Week-7 Challenge: DC Motor Control

1 ) Explain the following :

A) EXplain a matlab demo model named 'speed control of a DC motor using BJT H- bridge '.

• H bridge is a simple circuit that lets you control a DC motor to go backward or forward .

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 H-bridge consists of four BJT/ Diode pairs .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 operates 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 .

MODEL :

Explaination :

• The pulse generator is connected to mux block and the signal goes to switches ,which are further connected to demux block and extract off output signals as p1 , p2 , p3 ,p4 .
• when the input signal voltage is supplied with a pulse generator , the H bridge consist of 4BJT pairs , Q1 ,Q2 ,Q3 and Q4 respectively .
• Q1 and Q4 are forward monitering .
• Q2 and Q3 are reverse monitering.

B ) comment on the armature current shoot - up from the scope results .

• From the graph it can be seen that initially the armature current increases from 0 to 0.5 sec and at transition from positive to negative direction of rotation we observe a high surge in current , this is due to the transient switching occuring and we obsere a surge in current .

C ) Refer to the help section of 'the four - Quadrant chopper DC drive (DC7) block'. Compare it with the H - bridge model.

The four quadrant chopper DC drive block in matlab /simulink represents a four quadrant ,DC supplied chopper drive for DC motor . It feature closed loop speed control with four quadrent operation .

The close loop speed control will output the armature current of the machine . using a pi controller , the chopper duty cycle corresponding to the commanded armature current is derived . The duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.

Compare to other DC device, the main advantage of using the four quadren chopper device is that it can operate in all 4 quadrents . Each quadrent in its characteristics represents a different modde of operation . The first quadrent is forward motoring , the second is reverse braking , the third is reverse motoring and the forth is forward braking .Another advantages is that it uses high switching frequency DC-DC converters resulting in a lower armature current ripple is occure than compared to thyristor based DC drive .

The image below shows the DC7 block :

Detail circuit diagram for the DC7 block is shown below :

Comparision to H - Bridge

Both BJT H- bridge and 4 quadrant chopper have their advantages and disadvantages . H bridge like 4 quadrent chopper are also capable of the breaking operation without using air resistance or friction or physical brakes to bring the motor to rest .4 quadrant DC chopper are more commonly used in automobiles whereas BJT H- bridge are commonly used in robotics and electro - mechanical devices .

Unlike 4 quadrant DC choppers , BJT H- bridge are not capable of performing regenerative braking but they can perform dynamic braking . it is this difference that make 4 quadrent DC chopper useful in hyrid and electric vehicles.

In 4 quadrent DC choppers , the current signals are interrupted with one another to change the direction of rotation of the motor . This is done by speed reference or a drive cycle from which the speed values are converted to current signal which interrupt one another to change the rotation of the direction of the motor . However in BJT H -bridge , the polority of the current is reversed .

The current signal in 4 quadrent DC choppers are chopped meaning that they are not continuous , they are discrete and so take less time for simulations .But in BJT H-bridge , the current values are not chopped , meaning that they are continuous and so take more time for simulations .

In 4 quadrent DC chopper , a PI controller is used to control the speed and the current and in H-bridge , this is not implemented .

3 ) Explain in breif about operation of BLDC motor .

A Brushless DC motor , BLDC accomplishes commutation electronically using rotor position eedback to determine when to switch the current . The BLDC motor is electrically commutated by power switches instead of brushes .

There are thee types of the BLDC motor :

- Single phase

- Two phase

- Three phase

Stator for each type has the same number of windings . The rotot has permanent magnets to form two magnetic pole pairs , and surrounds the stator ,which has the windings.

Working of BLDC motor :

BLDC motor operation is based on the attraction or repulsion between magnetic poles . USing the three-phase motor , the process starts when current flows through one of the three stator winding and generate a magnetic pole that attracts the closest permanent magnet of opposite pole .

The rotor will move if the current shifts to an adjacent winding . Sequentially charging each winding will causes the rotor to follow in a rotating field . The torque in this example depends on the current amplitude and the number of turns on the stator winding , the strenght and size of the perment magnets , the air gap between te rotor and the winding , and the length of the rotating arm .

Advantages of BLDC motor :

• Higher reliability
• Higher efficiency
• Lower acoustic noise
• smaller and lighter
• greater dynamic response
• better speed vs torque characteristics
• high speed range
• longer life