Four quadrant operation of PMDC motor by using single-phase Dual Converter.

Experiment No.: 20

Aim of the Experiment:

                            Four Quadrant operation of PMDC motor using Single-phase Dual converter.

Objective:

                Measurement of different parameters experimentally such as speed torque current of DC motor drive.

Apparatus Required:

Sl. No.	Name	Specification	Remarks
1	Single Phase Dual Converter Kit		01 nos.
2	PMDC Motor with spring load	0.5HP, 180V, 2.1A 1500 rpm	01 nos.
3	CRO		01 nos.
4	Tachometer	Digital Type	01 nos.

Circuit Diagram:

Theory:     

A dual converter consists of two converters (single phase or three phases), both either fully controlled or half controlled connected to the same load. The purpose of a dual converter is to provide a reversible DC voltage to the load, typically a separately excited DC motor. When field excitation is maintained in the same direction with change in armature voltage polarity, the direction of rotation of the DC motor can be changed. The two modes of its operation are non-circulating current mode and the circulating current mode. In the former, only one of the bridges is triggered. When reversal of output is required, the firing pulses for the conducting bridge are stopped and the second bridge is triggered. When reversal of output is required, the firing pulses for the conducting bridge are stopped and the second bridge is gated. Since the conducting SCR 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 input will be sorted through the two bridges. In order to avoid shorting of the bridges a sufficient dead band is included while transferring from one direction to the other so that thyristors get sufficient time to get turned OFF. In the circulating current mode, both bridges are gated simultaneously, one operating in the rectifying mode and other in the inverting mode to avoid short circuit. The logic and control circuitry required for circulating current is quite complex. The main advantage of the circulating scheme is the rapidity with which the phase reversal of the output current can be obtained. The need for a less sophisticated system and consequent low cost of the system has made the non-circulating current mode most popular and practically this approach is used in all dual converters. 

Dynamic Breaking:

When the armature supply of a running DC motor is disconnected and the armature is shunted by a resistor, a generating torque is developed by the motor, and it breaks on its own. The main advantage of this system lies in the smallest time interval during which the motor can be brought to rest. In more sophisticated systems the stored mechanical energy instead of being dissipated is returned to the AC mains by using what is called a regenerative braking system for dual converters. In this system when the motor is required to be braked, the pulses for the bridge working as converter are blocked and the other bridge is operated in inverter mode. Thereby facilitating conversion of DC power into AC. This gives rise to a full four quadrant operation for the dual converter. In this particular setup, we are using dynamic braking with the help of a resistor (lamp load for visual).

Procedures:

NON-CIRCULATING MODE: – (Selection switch in UPWARD position)

1. Ensure pot P1 in zero position and switch on LEFT hand side of panel in (NORMAL) position.
2. Connect the motor with the help of a 12 pin johson socket.
3. Turn ON the SW1 switch for the electronic control circuit.
4. Connect CRO and observe all waveforms for TP1, TP2, TP3 & TP4 for both +ve & -ve outputs, by rotating pot P1 in clockwise and anticlockwise direction respectively. If you observe the waveform at TP5 & TP6 you can see phase shifted triggering pulses for thyristor.
5. Bring pot P1 to zero position & turn ON switch SW2 for power circuit, the thyristor circuit is energized & you can observe the lamp intensity control. When you move pot P1 in clockwise & anticlockwise direction, voltage polarity is indicated by voltmeter. You may observe o/p waveform across the attenuated test point for o/p. 
6. You must not observe the waveforms for triggering circuit & output simultaneously on CRO.
7. Now turn OFF the power circuit & connect the motor to the output socket with no load on the motor. Rotate the pot P1 to zero position & turn it in clockwise direction the motor starts rotating in clockwise direction. Load the motor & observe the effect on the o/p waveform across attenuated output test points.
8. Dynamic Braking: keep motor running at about 100 V with negligible load on the motor. Select dynamic braking mode. The armature will be disconnected from the power circuit & connected to the lamp load which is located on the right-hand side socket. Thus stored KE will be converted into electric energy & lamp will glow momentarily & motor comes to halt immediately. If the lamp is removed & the same process is repeated with the motor running, you notice that the motor takes a longer time to stop.
9. Now throw the switch in a downward (NORMAL) position. This switch should be normally in a downward position.
10. You can repeat the sequence when you rotate the motor in anticlockwise direction also.
11. When you are working in NON-CIRCULATING MODE, LEDs indicating either +ve or –ve bridge output will be switched on.

CIRCULATING MODE: – (Selection switch in DOWNWARD DIRECTION)

1.  Repeat all the steps from 1 to 11
2. When you are working in CIRCULATING MODE: – LEDs for both +ve or –ve bridges will be switched on. This indicates that both bridges are fired one in inverter mode and other in converter mode.

Observation Table: With No-Load (Clockwise Direction)

Sl. No	Output Voltage (V) in Volts.	Output current (I) in amps.	Speed (N) in RPM	W1 in Kg.	W2 in Kg.	W=(W1-W2) in Kg.	Torque (T) in N-m
1
2
3
4
5
6

Observation Table: With No-Load (Counter-clockwise Direction)

Sl. No	Output Voltage (V) in Volts.	Output current (I) in amps.	Speed (N) in RPM	W1 in Kg.	W2 in Kg.	W=(W1-W2) in Kg.	Torque (T) in N-m
1
2
3
4
5
6

Observation Table: With Load (Clockwise Direction)

Sl. No	Output Voltage (V) in Volts.	Output current (I) in amps.	Speed (N) in RPM	W1 in Kg.	W2 in Kg.	W=(W1-W2) in Kg.	Torque (T) in N-m
1
2
3
4
5
6

Observation Table: With Load (Counter-clockwise Direction)

Sl. No	Output Voltage (V) in Volts.	Output current (I) in amps.	Speed (N) in RPM	W1 in Kg.	W2 in Kg.	W=(W1-W2) in Kg.	Torque (T) in N-m
1
2
3
4
5
6

Conclusion:

Written by students.