Week-7 Challenge: DC Motor Control
1. Explain a MATLAB demo model named 'Speed control of a DC motor using BJT H-bridge
Answer: 'Speed control of a DC motor using BJT H-bridge.
H-bridge in general an H-bridge is a rather simple circuit,containing four switching element with the load at the center, in an H-like configuration.
basic operation of H brifge circuit diagram has given below
When the positive gate signal is given the switches QI and Q4 are turn-on and the motor rotates in forward direction.Dland D4 are in off condition. During changing of the positive to negative time period the DI and D4 are in on condition at that time the switch QI and Q4 are off condition.
whenthe negative gate signal is given the switch Q3 and Q2 are turn-on and the motor rotates in reverese direction and D3 and D4 are in off condition. During changing of the negative to positive time period the D2 and D3 are in on condition at that time the switch Q3 and Q4 are off condition.We cannot trigger the two switches of the same leg.lt creates a low resistance path and switches gets damaged.
Simulink model:
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 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).
Related
DC Motor used in the simulink model:
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 QI and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when QI and Q4 are switched off When Q2 and Q3 ore fired a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and 03. are switched off.
The Pulse gnerator given by a pulse width of 75% and period of 2e-3 seconds. The motor parameters of the torque and field voltage and armature voltages are taken as default values of the model.
Click on the run icon. the output waveforms of the diode current,IGBT current from the current scopes as shown below:
From the waveform i had notice that the o to 0.5 time the the IGBT current is zero and the diode current is on after the 0.5 to 1 rime the IGBT starts the current flow the diode current is zero.
Now we will take the output waveform of the load torque, armature current and speed.
From the above graph the speed is starts from zero and the reaches up to the maximum speed of (1000rpm) and decreases after 0.5 time and reaches to the (-1000rpm).
The armarture current is starts from o and increasesup to the (approx 45) and decreases after immediately and reaches to the nearer to the zero value.when the speed decreases the armature current is also starts flow in the reverese direction.when the speed comes to the nearer value of (-1000 rpm) the armture curent is flow in neagative direction nearer to the zero value.At the starting of the armature current i had noticed a spike in the current flow and also in the negative direction of the current flow.
The load torque follows the speed waveform in the positive direction.when the negative direction it follows the armature current current and the speed waveform.
By doing these process the speed control of DC motor using BJT H
bridge model and the output waveforms are obtained.
1B. Comment on the armature current shoot-up from the scope results.
From the above the armature current shoot-up at the starting of the armature current a sudden rise in the waveform we had noticed To reduce the sudden rise in the armature current we will the reduce the duty cycle of the pulse width modulation from (75% to 50%).
The output of the waveform are shown below.
The difference between the above two graph armature current in the second graph we observe that there is no sudden rise at the starting of the armature current. If we further reduced the pulse width modulation the negative of the rise will get reduced.
From the above graph it is clear that if we reduced the duty cycle ratio than we obtain a smooth variation in the output graph. The above graph PWM is 40%. and the motor details are default of the model. The torque and speed also varies the smoothly.
1C. Refer to the help section of 'The Four-Quadrant Chopper DC Drive (DC7) block. Compare it with the H-bridge model.
Chopper: Chopper is a static device(switch) used to obtain the variavle dc voltage from a source of constant dc voltage. The output voltage is less than the input voltage than the chopper is called step-down chopper. If the output voltage is greater than the input voltage than the chopper is called step-up chopper.
Chopper offers greater efficiency,faster response, lower maintainence,small size,smooth control. choppers are widely used in trolley-cars,battery-operated vehicles, traction-motor control. There are different types of the choppers are available according to the direction of output voltage and current
Class A Type A: First quadrant chopper. The output voltage and current both must be positive.
Class B type B: The output of the voltage is positive and the
current is negative
Class C/class D:Two quadrant chopper. The output voltage is positive and current can be positive or negative(class-C) or the output current is positive and the voltage can be positive or negative (class-D).
Class-E.Four quadrant chopper. The output voltage and current both can be positive or negative.
Type E or the fourth quadrant chopper consists of four semiconductor switches and four diodes arranged in antiparallel. The 4 choppers are numbered according to which quadrant they belong. Their operation will be in each quadrant and the corresponding chopper only be active in its quadrant.
