Introduction; What Is A Frequency Inverter - Mitsubishi Electric FR-A800 Series Beginner's Manual

Introduction

1 Introduction
1.1

What is a frequency inverter?

Asynchronous three-phase electric motors are simple, reliable and inexpensive, which makes them a
particularly popular choice for industrial applications.
The speed of an asynchronous three-phase motor is determined by two factors:
– The frequency of the three-phase current.
– The design of the motor winding (number of poles or pole pairs).
Since the frequency of the power supply is generally a constant 50Hz this means that the speed of the
motor is inherently fixed – you can only change it for different applications by changing the construc-
tion of the winding. Once that has been chosen the motor will always run at a fixed speed, for example
approximately 3,000 rpm or 1,500 rpm.
Providing more than one speed is only possible with "pole-changing" motors that have two sets of
windings (2 windings enable up to 4 different speeds). That is the end of the line, however. Neither
more speeds nor continuously-variable speeds are possible with pole-changing motors.
The solution to this problem is to use a frequency inverter, or inverter for short, which is a device that
converts the fixed voltage and frequency of the mains power supply into a variable voltage with a var-
iable frequency. It is installed between the mains supply and the motor and makes continuously- var-
iable speed adjustment possible, turning a standard motor with a single winding into a flexible var-
iable-speed drive system.
400 V 3~ 50 Hz
Output voltage
Inverters also have other benefits, including adjustable acceleration and braking times, torque boost-
ing, integrated electronic overcurrent protection and even integrated PID controllers, another ad-
vanced feature that has already been realised.
Frequency Inverters - Beginner's Guide
Inverter
R/L1
S/L2
T/L3
100 %
0 Output frequency (Hz)
U
V
W
The speed of the connected motor can be
adjusted continuously by changing the
output voltage and frequency of the in-
verter.
What is a frequency inverter?
Asynchronous 3~
motor
0 to 400 V 3~
0 to 50 Hz
1 - 1