Week-7 Challenge: DC Motor Control

Q.1 (a). Explain a MATLAB demo model named 'speed control of dc motor using BJTH - bridge ''

First of all , let us understand the basic concept of speed control using H bridge configration .

Most DC motors have two terminal because they are series type (armature and field winding are connectes in series)

DC motor .

Fig 1 : image showing H - Bridge circuit used for controlling direction of rotation of a DC motor shows every change

As shown in figure there are two terminal 'A' and 'B' of DC motor . now if we connect terminal A with +ve supply and

Terminal 'B' with , -ve supply or ground the current will rotate in one direction - say clockwise (CW) or forward direction

Now as shown in second figure shows change in supply terminal . Now B is connected with +ve and +A Connected to the

Terminal to the ground . the current will from the motor from B to A and motor will rotate in other direction (counter

clockwise direction - CCW or reverse) . The arrangement to get reversal supply is shown in right side of the figure ,

Four switches are connected in between +ve supply are ground and DC motor is connected in between two switches

As shown . Such circuit arrangement known as , (H - bridge circuit are most widely used in DC motor drives) . let us see

How it gives reverse supply to motor .

If SW1 and SW4 are passed are pressed simultaneously then current will flow from +ve , - SW1, -A-B SW4 - ground

So motor will gets revers direction supply and it will rotate in another direction .

Caution : are has to be taken that SW1 and SW3 or SW2 and SW4 must not be pressed simultaneously otherwise it will

Short circuit the supply .

Fig .2 : Circuit replaces the switches with NPN type transistor . Diodes are given for the current to free wheel as the

DC motor is a RLE load , where the current cannot change direction instantenously . we all know that transistor works

As as a switch . for NPN transistor if we give enough +ve input to base it will turn ON (Saturation region) and if we

Give 0 input it will be turned OFF .(cut off region) . so in this case circuit if Q1 and Q4 and turn ON simultaneously

the motor will rotate forward and if Q2 and Q3 turned ON then the motor will in the reverse direction ,

in order to varies speed of the DC motor we have to change the voltage applied to DC motor . this could be achieved

with the help of Pulse with modulation .

: Fig.3: Ground showing pulse width modulation for controlling speed of DC motor pulse with modulation for controlling

speed of dc motor .

Pulse width modulation means varying the width (duty) of pulse . width means ON time to , (T)on of pulse . The average

output of voltage (Vdc or Vavg) is given by equation .

Vdc = Ton/Ton+Toff *Vs

Vdc = n.Vs (where n is the duty cycle)

The average voltage directly depends upon Ton . As shown in the figure duty cycle 50% the average output voltage Vdc

is exactly the half of Vs. if duty is increased 75% , Vdc also increased 3/4th of Vs and if duty is decreased to 25% , Vdc

Thus as pulse width varies . so we have to apply PWM to DC motor to vary its speed ,

The example below shows the simulink model of an H - bridge used to generate a chopped voltage and to control the speed

of a DC motor easily to rotate to every half cycle .

Speed control of a DC Motor using BJT H - Bridge

The Bipolar Junction Transistor (BJT) when used for power switching applications , operates as on IGBT . when it is

Conducting (BJT operating in the saturated region ), a forward voltage Vf is developed between collector current

And emmitor current (in the voltage range if 1V) . Therefore the IGBT block can be used to model the BJT devices

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 used in the present model (5HP 24V 1750RPM). it simulate a fan type load (where load torque

is perportional to square of speed) . the armature mean voltage can be varied from 0 to 24 V when the duty cycle

(Specified in the pulse generateor block) , is varied from 0 to 100% .

NOTE : The DC motor used in the simulink model is a separately excited DC motor and NOT a DC generator or series

Motor . that is the reason why a DC voltage of 300V is given to the field windings .

Q.1(b) .Comment on the armature current shoot - up from the scope results .

Simulation output :

1.) Pulse Generator parameter :

Period : 2 milli second

Pulse width (%of period) :75%

Initially , there is a spike in armature currrent , this is because , in a DC motor .

Ia = V - Ea /Ra where Ia is the armature current V is the supply voltage , Eb is the back emf

And Ra is the armature resistence .

At starting Eb = 0 as speed = 0 starting .

Therefore Ia = V/Ra and Ra is a very small values . hence Ia = V/Ra and Ra is a very small values . Hence Ia is huge

And that reflects in the graph .

2.)

Period : 2 milli seconds

Pulse Width (%of period) :50%

Therefore when pulse width decreases, the voltage applied across the DC motor decreases and the armature current

finally decreases .

