Week-7 Challenge: DC Motor Control
Objective:
- To run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’. Modify the model such that armature current doesn’t shoot up when motor changes direction from forward to reverse.
- To understand the Four-Quadrant Chopper DC Drive (DC7) block and compare it with H-bridge model.
- To make a suitable EV model using DC7 block and make result report.
- Description of 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 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 uses the preset model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed).
The armature mean voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.
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.
- Initially, we run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’ with all it’s default values and different pulse width (duty cycle) 75%, 55%, and 45%.
- From this we get following three plot results-
(1) With 75% pulse width (2) With 55% pulse width
(3) With 45% pulse width
- From 1st plot we can see that the current plot has initially very high value and then becomes steady and after that shows same path in the -ve direction.
- In these three plots the value of this current shoot up is decreasing and the 3rd plot has almost smooth current flow.
- To get steadier and smoother current floe we need to decrease and control the duty cycle.
- This shoot up happens because of the switching in the circuit that cause heat losses during switching. To avoid these losses, we use HEAT SINK material to dissipate the heat. From this the motor can change its direction of rotation easily and very smoothly.
(2) Description of Four-Quadrant Chopper DC Drive (DC7) 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 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.
Comparison:
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Four-Quadrant Chopper DC Drive |
H-bridge model |
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To change the motor direction the polarity is changed |
To change the motor direction the current signal has to be interrupted. |
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It has broader torque and power output ranges |
It uses single pulse generator which gives narrow torque and power output range. |
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Up to 400v can be achieved. |
Up to 350v can be achieved. |
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In this the current has chopped values. |
In this the current has continuous values. |
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Regenerative braking is possible |
Regenerative braking is not possible |
(3) EV model using DC7 block:
We simply make use of dc7_example MATLAB Command to open the model in our system. After that we have to make some modification as follows-
Modify the energy source at the input side:
This model modified with an input of a battery from the Simulink Library Browser.
- We took a simple DC source as our energy source and give the appropriate voltage value for an EV.
- We connect a bus selector with M port of this dc battery.
- Bus selector provides three output signals SOC, current, and voltage of battery. This can we seen from the scope2.
Reference Speed:
We have prepared our own drive cycle data and import it to this model and replace it with reference speed block.
Subsystem for Vehicle Load Characteristics:
We took the vehicle characteristics equation for the relating Torque and Speed of the vehicle.
Linear load torque block has been replaced with a subsystem model according to the load characteristics given by
Change the simulation time according to our drive cycle data.
This EV model of 4 quadrant dc motor drive helps to read the plots of duty cycle IGBT 1&4, 2&3, armature voltage and current, and motor speed from the scope output.
In the duty cycles IGBT 1&4 and 2&3 have initially step increment and decrement respectively to provide the suitable voltage to the motor.
After t=22 sec. the reverse direction in armature current can be clearly seen. This is where the braking torque is produced and the speed also decreases.
After this point the motor speed also started decreasing as it clearly matches the driving cycle data.
3) BLDC 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 windings 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.
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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.
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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.
MUST READ BLOG POSTS ON DC MOTORS
- Different Types of DC Motor
- What is Brushed DC motor
- Difference between Brushed and Brushless DC motor
- Working Principle of DC Motor
- Difference Between AC and DC Motor
- Applications of Dc motor
- Speed control of DC motor
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
- 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 absence of brushes
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
Applications of Brushless DC motor
Brushless DC motors (BLDC) use for a wide variety of application requirements such as varying loads, constant loads and positioning applications in the fields of industrial control, automotive, aviation, automation systems, health care equipments etc.
- 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.
Hope this article helps you to understand about the brushless dc motor working principle and applications. We hope that you found it interesting and that you will come back for more of our educational blogs.
Conclusion:
Thus, we have studied the comparison between Four-Quadrant Chopper DC Drive and H-bridge model and also run the dc motor simulation using BJT and dc7 block. Studies the results from a EV model using dc7 block.
Reference:-
https://www.mathworks.com/help/physmod/sps/examples/dc7-four-quadrant-chopper-200-hp-dc-drive.html