Week-7 Challenge: DC Motor Control
AIM:
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.
C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
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)
3. Explain in a brief about operation of BLDC motor.
1a.) Explain a MATLAB demo model named 'Speed control of a DC motor using BJT H-bridge'. First of all, let us understand the basic concept of speed control using H bridge configuration. Most DC motors have two terminals because they are series type (armature and field windings are connected in series) DC motors.
THEORY:
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 IV). 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 pre-set model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed). The armatures 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: Ql 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 01-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.
When the simulation is run the motor starts in the positive direction with a duty cycle ratio75%. At t = 0.5 sec the armature voltage is suddenly reversed and the motor runs in the negative direction.
Chopper: A DC chopper is also known as DC- DC converter. It is a static device which is used to obtain a variable DC voltage from a constant DC Voltage source. A DC-to-DC converter is an electronic circuit or electromechanical device that converts a source of direct current (DC) from one voltage level to another. It is a type of electric power converter. Power levels range from very low (small batteries) to very high (high-voltage power transmission). Choppers are of two types- Step Down chopper and Step-Up choppers.
Process:
1. Explain and Run MATLAB demo 'Speed control of a DC motor using BJT-H Bridge'. The armature current shoot-up from the scope results is to be observed.
• Open the MATLAB and In the Help documentation type Speed Control of a DC motor using BJT -H Bridge
• Results tab opens, and then select the model and click Open Model.
As shown in figure there are two terminals 'A' and 'B' of DC motor. Now if we connect terminal A with +Ve supply and terminal B with -Ve supply or ground the current will flow from motor from A to B and motor will rotate in one direction - say clockwise (CW) or forward direction. Now as shown in second figure we change the supply terminals. Now B is connected with +Ve and A is connected to ground. The current will flow from motor from B to A and motor will rotate in other direction (counter clockwise - CCW or reverse). The arrangement to get reversible supply is shown in right side of figure. Four switches are connected in between +Ve supply and ground and DC motor is connected in between two switches as shown. Such circuit arrangement is known as H-bridge because it looks like letter 'H' (H-bridge circuits are most widely used in DC motor drivers). Let us see how it gives reverse supply to motor. If SW1 and SW4 are pressed simultaneously then current will flow from +Ve - SW1 - A - B - SW4 - Gnd. So motor will rotate in one direction. Open (release) SW1 and SW4 to stop motor. Now if SW2 and SW3 are pressed current will flow from +Ve - SW2 - B - A - SW3 - Gnd. So, motor gets reverse supply and it will rotate in another direction.
The circuit replaces the switches with NPN type transistors. Diodes are given for the current to free wheel as the DC motor is a RLE load where the current cannot change direction instantaneously. We all know that transistor works as switch. For NPN transistor if we give enough +Ve input to base it will turn ON (Saturation region) and if we give 0 input it will be turned OFF (Cut off region). So in this circuit if Q1 and Q4 are turned ON simultaneously the motor will rotate forward and if Q2 and Q3 are turned ON then motor will rotate in reverse direction. In order to vary the speed of the DC motor, We have to change the voltage applied to the DC motor. This could be achieved with the help of Pulse width modulation.
Pulse width modulation means varying the width (duty) of pulse. Width means ON time Ton of pulse. The average output voltage (Vdc or Vavg) is given by equation
Vdc= Ton/Ton+Toff * Vs
Vdc= δ*Vs is the duty cycle.
The voltage directly depends upon Ton. As shown in above figure if duty is 50% the average output voltage Vdc is exactly the half of Vs. If duty is increased to 75%, Vdc also increases to 3/4th of Vs and if duty is decreased to 25%, Vdc reduces to 1/4th of Vs. Thus, as pulse width varies the average output voltage varies. So, we have to apply PWM to DC motor to vary its speed.
The example below shows the Simulink model of an H-bridge used to generate a chopped voltage and to control the speed of a DC motor.
The link shows the simulink model https://drive.google.com/file/d/1gAyu-zfIYmfIQLhRVVTC0gVeVR70zZzW/view?usp=sharing
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 IV). 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 Q1and 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.
