Solid-State Drives and Microprocessor-Based Controllers

What Is a VVVF Drive and Why Does It Matter?

Older traction elevators used resistance banks or motor-generator sets to control speed. Modern elevators rely on a variable-voltage variable-frequency (VVVF) drive, sometimes called an AC drive or inverter drive. The VVVF drive accepts incoming AC power, converts it to DC inside a rectifier stage, then inverts it back to AC at precisely the voltage and frequency commanded by the controller. Because the speed of an AC induction motor is directly related to the frequency of its supply, the drive can ramp the motor smoothly from zero to full speed and back again—producing the soft starts and stops that passengers feel as comfort and that equipment feels as reduced mechanical stress.

Inside the Drive: Three Main Stages

1. Converter (Rectifier) Stage. The drive's input section takes three-phase line voltage and rectifies it into a DC bus voltage. Diode bridges or active transistor bridges are used depending on drive design. The DC bus is filtered by large capacitors that store energy and smooth out ripple.

2. DC Bus. The intermediate DC bus links the converter to the inverter. Bus voltage is monitored continuously. Regenerative drives can feed energy back to the building power system during descent or braking, improving energy efficiency.

3. Inverter Stage. Insulated gate bipolar transistors (IGBTs) switch the DC bus on and off thousands of times per second using pulse-width modulation (PWM). By varying the width of each pulse, the inverter synthesizes an AC waveform of any desired voltage and frequency. The microprocessor controller sends speed commands; the drive translates them into gate-firing signals for the IGBTs.

Key Drive Parameters

Drives are programmed through a keypad interface or software tool. Parameters are stored in non-volatile memory. The most important categories for elevator work include:

• Acceleration and deceleration ramp times – how quickly the output frequency rises or falls, directly affecting ride comfort and leveling accuracy.
• Maximum frequency / maximum speed – must match the motor nameplate and the contract speed of the elevator.
• Current limits – protect the motor during heavy loads or stall conditions.
• Motor data – nameplate voltage, current, frequency, number of poles, and rated speed allow the drive to perform motor self-tuning (auto-tune) for optimum torque control.
• Fault trip levels – thresholds at which the drive shuts down and logs a fault code (e.g., overcurrent, overvoltage, undervoltage, overtemperature).

Never change parameter settings without written authorization from the authority having jurisdiction and in accordance with the manufacturer's documentation. Incorrect ramp settings can cause a car to overshoot a floor or slam into a buffer.

The Microprocessor Controller Board

The main processor board is the brain of the elevator system. It runs the operating software that handles every decision the elevator makes:

• Dispatching logic – decides which calls to answer and in what order, using algorithms based on hall calls, car calls, direction of travel, and car position.
• Door timing – controls how long doors remain open, detects obstructions via the door detector circuit, and initiates door re-opening cycles.
• Fault logging – records fault events with time stamps in non-volatile memory. Mechanics retrieve this log during troubleshooting to reconstruct what happened before a shutdown.
• Safety chain monitoring – continuously reads the status of every safety input (pit stop switch, governor switch, door interlock contacts) and halts the car if any input opens unexpectedly.

Input/Output Boards and the Serial Bus

The processor board communicates with the physical world through I/O boards that contain relay outputs, digital inputs, and analog channels. Hall call buttons, landing door interlocks, position encoder signals, and load-weighing inputs all connect to I/O boards. The I/O boards and the drive interface board exchange data with the main processor over an internal serial bus (often a proprietary protocol or a standard such as CAN bus). This reduces wiring compared to hard-wired relay logic and allows the controller to send and receive dozens of signals over a few conductors.

The drive interface board translates the processor's speed command (often a digital serial message or a ±10 V analog signal) into the format the VVVF drive expects, and returns drive status and fault data back to the processor.

A Complete Up-Call Sequence

1. Passenger presses the hall up button → I/O board reads the input and sends the call to the processor.

2. Processor dispatching algorithm selects the car and assigns a destination floor.

3. Processor commands doors to close → door operator receives the signal via I/O board.

4. Processor confirms the safety chain is intact and releases the brake.

5. Processor sends a speed profile command to the drive interface board.

6. Drive interface board forwards the speed reference to the VVVF drive.

7. VVVF drive ramps up frequency and voltage → motor accelerates the car.

8. As the car approaches the destination floor, the processor commands a deceleration ramp → drive reduces output frequency smoothly.

9. Car levels at the floor; brake sets; processor commands doors to open.

10. Fault log records the run data; processor waits for the next call.

Safety Reminders

Before working on any controller cabinet or drive: follow lockout/tagout (LOTO) procedures in accordance with OSHA requirements and your employer's energy-control program. The DC bus capacitors in a VVVF drive can hold a lethal charge for several minutes after input power is removed—always verify that bus voltage has discharged to a safe level with a calibrated meter before touching internal components. Wear appropriate PPE including arc-flash-rated gear when the hazard assessment requires it. Never bypass or jumper safety chain inputs to test drive parameters while the car is in service.