08 Dual Converter

8/18/2019 08 Dual Converter

1/12

Dual Converter

The circuit diagram for a dual converter is shown below:

It has two back to back connected full-converters; the full-converters could be in 1-phase or 3-phase

configuration. Here, two 3-phase converters have been shown. Normally, a dual converter is used for

obtaining 4 quadrant operation of a DC motor drive. When you have an uncontrolled or semicontrolled

converter, neither the voltage nor the current can reverse in the DC link. This allows only Quadrant I

operation where both V and I are positive. If V is plotted along Y axis and I along X axis, then a fully-

controlled converter will allow reversal of voltage when the firing angle becomes > 90˚; however, the

current cannot reverse. This will allow two quadrants of operation with the voltage being positive or

negative in the DC link, whereas the current always being positive. A back to back connected full-

converter combination which is also called a dual converter can allow the reversal of both current and

voltage and hence can give 4 quadrant operation.

Both converters are connected to a common 3-phase supply. When the converters are connected in

anti-parallel, their average voltages should be equal to each other.


2/12

i.e., Vd1=-Vd2;

so, (3Vm/π)Cosα1 = - (3Vm/π)Cosα2

where α1 and α2 are the firing angles of converters 1 and 2 respectively and Vm is the line to line

voltage peak.

Thus Cosα1 + Cosα2 = 0; thus making α1 =180˚ - α2

If α1 < 90˚, then α2 is more than 90˚; in this case, converter 1 works in rectifying mode and converter 2

in inverting mode. During this time, motor is supplied from converter1 and converter 2 works as an

inverter just circulating the current between the 2 converters. The waveforms for the two converter

outputs for α1=60˚ and α2=120˚ are shown in the figure below. It clearly shows that the average values

of Vd1 and –Vd2 are the same; but, instantaneous values are different. So, if these terminals are tied up

together, it will result in huge circulating current between the two converters. So, a DC link inductor is

connected between the two converters as shown in the circuit diagram.


3/12

A dual converter will allow a smooth transition of the DC motor from forward motoring to reverse

motoring through regenerative braking action. Initially let’s say the motor is operating with a firing angle

α1=45˚; this will make the motor run at a particular speed w1. Now to make the rotor reverse its

direction of rotation, the firing angle will be slowly increased such that Eb becomes > Va. If the supply is

a 3-phase 400V 50Hz one, Va will be about 381 V at a firing angle of 45˚. Let’s say the back emf of the

motor is 375V at a particular speed and torque. Now, if the firing angle is increased to a value such that

Va


4/12

Fig. 3.13 a


5/12

One of the problems with the non-circulating current mode is that smooth transition from forward

motoring to reverse motoring cannot take place effectively in this case. Also, the current can become

discontinuous and hence the straight-forward relationship between Vdc and α may not be followed.

However, there is no need for a bulky DC link reactor; corresponding cost, space etc. are reduced. The

nose associated with the reactor is also not there.


6/12


7/12


8/12


9/12


10/12


11/12


12/12

Documents

08 Dual Converter