Single Phase Dual Converter Converter fed DC motor drive

Advanced Power Electronics

Aim: To study the Operation of a Single Phase Dual Converter Fed DC motor drive

Objectives: To study 4 quadrant operation of a dc drive using single phase dual converter

Apparatus: 1. Experimental Kit

2. Connecting Probes

3. DSO

4. DMM

Pre Test

Video:

Theory:

Experimental Setup

CIRCUIT DESCRIPTION:-

Procedure:

OBSERVATION TABLE:

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Dual converter, the name itself says two converters. It is really an electronic converter or circuit which comprises of two converters. One will perform as a rectifier and the other will perform as an inverter. Therefore, we can say that double processes will occur at a moment. Here, two full converters are arranged in anti-parallel pattern and linked to the same dc load. These converters can provide four quadrant operations.

Single Phase Dual Converter Controlled DC Drive:

· Single-phase full converters with inductive loads allow only two-quadrant operation.

· If two of these converters are connected back to back, both the output voltage and the output current can be reversed.

· This system will provide four-quadrant operation and it is called a dual converter.

· The delay angles are controlled such that one converter operates as a rectifier and the other converter operates as an inverter.

· However, both converters produce the same average output voltage.

· Since the instantaneous output voltages of the two converters are out of phase, there will be an instantaneous voltage difference between the two converters.

· This will result in a circulating current between the two converters.

· The dual converters can be operated with or without a circulating current.

· In case of operation without the circulating current, only one converter operates at a time and carries the load current.

Modes of Operation of a Dual Converter:

1. Non Circulating Mode:

In this mode only one of the bridges is triggered. When reversal of output voltage is required , the firing pulses for the conducting bridge are stopped. & the second bridge is gated. Since the conducting SCR’s in the first bridge will turn off only when the current goes to zero , a small dead time must be allowed before the second bridge is gated. Otherwise , the AC voltage will be shorted through the two bridges.

2. Circulating Mode:

In this mode ,both bridges are gated simultaneously, one operating in the rectifying mode & the other in the inverting mode to avoid short circuits. We have used this mode of operation for this kit. This Scheme requires fully controlled bridges. If firing angle of one bridge( in rectifier mode) is 30 ⁰then for other bridge it will be 180-30= 150 ⁰ (in inverter mode). If reversal of o/p voltage polarity is required then the firing angles of the two bridges are changed simultaneously such that bridge 2 work as a rectifier & bridge 1 as inverter. The main advantage of the circulating current scheme is the rapidity with which the phase reversal of the o/p current can be obtained. However this scheme will produce a continuous flow of circulating current between the two bridges resulting in increased power losses. To

In our kit we have provided DC motor Drive for 110 V, 100W DC motor. a step down transformer ( 230 V to 110V) is used to give supply to both the bridges. There is one more transformer for control circuit.( 18V secondary) power supply ( +12V & -12V) .

This kit consists of two Full converters which are connected in anti parallel way. The thyristor used is TYN612.The detailed circuit is show in fig. This is a Circulating type of Dual Converter. Full converter can provide two quadrant operation. Thus by connecting two full converters in opposite direction we can achieve four quadrant operation. A DC motor is connected between the two full converters. The firing angle of the two converters can be varied by varying reference voltages using the pots provided for two converters separately. By adjusting the firing angles we can operate the DC drive in forward and reverse motoring.

Control Signal:

There are two converters. Obviously since converter-I and converter-II are connected in anti-parallel they must produce the same dc voltage. This requires that the firing angles of these two converters be related as: α₂= π – α₁

Here firing angle for the two Converters is provided separately. Here firing angle of two converters can be adjusted by varying the reference voltage using pot. To get variable speed firing angles of converters are changed. A RAMP Generator is derived from Control circuit transformer. It is given to non-inverting terminals of OPAMP1 and OPAMP2 used as a comparators. For inverting terminals variable dc voltages are given separately. The reference voltage to comparator 2 is adjusted using substractor circuit so that α₂= π – α₁.At the o/p of two comparators we get firing Angles. As dc voltage varies , we get variable firing angles for two full converters. These o/p of comparators are ANDed with the o/p of 555 based oscillator . At the o/p of AND gate we get firing pulses. This is required because we have used pulse transformer for isolation purpose.

Switch on the power supply.
Observe O/P of RAMP Generator on DSO.
Observe O/P of both comparators on Dual Scope DSO. By varying nearby pot observe change in firing angles.
Observe O/P of AND gate & observe that there are no. of pulses within firing Angle
Connect firing pulses to respective bridges as shown in connection Diagram. & Motor to O/P of dual converter. Now vary the firing angle from minimum to maximum. Take readings of O/P DC voltage & RPM of motor at regular intervals. For RPM measurement observe o/p of optical sensor on CRO & measure it’s frequency. Calculate RPM = F * 60/20
Plot the graph of O/P DC voltage Vs RPM of motor for Forward and Reverse motoring.

Forward Motoring

Reverse Motoring

Sr.No.

Toff(msec) of Conv.1

Toff(msec) of Conv.2

a1 (Angle)

a2 (Angle)

Voltage across motor

Speed(RPM)