Variable-Frequency Drive (VFD) Principles: V/Hz, Volts-per-Hertz, and PWM

What a VFD Does and Why It Matters

A variable-frequency drive (VFD) lets you control the speed of an AC induction motor by changing both the frequency and the voltage of the power supplied to it. Instead of running a motor at one fixed speed, a VFD allows smooth ramp-up, precise speed control, and energy savings — critical capabilities in automated manufacturing and marine systems. Understanding how a VFD works internally makes you a more effective technician when you need to commission, troubleshoot, or safely service one.

The Three Stages of a VFD

Every VFD converts power in three sequential stages. Trace the power flow from left to right through these blocks.

Stage 1 — Converter (Rectifier): Incoming three-phase AC (typically 480 V, 60 Hz in US industrial plants) enters a bridge rectifier made of diodes. The diodes allow current to flow in only one direction, converting the alternating waveform into a pulsating DC voltage. The result is not yet smooth — it ripples at six times the line frequency for a three-phase, full-wave bridge (360 Hz ripple on 60 Hz supply).

Stage 2 — DC Bus (Capacitor Bank): Large electrolytic capacitors smooth the pulsating DC into a relatively steady DC bus voltage. On a standard 480 V AC drive, the DC bus sits at roughly 650–680 V DC under normal conditions. This bus is the energy reservoir the inverter draws from. A pre-charge resistor (or pre-charge circuit) is connected in series with the bus during power-up to limit the inrush of current that would otherwise surge into the uncharged capacitors and damage them or blow fuses. Once the capacitors are charged to near full voltage, a contactor bypasses the resistor and normal operation begins. The DC bus test points are the measurement locations you will use with a meter to confirm bus voltage — always treat these points as energized even after the drive is powered off, because capacitors store lethal charge.

Stage 3 — Inverter (IGBT Switches): Insulated-gate bipolar transistors (IGBTs) are arranged in three pairs (one pair per output phase). By switching each IGBT on and off thousands of times per second under control of the drive's microprocessor, the inverter reconstructs a synthetic AC output whose frequency and voltage are both adjustable. This switching technique is called pulse-width modulation (PWM).

Pulse-Width Modulation (PWM)

PWM works by varying the width of each voltage pulse while keeping the DC bus voltage fixed. The average voltage seen by the motor over many switching cycles approximates a sine wave. If you look at the raw output on an oscilloscope, you see a series of rectangular pulses at DC bus voltage levels, not a true sine wave. The motor's own inductance acts as a low-pass filter and smooths the current into something close to sinusoidal. The rate at which the IGBTs switch is called the carrier frequency (also called switching frequency), typically 2–16 kHz. Higher carrier frequencies produce quieter motor operation (less audible whine) but generate more heat in the drive and in the motor, and cause greater stress on motor insulation — especially over long cable runs.

V/Hz Ratio: Keeping Flux Constant

An AC induction motor develops torque through electromagnetic flux in its core. The flux is proportional to voltage divided by frequency (V/Hz). If you reduce frequency to slow the motor but do not also reduce voltage by the same proportion, the core saturates, draws excessive current, overheats, and can be damaged. If you reduce voltage too much relative to frequency, flux weakens and torque drops.

The solution is to maintain a constant V/Hz ratio at all speeds below base speed (the nameplate frequency, typically 60 Hz). For example, a 460 V / 60 Hz motor has a V/Hz ratio of approximately 7.67 V/Hz. At 30 Hz, the drive outputs roughly 230 V — half the voltage at half the frequency. The ratio stays constant, flux stays constant, and rated torque capability is maintained.

Above base speed, voltage cannot increase beyond the drive's supply limit, so the drive holds voltage constant while continuing to raise frequency. In this field-weakening region, flux and torque decrease. Most general industrial applications stay at or below base speed.

Why Long Cable Runs Require Derating

When PWM pulses travel down a long motor cable, the cable's capacitance and inductance cause voltage reflections. These reflections can produce voltage spikes at the motor terminals significantly higher than the DC bus voltage, stressing the motor's winding insulation. Drive manufacturers publish derating guidelines and maximum cable length specifications. For long runs, output reactors or dV/dt filters are added between the drive and motor.

Safety: Work Near VFDs

VFDs present serious electrical hazards.

• Lockout/Tagout (LOTO): Follow OSHA 29 CFR 1910.147. De-energize the upstream disconnect, apply your personal lock, then wait the manufacturer's specified discharge time (often 5–10 minutes or more) for the DC bus capacitors to discharge before opening the drive enclosure. Verify absence of voltage on the DC bus with a properly rated meter before touching internal components.
• NFPA 70E: Identify arc-flash and shock boundaries before any energized work. Use appropriate PPE (rated gloves, face shield, arc-rated clothing) if energized measurement is required on the DC bus test points.
• NFPA 79: Governs electrical safety of industrial machinery, including proper enclosure ratings and wiring practices for drive installations.
• Machine Guarding: Under OSHA 1910.212, ensure that any driven machinery is guarded before the VFD is enabled.
• Never assume the drive output is de-energized just because the motor has stopped — the DC bus may still hold lethal voltage.