Week-7 Challenge: DC Motor Control
Answer1:
A:
SPEED CONTROL OF DC MOTOR USING BJT H BRIDGE:
A DC motor spins either backward or forward, depending on how you connect the plus and the minus.
If you close switch 1 and 4, you have plus connected to the left side of the motor and minus to the other side. And the motor will start spinning in one direction
If you instead close switch 2 and 3, you have plus connected to the right side and minus to the left side. And the motor spins in the opposite
direction.
If you add diodes in the reverse direction for the transistors, you give a path for the current to take to release this energy
The model of speed control of dc motor using bjt h bridge is
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. and motor operates in forward motoring region.
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. and motor operates in reverse motoring region.
The DC Motor parameters are:1750rpm.
Field=300V; 5HP 240V
the control signals fot the switches are given from P1 P2 P3 P4
Which are in turn controlled by a pulse generator as shown in below
the IGBT and Diode currents are
The speed and armature current and Load torque waveforms are:
the speed is reached to 945 rpm in 0.145seconds.
and decelarates from 0.5 seconds. at 0.54 seconds the motor runs in backward.
B:
The Speed and Armature current is obtained by the DC Motor and Load Torque is Calculated by load torque proportional to square of speed. In
Starting point, there is a sudden Shootup is acted. In this Scope shows that speed is increased in certain period of Time and at that time
armature current is positive whereas Load Torque also moves forward. After the time period of 0.5 sec which is given in Pulse generator (step
time), sudden shoot-up is acted and speed gets decreased inthat time armature current is in negative terminal whereas Torque act as reverse
action. Here the sudden shoot up is acted while Armature current is processed at a positive and negative terminal.
One simple and easy way to control the Speed of a Motor is to Regulate the Amount of Voltage across its Terminals and this can be achieved
using "Pulse Width Modulation" (PWM Technique).
For Experimental Section we will reduce the Pulse width of Pulse generator block as 50% at the same time period.
The Inputs of P1, P2, P3 and P4 are also changed according to the drive cycle. When the Q1 and Q4 switch is ON the Motor is generated as
Forward direction that state Q3 switch and Q2 switch is in OFF.
When the Q2 and Q3 switch is ON the Motor is generated as reverse direction that state Q1 and Q4 switch is in OFF Condition.
Here we see that the sudden shoot up is not acted by comparing with 75 of pulse width. Same cycle time is followed in this experiment also. In this observation starting current is generated as Diode current for time period of 0.5 sec, then IGBT current is acted while diode current is being stopped. This graph shows when the switch Q3 is OFF on the starting stage of current flow and Diode current is ON at that stage. During the 0.5 sec of time D3 switch is ON that is given according to the Pulse generator and after the time period of 0.5 sec D3 switch is OFF. An IGBT current is acted after 0.5sec and Q3 switch remains ON. There will be little variation as shown in graph at the starting point of current but the complete reduction of Shoot-Up are not held here.
C:
The Four-Quadrant Chopper DC Drive (DC7) block:
An Example of DC7 : 4 quadrant chopper 200HP DC DRIVE:
COMPARISION:
1. In four Quadrant Chopper DC drive distributes required current supply for each sector and it consist of 4 quadrants like Forward Motoring,
Regenerative Braking, Reverse Motoring and Reverse Regenerative Braking. Whereas in H-bridge supplies an irregular current for each sector
that causes voltage Fluctuation in armature current, it consist of 2 Operation namely they are Forward Motoring, Reverse Motoring.
2. In Modern Battery Technology Charger use for EV charging uses H-bridge circuit. Whereas a Four Quadrant Operation is required in Industrial
as well as Commercial application.
3. The efficiency is high in Chopper compare with H-bridge and Maintenance is less than H-bridge.
4. In Robotic Technology, H-bridge circuit is used; some other application of Four Quadrant Operation is Engine Test Loading Systems.
5. In H-bridge we need to change Polarity which turns into opposite direction to control the motor whereas in chopper model we need to
interrupt one signal under control of another.
DC 7 four quadrant chopper BJT H-Bridge Drive
It can be operated in 4 ways motoring and braking in both directions. It is used for changing the direction of motor (forward&reverse).
Regenerative Braking can be Achieved. Regenerative Braking cannot be available.
the source is Ac supply which is rectified into The source is fixed Dc supply
DC supply.
the motor ref. speed can be fed The motor speed reference cannot be fed
have comparatively less shoot up the arm.current shoot up is possible
better control over the vehicle less control over the vehicle
here signals are discrete here signals are continuous
Answer2:
Modelling of two quadrant chopper:
here we are modelling Type - C chopper
The type C chopper is parallel combination of type A and type B chopper. The output voltage is always positive but output current either
positive or negative in this type of chopper therefore it works in first as well as second quadrant.
when ch1 is closed, the flow of current to the motor is shown in the below image,
the wave forms are shown below
the gate currents are given from a pulse generator and using one not gate so that both mosfets will not triggered at a time.
reference: https://www.myelectrical2015.com/2017/03/type-c-chopper.html
Answer3:
What is a Brushless DC motor (BLDC)?
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 an 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 mechanical commutation system. BLDC motors are also referred to as trapezoidal permanent magnet motors.
Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with a commutator on the rotor to form an electric path between a DC electric source and rotor armature windings, BLDC motor employs electrical commutation with a 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 silicon controlled rectifiers at the correct rotor position in such a way that the 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 per cent, whereas brushed type DC motors are 75 to 80 per cent efficient. There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.
Construction of Brushless DC motor
In this motor, the permanent magnets are attached to the rotor. The current-carrying conductors are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy.
The main design difference between brushed and brushless motors is the replacement of a 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.
Construction of BLDC motor
The Brushless motor does not have any current-carrying commutatorThe 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 “Outrunner” style or “Inrunner” style.
Outrunner – The field magnet is a drum rotor that 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 outrunner style but is capable of spinning at very high rpm.
Brushless DC Motors Introduction for Next-Generation Missile Actuation Systems Outline | Analog Devices
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 rotors are most commonly used.
The construction of this motor has many similarities to the three-phase induction motor as well as the conventional DC motor. This motor has stator and rotor parts as like all other motors.
The stator of a BLDC motor is made up of stacked steel laminations to carry the windings. These windings are placed in slots that 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 a three-phase star connected stator.
Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. 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 automation 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. 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 the 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 a 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 the BLDC motor is controlled electronically, the stator windings should be energized in sequence to rotate the motor. Before energizing a particular stator winding, acknowledgement of the rotor position is necessary. So the Hall Effect sensor embedded in the 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 responses.
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 is placed in a magnetic field it experiences a force. As a consequence of the reaction force, the magnet will experience an equal and opposite force. In the case of, 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 an 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 a trapezoidal shape.Due to the force of interaction between the electromagnet stator and the permanent magnet rotor.
consider the figure below in which the 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 the motor to rotate.
Observe that the 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). In this way motor moves in a clockwise direction.
How to Power and Control Brushless DC Motors | DigiKey
Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; 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 the 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 transistors or other solid-state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of a MOSFET bridge (also called an 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 the 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 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
The electronic controller required to 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 equipment 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