Page 335 - Electronic Mechanic - TP - Volume - 2

ELECTRONICS MECHANIC - CITS

EXERCISE 223: Study the AC drive set up and its connection

Objectives

At the end of this exercise you shall be able to:
• to study the AC drive set up
• to study the AC drive connection.

Requirements

Tools/ Equipments/ Instruments Materials/ Components
• Combination Plier 250mm - 1 No. • PLC-Any Brand-Dismountable
• Screw Driver set 150mm - 1 No. latest specification - 1 No.
• Wire Stripper - 1 No.

Procedure

The AC drive industry is growing rapidly and it is now more important than ever for technicians and maintenance
personnel to keep AC drive installations running smoothly. AC drives change the speed of ac motor by changing
voltage and frequency of the power supplied to the ac motor. In order to maintain proper power factor and reduce
excessive heating of the motor, the name plate volts/hertz ratio must be maintained

AC Drive Operation principle
For understanding the basic principles behind AC drive operation requires understanding three basic section of
AC drive: the Rectifier unit, DC Bus and the Inverter unit.

The supply voltage is firstly pass through a rectifier unit where in gets converted into AC to DC supply, the three
phase supply is fed with three phase full wave diode where it gets converts into DC supply. The DC bus comprises
with a filter section where the harmonics generated during the AC to DC conversion are filtered out. The last
section consists of an inverter section which comprises with six IGBT where the filtered DC supply is being
converted to quasi sinusoidal wave of AC supply which is supply to the ac motor connected to it.
From the ac motor working principle, we know that the synchronous speed of motor (rpm) is dependent upon
frequency. Therefore by varying the frequency of the power supply through AC drive we can control the synchronous
motor speed.
Basic of AC drives:
AC drive, inverter, converter and variable frequency drive are all terms that are often used in reference to equipment
designed to control the speed of an AC motor. AC drives consist of the following sections. Power Section The
power section of a basic AC drive circuit is composed of the following components.

The rectifier, often called a converter, with power semiconductor devices, coverts incoming AC power to DC.
rts the DC power to AC for variable speed control.
Constant V/F Ratio Operation
All AC drives maintain the output voltage - to - frequency (V/f) ratio constant at all speeds for the reason that
follows. The phase voltage V, frequency f and the magnetic flux ϕ of motor are related by the equation:
V = 4.444 f N ϕm (or) V/f = 4.444×N ϕm
Where N = number of turns per phase.
ϕm = magnetic flux
Formula:

Speed (rpm) = Frequency (Hertz) x 120 / No. of poles.

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