Page 206 - Root Cause Failure Analysis

16

INVERTERS

Inverters control the output speed of alternating current (AC) motors. While the basic
function of all inverters is the same, the approach varies with the type of inverter.

CONFIGURATION

Two basic types of inverters commonly are used in industrial applications: voltshertz
and vector control.

Vo/ts/Hertz Control
Traditionally, a voltshertz speed-control device uses a volts-per-hertz (V/Hz) control-
ler, which uses a mechanical-reference command taken from a shaft encoder, or
resolver, to vary the voltage and frequency applied to the motor. By maintaining a
constant V/Hz ratio, the inverter drive controls the speed of the connected motor.
Figure 16-1 shows how this type of controller limits current frequency to the motor.

Inside the drive shown in Figure 16-1, a current-limit block monitors motor current and
alters the frequency command when the motor current exceeds a predetermined value.
Early V/Hz inverters were sensitive to variations in applied load and could not maintain
consistent speed control in applications subjected to frequency-load variations.

The introduction of slip compensation, a feature added to later V/Hz models, altered
the frequency reference to keep the actual motor speed close to the desired speed dur-
ing load changes. The slip-compensation module compares the deviation between
actual and no-load speed of the motor and enters a correction factor to the inverter
drive. This factor compensates for the variation in speed, or slip, caused by load
changes.

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