Week-7 Challenge: DC Motor Control
ANS 1:
THEORY -
Pulse Width Modulation (PWM)
A far better method of controlling DC motors is to use pulse width modulation or PWM.
With PWM the motor is sent a series of pulses. Each pulse is of the full voltage that the motor can handle so a 6-volt motor will be sent 6-volt pulses while a 12-volt motor will be sent 12 volt pulses. The width of the pulses are varied to control the motor speed, pulses with a narrow width will cause the motor to spin quite slowly. Increasing the pulse width will increase the speed of the motor, as illustrated below.
In order to stop the motor completely you just stop pulsing it, essentially sending it zero volts. To run it at full speed you send it the full voltage, again without pulsing it.
H-Bridge
Now that you know how DC motors work, how you can reverse their direction by changing polarity and how you can change their speed using pulse width modulation, let’s examine an easy way to do this using a very common circuit configuration called an “H-Bridge”.
An “H-Bridge” is simply an arrangement of switching the polarity of the voltage applied to a DC motor, thus controlling its direction of rotation. To visualize how this all works I’ll use some switches, although in real life an H-Bridge is usually built using transistors. Using transistors also allows you to control the motor speed with PWM, as described above.
In the first diagram we can see four switches which are all in the open or “off” position. In the center of the circuit is a DC motor. If you look at the circuit as it is drawn here you can distinctly see a letter “H”, with the motor attached in the center or “bridge” section – thus the term “H-Bridge”.
If we close (i.e. turn on) two of the switches you can see how the voltage is applied to the motor, causing it to turn clockwise
Now we’ll open those switches and close the other two. As you can see this causes the polarity of the voltage applied to the motor to be reversed, resulting in our motor spinning counter clockwise.
This is pretty simple but effective. In fact if all you need to do is design a circuit to drive the motor full-speed in either direction you could actually build this as shown here, using a 4PDT (4 Pole Double-Throw) center-off switch. But of course we want to control the motor using an Arduino, so an electronic circuit where the switches are replaced by transistors is what we need.
SIMULATION RESULTS:
Speed control of a DC motor using BJT H-bridge
SIMULATION OF THE MODEL WITHOUT MODIFICATIONS:
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.
RESULTS:
FOR 75% DUTY CYCLE OF THE SUPPLY INPUT VOLTAGE WE OBSERVE A VERY SHARP RISE IN THE CUREENT WHENEVER THE MOTOR DIRECTION CHANGES
NOW REDUCING THE DUTY CYCLE TO 50% IN PULSE GENERATOR BLOCK
50% DUTY CYCLE:
FROM THE PLOT WE CAN OBSERVE THAT REDUCING DOWN THE DUTY CYCLE TO 50% BRINGS DOWN THE ARMATURE CURRENT SIGNIFICANTLY BUT REDUCING THE DUTY CYCLE FURTHER WILL TOTALLY ELIMINATE THE CURRENT SHOOT.
35% DUTY CYCLE
25% DUTY CYCLE
FOR 25% DUTY cycle the armature current drops to a negligible value
CONCLUSION:
As the change in duty cycle is lowered the speed, torque and hence the armature current of the DC motor reduces.
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ANS 2:
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.
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, i.e., in motoring and braking operations.
The product of angular speed and torque is equal to the power developed by a motor. For the multi-quadrant operation of drives, the following conventions about the signs of torque and speed are used. When the motor is rotated in the forward direction the speed of the motor is considered positive. The drives which operate only in one direction, forward speed will be their normal speed.
In loads involving up and down motions, the speed of the motor which causes upward motion is considered to be in forward motion. For reversible drives, forward speed is chosen arbitrarily. The rotation in the opposite direction gives reverse speed which is denoted by a negative sign.
The rate of change of speed positively in the forward direction or the torque which provides acceleration is known as Positive motor torque. In the case of retardation, the motor torque is considered negative. Load torque is opposite to the positive motor torque in the direction.
The figure below shows the four-quadrant operation of drives:
In the I quadrant power developed is positive and the machine is working as a motor supplying mechanical energy. The I (first) quadrant operation is called Forward Motoring. II (second) quadrant operation is known as Braking. In this 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.
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 III (third) quadrant operation is known as the reverse motoring. The motor works, in the reverse direction. Both the speed and the torque have negative values while the power is positive.
In the IV (fourth) quadrant, the torque is positive, and the speed is negative. This quadrant corresponds to the braking in the reverse motoring mode.
SIMULATION RESULT:
- The duty cycles of IGBTs 1 &4 are exactly opposite to that of the duty cycles of IGBTs 2&3 as the pulse of both 1&4 are opposite to 2&3
- The desired controlled armature current is obtained an follows the reference current with lesser ripples.
Comparison of Four-Quadrant Chopper DC Drive (DC7) with H-bridge model
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Four-Quadrant Chopper DC Drive |
H-bridge model |
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· Here the speed reference input is used to change the direction of the motor rotation. · Motor speed is controlled using the speed reference values · It can operate in all four modes (forward motoring, reverse motoring, forward braking and reverse braking). · Regenerative braking is possible · Switching is smoother and faster with better control capability. · Snubber limits or stops (snubs) switching voltage amplitude and its rate of rise, therefore reducing power dissipation · Current shoot up is minimal and the stability is better
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· whereas in the BJT H-model, the polarity is reversed to change the direction of the motor rotation. · Motor speed is controlled using PWM by changing the pulse width. · It can only operate in forward and reverse modes. · No regenerative braking is possible. · Switching is less smooth and faster with moderate control capability · No snubber is used hence, switching losses are greater. · The current shoots are very high and stability is less. |
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ANS 3:
EV model using four quadrant chopper DC drive DC7 block :
VEHICLE TORQUE-SPEED CHARACTERISTICS:
The linear load torque is replaced with thewith the vehicle load characteristics given by,
`τ=24.7+0.0051ω^2`
The above equation is realised into simulink block model subsystem as shown:
Connecting this subsystem to the main model as shown:
Now we run the simulation and observe the result:
- The Duty Cycle plot of IGBT 1&4 follows the same relation as that of motor speed, i.e. it increases during acceleration and decreases during deceleration.
- The Duty Cycle plot of IGBT 2&3 follows the opposite relation to IGBT 1&4, thus with motor speed as the pulse values are opposite
- The average armature voltage remains constant throughout the operation or experiences a negligible change.
- The armature voltage increases with increasing speed and decreases with decreasing speed.
REFERENCES:
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ANS 4:
OPERATION OF 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.
- 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
- 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.
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- 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.
REFERENCE:
https://robu.in/brushless-dc-motor-working-principle-construction-applications/