Note: The DC motor used in the Simulink model is a separately excited DC motor and NOT a DC series motor. That is the reason why a DC voltage of 300V is given to the field windings.
1.b) Comment on the armature current shoot-up from the scope results. Simulation Output:
i.) Pulse Generator Parameters: Period: 2 milli seconds
Pulse Width (% of period): 75%
The link shows the output of scope https://drive.google.com/file/d/1lRimhg4wKNPGIHKI-BrdD5DO4x2ZrGd-/view?usp=sharing
2) Pulse Generator Parameters: Period: 2 milli seconds
Pulse Width (% of period): 55%
2) Pulse Generator Parameters: Period: 2 milli seconds
Pulse Width (% of period): 45%
1.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. 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 Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with four-quadrant operation. The speed control loop outputs the reference armature current of the machine. Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived. This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.
The Four-Quadrant Chopper DC Drive block uses these blocks from the Electric Drives/Fundamental Drive Blocks library:
• Speed Controller (DC)
• Regulation Switch
• Current Controller (DC)
• Chopper
The Stability of the voltage and current supply is higher in the four-quadrant compared to BJT as the four quadrant uses the step-by-step ramping strategy for the change in the motor direction whereas the stability is lost.
• Regenerative braking is possible in four quadrants but not in BJT.
• It is widely used in automobile sector to control DC motor speed and direction in Four quadrant. Whereas the BJT is used in robotics and electro-mechanical devices to control DC motor speed and direction.
• The current values are continuous in BJT and In Four Quadrants; it is chopped so that the simulation takes less time.
• To change the direction of the motor, current signals are interrupted in four quadrants. Whereas in BJT the polarity of the current needs to be changed
. • The main advantage of Four quadrant is that it operates in all the four quadrants but BJT can movie only is two directions.
• Four Quadrants provides better efficiency and operates at high power levels when compared to BJT that operates at moderate power levels.
SIMULINK MODEL FOR 4 QUADRANT CHOPPER
THE LINK SHOWS THE SIMULINK MODEL https://drive.google.com/file/d/1iEYLGNNJdcHONeZsDYZoJKoRbGGhYunQ/view?usp=sharing
H bridge
Only Two quadrants of operation are possible.
They are Forward motoring and reverse motoring
Higher armature current ripple.
Less complex Simulation is continuous and simulation time is more Widely Used in Robotics.
Four Quadrant chopper
DC drive It can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration)
Lower armature current ripple due to high switching frequency of DC-DC converters More complex
Discrete and Simulation time is less Widely used in Automobile.
RESULT OF MODEL SIMULINK
2)Develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results
Type C chopper or Two Quadrant class A chopper:
Class-C or Type-C Chopper is a category of chopper which can operate in first as well as second quadrant. This basically means that, the power can either flow from source to load or load to source in this chopper. This kind of chopper is also known as Two Quadrant Class-A chopper.
Operation or Working of Class-C or type C:
As we know that, a Class A and Class 8 chopper operates in first and second quadrant respectively. Therefore, if we connect both these types of choppers in parallel then it is possible to have chopper operation in first as well as second quadrant. In fact, Class-C or Type-C chopper is obtained by the parallel connection of Class-A and Class-8 chopper. Figure below shows the circuit diagram of this type of chopper.
We can notice that chopper CH1, free-wheeling diode (FD) and load are forming Class-A Chopper whereas chopper CHZ D2 and Load are forming Class-B chopper. Both these choppers are connected in parallel. To obtain first quadrant operation we should switch ON chopper CHI and for getting second quadrant operation we should switch ON chopper CH2. Let us discuss this in detail.
