Week-7 Challenge: DC Motor Control
Q1) Run the MATLAB model demo for speed control of DC motor using BJT H bridge
A) 1) Import the model from the Documentation Help of MATLAB .
2) open the model in simulink , the model previes as below
a) The above is the example of H bridge used to generate a chopped voltage and to control the speed of the DC motor
b) The imported model of this BJT H-Bridge mainly consists of the blocks DC motor, pulse generator, armature voltage control, IGBT transistor, diodes, scope for plotted results and other constant block and gain blocks to calculate the torque output of the motor.
c) So in this model it mainly describes or output the armature current, load torque, speed and also currents of IGBT and diode.
d) The working of this model is that a motor rotates in the forward direction and then in the reverse direction.
e) So to decrease the peak change in the current we need to change the input signal generator's duty cycle to decrease the sudden shoot in the current.
f) The Default Duty cycle of the model is 75 %
The results for the default Duty cycle is shown below
The results we can see here are armature current, Angular speed & Torque
As you can see the Armature current changes its Direction as soon as the speed of the motor reverse and there is a sudden drop
Now Let us change the duty cycle value to 45 % to observe total supply voltage the peak shoot in the armature current will be normalized.
if we need to switch the motor operations like forward braking, reverse braking we need to change the supply to switches and diodes according to multi quadrant operation of a DC motor
The output of the Armature current, speed and torque results were given below :
i) So from changing the duty cycle to about 45% we can see the reduce in the Armature current shoot peak.
ii)If the duty cycle is go on increasing the output voltage coming from the bridge circuit to the motor will be getting equal to the input voltage of motor.
iii) And also the motor duty cycle % cannot be less than 30% as it decreases the ability of motor to have rotations.
Q1C) Refer to help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with H-bridge model?
A) CHOPPER DRIVE :
A chopper drive-based speed control is widely preferred as far as speed control is concerned. Choppers are mostly applied in electric cars, conversion of wind and solar energy, and DC motor regulators.
A chopper uses the high speed to connect and disconnect from a source load.
A fixed DC voltage is applied intermittently to the source load by continuously triggering the power switch ON/OFF. The period for which the power switch stays ON or OFF is referred to as the chopper’s ON and OFF state times, respectively
a) The four Quadrant DC chopper drive (DC7 ) model
First-Quadrant: Forward Motoring
The chopper-1 & chopper-4 are switched ON, in which the current flows from Chopper-1 to chopper-4 through inductance (which stores energy) the load current and armature current are same and positive thus the motor rotates in forward direction. If the chopper-1 is switched OFF, the energy stored in the inductance flows to chopper-4 which activates the D2 and forms closed loop
Second-Quadrant: Forward Regenerative Braking
The chopper-2 is switched ON D4 is activated, the current flows from D4 to Chopper-2 through inductance (which stores energy) which is in opposite direction (negative current) and the voltage is positive thus the motor stops in forward direction. If the chopper-2 is switched OFF, the D4 & D1 are conduct current.
Third-Quadrant: Reverse Motoring
The chopper-2 & chopper-3 are switched ON, in which the current flows from Chopper-3 to chopper-2 through inductance (which stores energy) the load current is negative and armature current and also the voltage is negative which is less than the back emf thus the motor rotates in reverse direction.
If the chopper-3 is switched OFF, the energy stored in the inductance flows to chopper-2 which activates the D4 and forms closed loop.
Fourth-Quadrant: Reverse Regenerative Braking
The chopper-4 is switched ON D2 is activated, the current flows from Chopper-4 to D2 through inductance (which stores energy) which is in same direction (positive current) and the voltage is negative thus the motor stops in reverse direction. If the chopper-4 is switched OFF, the D2 & D3 are conduct current.
Visual representation:
Class E - Chopper characteristics:
1. Regenerative braking is possible.
2. Widley used in automobiles to control DC motor speed and direction.
3.This cannot be operated by the signal generator
4. Maintainance for the chopper drives will be more as these are controlled by many switches individually
5. These will be used in the small machines such as robotic applications
6. This work more efficiently as the change in the frequency is more here
7. Managing in changing the signals were not as ease as in the H-bridge.
H- BRIDGE characteristics :
1. Regenerative braking is not possible but dynamic braking can be achieved.
2. Widley used in robotics and electro-mechanical devices to control DC motor speed and direction.
3. This cab be operated with the signal generated
4. The maintainance for the H-bridge circuit is less than the chopper circuit as this is run by signal modulation
5. These needs more space to adjust but have a light weight considering in the whole.
6. Efficiency of H-Bridge is less than the Chopper circuit.
7. Ease of operation is better than the Four_Quadrant Chopper DC Drive.
Q2) Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.
Circuit diagram for 2 quadrant chopper is
2 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 V as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage v will be equal to V . The direction of the load current i will be reversed. The current i 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 chopper 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.
Matlab simulink model for the 2 quadrant chopper will be as below
Types of block used in this model are
1) Mosfets -2
2) Diodes - 2
3) RLC Series branch -1
4) Current and voltage measure ment blocks -1 ea
5) Pulse generator
6) NOT
7) Dc Voltage source -2
These blocks need to be connected as above diagram
specifications to be given to the below blocks
Pulse generator :
DC Voltage source :
DC Voltage source 1 :
RLC Series branch :
Result :
1) 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.
