Drive Architecture

An induction motor's speed is set by the supply frequency and its pole number. To change the speed, change the frequency — which is exactly what a variable frequency drive does.

Three stages

• Rectifier — converts the incoming fixed-frequency AC to DC. Usually a six-pulse diode bridge; twelve-pulse and eighteen-pulse arrangements and active front ends are used where harmonic performance matters.
• DC link — capacitors that smooth the DC and store energy. These capacitors hold a lethal charge after the supply is removed, which is why a defined discharge time must elapse before the drive is opened, verified by measurement, not by the clock alone.
• Inverter — power transistors, normally IGBTs, switched rapidly to synthesise a variable-frequency, variable-voltage output.

Pulse width modulation

The inverter does not produce a sine wave. It produces a train of DC pulses of varying width, switched at a carrier frequency of a few kilohertz, whose average over time follows a sine wave. The motor's inductance smooths the current into something close to sinusoidal.

The carrier frequency is a trade-off. Higher carrier gives smoother current, less motor noise and slightly better motor performance, at the cost of higher switching losses in the drive and worse effects on motor insulation and cable. Lower carrier is quieter for the drive and harder on the motor's audible noise.

Regeneration

A standard drive can motor but cannot return energy to the supply. An overhauling load — a descending hoist, a decelerating high-inertia fan — pushes energy back into the DC link and raises its voltage until the drive trips on overvoltage. The remedies are a longer deceleration ramp, a braking resistor that dissipates the energy as heat, or a regenerative front end that returns it to the supply. Choosing between them is an application question that must be settled at design, not discovered at commissioning.