Variable Frequency Drive

What is Variable Frequency Drive?

The frequency converter is also called as variable speed drive or variable frequency drive. 

This drive basically consists of two parts

( 1 ) First part which converter AC to DC and

( 2 ) converts DC into AC.

Power Factor Improvement in the Converter

Power factor of the Phase Controlled Converter

The operation of phase-controlled converter is very simple, reliable and less costly. The commutation circuit is not necessary in this type of converter. The power factor becomes low when the output voltage less than the supply voltage (particularly when the firing angle is high).

Speed Control of Universal Motor

• The speed control of universal motor is shown in the figure A. 
• The single phase bridge rectifier converts alternating voltage into direct voltage.
•  The zener diode provides constant voltage to RC circuit. 
• The charging of capacitor is done through variable resistor R. 

Phase Locked Loop Technique

• The speed control of DC motor may be open loop or closed loop. 
• The accurate speed control of DC motor is done by analog servo feedback closed loop system. 
• The speed of DC motor is measured by tachometer. 
• The error signal is generated by comparing speed of AC techno generator and reference signal. 
• The speed of DC motor may be not accurate when it is measured by analog device and its speed regulation may be greater than 0.2%. 
• This type of speed control is not accurate in the industrial drive. 
• As the analog device measurement is not accurate, the phase locked loop control system is used for DC motor speed control. 
• The speed of motor is converted into digital pulse train and it is compared with input reference signal.

Phase locked loop system

• The block diagram of DC motor speed control by phase locked loop system is shown in the figure A.  
• The speed of motor is converted into digital pulse train and it is compared with input reference signal in this method. 
• The phase detector compares two signals and generates output accordingly. 
• The output of the phase detector feeds at low pass filter. 
• The high frequency components are filtered partly or completely by low pass filter and it generates continuous DC signal. 
• The output of the low pass filter is given to the converter input. 
• When the reference speed and speed of DC motor are equal, both frequencies are locked or synchronize. 
• The phase detector generates constant voltage at output and it is directly proportional to phase difference between two quantities. 
• Therefore the speed of motor is constant with reference of load. 
• If there is change in speed due to any disturbance, the output of the AC techo generator changes accordingly. 
• This will change the output of phase detector but the reference signal and feedback signals are always locked. 
• The response of phase locked loop is very fast, ideally it is nil but actually it is 0.002%. 
• The application of phase locked loop is to control DC motor drive and synchronize communication system.

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Three Phase Welding

• The main disadvantage of single phase welding system is that it will draw high current in small duration of time. 
• This will result in large voltage drop and it affects supply system of all the other electrical equipments which are connected through it. 

Resistance Welding

• The process of joining two metal plates is called as resistance welding. 
• The step down transformer is used to pass current through two metal plates. 
• The mechanical pressure is given to two metal plates between two electrodes as shown in the figure A.

Resistance welding using Triac - Diac circuit

• The power circuit for the resistance welding using TRIAC – DIAC is shown in the figure A. 
• The current passes through transformer primary winding when TRIAC is turned on. 

Firing angle Control of SCR By the UJT Full Wave Circuit

Firing of SCR by UJT full wave circuit

The firing of SCR is done by various method. Firing of SCR is done by UJT full wave is explained here. The firing of SCR is done by UJT half wave circuit is one of the methods.

Firing of SCR By the UJT half Wave Circuit

Firing of SCR by UJT half wave circuit

The firing of SCR is done by various method. Firing of SCR is done by UJT half wave is explained here. The firing of SCR is done by UJT half wave circuit is one of the methods.

RC Phase Shift Control

SCR Firing angle control by RC phase shift control

The firing angle control of SCR is done by RC phase shift control. The working of RL phase shift control is explained here.

RL Phase Shift Control

SCR Firing angle control by RL phase shift control

The firing angle control of SCR is done by RL phase shift control. The working of RL phase shift control is explained here.

RL phase shift control: Working

• Figure A shows circuit for the RL phase shift control. It consists of one step down transformer which consists of center taped secondary winding.