the floors passengers want to go to. It also has the door open and close buttons and the alarm button, and sometimes key switches for special modes.
Other lifts in the group
Where several lifts serve the same floors, their controllers are linked by group control. It allocates each landing call to the car that can answer it best, so the lifts share the work instead of all chasing the same calls.
Controller
The controller is the decision-maker. On modern lifts it is a microprocessor system; on older lifts it may be a panel of relays. It keeps a list of calls, decides where the car goes next, and tells the drive, doors and indicators what to do. It also records faults, which engineers read when diagnosing a problem.
Safety circuit
The safety circuit is a chain of safety switches: the landing door locks, the car door contact, the limit switches, the governor switch, the stop switches and more. The controller monitors it, but the circuit itself cuts power to the motor and lets the brake apply if any switch opens, whatever the controller is doing.
Position and load sensors
A positioning system tells the controller where the car is, using magnets, vanes or a coded tape in the shaft, or an encoder on the machine. A load-weighing sensor measures the load in the car. On a lift built to BS EN 81-20 it must stop the lift starting once the load is 10 per cent over the rated load, or 75 kg over if that is more. Many lifts also use it to pass landing calls when the car is full.
Drive
The drive supplies power to the motor and controls its speed. Most modern lifts use a variable-voltage, variable-frequency (VVVF) drive, also called an inverter, which follows a speed profile set by the controller so the car accelerates, travels and slows smoothly.
Door operator
The controller tells the door operator when to open and close the doors, and how long to hold them open. The operator reports back when the doors are fully open or closed, and the light curtain reports if anything is in the way. Read more about lift doors.
Indicators and gongs
Position indicators, direction arrows, gongs and spoken announcements tell passengers where the car is, which way it is going and which lift is arriving. Clear visual and audible signals matter for everyone, and especially for people with impaired sight or hearing.
Motor and brake
The drive runs the motor, and the brake is released only when the controller and the safety circuit allow the car to move. Whenever the car stops, or the power or safety circuit is interrupted, the brake applies and holds the car.
Alarm and monitoring
The alarm button connects trapped passengers to a rescue service through two-way communication. It has its own emergency power supply so it keeps working in a power cut. Under BS EN 81-28, if that supply can no longer keep the alarm working for an hour, including 15 minutes of speech, the system must tell the rescue service. Many lifts also send fault and status data to their maintenance provider for remote monitoring. It is drawn with a dashed line because it works independently of the lift’s normal control.
Drives: how the motor is controlled.
The drive is what makes a modern lift ride smoothly. It takes a speed profile from the controller and supplies the motor with exactly the power it needs at each moment, so the car accelerates gently, travels at its rated speed and slows to a precise stop at the floor.
Most lifts installed or modernised in recent decades use a VVVF drive. Older lifts may still have earlier technology:
| Drive | How it works | What it means in practice |
|---|---|---|
| Two-speed AC | A motor with fast and slow windings, switched by contactors | Noticeable jolts at start and stop, and less accurate levelling |
| DC with motor-generator set | A motor-generator set feeds a DC lift motor at variable voltage | Smooth but energy-hungry, with a lot of rotating equipment to maintain |
| DC thyristor | Electronics control a DC lift motor directly | Smoother and more efficient than a motor-generator set, but now old technology |
| VVVF (inverter) | Electronics vary the voltage and frequency supplied to an AC motor | Smooth ride, accurate levelling and lower energy use |
Some drives are regenerative. When the motor is being driven by the load, for example a full car going down or an empty car going up, it acts as a generator. A regenerative drive feeds that energy back into the building’s supply instead of burning it off as heat.
Relays, microprocessors and proprietary systems.
Older controllers used banks of electromechanical relays, wired so that the lift’s logic was built into the wiring itself. Many still work, but they are bulky, wear out and are hard to diagnose. Modern controllers are microprocessor systems that record faults and can be monitored remotely.
Some controllers are proprietary: diagnosing or adjusting them needs the manufacturer’s own tools or software. Others are designed so that any competent maintenance company can service them. That choice matters for the life of the lift, because it affects who can maintain it and how easily parts can be obtained.
Special operating modes.
As well as normal service, a controller has modes for particular situations:
- Inspection: an engineer on the car roof or in the pit takes control using inspection controls, and the car moves only at slow speed while a button is held.
- Fire recall: when the building’s fire detection system signals the lift, it returns to a designated floor so passengers can leave, and then stops answering normal calls.
- Firefighters lift operation: a firefighters lift can be taken over by firefighters with a switch at the fire service access level, and then answers only their commands.
- Independent service: a key switch takes the car out of normal service, for example to move furniture, so it answers only calls from inside the car.
- Out of service: if the controller detects a fault it cannot recover from, it takes the lift out of service and records the fault.
How a lift should behave in a fire is set out in BS EN 81-73, and the additional requirements for firefighters lifts in BS EN 81-72.
When a controller is replaced.
Controllers usually wear out or become obsolete before the heavy parts of a lift, such as the guide rails and the machine. Signs that a controller may need replacing include:
- repeated faults that are hard to trace or keep coming back;
- replacement parts that are no longer made, or only available second-hand;
- a rough ride, poor levelling or slow door operation that adjustment cannot fix;
- no way of adding features the building now needs, such as remote monitoring or better accessibility.
Replacing the controller, usually with a new drive, is often the core of a modernisation. It can transform how a lift rides and how reliable it is, while keeping parts that still have years of life in them. Our lift modernisation page explains how we plan these upgrades.
Engineer’s note
When comparing quotes for a new controller, ask whether it is proprietary or non-proprietary, who will be able to maintain it, and how long the manufacturer commits to supplying parts. Those answers affect your maintenance costs for years.
Check your understanding.
Three quick questions on this lesson. Choose an answer to check it, or open the answer below each question.
-
What does a VVVF drive do?
Show the answer
B. It varies the voltage and frequency supplied to the motor to control its speed. A variable-voltage, variable-frequency drive controls the speed of an AC motor by changing the voltage and frequency it receives, giving smooth starts, stops and accurate levelling.
-
In a group of lifts, what decides which car answers a landing call?
Show the answer
B. Group control, using each car’s position, direction and existing calls. Group control shares landing calls between the lifts, sending the car that can answer each call best.
-
What happens if a safety switch opens while the lift is running?
Show the answer
B. The safety circuit cuts power to the motor and the brake applies. The safety circuit is wired so that opening any switch stops the lift: power to the motor is cut and the brake applies.
Sources.
The primary sources for this lesson. We checked them when the lesson was last updated on 18 October 2026. How we research and check our lessons.
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Related reading
Is your controller getting old?
Talk to an engineer about parts availability, reliability and your options for an upgrade.
Common questions.
Quick answers to common questions about this topic.
What does a lift controller do?
A lift controller registers calls from the landing and car buttons, decides which floor to serve next, commands the drive to run the motor, opens and closes the doors and drives the indicators. It also monitors the safety circuit and records faults.
What is a VVVF drive on a lift?
VVVF stands for variable voltage, variable frequency. A VVVF drive controls the speed of the lift motor by varying the voltage and frequency it supplies, which gives smooth acceleration, accurate levelling and lower energy use than older drives.
Where is the lift controller?
On a lift with a machine room, the controller is in the machine room next to the machine. On a machine room-less lift, it is usually in a locked cabinet at a landing, often at the top floor.
When should a lift controller be replaced?
Common reasons include parts that are no longer available, repeated faults, poor levelling, a rough ride or the need for features such as remote monitoring or better accessibility. An engineer’s survey will show whether a controller replacement, or a wider modernisation, makes sense.