Motors & Drives

Variable-frequency drives (VFDs)

A variable-frequency drive (VFD) — also called an adjustable-speed drive (ASD), inverter, or AC drive — controls the speed of an AC induction motor by varying the frequency and voltage of the power supplied to it. VFDs are among the most energy-efficient investments in industrial automation: in fan and pump applications, power consumption follows the affinity laws, meaning that reducing speed to 80% of rated speed reduces power consumption to approximately 51% (the cube of the speed ratio).

VFD architecture

A typical VFD has three stages:

1. Rectifier: Converts incoming AC mains (single-phase or three-phase) to DC using diodes or thyristors. Most modern drives use a diode bridge rectifier.
2. DC link (bus): Filters and stores the rectified DC voltage using large electrolytic capacitors. The DC bus voltage is approximately √2 × the AC line voltage (e.g., 565 V DC for a 400 V AC supply).
3. Inverter: Converts DC back to AC at the desired frequency and voltage using pulse-width modulation (PWM). Six IGBTs (Insulated Gate Bipolar Transistors) switch rapidly to synthesise the output waveform.

The output is not a pure sine wave — it is a series of high-frequency pulses that approximate a sine wave at the target frequency. The motor's inductance integrates these pulses into approximately sinusoidal current. This PWM switching produces audible motor noise and can cause issues with long cable runs (reflected voltage waves) — requiring output reactors or dV/dt filters when cable lengths exceed drive manufacturer recommendations (typically 30–100 m).

Control modes

V/Hz (scalar) control maintains a constant voltage-to-frequency ratio across the speed range. It is simple, stable, and suitable for centrifugal fans and pumps where precise speed regulation is not required. Torque accuracy degrades at low speeds because stator resistance becomes significant relative to the output impedance.

Open-loop vector control (also called sensorless vector) uses motor model calculations to estimate the rotor flux position and control torque independently of speed. It provides better low-speed torque and tighter speed regulation than V/Hz without requiring a feedback encoder. Most mid-range drives support this mode.

Closed-loop vector control adds an encoder on the motor shaft for precise position feedback. It provides the highest accuracy — speed regulation to ±0.01% — and is used for winding, draw, and synchronisation applications.

Common VFD faults

Overvoltage (OV)

An overvoltage fault trips the drive when the DC bus voltage exceeds a threshold (typically ~800 V DC for a 400 V supply). The most common cause during normal operation is regeneration — the motor acting as a generator during deceleration, pumping energy into the DC bus faster than the drive's braking circuit can dissipate it. Solutions include extending the deceleration ramp, adding a braking resistor, or using a regenerative (active front end) drive. See the dedicated VFD overvoltage fault guide for detailed diagnosis.

Overcurrent (OC)

An overcurrent fault trips when output current exceeds the drive's current limit (typically 150–200% of rated current). Causes include mechanical jam, too-fast acceleration ramp, a motor winding fault, or attempting to start into a rotating load at the wrong phase. Diagnosis starts with checking whether the fault occurs at start, during acceleration, or at steady state.

Undertemperature / overtemperature

VFD power modules require adequate cooling. Overtemperature faults are caused by blocked cooling fans, excessive ambient temperature, inadequate enclosure ventilation, or prolonged operation above the drive's continuous current rating. Check the drive's derating curves in the technical manual — most drives derate their rated current above 40°C ambient.

Servo and stepper motor systems

In applications requiring precise position control — CNC axes, pick-and-place, robotics, packaging — servo and stepper motor systems are used instead of VFD-driven induction motors.

A stepper motor moves in discrete angular steps (typically 1.8° per step, 200 steps/revolution) by energising its windings in sequence. Open-loop operation (no encoder) is possible because the motor moves a predictable angular amount per pulse. Steppers provide high holding torque at standstill and low cost, but torque falls sharply above a few hundred RPM and missed steps (stall) are not detected in open-loop operation.

A servo motor is a permanent-magnet synchronous motor (PMSM) or brushless DC motor paired with an encoder and a servo drive (amplifier). The drive closes a position and velocity loop in real time, applying whatever current is needed to move the motor to the commanded position. Servo systems provide high accuracy across their full speed range, detect and recover from disturbances, and maintain torque at high speeds. They cost significantly more than equivalent stepper systems.