Week-7 Challenge: DC Motor Control
EV BATCH 16 LIVE SESSION BATCH
AIM :
(1)To run the Matlab demo script ‘Speed control of a DC motor using BJT H-bridge’ .
(2) To Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.
(3) To Explain in a brief about operation of BLDC motor.
ANSWERS :
Choppers : the DC motor is used in many industrial applications requiring variable speed and load characteristics due its ease of control. A chopper drive based speed control is widely referred as far as speed control is concerned. Toppers are mostly applied in electrical 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 or off. The period for which the power 60 is on or off is referred to as the chopper on or off state times respectively. A DC to DC converter aka DC chopper is a static device which is used to obtain a variable DC voltage from a constant DC voltage source. Top Savli used in the trolley car battery operated vehicles traction motor control control of large number of DC motors etc.
Power switches : when it comes to EV application the speed of the motor mainly depends on controlling the voltage and current flowing to the motor. lTo ensure a rotating field in the motor continuously not has to be switched continuously by controlling the direction and timing of the current into the coil. In order to achieve this inverters are used to adjust the voltage in the coil and to control the magnitude of current. Pulse width modulation is the technique used in power switches which alters the voltage level by listening or reducing the pulse on time.
Transistor :
A transistor is a semiconductor device used to amplify of which electronic signals and electrical power. It is composed of semiconductor material easily with at least three terminals of connection to an external circuit . A voltage or current applied to one pair of the transistor is terminus controls the current to another pair of terminal. Because of the control output power can be higher than the control input power a transistor can amplify a signal.
Bipolar junction transistor :
A bipolar junction transistor(BJT) is a type of assistance that uses both electrons and holes as a charge carriers. Unipolar transistors such as field effect transistor use only one kind of charge carrier. A bipolar transistor allows a small current injected set one of its terminal to control a much larger current flowing between the two other terminal making the device capable of switching or amplification.
Bipolar junction transistor is use to junction between two semi conductor types n type and P type Which are regions in a single crystal of material. The junction can be made in several different ways suggest changing the grouping of the semiconductor materials as it is grown by depositing metal pellets to form alloys Junction or by such methods as diffusion of n type and p type doping substances into the crystal. The collector emitter current can be viewed as being controlled by the base emitter current or by the base emitter voltage these views are related by the current voltage relation of the base editor Junction which is the usual exponential current voltage curve of a PN junction.
Speed control of DC motor using BJT-H bridge :
The above figure shows the block diagram of the DC-DC chopper for the motor control in this model it is implemented as 2 quadrant operation that is forward motoring and reverse Motoring. You are they are using for IGBTs as the power conditioning devices by varying the duty cycle we can achieve the desired operation of motor. The motor is wound Field DC machine axis is provided to the field connection so that the machine can be used as a separately excited shunt connected or a series connect a DC machine. The controlling IGVTs all done through the controlling circuit which shows at bottom of the right corner at first motor is working as a forward motoring with chopper one and four are active after .5 seconds for reverse motoring More I GBT3 and two are operated using pulse generator.
Motor and simulation specifications
Power = 5HP
voltage = 240V
Rated speed = 1750RPM
Motor starts in a positive direction with a duty cycle of 75%
at t = 0.5 seconds a armature voltage is reversed and motor runs in the negative direction
OUTPUT : 
From the output we can see that there are less ripples in armature current.
Now by changing the duty ratio from 75% to 45% we get the following outpu
By reducing the duty ratio or metre current shoot up can be completely optimise but there will be considerable fall in the load torque and hence the speed. No load torque of the motor has been reduced from 6Nm to 2 Nm.
No load speed is reduced from 900 rpm to 400 rpm.
CONCLUSION : reducing the duty ratio of the power model ultimately reduce is the effect of shooting armature current but reduces the supply voltage into the motor and results in the reduction of the rated speed and torque of the motor.
