Week-7 Challenge: DC Motor Control

1) A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

ans=

Model:

- H bridge consists of 4 BJT and diode pairs. Two transistors are switched on simulteneously. Q1 and Q4 or Q2 and Q3.

-When Q1 and Q4 are fired a positive voltage applied across motor and diodes. D2 and D3 operates as free wheeling diodes when Q1 and Q4 are switched off. Similarly when Q2 and Q3 are fired , a negative voltage is applied to the motor and diodes.D1 and D4 operates as free wheeling diodes when Q2 and Q3 are switched off.

output:

- There is armature current shootup intially, this is because of reversal of armature voltage.

DC 7- Four quadrant chopper:

output:

-Compared to other DC drives the main advantages of using the four quadrant chopper DC drive is that it can operate in all four quadrants. Each quadrant in its characteristics represents a different mode of operation.

- First quadrant is forward motoring, sencond is forward braking, third reverse motoring and forth is reverse braking.

- Another advantage is that it uses high switching frequency DC-DC converters resulting in lower armature current ripple is occurs than compared to thyristor based DC drives.

-Unlike 4 quadrant DC chopper BJT H-bridges are not capable of performing regenerative braking but performs dynamic braking.

2) Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

ans=

model

Two quadrant chopper DC drive uses

speed controller(DC)

Regulation switch

Chopper

- Two quadrant chopper(DC6)block represents two quadrant, DC supplied, chopper drive for DC motors. This drive features closed loop speed control with two quadrant opertion.

-Due to use of high switching frequency DC -DC converters a lower armature current ripple is obtained. However for 2 quadrant DC drives, reversible and regenerative operations cannot be obtained.

3) Explain in a brief about operation of BLDC motor.

ans =

BLDC MOTOR:

- A BLDC accomplishes commutation electronically using rotor position feedback to determine when to switch the current.

- The BLDC motor is electrically commutated by power switches instead of brushes.

-There are three types of BLDC motor:

signal phase.

Two phase .

Three phase

- BLDC operation is based on attraction and repulsion between magnetic poles.

- Advantages:

1) Higher efficiency

2) Reliability

3) Lower acoustic noise

4) smaller and lighter

5) Greater dynamic response

6) Better speed versus torque characteristics.