Week-7 Challenge: DC Motor Control
AIM:
1. To explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’ and ‘The Four-Quadrant Chopper DC Drive (DC7) block’ and Compare it with the H-bridge model.
2. To explain the operation of the BLDC motor.
1. Speed control of a DC motor using BJT H-bridge
The Bipolar Junction Transistor (BJT) is used for power switching applications and operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage is developed between collector and emitter (in the range of I V). Therefore, the IGBT block can be used to model the BJT device The IGBT block does not simulate the gate current controlling the BJT or IGBT.
The switch is controlled by a Simulink signal (1/0).
The DC motor used in the present model is (5 HP 24V 1750 rpm).
The armature means voltage can be varied from 0 to 240V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.
It simulates a fan type load (where Load torque is proportional to the square of speed).
The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models).
Two transistors are switched simultaneously: QI and Q4 or Q2 and Q3.
Switches QI and Q2=Forward Motoring Q2 and Q3=Reverse Motoring.
When Ql and Q4 are fired, a positive voltage is applied to the motor, and diodes D2-D3 operate as free-wheeling diodes
when Ql and Q4 are switched off. When Q2 and Q3 are fired, a negative voltage is applied to the motor, and diodes DI-D4 operate as free-wheeling diodes
when Q2 and Q3 are switched off. 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 V). Therefore, the IGBT block can be used to model the BJT device. The IGBT block does not simulate the gate current controlling the BJT or IGBT. The switch is controlled by a Simulink signal (1/0).
When QI and Q4 are switched off. When Q2 and Q3 are fired, a negative voltage is applied to the motor, and diodes DI-D4 operate as free-wheeling diodes when 2 and Q3 are switched off. A Switch block at bottom of the circuit Splits the Power Supply into 4 parts namely they are LLP) (p2) [p3) [p4].
These inputs are in the Goto form block. That block is used to send a signal to "From Block that has 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 tags are enclosed in the bracket and Scoped tag names are enclosed in braces 0)
Therefore these 4 types of Input are connected to transistor QI Q2, Q3, and Q4 through "From Block"; An From block' Receives. the Signal from the "Goto Block' with the specified tag The block icon displays the selected.
The Speed and Armature current is obtained by the DC Motor and Load Torque is Calculated by load torque proportional to the square of the speed Starting point, there is a sudden Shoot up is acted.
This Scope shows that speed is increased in a certain period and at that time armature current is positive whereas Load Torque also moves forward.
After 0.5 sec which is given in Pulse generator (step time), sudden shoot-up is acted and speed gets decreased in that time armature current is in negative terminal whereas Torque act as reverse action. Here the sudden shoot-up is acted while the Armature current is processed at a positive and negative terminal.
b) The Four-Quadrant Chopper DC Drive (DC7) block.
Model Description:
In this design, a 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.
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 to obtain the electromagnetic torque needed to reach the desired speed.
The speed reference change rate follows acceleration and deceleration ramps 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 behavior, the instantaneous pulse values of IGBT devices1 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. To limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.
2. Operation of the BLDC motor.
CONSTRUCTION:
In this motor, the permanent magnets are attached to the rotor. The current-carrying conductors are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy. The main design difference between brushed and brushless motors is the replacement of a 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.
in slots that are axially cut along the inner periphery of the stator. These windings can be arranged in either star or delta. However, most BLDC motors have a three-phase star-connected stator. Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. To form an even number of poles, each of these windings is distributed over the stator periphery.
WORKING:
BLDC motor works on the principle similar to that of a conventional DC motor, ie., the Lorentz force law which states that whenever a current-carrying conductor is placed in a magnetic field it experiences a force. AS a consequence of the reaction force, the magnet will experience an equal and opposite force. In the case of, the 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 an 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 a trapezoidal shape Due to the force of interaction between the electromagnet stator and the permanent magnet rotor.
In the figure above; in which the 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 aligns with stator poles causing the motor to rotate. Observe that the 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). In this way motor moves in a clockwise direction.
Advantages of Brushless DC motor
Less overall maintenance due to the 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 a permanent magnet rotor has a High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors.
Long-life as no inspection and maintenance is required for commutator system
Higher dynamic response due to low inertia and carrying windings in the stator Less electromagnetic interference
Low noise due to the absence of brushes
Limitations of Brushless DC motor
Need for additional sensors
These motors are costly
The electronic controller required to Control this motor is expensive Requires complex drive circuitry
Applications of Brushless DC motor
Computer hard drives and DVD/CD players Electric vehicles, hybrid vehicles, and electric bicycles Industrial robots, CNC machine tools, and simple belt-driven systems Washing machines, compressors and dryers Fans, pumps, and blowers
Brushless DC motors (BLDC) are used for a wide variety of application requirements such as varying loads constant and positioning applications in the fields of industrial control, automotive, aviation, and automation systems. heating cure equipment etc.