Case 1• When CH1 is switched ON/OFF
When chopper CHI is switched ON, source Vs directly gets connected to the load and hence, load voltage V, is equal to source voltage. The direction of load current is from source to load as shown in the circuit diagram which is assumed positive. When CHI is switched OFF, the free-wheeling diode FD comes into the circuit as it gets forward biased and hence shorts the load. Therefore, the output voltage V, becomes zero. However, the i, continues to die down through the FD and L in the same direction as shown in circuit diagram. Thus, the average output voltage V, and current 10 are positive and hence operation of chopper is in first quadrant
Case 2 When CH2 is switched ON/OFF
When chopper CH2 is switched ON, load DC source E drives current through CH2 and load. The direction of this current i, will be opposite to that shown in circuit diagram and hence is assumed negative. Output voltage V, is zero during this time. When CH2 is made OFF, diode D2 gets forward biased and hence the current into the source from the load. The output voltage is Vs in this time as the load is connected to the source through D2 during OFF time of chopper CH2. Thus, the load current is always negative i.e. operation of chopper is within second quadrant. The average load voltage is always positive but the average load current may be positive or negative. Therefore, power flow may be from source to load (first quadrant operation) or load to source (second quadrant operation). The operating region of Class-C or Type-C chopper is shown below by hatched area.
SIMULINK OUTPUT
3. Explain in a brief about operation of BLDC motor.
Brushless DC electric motors also known as electronically commutated motors (ECMs, EC motors). Primary efficiency is a most important 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 :
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.
Stator
A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor) or synchronous DC motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic closed looped controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor. This control system is an alternative to the mechanical commutators (brushes) used in many conventional electric motors. The construction of a brushless motor system is typically similar to a permanent magnet synchronous motor (PMSM), but can also be a switched reluctance motor, or an induction (asynchronous) motor. They may also use neodymium magnets and be outrunners (the stator is surrounded by the rotor), in runners (the rotor is surrounded by the stator), or axial (the rotor and stator are flat and parallel). The advantages of a brushless motor over brushed motors are high power-to-weight ratio, high speed, nearly instantaneous control of speed (rpm) and torque, high efficiency, and low maintenance. Brushless motors find applications in such places as computer peripherals (disk drives, printers), hand-held power tools, and vehicles ranging from model aircraft to automobiles. In modern washing machines, brushless DC motors have allowed replacement of rubber belts and gearboxes by a direct-drive design.
WORKING
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 BLDG 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 aligns with stator poles which causes the motor to rotate.
Types of Brushless DC Motors
Basically, BLDC are of two types, one is outer rotor motor and other is inner rotor motor.
The basic difference between the two is only in designing, their working principles are same. Inner Rotor Design In an inner rotor design, the rotor is located in the centre of the motor and the stator winding surround the rotor. As the rotor is located in the core, rotor magnets do not insulate heat inside and heat get dissipated easily. Due to this reason, inner rotor designed motor produces a large amount of torque and validly used.
Outer Rotor Design In outer rotor design, the rotor surrounds the winding which is located in the core of the motor. The magnets in the rotor trap the heat of the motor inside and do not allow to dissipate from the motor. Such type of designed motor operates at lower rated current and has low cogging torque.
Advantages of Brushless DC Motor
The advantages of a BLDC motor are:
I. Brushless motors are more efficient as its velocity is determined by the frequency at which current is supplied, not the voltage.
2. As brushes are absent, the mechanical energy loss due to friction is less which enhanced efficiency.
3. BLDC motor can operate at high-speed under any condition.
4. There is no sparking and much less noise during operation.
5. More electromagnets could be used on the stator for more precise control.
6. BLDC motors accelerate and decelerate easily as they are having low rotor inertia.
7. It is high performance motor that provides large torque per cubic inch over a vast speed range.
8. BLDC motors do not have brushes which make it more reliable, high life expectancies, and maintenance free operation. 9. There is no ionizing sparks from the commutator, and electromagnetic interference is also got reduced.
10. Such motors cooled by conduction and no air flow are required for inside cooling.
Disadvantages of Brushless DC Motors The disadvantages of a (ADC motor are:
1. BLDC motor cost more than a brushed DC motor.
2. The limited high power could be supplied to BLDC motor, otherwise, too much heat weakens the magnets and the insulation of winding may get damaged.