Week-7 Challenge: DC Motor Control
AIM: To understand the dc control.
OBJECTIVES:
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.
3. Explain in a brief about operation of BLDC motor.
OBJECTIVE: 1.
A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
An H-bridge is a simle circuit that lets us to control a dc motor to go backward or forward.
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:
the switching elements (Q1...Q4) are usually bi-polar or FET transistors, in some high-voltage applications IGBTs. Integrated solutions also exist but whether the switching elements are integrated with their control circuits or not is not relevant for the most part for this dicussion. The diodes (D1..D4) are called catch diodes and are usually of a schotty type.
model is uploaded below;
The simulation of an H-bridge used to generate a chopped voltage and to control the speed of a DC motor. Gilbert Sybille (Hydro-Quebec).
Description
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.
Simulation
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.
WORKING:
- In the above circuit, the input voltage is generated from the pulse generator where the amount of supply and duty cycle is assigned in the pulse generator.
- Then the switch block is connected to pulse generator through an armature voltage control block.
- The H- bridge consists of 4 BJT/ diode pairs. Two transistors are
- The H-bridge consists of four BJT/Diode pairs. 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 positive voltage is applied motor and diodes D.
SIMULATION MODEL AND RESULTS ARE UPLOADED BELOW:
1)B. Comment on the armature current shoot-up from the scope results.
From armature current graph, we can see that, at starting there is sudden spike increase in current at 0 sec also and at 0.5sec also means at transition from postive to negative direction of rotation we observe a high surge in current, this is due to the transient switching occuring in thr circuit.
as there is already current flowing in the circuit and on changing of switches after 0.5sec the current in the circuit adds to the changed direction of current and we observe a surge in current.
the surge in the current can be reducing byy the duty cycle in above results, the duty cycle selected as 75%.
By changing the duty cycle from 75% to 50%, the current surge can be reduced tremendously.
Thus we can find that, the armature current spike is reduced by changing the duty cycle from 75%to 50%.
1).C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
The four-quadrant chopper DC drive (DC7) block represents a four-quadrant dc supplied chopper, 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. the duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.
The machine is separately excited with a constant DC field voltage source. there is thus field no field voltage control. by default, the field current is set its steady state value when a simulation is started.
The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a constant DC voltage source. armature current oscillators are reduced by a smoothing inductance connected in series with the armature circuit.
WORKING OF FOUR QUADRANT CHOPPER:
The circuit of a four quadrant chopper or class-E chopper basically consists of four semiconductor switches CH1 TO CH4 and four diodes D1 to D4. The four diodes are connected in anti-parallel. the circuit diagram of this type of chopper is shown below.
In the above circuit diagram, the choppers are numbered CH1 to CH4. For first quadrant operation CH2 is made ON and so on. To better understand the working of four quadrant chopper, we will dicuss its operation separately for each quadrant.
FIRST QUADRANT OPERATION:
For first quardant operation, CH4 is kept ON, CH3 is kept OFF and CH1 is operated when both CH1 AND CH4 are ON Simulationeously, the load gets directly connected to the source voltage. and hence the output voltage becomes equal to the source voltage. This essentially means that Vo=Vs. It may be noted that the load current flows from source to load as shown by the direction of io.
When CH1 is switched OFF, the load current free wheels through CH4 and D2. During this period the load voltage and current remains positive.
- First Quadrant
During the first quadrant operation the chopper CH4 will be on . Chopper CH3 will be off and CH1 will be operated. AS the CH1 and CH4 is on the load voltage v0 will be equal to the source voltage Vs and the load current i0 will begin to flow . v0 and i0 will be positive as the first quadrant operation is taking place. As soon as the chopper CH1 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+L.di/dt) will be more than the source voltage Vs . 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 CH1 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 Vs and v0 and i0 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 CH1, 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 L 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 negative but the load current is positive. The chopper acts as a step up chopper as the power is fed back from load to source.
2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.
