Week-7 Challenge: DC Motor Control

 1.

attached Screenshot of Simulation Diagram & Plot -  Speed control of a DC motor using BJT H-bridge & (BJT_H_bridge_75% Pulse width) 

Explanation,

The Bipolar Junction Transistor (BJT), when used for power-switching applications operates as an IGBT Switch. when it is conducted in a saturation region. forward voltage is developed between the emitter and collector.

Operation of H-bridge,

The H-bridge consists of four BJT/Diode coupled. Two transistors are switched simultaneously- (Q1, Q4) or (Q2,Q3). When Q1 and Q4 are ON, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when Q1 and Q4 are switched off and develop Forward Motoring. When Q2 and Q3 are fired, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off and develop Reverse Motoring.

Comparison of Four-Quadrant Chopper DC Drive and H bridge model.

Four-Quadrant Chopper DC Drive, 

1. To change motor direction the polarity is changed.

2. It has broader torque and power output range.

3. Upto 400V can be acheived.

4. Current is chopped values.

5. Regenerative braking is possible.

H bridge model,

1. To change motor direction the current signal has to be interrupted.

2. It uses single pluse generator which gives narrow torque and power outage range.

3. upto 350V can be acheived.

4. Current is continuous value.

5. Regenerative braking is not-possible.

 Four-Quadrant Chopper DC Drive (DC7) block 

The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source. The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz voltage source. In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC7 block.

The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed.

The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system.

The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation). For proper system behavior, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3. This generates the average armature voltage needed to obtain the desired armature current.

In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

2. 2-quadrant chopper using Simulink & explain the working of the same with the relevant results. 

Working of Two Quadrant Chopper,

In this type of Chopper, the average voltage will be always positive but the average load current might be positive or negative. The two Switches CH1 and CH2 should not be turned on simultaneously as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

We will always get a positive output voltage V₀  as the freewheeling diode FD is present across the load. When the chopper CH1  is on the current flows from source to load. The inductor stores energy. When Ch1 turn OFF  the freewheeling diode starts conducting and the output voltage v₀ will be equal to V_s. The direction of the load current i₀ will be reversed. The current i₀  will be flowing towards the source. The load current will be negative when the CH2 is on. The current completes its path through diode D2.

Operation,

First Quadrant Chopper,

For first quadrant operation, CH4 is kept ON, CH3 is kept OFF and CH1 is operated. When both CH1 & CH4 are ON simultaneously, the load gets directly connected to the source and hence the output voltage becomes equal to the source voltage. This essentially means that vo = vs. It may be noted that the load current flows from source to load as shown by the direction of io.    

Fourth Quadrant Chopper,

fourth quadrant operation, CH4 is operated while keeping CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be reversed.When CH4 is turned ON, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is ON. 

Result,

Simulink model (model file, Plot and screenshot Attached),

First and Fourth Quadrant Chopper Control

Parameters Details,

Iref = 40

Transistor (Switching Device) = IGBT

Conclusion (Attached concl. fig)

 The operation of a First and Fourth quadrant chopper is summarized in the figure below,

3. Operation of BLDC motor,

BLDC motor, the current carrying conductor is stationary while the permanent magnet moves. When the Supply Voltage Vs is applied to stator, it becomes electromagnet and start producing Uniform in air gap.Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate.Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction.