JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III POWER SEMICONDUCTOR DRIVES Previous Question Paper (August - 2022 - R18)

A DC series motor fed by a single-phase fully controlled converter exhibits distinct speed-torque characteristics based on the firing angle (α) and the conduction mode. The converter provides a variable average output voltage, which directly controls the motor speed.

The average output voltage of a single-phase fully controlled converter is given by:

The speed-torque characteristics are analyzed across the following operating modes:

1. Continuous Conduction Mode (CCM): At low firing angles, the armature current remains continuous. The speed-torque curve is relatively stiff. As torque increases, the speed drops due to the voltage drop across the armature resistance and the effect of the series field. The speed is inversely proportional to the square root of the torque in the ideal case, but the converter's internal resistance and the firing angle modify this relationship.
2. Discontinuous Conduction Mode (DCM): At higher firing angles or light loads, the current becomes discontinuous. In this mode, the speed-torque characteristics become significantly softer (steeper drop in speed with increasing torque). The output voltage increases as the load current decreases, leading to high no-load speeds, which can be dangerous for series motors.
3. Quadrant Operation: A single-phase fully controlled converter typically operates in the first quadrant (motoring). By controlling the firing angle α, the motor speed can be varied from zero to rated speed at a constant torque or variable torque depending on the load.

Key observations for the series motor drive include:

• The series field flux is dependent on the armature current, causing the speed to be highly sensitive to load variations.
• As α increases, the average voltage decreases, shifting the speed-torque curves downward, resulting in lower speeds for a given torque.
• The presence of the converter introduces ripple in the armature current, which increases motor heating and causes torque pulsations, though the high inductance of the series field helps in smoothing the current.

A three-phase semi-controlled converter (also known as a half-controlled converter) consists of three thyristors and three diodes. It is used to drive a DC separately excited motor, providing unidirectional voltage and current, which allows for single-quadrant operation.

In this configuration, the three thyristors (T1, T2, T3) are connected to the three phases of the supply, and the three diodes (D1, D2, D3) provide the return path. The output voltage is controlled by varying the firing angle α of the thyristors. The average output voltage is given by:

where V_m is the peak phase voltage. The operation is divided into two modes: the conduction mode where the thyristor and diode conduct, and the freewheeling mode where the load current freewheels through one thyristor and one diode.

The voltage and current waveforms are characterized by the following features:

• The output voltage waveform consists of segments of the line-to-line voltages.
• For firing angles α ≤ 60°, the output voltage is continuous.
• For firing angles α > 60°, the output voltage becomes discontinuous, and the freewheeling diode (or the inherent freewheeling action of the converter) keeps the load voltage at zero during the off-period.
• The armature current waveform is smoother than that of a single-phase converter due to the higher ripple frequency (three times the supply frequency).
• The field current remains constant as the motor is separately excited.

The semi-controlled converter is advantageous because it eliminates the possibility of negative output voltage, preventing the motor from entering regenerative braking mode, and it is more cost-effective than a fully controlled converter.