Question

A 4-quadrant is known as a versatile singe-phase converter that can generate any periodic output voltage of almost any frequency, phase angle, amplitude and wave shape. Explain how this can be achieved?

Answer 1

Single phase dual converter drives-

The circuit diagram is shown below.

The armature is supplied from two converters and field from another full converter as shown in the fig above. Let's considered armature part first-

Let the firing angles be _A and B? for the two A and B converters.The firing angles are adjusted such that firing angle of converter 1 (_A) + firing angle of converter 2 ((_B) = 180^o.

Converter 1(A) provides operation in I and IV quadrant converter 2 operates in ( Ii and III) quadrant respectively. Let's see converter wise operations which are as follows-

When converter A is operating -

If the firing angle is varied from 0- ?/2 the machine operates as motor in forward direction . Converter A acts as rectifier if the firing angle is varied from ?/2 -? converter operates as inverter.And the machine operates in reverse breaking mode. When converter A is operating current flows from top to bottom direction.

when converter B is operating -

If the firing angle is varied from 0-?/2 the conVverter operates as inverter and machine operates as forward breaking . If the firing angle is varied from ?/2-? the converter operates as rectifier and machine operates in reverse motoring , the current flows from bottom to top direction.

When the direction of rotation o motor iS to be reversed it is required to see that the first converter is turned OFF and the motor has attained zero speed and current becomes zero and then only the second converter is made to operate.

The V and I plane is shown below-

The output voltage V????o? = 2V_m/? Cos? ( _A) for 0<A?<?

The fIring angle of convrter B will be (? - _A) and the converter B is operating armature voltage is given by -

V????o? = 2V_m/? Cos?  ( _B )

Hence the output voltage widely depends on the firing angles of the converter. The association of output voltage with the firing angle alpha shows that the output voltage frequency majorily depends upon the firing angle as well as the !agnitude of output voltage is also firing angle dependant. By assuming different values of firing angle the voltage waveform varies with frequency .Also the converter group decides the mode of operation of the motor . Hence these are more versatile and used in major applications includes -

• Direction and Speed control of DC motors
• Industrial variable speed DC drives.