JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year POWER SEMICONDUCTOR DRIVES Previous Question Paper (August/September - 2021 - R18)

A separately excited DC motor fed by a single-phase fully controlled converter operates in different quadrants depending on the firing angle (α) and the back EMF (E). The converter acts as a controlled rectifier, where the average output voltage is given by:

The operation is analyzed as follows:

• Motoring Mode (Quadrant I): In this mode, the firing angle α is maintained between 0° and 90°. Consequently, the average output voltage V_o is positive and greater than the back EMF E. The armature current I_a flows from the converter to the motor, and the motor develops positive torque in the direction of rotation. The power flows from the AC source to the DC motor.
• Plugging (Reverse Braking) Mode: Plugging is a form of electrical braking where the motor terminals are reversed to bring the motor to a quick stop. In a converter-fed drive, this is achieved by shifting the firing angle α beyond 90°.
  • When α > 90°, the converter output voltage V_o becomes negative.
  • The back EMF E, which remains positive due to the inertia of the motor, now acts in series with the negative converter voltage.
  • The total voltage in the armature circuit becomes (V_o + E), resulting in a large reverse current.
  • This reverse current produces a braking torque opposite to the direction of rotation, causing rapid deceleration.
  • Because the converter cannot feed power back to the AC source (it is a non-regenerative rectifier), the energy is dissipated as heat in the armature circuit resistance.

In summary, the single-phase fully controlled converter allows for smooth speed control in the motoring region (0° ≤ α < 90°) and provides a braking mechanism through plugging by operating in the inverter mode (90° < α ≤ 180°) where the converter acts as a load, dissipating the kinetic energy of the motor.

A three-phase fully controlled rectifier (also known as a six-pulse converter) fed DC series motor provides a highly efficient means of speed control by varying the average output voltage applied to the motor terminals. In continuous conduction mode, the armature current remains non-zero throughout the entire cycle, which is typical for inductive loads like DC motors.

The average output voltage V_o of a three-phase fully controlled bridge rectifier is given by:

where V_m is the peak phase voltage and α is the firing angle. Since it is a series motor, the armature current I_a flows through the field winding, creating flux Φ proportional to I_a. The motor back EMF E_b is given by:

The steady-state voltage equation is:

Operation Details:

• The rectifier uses six thyristors (T1-T6) triggered in sequence at 60° intervals.
• For α = 30°, the output voltage waveform consists of six pulses per cycle of the supply frequency.
• The load current ripple is significantly reduced compared to single-phase converters due to the higher pulse frequency (6 times the supply frequency).
• The motor speed is controlled by adjusting α; increasing α reduces V_o, thereby decreasing the motor speed.

Waveform Description (α = 30°):

• Output Voltage: The waveform follows the envelope of the line-to-line voltages. At α = 30°, the conduction starts at 30° after the natural commutation point of each thyristor pair. The waveform shows a ripple frequency of 300 Hz (for 50 Hz supply).
• Output Current: Due to the high inductance of the series field winding and armature, the current I_a is nearly constant with a small ripple component, confirming continuous conduction.