1.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 DC chopper DC drive (DC7) block represent a four quadrant , DC - supplied , chopper can be

(DC - DC PWM converter) .drive for the DC motor . This drive features closed - loop speed control with four - quadrant

The speed control loop outputs the reference armature currrent of the machine using a PI current controller . the chopper

Duty cycle corresponding to the commanded armature current is derived. this duty cycle is derived PWM signal controller

The Quadrant chopper of DC Drive block uses these blocks from the electrcic drives/Fundamental drives Blocks library..

• Speed controller (DC)
• Regulation Switch
• Current controller
• Chopper

• The above figure shows the schematic of the 4 quadrant chopper DC drive

H - Bridge

Only two quadrant of operation are possible they are forward motoring and reverse motoring

Higher armature current ripple .

Less complex ,simulation is continuous and simulation time is more widely used in robotics .

Q.2) Developed a 2 - quadrant chopper using simulink and explain the working of the same

with the revelent result .

• Type C chopper or two Quadrant class A chopper .

Class - C or Type - C Chopper is a category of a chopper which can be operate in first as well as second quadrant

This basically means that , the power can either flow from source to load to source in in this chopper . this kind of

Chopper is also known as Two quadrant Class - A chopper .

Operation or working of class - C or Type - C :

As we know that , a class A and Class B chopper operates in first and second quadrant respectively , there if we connect

Both these types of in parallel then it is possible to have chopper in parallel then it is possible to have chopper operation in

First asa well as second quadrant , in fact Class C or Type - C chopper is obtained by the parallel connection of class - A

and Class - B chopper , Figure below shows the circuit diagram of this type of chopper .

We can notice that chopper CH1 , free - wheeling diode (FD) and load forming Class - A chopper whereas chopper

CH2 , D2 and load are forming class - B chopper , both these choppers are connected in parallel. To both first quadrant

Operation we should swith ON chopper CH1 and for getting second quadrant operation we should switch ON chopper

CH2 . let us consider in detail.

Case :1,When CH1 is Switched ON/OFF

When chopper CH1 is switched ON , source Vs directly gets connected to the load and hence load voltage Vo is equal to

Source to load as shown in figure source to the load as shown in the circuit diagram which ic assumed positive of the

operation of chopper is first quadrant .

Case ;2 : When CH2 is Switched ON/OFF

When load chopper CH2 is switched ON , load DC source E drives current through CH2 and load . The direction of this current

io will be positive to that shown in circuit diagram and hence is assuemd negative , output voltage Vo is zero during this time

When CH2 is mode OFF , Diode D2 gets forward biased and hence current in to the source from the load , the output

Voltage is Vs in this time as load is connected to source through D2 during current always negative , i.e operation of chopper

is with in second quadrant .

The average load voltage is always positive but tha average load current may be positive / negative , therefore power

flow may be form source to load (first quadrant operation). The operating region of class- C OR Type Chopper is shown

below hatched area..

• Caution : Chopper CH1 and CH2 should not be ON simultaneously as this whould be lead to direct short circuit on the

Supply lines .

APPLICATION : Class - C or Type - C choppers are used in motoring and regenrative breaking of DC motor ,

Realization using Simulink :

Hence the simulink model corresponding to the circuit diagram is built using basic simulink blocks . The pulse generators

is used with the NOT gate to prevent misfiring of the MOSFETs so that they do not conduct at the same time .

PULSE GENERATOR PARAMETERS :

An amplitude of 20 is choosen so that we could see the pulses clearly .

DC voltage parameter :

Source voltage = 24V

Load Side = 12V

Simulink Output :

Hence , we could see clearly that the voltage is always positive but the current becomes both positive and negative . hence

The type C chopper simulated and verified .

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

Brushless DC electric motors also known as electronically commutated motor (ECMs , EC motors). primary efficiency

is a most important feature for BLDC motor . Because the rotor is the sole bearer of the magents and it doesn't require

Any power i.e no connection , no commutator and no brushes .

CONSTRUCTION : -

on the stator . they use electrically communication to convert electrical energy into mechanical energy . the main design

diffrences brushless motor is the replacement of mechanical with the electrical switch circuit with the help of electrical as

well as magnetic effect of current easily to moderate variable supply .

WORKING :

BLDC motor works on the principle similar to that of brushed DC motor . Lorentz force law which states that whenever

a force current carrying a conducter is stationary and varying .

when the stator coil get a supply from source , it becomes electromagnet and starts producing the uniform field in the

Air switching makes the generate an AC voltage waveform with trapezoidal shape due to the force interaction to the

Rotate to the rotor , the rotor continues to rotates .