Now Referring to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
The fourth quadrant chopper Drive Block represent a 4 quadrant and DC supplied chopper drive for DC motors. Destroy features close loop speed control with four quadrant operation. Speed control loop output is the reference armature current of the machine. Using a pi current controller the chopper duty cycle corresponding to the commander or make a current is derived. Duty cycle is then compared with a sawtooth carrier signal to opt in the required PWM signals on the top. Main advantage of this drug compared with other disease price is that it can operate in all four quadrants that is power motoring, reverse generation reverse motoring and forward regeneration. In addition due to the use of high switching frequency DC to DC converters on Low were armature current triple is opting however for switching devices are required which increases the complexity of the drive system.
Comparison between H Bridge and four quadrants chopper DC drive :
H BRIDGE 4 Quadrant DC drive chopper
(1)Only two quadrants of operation of possible it can operate in all four quadrants i.e forwardmotoring,
which are power motoring and reverse motoring reverse regeneration, reverse motoring and forward Regeneration.
(2) higher armature current ripple lower or major current ripple due to high switching frequency of DCDC converter
(3) less complex more complex
(4) simulation is continues and simulation time is more discrete and simulation time is less widely used in
Widely used in robotics. automobiles. QUESTION (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 )
2 Quadrant chopper :
In this chopper the average voltage will be always positive but the average load current might be positive or negative. The two switches CH1 CH2 should not be turn on simultaneously as the combined action because a short-circuit in supply lines more regenerative braking and murdering this type of chopper consideration is used. We will always get a positive output voltage Vo 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 is turn off 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. The load current will be negative when does CH2 is on. The current complete its part through diode D2.
For discussing on current operation of DC motor go for type E or the 4 quadrant top consisting of four semiconductors which is in full diodes arranged in antiparallel. The four choppers are numbered according to which quadrant they belong. The operation will be in each quadrant in the corresponding chopper only is active in its quadrant.
Simulink Model :
OUTPUT :
QUESTION (3) Explain in a brief about operation of BLDC motor.
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 DC electric power supply. It uses an electronic 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 and t speedorque of the motor. This control system is an alternative to the mechanical commutator (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 motor. They may also use neodymium magnets and be outrunners (the stator is surrounded by the rotor),inrunners (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.
Construction of BLDC Motor
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.
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. For industrial applications and automated systems 100 V or higher rating motors are used.
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.
Working Principle and Operation of BLDC Motor
BLDC motor works on the principle similar to that of a conventional DC motor, i.e., 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 case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.
When the stator coils are electrically switched by a supply 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.
Consider the figure below in which motor stator is excited based on different switching states. 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 causing motor to rotate.
Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction. the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.
Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.
Brushless DC Motor Drive
As described above that the electronic controller circuit energizes appropriate motor winding by turning transistor or other solid state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of MOSFET bridge (also called as inverter bridge), electronic controller, hall effect sensor and BLDC motor.
Here, Hall-effect sensors are used for position and speed feedback. The electronic controller can be a microcontroller unit or microprocessor or DSP processor or FPGA unit or any other controller. This controller receives these signals, processes them and sends the control signals to the MOSFET driver circuit.
In addition to the switching for a rated speed of the motor, additional electronic circuitry changes the motor speed based on required application. These speed control units are generally implemented with PID controllers to have precise control. It is also possible to produce four-quadrant operation from the motor whilst maintaining good efficiency throughout the speed variations using modern drives.
Advantages of BLDC Motor
- It has no mechanical commutator and associated problems
- High efficiency 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
- Quite operation (or low noise) due to absence of brushes
Disadvantages of Brushless Motor
- These motors are costly
- Electronic controller required control this motor is expensive
- Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors
- Requires complex drive circuitry
- Need of additional sensors
Applications of Brushless DC Motors (BLDC)
Brushless DC Motors (BLDC) are used 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. Some specific applications of BLDC motors are
- 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.