Week-7 Challenge: DC Motor Control

 First of all, let us understand the basics concept of speed control using H bridge configuration.

Most DC motors have two terminals because they are series type (Armature and field windings are connected in series)DC motors.

As shown in figure there are two terminals '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 flow from motor from A to B and motor will rotate in one direction say clockwise(CW)or forward direction.Now as shown in second figure we change the supply terminals.Now Bis connected with +ve and A is connected to ground.The current will flow from motor from B to A and motor will rotate in other direction(counter clockwise CCW or revese).

The arrangement to get reversible supply is shown in right side of figure.Four switches are connected in between +ve supply and ground and DC motor is connected in between two switches as shown.such circuit arrangement is known as H-bridge because it looks like letter 'H'(H-bridge circuits are most widely used in DC motor drivers).Let us see how it gives reverse supply to motor.

If SW1 and SW4 are pressed simultaneously then current will flow from+ve -SW1-A-B-SW4-Gnd.so motor will rotate in one direction.Open(release)SW1 and SW4 to stop motor.Now if SW2 and SW3 are pressed current will flow from+ve -SW2-B-A-SW3-Gnd.so motor gets reverse supply and it will rotate in another direction.

(caution:case has to be taken that SW1 and SW3orSW2andSW4 must not be pressed simultaneously otherwise it will short circuit the supply).

The circuit replaces the switches with NPN type transistors.Diodes are given for the current to free wheel as the DC motor is a RLE load where the current cannot change direction instantaneously. We all know that transistorworks as switch.For NPN transistor if we give enough +ve input to base it will turn ON(saturation region)and if we give0 input it will be turned OFF (cut off region).so in this circuit if Q1 and Q4 are turned on simultaneously the motor will rotate forward and ij Q2 and Q3 are turned on then motor will rotate in reverse direction.

In order to vary the speed of the DC motor,we have to change the voltage applied to the DC motor.This could be achieved with the help of pulse width modulation.

Pulse width modulation means varying the width(duty)of pulse.width means ON time Ton of pulse.The average output voltage(Vdc or Vavg) is given by equation

Vdc=Ton/Ton+Toff*Vs

vdc=d*Vs

d is the duty cycle

The voltage directly depends upon Ton.as shown in above figure if duty is 50% the average output voltage Vdc is exactly the half of Vs.if duty is increased to 75%,Vdc also increases to3/4th of Vs and if duty is decreased to 25%,Vdc  reduces to 1/4th of Vs.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.

The Bipolar junction Transistor(BJT)when used for power switching applications ,operates as an IGBT.when it is coducting (BJT operating in the saturated region),a forward voltage Vf is emitter(in the range of1 V0.Therefor,the IGBT block can be used to model the BJT device.

The IGBT does not simulate the gate current controlling the BJT or IGBT.The swith is controlled by a simulink signal(1/10).The DC motor uses the present model (5HP 24v 1750 rpm).it simulates a fan type load(where load torque is propotional to suare of speed.)The armature mean voltage can be varied from 0to 240v when the duty cycle(specified in the pulse  generator block)is varied from 0to 100%.

The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models).Two transistors are switched simultaneously:Q1andQ4orQ2andQ3.when Q1 and Q4 are fired.apositive voltage is appliedto the motor and diodes when D2-D3operate as free-wheeling diodes when Q1andQ4 are switched off.When Q2 ANDQ3 are fired,a negative voltage is applied to the motor and dioded when Q2andQ3are switched off.

Note:The Motor used in the simulink model is a separatelyexcited DC motor and NOT a DC series motor.That is the reason why a DC voltage of a 300V is givento the field windings.

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

simulation output:

1)Pulse Generator Parameters:

period:2 milli seconds

pulse width (%of peroid):75%

initially, there is a spike in armature  current.This is because , in a DC motor,

Ia=V-Eb/Ra

where

Ia is the armature current

V is the supply voltage

Eb is the back emf and

Ra is the armature resistance.

At starting.Eb=0 as speed=0 at starting .

Therfore

Ia=V/Ra

and Ra is avery small value.Hence

Ia is huge and that reflects in the graph.

2)perio:2 milli secondspulse width(%of peroid):50%

Thereefor when pulse width decreases,the voltage applied across the DC motor decreses and the armature current finally decreases.

1.c)Refer to the help section of the four-quadratic chopper DC drive(DC7).block compare it with H-bridge model.