2) however in current waveform , the current increases and decreases. the current may be positive as well as negative.
and
3) The pulse amplitude is 10 thus from pulse waveform we can see that the pulse amplitude is 10.
Q3) Explain in a brief about operation of BLDC motor
A) BLDC MOTOR
Brushless DC electric motors also known as electronically commutated motors .A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electricity and it accomplishes electronically controlled commutation system (commutation is the process of producing rotational torque in the motor by changing phase currents through it at appropriate times) instead of a mechanically commutation system. BLDC
motors are also referred as trapezoidal permanent magnet motors.
Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with commutator on the rotor so as to form an electric path between a DC electric source and rotor armature windings, BLDC motor employs electrical commutation with permanent magnet rotor and a stator with a sequence of coils. In this motor, permanent magnet (or field poles) rotates and current carrying conductors are fixed
The armature coils are switched electronically by transistors or SCR at the correct rotor position in such a way that armature field is in space quadrature with the rotor field poles. Hence the force acting on the rotor causes it to rotate. Hall sensors or rotary encoders are most commonly used to sense the position of the rotor and are positioned around the stator. The rotor position feedback from the sensor helps to determine when to switch the armature current.
This electronic commutation arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more reliable and less noisy operation is achieved. Due to the absence of brushes BLDC motors are capable to run at high speeds. The efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient. There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.
In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature winding 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.
BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as single-phase, two-phase, or three-phase motors. However, three-phase BLDC motors with permanent magnet rotor are most commonly used.
The construction of this motor has many similarities of three phase induction motor as well as conventional DC motor. This motor has stator and rotor parts as like all other motors.
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.
Stator
Stator of a BLDC motor made up of stacked steel laminations to carry the windings. These windings are placed in slots which are axially cut along the inner periphery of the stator. These windings can be arranged in either star or delta. However, most BLDC motors have three phase star connected stator.
Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. In order to form an even number of poles, each of these windings is distributed over the stator periphery.
The stator must be chosen with the correct rating of the voltage depending on the power supply capability. For robotics, automotive and small actuating applications, 48 V or less voltage BLDC motors are preferred.
Rotor
BLDC motor incorporates a permanent magnet in the rotor. The number of poles in the rotor can vary from 2 to 8 pole pairs with alternate south and north poles depending on the application requirement. In order to achieve maximum torque in the motor, the flux density of the material should be high. A proper magnetic material for the rotor is needed to produce required magnetic field density.
ferrite magnets are inexpensive, however they have a low flux density for a given volume. Rare earth alloy magnets are commonly used for new designs. Some of these alloys are Samarium Cobalt (SmCo), Neodymium (Nd), and Ferrite and Boron (NdFeB). The rotor can be constructed with different core configurations such as the circular core with permanent magnet on the periphery, circular core with rectangular magnets, etc.
Hall Sensors
Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the commutation of BLDC motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor. Before energizing a particular stator winding, acknowledgment of rotor position is necessary. So the Hall Effect sensor embedded in stator senses the rotor position.
Most BLDC motors incorporate three Hall sensors which are embedded into the stator. Each sensor generates Low and High signals whenever the rotor poles pass near to it. The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor’s response.
The underlying principles for the working of a BLDC motor are the same as for a brushed DC motor; i.e., internal shaft position feedback. In case of a brushed DC motor, feedback is implemented using a mechanical commutator and brushes. With a in BLDC motor, it is achieved using multiple feedback sensors. The most commonly used sensors are hall sensors and optical encoders. Hall sensors work on the hall-effect principle that when a current-carrying conductor is exposed to the magnetic field, charge carriers experience a force based on the voltage developed across the two sides of the conductor.
If the direction of the magnetic field is reversed, the voltage developed will reverse as well. For Hall-effect sensors used in BLDC motors, whenever rotor magnetic poles (N or S) pass near the hall sensor, they generate a HIGH or LOW level signal, which can be used to determine the position of the shaft.
Advantages of BLDC motor :
1) Less overall maintenance due to absence of brushes
2) Reduced size with far superior thermal characteristics
3) Higher speed range and lower electric noise generation.
4) It has no mechanical commutator and associated problems
5) High efficiency and high output power to size ratio due to the use of permanent magnet rotor
6) High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed
7) Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors.
8) Long life as no inspection and maintenance is required for commutator system
9) Higher dynamic response due to low inertia and carrying windings in the stator
10)Less electromagnetic interference
11) Low noise due to absence of brushes
Dis advantages of BLDC motor :
1) These motors are costly
2) Electronic controller required control this motor is expensive
3) Requires complex drive circuitry
4) Need of additional sensors
Applications of BLDC motor :
1) Computer hard drives and DVD/CD players
2) Electric vehicles, hybrid vehicles, and electric bicycles
3) Industrial robots, CNC machine tools, and simple belt driven systems
4) Washing machines, compressors and dryers
5) Fans, pumps and blowers