Week-7 Challenge: DC Motor Control

Problem 1

AIM : The MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge

The basic 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.

H-Bridge circuit used for controlling direction of rotation of a DC Motor

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 B is 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 reverse).

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: care has to be taken that SW1 and SW3 or SW2 and SW4 must not be pressed simultaneously otherwise it will short circuit the supply)

H-Bridge built by transistor for controlling DC motor speed and rotation

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 transistor works as 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 circuit if Q1 and Q4 are turned ON simultaneously the motor will rotate forward and if 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 for controlling speed of DC Motor

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

`V_(dc)= T_(on)/(T_(on)+T_(off))* V_s`

`=> V_(dc)= delta*V_s` where `delta` 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 to 3/4^th of Vs and if duty is decreased to 25%, Vdc reduces to 1/4^th of Vs. Thus as pulse width varies the average output voltage varies. So we have to apply PWM to DC motor to vary its speed.

Block Model

Speed Control of a DC Motor Using BJT H-Bridge

The BJT function is used for the power switching operations, it operates as an IGBT. When its lead to operate by using the BJT (saturated region) a forward voltage (Vf) is developed between the collector and emitter (1V).Here the IGBT block can be used to model the BJT device.

The IGBT function doesn’t simulate the gate current controlling the BJT or IGBT. The switch is controlled by the voltage device .BJT requires the base input current which is supplied continuously to maintain the enough quantity of saturation period. The armature voltage can be varied from 0 to 230V 50hz .The Dc motor model (5HP 24HV 1750RPM).

As we can observe the H-bridge consists of four Diodes

The two transistors are existing by the switching operations that is the Q1 and Q4 diode and Q2 and Q4 diode.

Here the Q1,Q2 shouldn’t trigger at once and also Q3,Q4 shouldn’t trigger at a time it leads to burn ar short circuit.

When we trigger on the bridge

The Q1 and Q4 is turned on the positive voltage is applied to the DC motor and to the diodes D1,D4 to opearate as free wheeling diode as in its forward direction. At this period the Q2 and Q3 are turned off in its direction

The Q2 and Q3 is turned on the negative voltage is applied to the DC motor and to the diodes D2,D3 to opearate as free wheeling diode as in its Reverse Direction. At this period the Q1 and Q4 are turned off in its direction.

b) AIM : Comment on the armature current shoot-up from the scope results.

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

`I_a= (V-E_b)/R_a` where `I_a`is the armature current, `V`is the supply voltage , `E_b`is the back emf and `R_a`is the armature resistance.

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

Therefore `I_a= V/R_a` and `R_a`is a very small value. Hence `I_a`is huge and that reflects in the graph.

C) AIM : 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 coverter.It is a closed loop speed controller,the output of the speed control reference the armature current of the machine.

Difference between H-bridge and Four Quadrant 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 ripple.	Lower armature current ripple due to high switching frequency of DC-DC coverters
Less complex	More complex
Simulation is continuous and simulation time is more	Discrete and Simulation time is less
Widely Used in Robotics	Widely used in Automobile

Problem 2

AIM : To develop a 2-quadrant chopper using simulink & explaining the working of the same with the relevant results

As we know that, a Class A and Class B 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 operation 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 switch ON chopper CH1 and for getting second quadrant operation we should switch ON chopper CH2. Let us discuss this in detail.

Case 1: When CH1 is switched ON/OFF

When chopper CH1 is switched ON, source V_s directly gets connected to the load and hence, load voltage V_o is equal 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 voltage V_o becomes zero. However, the i_o continues to die down through the FD and L in the same direction as shown in circuit diagram. Thus, the average output voltage V_o and current I_o are positive and hence operation of chopper is in first quadrant.

Case 2: 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 i_o will be opposite to that shown in circuit diagram and hence is assumed negative. Output voltage V_o is zero during this time. When CH2 is made OFF, diode D2 gets forward biased and hence the current into the source from the load. The output voltage is V_s 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 i.e. 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 hatched area.

Simulink block

Scope

The Choppers CH1 and CH2 should not be ON simultaneously as this would lead to direct short circuit on the supply lines.

and its application Class-C or Type-C choppers are used for motoring and regenerative breaking of DC Motors.

Problem 3

AIM : Brushless DC motor

It is an electric motor and it is also called electronically commutated motors or synchronous dc motor ,it is a synchronous motor using a direct current electric power supply.It is essentially flipped inside out,eliminating the need for brushes to flip the electromagnetic field.

In brushless Dc motors,the PERMANAENT MAGNETS are on the rotor and the electromagnets are on the stator.

The electronic commutation arrangement eliminates the commutator arrangement and brushes in a dc motor,hence it has more reliable and less noisy operation is achieved.

Here due to the absence od BLDC motors are capable to run at high speeds. The efficiency of BLDC motors is typically 85 to 90 percent,where as brushed type DC motors are 75 to 80% efficient

These are the wide varieties of BLDC motors are available ranging from small power range to fractional horsepower .

Construction

It has many different physical configurations .Depends on the stator windings ,these are arranged as single phase ,two phase ,three phase BLDC motors with permanent magnet rotor (commonly used)

Now, this is constructed with the three phase induction motor as well as conventional DC motor.The parts are

• Stator
• Rotor
• Hall effects
• Permanant Magnet Rotor

Working

The BLDC motor works on principle similar to that of conventional DC motor the lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force.

As the result of the reaction force,the magnet will experience an equal and opposite force.Incase BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.

When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform 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 continues to rotate.

Consider the figure below in which motor stator is excited based on different switching states.

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 poles align with stator poles causing motor to rotate.

Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction

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Here, one might get a question that how we know which stator coil should be energized and when to do.

This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.

Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.