Week-7 Challenge: DC Motor Control

Ans -3 The BLDC motor is nothing but Brushless Dc 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 magnets and it doesn't require any power. i.e. 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, rotary encoders or a Hall sensor.

Construction of Brushless DC motor

In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature winding are located on the stator. They use 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 of synchronous motor in the sense 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.

The layout of a DC brushless motor can vary depending on whether it is in “Out runner” style or “Inrunner” style.

• Outrunner – The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.

• In runner – The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the out runner style, but is capable of spinning at very high rpm.

Working Principle of Brushless DC motor

BLDC motor works on the principle similar to that of a Brushed Dc motor. The Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In the BLDC motor, the current carrying conductor is stationary and the permanent magnet is moving.

When the stator coils get a supply from 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.

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 which causes the motor to rotate.

Advantages of Brushless DC motor

• Less overall maintenance due to absence of brushes
• Reduced size with far superior thermal characteristics
• Higher speed range and lower electric noise generation.
• It has no mechanical commutator and associated problems
• High efficiency and high output power to size ratio due to the use of permanent magnet rotor

Limitations of Brushless DC motor

• These motors are costly
• Electronic controller required control this motor is expensive
• Requires complex drive circuitry
• Need of additional sensors

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

THis model is opened by command - power_Hbridge .

Model:

A)Working of H-bridge model:

• Here H-bridge DC chopper circuit is designed to control the speed of the motor.
• IGBT acts as BJT in power switching devices.
• Four IGBT switches are used for inverting the DC voltage 0 to 240 v supply.The current flows from collector(C) to emitter(E).
• The gate are controlled by the pulses(0 and 1) according to the armature voltge of motor given by pulse generator.

• If Switch 1 and 4 is closed, this will cause the current to flow from the source, through Switch 1 and then through the load, then through Switch 4, then down to the ground.This is known as Forward motion of the motor.

  - If Switch 2 and 3 is closed, this will cause the current to flow from the source, through Switch 3 and then through the load, then through Switch 2, then down to the ground.This is known as Reverse motion of the motor.

  - If Switch 1,3 or 2,4 is Closed, then both sides of the motor have same voltage and this is called as Brake mode.

  - If Switch 1,2 or 3,4 is closed, then it will cause a short and burn out the H-bridge.

• Here run this demo model and study the results from scopes.

Speed of motor:

• Initially increases to 0 to nearly 900 rpm upto 0.5 secs(Forward motoring action takes place
• Then speed increases from 0 to -900 rpm at upto 1 sec( Reverse motoring action takes place).

Load torque of motor:

• Initially torque increase from 0 to 0.5 secs in forward motoring action.
• Then torque increases in reverse direction in reverse motoring action upto 1 sec.
• Load Torque proportional to
  square of speed
  T=k*w^2

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

Here the initial current shoot up is due to sudden increase in voltage which in turns regulated by duty cycle.In this case motor driven in 75% of duty cycle. so voltage will be 0.75 *240 = 180 V

Lets decrease the duty cycle and see what happens :

So as we decrease the duty cycle percentage the voltage decreases and so speed and hence the armature current also decreases . the small shoot up is there which can also be reduced by reducing the duty cycle .

Now keeping the Duty cycle percentage 30% and lets see what happens :

Conclusion:

Thus to reduce the shootup armature currents we have to change the duty cycle gradually from 0 to 100%,that inturns increases voltage gradually and speed also changes accordingly.

C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

Four Quadrant Operation of any drives or DC Motor means that the machine operates in four quadrants. They are Forward Braking, Forward motoring, Reverse motoring and Reverse braking. A motor operates in two modes – Motoring and Braking. A motor drive capable of operating in both directions of rotation and of producing both motoring and regeneration is called a Four Quadrant variable speed drive.

circuit Diagram of fourquadrant chopper

COMPARISON OF FOUR-QUADRANT CHOPPER DC DRIVE (DC7) WITH H-BRIDGE:

In four quadrant chopper DC drive distributes required current supply for each sector and it consist of four quadrants like Forward Motoring, Forward Regenerative Braking, Reverse Motoring and Reverse Regenerative Braking. whereas H-bridge supplies an irregular current for each sector that causes voltage fluctuation in armature current. It consist of two operations they are Forward motoring, reverse motoring.

In modern battery charger for electrical vehicles, H-bridge circuits are used. A four-quadrant operation is required in industrial as well as commercial applications.

The efficiency is high in chopper compare with H-bridge, and maintenance is less than H-bridge.

In Robotics Technology, H-bridge circuits are used. Some other applications of four quadrant operation is engine test loading systems.

The regenerative braking is not possible in H-bridge while in chopper it is possible.

The most important application of the H-Bridge circuit as the motor driver circuit. H-bridge circuits can be placed in small places. Four quadrant chopper is lightweight in nature.

In H-bridge we need to change polarity which turns into opposite direction to control the motor whereas in chopper model we need to interrupt one signal under control of another.

Ans -2 . Develop a 2-quadrant chopper using simulink

Scope :

explain 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.

Carefully observe the circuit diagram. You will 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

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. In fact, this is the Class-B mode of operation.

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. In fact, this is the Class-B mode of operation.

Basic conclusion -

• The output voltage V_o is zero when chopper CH2 is ON or free-wheeling diode FD conducts.
• The output voltage V_o is equal to source voltage V_s when chopper CH1 is ON or diode D2 conducts.
• The load current flows in the direction shown in circuit diagram i.e.i_o is positive when CH1 is ON or FD conducts.
• The load current flows opposite to the direction shown in circuit diagram i.e.i_o is negative when CH2 is ON or D2 conducts.