• First Quadrant
During the first quadrant operation the chopper CH4 will be on Chopper CH3 will be off and CHI will be operated. AS the CHI and CH4 is on the load voltage vo will be equal to the source voltage V₁ and the load current io will begin to flow vo and lo will be positive as the first quadrant operation is taking place. As soon as the chopper CHI is turned off, the positive current freewheels through CH4 and the diode D2. The type E chopper acts as a step-down chopper in the first quadrant
. Second Quadrant
In this case the chopper CH2 will be operational and the other three are kept off. As CH2 is on negative current will starts flowing through the inductor L. CH2 ,E and D4. Energy is stored in the inductor L as the chopper CH2 is on. When CH2 is off the current will be fed back to the source through the diodes D1 and D4. Here (E+Ldi/dt) will be more than the source voltage V₁ In second quadrant the chopper will act as a step-up chopper as the power is fed back from load to source.
Third Quadrant
In third quadrant operation CHI will be kept off, CH2 will be on and CH3 is operated. For this quadrant working the polarity of the load should be reversed. As the chopper CH3 is on, the load gets connected to the source V, and vo and io will be negative and the third quadrant operation will takes place. This chopper acts as a step-down chopper.
Fourth Quadrant
CH4 will be operated and CHI, CH2 and CH3 will be off. When the chopper CH4 is turned on positive current starts to flow through CH4, D2,E and the inductor I will store energy. As the CH4 is turned off the current is feedback to the source through the diodes D2 and D3, the operation will be in fourth quadrant as the load voltage is negdave but the load current is positive. The chopper acts as a stop up chopper as the power is fed back from load to source.
The four quadrant chopper waveforms are shown
The simulink model of dc7 chopper
The detail description of the dc block The armature voltage is provided by an IGBT converter controlled by two Pl 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. Comparision of Four quadrant chopper and H-bridge
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)
A 2-quadrant chopper :
In the 2-quadrant chopper are two types i) Class-c and ii) Class-D.
This is class -c Chopper the output of the voltage is positive and the output of the current is positive and negative.Type C chopper is obtained by connecting type-A and type -B choppers in parallel. We will always get a positive output voltage Vo as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage vo will be equal to V, The direction of the load current lo will be reversed. The current lo will be flowing towards the source and it will be positive regardless the chopper is on or the FD conducts. The load current will be negative if the chopper is or the diode D2 conducts. We can say the chopper and FD operate together as type-A choppes in first quadrant in the second quadrant, the chopper and D2 will operate together as type -B chopper
The average voltage will be always positive but the average load current might be positive or negative. The power flow may be life the first quadrant operation ie from source to load or from load to source like the second quadrant operation. The two choppers should not be turned on simultaneously as the combined action my cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.
For the class-d chopper the output current is postive and the output voltage is positive and negative.
The circuit diagram of the type D chopper is shown in the above figure. When the two choppers are on the output voltage vo will be equal to Vs. When Vo = - -Vs the two choppers will be off but both the diodes D1 and D2 will start conducting. Vo the average output voltage will be positive when the choppers turn-on the time Ton will be more than the turn off time Toff its shown in the wave form below. As the diodes and choppers conduct current only in one direction the direction of load current will be always positive.
The waveforms of the positive and negative operations are given below:
Simulink model of the class-D type chopper:
1.Click on the matlab Click on the simulink model and click on the library browser.
2.Select the blocks of the Mosfet, diode,scope,inductor (load), pulse generator, dc supply voltage and current
measurement.
3. The blocks of the simulink model can be arranged in a passion that the class-D type chopper as shown below
We will assign the values of pulse genertor
output
3. Explain in a brief about operation of BLDC motor.
A. A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor). A BLDC motor is also known as the permanent magnet synchronus motor (PMSM) There are two types of BLDC motors are present i) 3-phase trapeziodal BLDC motor.ii) Sinusidal type BLDC motor Most of the BLDC motors are 3-phase type.
As shown in the above figure the brushless DC motor has only two basic parts; rotor and the stator. The rotor is the rotating part and has rotor magnets whereas stator is the stationary part and contains stator windings In BLDC permanent magnets are attached in the rotor and move the electromagnets to the stator The high power transistors are used to activate electromagnets for the shaft turns. The controller performs power distribution by using a solid state circuit.
In brushless motors, permanent magnets rotate around a fixed armature and overcome the problem of connecting current to the armature. Commutation with electronics has a large scope of capabilities and flexibility. They are known for smooth operation and holding torque when stationary.
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/Outer motor - 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/Inner motor- 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.
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.