CIRCUIT DIAGRAM FOR TWO QUADRANT CHOPPER:
Two quadrant chopper is also called as type c chopper. 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 Vs. The direction of the load current io will be reversed. The current io 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 on or the diode D2 conducts. We can say the chopper and FD operate together as type -A in the first quardent. In the second Quardent, the chopper and D2 will operate together as type-B chopper.
The chopper permits a change in the polarity of terminal voltage keeping the direction of current constant. S1 and S2 are the choppers working as electronic switches and D1 and D2 are the feedback diodes. Assuming continuous conduction, both S1 and S2 operate simultaneously.
The Two-Quadrant Chopper block represents a two-quadrant controlled chopper for converting a fixed DC input to a variable DC output. The block contains two switching devices. Options for the type of switching devices are: GTO — Gate turn-off thyristor.
TWO QUADRANT CHOPPER MODEL USING SIMULINK:
Lets us take two mosfet blocks and diodes blocks and conect it as show in circuit diagram.
The dc voltage source is connected as shown.
Lets the voltage of dc voltage source be 24 volts.
Lets us consider an series RLC branch block and select inductor L. to consider LE load i.e motor load, and dc voltage source of 12volts is also connected in series with inductor.
The pulse generator block is used to create gate pulse. the not gate is used in the model so that the two mosfet will not get closed at the same time otherwise short circuit will occur. due to use of not gate, when one mosfet is ON the other mosfet will get off and when other is ON the first mosfet will get off.
Pulse generator.
SCOPE RESULTS:
The voltage waveform is not going less than zero. the minimum value of voltage is zero and maximum value is always in the positive direction.
However in current waveform, the current increases and decreases. the current may be positive as well as negative.and the pulse amplitude is 10 thus from pulse waveform we can see that the pulse amplitude is 10.
3. Explain in a brief about operation of BLDC motor.
BRUSHLESS DC MOTOR: A brushless DC motor (also known as a BLDC motor or BL motor) is an electronically commuted DC motor which does not have brushes. The controller provides pulses of current to the motor windings which control the speed and torque of the synchronous motor.
Speed of a brushless dc motor can be controlled by controlling the input dc voltage / current. The higher the voltage more is the speed. Many different control algorithms have been used to provide control of BLDC motors. The motor voltage is controlled using a power transistor operating as a linear voltage regulator.
Construction of BLDC Motor
The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit. Keeping that in mind, 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 Motors are available in three configurations: single phase, two phase and three phase. Out of these, the three phase BLDC is the most common one.
The following image shows the cross-section of a BLDC Motor.
As you can see in the image, a BLDC Motor consists of two main parts: a stator and a rotor.
Stator
The structure of the stator of a BLDC Motor is similar to that of an induction motor. It is made up of stacked steel laminations with axially cut slots for winding. The winding in BLDC are slightly different than that of the traditional induction motor.
WORKING: A brushed DC motor has permanent magnets on the outside of its structure, with a spinning armature on the inside. ... In brushless DC motors, the permanent magnets are on the rotor, and the electromagnets are on the stator. A computer then charges the electromagnets in the stator to rotate the rotor a full 360-degrees.
isadvantages: The friction of the brushes sliding along the rotating commutator segments causes power losses that can be significant in a low power motor. The soft brush material wears down due to friction, creating dust, and eventually the brushes must be replaced.
HALL SENSER: The purpose of moving a motor is controlling speed and torque. Speed in a BLDC motor is directly proportional to the voltage applied to the stator. ... The Hall effect sensors that are used to detect the position of the rotor are also used to detect speed by measuring the time it takes for the sensors to switch.
OPERATION OF BLDC 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.
The motor operation is controlled using an optocoupler and MOSFET arrangement, where input DC power is controlled through the PWM technique from the microcontroller.
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.
CONCLUSION:
1) MATLAB demo model named Speed control of a DC motor using BJT H- bridge is studied and the armature current shoot- up is studied and 'the four quadrant chopper DC drive (DC7) block is also studied and compare it with the H-Bridge model.
2) 2-quadrant chopper using simulink model is made and the various results obtained is studied.
3) Operation of BLDC motor is studied.