Week-7 Challenge: DC Motor Control

Ans.1A)

MATLAB Model :

WORKING:-

In the above Circuit the Input voltage is generated from the Pulse generator where the amount of supply and duty cycle is assigned in the Pulse generator. Then the switch block is connected to Pulse generator through armature voltage control block.

The H-bridge consists of 4 BIT/Diode pairs (BJT Simulated by IGBT Models). Two transistors are switched simultaneously Qland Q4 or Q2 and Q3.

When Q1 and Q4 are fired; a positive voltage is applied to Motor and Diodes D2-D3 operate as free-wheeling diodes, when Q1 and Q4 are switched OFF.

When Q2 and Q3 are fired, a negative voltage is applied to Motor and Diodes D1-D4 operates as free-wheeling diode, when Q2 and Q3 are switched OFF.

A Switch block at bottom of circuit Splits the Power Supply into 4 parts namely they are [[pl] (p2) [p3) (p4)]. These inputs are in Goto form block.

That block is used to send signal to "From Block" that have the specified tag. If the tag visibility is "scoped", then a Goto tag visibility block must be used to define the visibility of that particular tag. The block icon displays the selected tag name (local tag are enclosed in bracket "[]" and Scoped tag names are enclosed in braces"{}").

Therefore these 4 types of Input are connected to transistor Q1, Q2, Q3, and Q4 through "From Block": An "From block" Receives the Signal from the "Goto Block" with the specified tag.

Q1 and Q4 = Forward Motoring,

Q2 and Q3 = Reverse Motoring

This formation is called as H - Bridge, An H- Bridge is an arrangement of transistor that allows an circuit full control over standard electric DC motor.

Ans.1B)

Simulation of the model with by default in the pulse generator block:

The Default value of pulse width is 75.

From the result it is seen that the armature current shoots up when the motor is made to change direction at 0.5s of the simulation.

Simulation of the model with change in the pulse generator block:-

The default value of pulse width 75 is reduced to 50 and then the model is simulated.

From the result we can see that T = 0.5 of the simulation the motor changes direction and the current shoot is reduced accordingly. Due to the reduced pulse width, the speed of the motor and the load torque is also reduced.

Simulation of the model with change in the pulse generator block:-

The pulse width is again reduced from 50 to 25 percent and then simulated.

From the result it is see that, the current does not shoot up when the motor changes direction T= 0.5s of the simulation. The speed of the motor and the load torque in of the motor are low.

From all the simulations it is concluded that, when the duty cycle of the pulse generator block is low then the speed, current and load torque is also low accordingly.

Ans.1c)

Fourth Quadarant Chopper 200 HP DC7 operation of DC Motor:-

• The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source.

• The armature voltage is provided by an IGBT converter controlled by two Pl regulators.

• The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz voltage source.

• In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC7 block.

• The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference (input.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed.

• The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system.

• The current regulatory controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation).

• For proper system behavior, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3.

• This generates the average armature voltage needed to obtain the desired armature current. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

Results:-

Comparison with H-Bridge:-

1. In four Quadrant Chopper DC drive distributes required current supply for each sector and it consist of 4 quadrants Like Forward Motoring Regenerative Braking, Reverse Motoring and Reverse Regenerative Braking, Whereas in H bridge supplies an irregular current for each sector that causes voltage Fluctuation in armature current it consist of 2 Operation namely they are Forward Motoring, Reverse Motoring.

2. In Modern Battery Technology Charger use for EV charging uses H-bridge circuit Whereas a Four Quadrant Operation is required in Industrial as well as Commercial application.

3. The efficiency is high in Chopper compare with H-bridge and Maintenance is less than H-bridge.

4. In Robotic Technology, H-bridge circuit is used some other application of Four Quadrant Operation is Engine Test Loading Systems.

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

Ans.2)

Circuit diagram for two quadrant chopper:-

This type of Chopper, the average voltage will be always positive but the average load current might be positive or negative. The two Switches CHI and CH2 should not be turned on simultaneously as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

We will always get a positive output voltage Vo as the freewheeling diode FD is present across the load. When the chapper CH1 is on the current flows from source to load. The inductor stores energy. When Cht turn OFF the freewheeling diode starts conducting and the output voltage vo will be equal to V, The direction of the load current io will be reversed. The current ip will be flowing towards the source. The load current will be negative when the CH2 is on. The current completes its path through riode D2.

