Week-7 Challenge: DC Motor Control
1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
B. Comment on the armature current shoot-up from the scope results.
- Speed Control of a DC Motor Using BJT H-Bridge
The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage Vf is developed between collector and emitter (in the range of 1 V). Therefore, the IGBT block can be used to model the BJT device.
The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models). Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired, a positive voltage is applied to the 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 the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.
Settings in the Pulse generator 500Hz with duty cycle 75%
The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.
Scope - shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.
Scope after running the model
When the duty cycle is decreased to 50%. Scope is obtained as below.
No Change in armeture current with 50% duty cycle at initial stage. But after 0.5sec the armeture current goes in negative direction when the armeture voltage is reversed.
B. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model
4 Quadrant opration of drive
Foward Motoring - In the 1st quadrant power developed is positive and the machine is working as a motor supplying mechanical energy.
Foward Bracking - In 2nd quadrant, the direction of rotation is positive, and the torque is negative, and thus, the machine operates as a generator developing a negative torque, which opposes the motion. It is called Forward Bracking.
Reverse Motoring - The kinetic energy of the rotating parts is available as electrical energy which may be supplied back to the mains. In dynamic braking, the energy is dissipated in the resistance. The 3rd quadrant operation is known as the reverse motoring.
Reverse Braking - In the 4th quadrant, the torque is positive, and the speed is negative.
The below image shows the D7 Block
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 PI 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 (in p.u.) 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 regulator 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 behaviour, 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.
The output scope after running the model.
The 4 quadrant DC drives are used mainly due to their ability to work in 4 different quadrants of operation.It allows the controller to command the DC motor to rotate in clockwise and counterclockwise direction and perform braking.
In motoring mode - the machine works as a motor and converts the electrical energy into mechanical energy supporting its motion.
In braking mode - the machine works as a generator and converts mechanical energy into electrical energy and as a result, it opposes the motion. The Motor can work in both, forward and reverse directions that is in motoring and braking operations.
H Bridge concept
An H-bridge is a simple circuit that lets you control a DC motor to go backward or forward.
A DC motor spins either backward or forward, depending on how you connect the plus and the minus. If you close switch 1 and 4, you have plus connected to the left side of the motor and minus to the other side. And the motor will start spinning in one direction.
If you instead close switch 2 and 3, you have plus connected to the right side and minus to the left side. And the motor spins in the opposite direction.
Comparision - The 4 quadrant DC Drives distribute the required current supply for each sector and it consist of 4 quadrant i.e Foward Motoring Forward Braking Reverse motoring and Reverse Braking whereas H bridge supplies an irregular current for each sector that causes voltage fluctuation in armeture current. It has 2 operation Forward motoring and Reverse motoring.
In 4 Quadrant DC Chopper the current signal is interrupted with one another to change the direction of rotation of the motor. But in H bridge the current is changed to change the direction of rotation of the motor.
The H bridge circuit is used in robotics technology while 4 qaudrant DC chopper circuit are used in indutrial as well as commercial applications.
The effciency is high in 4quadrant DC chopper compare to H Bridge. Maintainance is less in H Bridge comparitively.
2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor )
Type C chopper is obtained by connecting type – A and type – B choppers in parallel. We will always get a positive output voltage V0 as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage v0 will be equal to Vs . The direction of the load current i0 will be reversed. The current i0 will be flowing towards the source and it will be positive regardless the chopper is on or the FD conducts. The load current will be negative if the chopper is or the diode D2 conducts. We can say the chopper and FD operate together as type-A chopper in first quadrant. In the second quadrant, the chopper and D2 will operate together as type –B chopper.
Circuit Diagram -
The average voltage will be always positive but the average load current might be positive or negative. The power flow may be life the first quadrant operation ie from source to load or from load to source like the second quadrant operation. The two choppers should not be turned on simultaneously as the combined action my cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.
First we add powergui block to take simulation in place we need a voltage and current measurement block we will be requiring a mosfest and pulse then add diode.We need Logical operation later on we will be converting this to NOT gate and we will be requiring series RLC branch which will be converted to inductor as our load and scope for results. We will arrange these according to our circuit diagram and enclosed the circuit as per abover diagram.The reason we we are giving NOT gate both switches does not conduct simultaneously if they do supply will be shot circuited.Now we will set the voltage in the source change the value of amplitude in pulse generator and period is 0.02 sec pulse width 50%. Then we run the program.
The voltage waveform is not going below0.The voltage is always in the +ve direction.However, we see the current increases and decreases. It goes to positive and negative based on the switching action. As we see the voltage is always positive and current can either positive or negative this type of circuit is called two quadrant type- C chopper.
3. Explain in a brief about operation of BLDC motor.
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
Working Principle and Operation of BLDC Motor
BLDC motor works on the principle similar to that of a conventional DC motor, i.e., 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 case 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.
The advantages of a BLDC motor are:
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.
The disadvantages of a BLDC motor are:
BLDC motor cost more than a brushed DC motor. Limited high power could be supplied to BLDC motor, otherwise, too much heat weakens the magnets and the insulation of winding may get damaged.