Week-7 Challenge: DC Motor Control
Question No.: 01
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.
C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
Introduction of Bipolar Junction Transistor:
BJT is a semiconductor device that is constructed with 3 doped semiconductor Regions i.e., Base, Collector & Emitter separated by 2 p-n Junctions.
Bipolar transistors are manufactured in two types, PNP and NPN, and are available as separate components, usually in large quantities.
The prime use or function of this type of transistor is to amplify current. This makes them useful as switches or amplifiers.
They have a wide application in electronic devices like mobile phones, televisions, radio transmitters, and industrial control.
Operation of Bipolar Junction Transistor
There are three operating regions of a bipolar junction transistor:
Active region: The region in which the transistors operate as an amplifier.
Saturation region: The region in which the transistor is fully on and operates as a switch such that the collector current is equal to the saturation current.
Cut-off region: The region in which the transistor is fully off, and the collector current is equal to zero.
Types of Bipolar Junction Transistors
There are two types of bipolar junction transistors:
- PNP Bipolar Junction Transistor
- NPN Bipolar Junction Transistor
Symbols:
PNP BJT
In PNP BJT, the n-type semiconductor is sandwiched between the two p-type semiconductors. The two p-type semiconductors act
as emitter and collector respectively while the n-type semiconductor acts as a base. This is shown in the figure below.
The current enters the transistor through the emitter such that the emitter-base junction is forward biased, and the collector-base junction is reverse biased.
NPN BJT
An NPN BJT, a p-type semiconductor is sandwiched between the two n-type semiconductors. The two n-type semiconductors act as
emitter and collector respectively while the p-type semiconductor acts as a base. This is shown in the figure below.
The current entering the emitter, base, and collector has the sign convention of positive while the current that leaves the transistor has the sign convention of negative.
Voltage, Charge Control and Current
The base-emitter current is controlled by the collector-emitter current. This conclusion is drawn by the current-voltage relation of the
base-emitter junction. Collector current has a base region where minority carriers are concentrated.
Transistor models such as the Glenn poon model are responsible for the distribution of the charge which explains the behavior of a transistor.
A. Explain a MATLAB demo model named ‘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 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 the square of speed).
The armature means 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.
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 01: (IGBT and Diode Current)
B. Comment on the armature current shoot-up from the scope results.
Scope 02: (RPM, Armature Current, and Load Torque) - Default Value
Scope 03: (RPM, Armature Current, and Load Torque) - PW 50%
The motor starts in a 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 a negative direction.
From Scope no. 02, we can see that the current plot has initially a very high value and then becomes steady and after that shows the same path in the negative direction.
The value of this current shoot-up is decreasing, and Scope No.03 has an almost smooth current flow.
To get steadier and smoother current flow we need to decrease and control the duty cycle.
C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
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. 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 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. To limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.
Scope: (Duty Cycle, Amature Voltage, Armature Current & Speed)
Four Quadrant Operation of DC Motor:
Four Quadrant Operation of any drive 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.
QUADRANT-I
When a motor draws power from the supply, the voltage and current are in the same direction.
Now the motor is said to be in motoring operation, its operation corresponds to the first quadrant.
In the first quadrant, power the developed is positive and the machine is working as a motor supplying mechanical energy. The I (first) quadrant operation is called Forward Motoring.
QUADRANT-II
If there is a possibility of reversing the current direction while keeping the voltage polarity the same, the motor is said to be operating in the second quadrant.
This happens when regenerative braking is applied to the motor.
During regeneration operation, reverse power flow will happen. ie, power will be fed back to the supply system.
The 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 dissipated the energy is dissipated in the resistance.
QUADRANT-III
If the voltage polarity and load current are reversed simultaneously, the motor’s operation will be in the opposite direction ( but the power is consumed by the load).
This operation corresponds to the third quadrant and even now it is said to be in motoring operation (in the reverse direction).
The third quadrant operation is known as reverse motoring. The motor works, in the reverse direction. Both the speed and the torque have negative values while the power is positive.
QUADRANT-IV
If the voltage polarity is reversed maintaining the current direction from source to load, the power is reversed and the operation corresponds to the fourth quadrant.
Now the motor is said to be in reverse regeneration operation (ie, regeneration in the reverse direction).
In the fourth quadrant, the torque is positive, and the speed is negative. This quadrant corresponds to braking in the reverse regenerative braking mode.
