Introduction To Variable-Frequency Drives - Hitachi CONVENTION 14 Instruction Manual

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 – textile and printing presses
Need to control acceleration and deceleration (torque)
Sensitive loads – elevators, food processing, pharmaceuticals
What is an Inverter
inverter
The term
interchangeable. An electronic motor drive for an AC motor can control the motor's
varying the frequency
speed by
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.
Power
Input
Converter
L1
L2
L3
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.
variable-frequency drive
and
of the power sent to the motor.
Variable-frequency Drive
Internal
DC Bus
Rectifier
are related and somewhat
Inverter
U/T1
V/T2
W/T3
1 − 19
1 19
Motor