The four-quadrant chopper DC drive (DC7)block represents a four-quadrant.DC-supplied,chopper(orDC-DC pwm converter)drive for DC motors.This drive features closed-loop speed ccontrol with four-quadrant operation.The speed control loop outputs the reference armature current of the machine.Using a PIcurrent controller,the chopper duty cycle corresponding to the commanded amature current is delivered.This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.

The Four-Quadrant chopper DC drive block uses these blocks from the electric Drives/Fundamental Drive Blocks library:

• speed controller(DC)
• Regulation switch
• Current controller(DC)
• chopper

The above figure shows the schematic of the 4 quadratic chopper DC drive

H bridge

Four Quadrant chopper DC drive

Only Two quadrants of operation are possible.They are Forward motoring and reverse motoring.

It can operate in all four quadrants (forward motoring reverse regeneration,reverse motoring,and forward regeneration)

Higher armature current ripples.

Lower armature current ripple due to high switching frequency of DC-DC converters

Less complex

more complex

Simulation is continuous and simulation time is more

Discrete and simulation time is less 

widely used in automobile

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

Type C chopper or Two quadrant class A chopper

Class-C or Type-C chopper is a category of  chopper which can operate in fist as well as second quadrant.This basically means that,the power can either flow from source to load or load to source in this chopper.This kind of chopper is also known as two quadrant class-A chopper.

operation of working of class-C or type C:

As we know that,a class A and classB chopper operates in first and second quadrant respectively.Therefore if we connect both these types of chopper in parallel then it is possible to have chopper operattion in first as 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 are forming class-A chopper whereas chopper CH2,D2 and load are forming Class-B chopper.Both these choppers are connected in parallel.To obtain first quadrant operation we should swith ON chopper CH1 and for getting second quadrant operation we should switch ON chopper CH2.

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 eual to source voltage.The direction of load current is from source to load as shown in the circuit diagram which is assumed positive.

When CH1 is switched off the free wheeling diode FD comes into the circuit as it gets forward biased and hence shorts the load.Therefore the output voltageVo becomes zero.However the Io continues to die down through the FD and Lin the same direction as shown in circuit diagram.Thus the average output voltageVo and current Io are positive and hence operation of chopper is in first quadrant. 

CASE2:when CH2 is switched ON/OFF:
When chopper CH2 is switched ON,load DC source E drives current through CH2 and load.The direction of this current Iowill be opposite to that shown in circuit.diagram and hence is assumed negative.Output Voltage Vo is Zero during this time.When CH2 is more OFF,diode D2 gets forward biased and hence the current in to the source from the load.The output voltage is Vs in this time as the load is connected to the source through D2 during OFF time of chopper CH2.Thus,the load current is always negative.operation of chopper is within second quadrant.

The average load voltage is always positive but the average load current may be positive or negative.Therefore,power flow may be from source to load (first quadrant operation)or load to source(second quadrant operation).The operating region of class-C or Type-C chopper is shown below by 

CAUTION:choppers CH1 AND CH2 should not be ON simultaneously as this would lead to direct short circuit on the supply lines.

Application:Class-C or Type-C choppers are useed for motoring and regenerative braking of DC motors.

Hence the simulink model corresponding to the circuit diagram is built using basic simulink blocks.The pulse generator is used with the Not gate to prevent misfiring of the MOSFETS so that they do not conduct at the same time which results in the short circuit.

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

DC voltage source parameters:

source side:24v

load side:12v

Hence we could see clearly that the voltage is always is always positive but the current becomes both positive and negative.Hence the type C chopper  simulated and verified.

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

Brushless DC  electric motors also known as electronically commutated motors (ECMs,EC motors).primary efficiency is a most importent feature for BLDC motors.Because the rotor is the sole bearer of the magnents and it doesnt require any power.no connections,no commutator and no brushes.in place of these,the motor employs control circuitry.To detect where the rotor is at certain times,BLDC motors employ along with controllers,relay encoders or a Hall sensor.

CONSTRUCTION:

In this motor,the permanent magnents attach to the rotor.The current carrying conductors or armature windings are located on the stator.They use electrical commutation to convert electrical commutation to convert electrical energy into mechanical energy .The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit.A BLDC Motor is a type synchronous motor in the sence that the magnetic field generated by the stator and the rotor revolve at the same frequency.

Brushless motor does not have any current carrying commutators.The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.

When the stator coils get a supply from source.it becomes electromagnet and starts producing the unifrom field in the 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 continuous 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 align with stator poles which causes the motor to rotate.