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Introduction to Variable-Frequency Drives
Getting Started
1–12
Introduction to Variable-Frequency Drives
The Purpose of Motor Speed Control for Industry
Hitachi inverters provide speed control for 3-phase AC induction motors. You connect
AC power to the inverter, and connect the inverter to the motor. Many applications
benefit from a motor with variable speed, in several ways:
• Energy savings - HVAC
• Need to coordinate speed with an adjacent process—textiles and printing presses
• Need to control acceleration and deceleration (torque)
• Sensitive loads - elevators, food processing, pharmaceuticals
What is an Inverter?
The term inverter and variable-frequency drive are related and somewhat interchange-
able. An electronic motor drive for an AC motor can control the motor’s speed by
varying the frequency of the power sent to the motor.
An inverter, in general, is a device that converts DC power to AC power. The figure
below shows how the variable-frequency drive employs an internal inverter. The drive
first converts incoming AC power to DC through a rectifier bridge, creating an internal
DC bus voltage. Then the inverter circuit converts the DC back to AC again to power the
motor. The special inverter can vary its output frequency and voltage according to the
desired motor speed.
The simplified drawing of the inverter shows three double-throw switches. In Hitachi
inverters, the switches are actually IGBTs (insulated gate bipolar transistors). Using a
commutation algorithm, the microprocessor in the drive switches the IGBTs on and off
at a very high speed to create the desired output waveforms. The inductance of the motor
windings helps smooth out the pulses.
Power
Input
Inverter
L1
Motor
L2
L3
Rectifier
Variable-frequency Drive
Internal
DC Bus
+
+
–
U/T1
V/T2
W/T3
Converter