FOUR QUADRANT CHOPPER CIRCUIT DIAGRAM:
FIRST QUADRANT(Forward Motoring)
When switch CH1 turned ON
In firstthe quadrant a switch CH1 operated, Switches CH1 and CH2 conduct, the output voltage V and the output current I both are positives, power flows from source to load and inductor stores energy while the motor rotates in the forward direction hence called forward motoring.
When switch CH1 turned OFF
While switch CH1 is turned OFF but switch CH4 and diode D2 conduct output current I is positive and the output voltage V becomes zero. An induction release energy and freewheeling action using diode D2 takes place, and the motor rotates in the forward direction hence called forward motoring.
SECOND QUADRANT(Forward Regenerative Braking)
When switch CH2 turned ON
Assume that the motor is running in the forward direction. Here switch CH2 operated, switch CH2 and diode D4 conducts, the output voltage V is zero and Eb is responsible for the negative output current I, machine behave as generator and inductor stores energy.
When switch CH2 turned OFF
While switch CH2 is turned off, diode D1 and diode D4 conduct, the output voltage V becomes positive and the output current I is negative, inductor release energy using diodes D1 and D4 where power flows from load to source and hence called as reverse braking.
THIRD QUADRANT(Reverse Motoring)
When switch CH3 turned ON
The polarity of back emf Eb must be reversed. Here switch CH3 operated, switches CH3 and CH2 conduct output voltage V and the output current I both are negative. Power flows from source to load and the inductor stores energy, therefore, the motor rotates in reverse direction hence called as reverse motoring.
When switch CH3 turned OFF
While switch CH3 is turned off but switch CH2 and diode D4 conducts, output current I is negative and the output voltage V becomes zero, inductor release energy and freewheeling action using diode D4 takes place hence the motor rotates in the reverse direction.
FOURTH QUADRANT(Reverse Regenerative Braking)
When switch CH4 turned ON
The polarity of back emf Eb must be reversed. let us assume that the motor is running in the reverse direction. Here switch CH4 operated, switches CH4 and diode D2 conduct, the output voltage V is zero and Eb is responsible for the positive output current I. This machine behaves as a generator and the inductor stores energy.
When switch CH4 turned OFF
While switch CH4 is turned off, diode D2 and diode D3 conduct, the output voltage V becomes negative, and output current I is positive, inductor release energy using diodes D2 and D3. Thus power flows from load to source and is hence called reverse braking.
COMPARISON OF FOUR-QUADRANT CHOPPER DC DRIVE (DC7) WITH H-BRIDGE:
In a four-quadrant chopper, the DC drive distributes the required current supply for each sector and it consists of four quadrants like Forward Motoring, Forward Regenerative Braking, Reverse Motoring, and Reverse Regenerative Braking. whereas H-bridge supplies an irregular current for each sector that causes voltage fluctuation in armature current. It consists of two operations they are Forward motoring, and reverse motoring.
In modern battery charger for electrical vehicles, H-bridge circuits are used. A four-quadrant operation is required in industrial as well as commercial applications.
The efficiency is high in chopper compared with H-bridge, and maintenance is less than in H-bridge.
In Robotics Technology, H-bridge circuits are used. Some other application of four quadrant operation is engine test loading systems.
The regenerative braking is not possible in H-bridge while in chopper it is possible.
The most important application of the H-Bridge circuit is the motor driver circuit. H-bridge circuits can be placed in small places. Four quadrant chopper is lightweight in nature.
In H-bridge we need to change polarity which turns in the opposite direction to control the motor whereas in the chopper model we need to interrupt one signal under the control of another.
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Question No. 02
Develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of the motor)
Model: (Refer File Name "TWOQUARDERENT.slx")
Scope Output: (Refer the file name "Scope - Two quadrant chopper.fig")
In this type of Chopper, the average voltage will be always positive but the average load current might be positive or negative. The two Switches CH1 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 V0 as the freewheeling diode FD is present across the load. When the chopper CH1 is on the current flows from source to load. The inductor stores energy. When Ch1 turns OFF 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. The load current will be negative when the CH2 is on. The current completes its path through diode D2.
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Question no.: 03
Explain in a brief about operation of BLDC motor.
Brushless DC electric motors are also known as electronically commutated motors (ECMs, EC motors).
Primary efficiency is the most important feature of 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 are attached 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 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.
The 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 “Outrunner” style or “Inrunner” style.
Outrunner – The field magnet is a drum rotor that rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.
Inrunner – The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the outrunner 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 is 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.
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