Week-7 Challenge: DC Motor Control
Q1.
A. The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage Vf is developed between collector and emitter (in the range of 1 V). Therefore, the IGBT block can be used to model the BJT device.
The IGBT block does not simulate the gate current controlling the BJT or IGBT. The switch is controlled by a Simulink® signal (1/0). The DC motor uses the preset model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed). The armature mean voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.
The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models). Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when Q1 and Q4 are switched off. 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.
B. I observed a sudden spike in the armature current graph during the transition at 0 seconds and 0.5 seconds. There was a huge surge in current during this period. To minimize this sudden increase in current, we can consider changing the duty cycle and with a decrease in duty cycle, the spike is minimized.
C.
Four-Quadrant Chopper DC Drive vs H-bridge model
To change the motor direction the polarity is changed To change the motor direction the current signal has to be interrupted.
It has broader torque and power output ranges It uses single pulse generator which gives narrow torque and power output range.
Up to 400v can be achieved. Up to 350v can be achieved.
In this the current has chopped values. In this the current has continuous values.
Regenerative braking is possible Regenerative braking is not possible
Q2.
The 2-quadrant chopper is also known as a Type C chopper. In this type, during both quadrants, the voltage remains positive. And a freewheeling diode is placed across the load. In the first quadrant, CH1 is on & CH2 is off, current starts to flow through CH1, L, E. If CH1 is off, the inductor discharges through FD(L-E-FD-L). The output voltage is equal to Vs, so both voltage & current will be positive. When CH2 is on & CH1 is off, current flows from E, L, CH2 & E. If CH2 is off, current flows through D2 to the source(E-L-D2-E). Regenerative braking occurs in the 2nd quadrant as shown below.
The voltage always remains positive. Current varies from positive to negative.
Q3.
In addition to the basic operation described earlier, here are some key points on the operation of a BLDC motor:
1. **Electronic Commutation**: BLDC motors are electronically commutated, meaning the switching of current in the motor's windings is done through an electronic controller rather than a mechanical commutator. This allows for greater control and efficiency in driving the motor.
2. **Three-phase System**: Most BLDC motors operate on a three-phase system, where the power is supplied to three windings with a phase difference of 120 degrees. This three-phase power delivery helps in producing a smoother rotational motion.
3. **Hall Effect Sensors or Sensorless Operation**: BLDC motors typically use Hall effect sensors to determine the rotor position and apply the current accordingly. However, sensorless methods, which infer rotor position through back EMF sensing or other techniques, are also employed in some designs to reduce cost and increase reliability.
4. **PWM Control**: Pulse Width Modulation (PWM) is commonly used to control the speed of BLDC motors. By adjusting the duty cycle of the PWM signal, the effective voltage applied to the motor can be varied, thereby controlling the speed.
5. **Efficiency and Maintenance**: BLDC motors are known for their high efficiency due to the absence of brushes, which reduces friction and wear. This results in lower maintenance requirements and longer lifespan compared to brushed motors.
6. **Applications**: BLDC motors find applications in a wide range of devices such as HVAC systems, electric vehicles, robotics, computer peripherals, and more. Their efficiency, reliability, and precise speed control make them suitable for various industries.
These additional points further highlight the unique characteristics and advantages of BLDC motors in modern industrial and consumer applications.