Two quadrant chopperv using simulink:-

To start with the two-quadrant chopper drive, we have first given the input from the DC source voltage.The voltage value is kept to be 24V.

For Power switches or transistors, we will be using the MOSFET (metal-oxide-semiconductor- field-effect transistor) coupled with the Diode in parallel. There will be two pairs of these switches and diode so that we can operate the model in 2- quadrants and all these are connected accordingly.

Now, the pulse generator with 50% duty cycle, 0.3 sec, and 5 as amplitude. The output of the generator is connected to the NOT operator which ensures that only one switch is working at a time. Then it is further connected to the gate of the switches.

The inductor is provided which will store the current when switch 1 is turned off. Then for the load, we have used another dc source with a voltage of 12V as shown in the below diagram.

To measure the current and voltage, the measuring block is used whose output can be visualized using the scope.

We need the PowerGui block to simulate any Simulink model containing Simscape Electrical Specialized Power Systems blocks. It stores the equivalent Simulink circuit that represents the state-space equations of the model.

Output result:-

The voltage waveform the minimum value of voltage is zero and maximum value is always in the positive direction. However in current waveform, the current increases and decreases, the current may be positive as well as negative.and the pulse amplitude is 5 thus from pulse waveform we can see that the pulse amplitude is 5.

Ans.3)

A brushless DC motor (also known as a BLDC motor or BL motor) is an electronically commuted DC motor which does not have brushes. The controller provides pulses of current to the motor windings which control the speed and torque of the synchronous motor.

These types of motors are highly efficient in producing a large amount of torque over a vast speed range. In brushless motors, permanent magnets rotate around a fixed armature and overcome the problem of connecting current to the armature. Commutation with electronics has a large scope of capabilities and flexibility. They are known for smooth operation and holding torque when stationary.

Functions:-

Brushless DC motor has only two basic parts: rotor and the stator. The rotor is the rotating part and has rotor magnets whereas stator is the stationary part and contains stator windings. In BLDC permanent magnets are attached in the rotor and move the electromagnets to the stator. The high power transistors are used to activate electromagnets for the shaft turns. The controller performs power distribution by using a solid-state circuit.

Working Principle of BLDC Motor:-

BLDC motor works on the principle similar to that of a Brushed DC motor. The fleming left hand rule 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.

Types of Brushless DC Motors :-

BLDC are of two types, one is outer rotor motor and other is inner rotor motor. The basic difference between the two is only in designing, their working principles are same.

Inner Rotor Design:-

In an inner rotor design, the rotor is located in the centre of the motor and the stator winding surround the rotor. As the rotor is located in the core, rotor magnets do not insulate heat inside and heat get dissipated easily. Due to this reason, inner rotor designed motor produces a large amount of torque and validly used.

Outer Rotor Design:-
In outer rotor design, the rotor surrounds the winding which is located in the core of the motor. The magnets in the rotor trap the heat of the motor inside and do not allow to dissipate from the motor. Such type of designed motor operates at lower rated current and has low cogging torque.

Advantages of Brushless DC Motor :-

• Brushless motors are more efficient as its velocity is determined by the frequency at which current is supplied, not the voltage.
• As brushes are absent, the mechanical energy loss due to friction is less which enhanced efficiency
• BLDC motor can operate at high-speed under any condition.
• There is no sparking and much less noise during operation.
• More electromagnets could be used on the stator for more precise control.
• BLDC motors accelerate and decelerate easily as they are having low rotor inertia.
• It is high performance motor that provides large torque per cubic inch over a vast sped rang.
• BLDC motors do not have brushes which make it more reliable, high life expectancies, and maintenance free operation.
• There is no ionizing sparks from the commutator, and electromagnetic interference is also get reduced.
• Such motors cooled by conduction and no air flow are required for inside cooling.

Disadvantages of Brushless DC Motors:-

• BLDC motor cost more than a brushed DC motor.
• The limited high power could be supplied to BLDC motor, otherwise, too much heat weakens the magnets and the insulation of winding may get damaged.