Week-7 Challenge: DC Motor Control
1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
Answer:
The demo model is explained about "Speed control of DC motor" using BJT H-Bridge with PWM technique.
PWM technique:
Pulse width modulation (PWM) is a modulation technique used to generate variable-width pulses with some amplitude.
Duty cycle = Ton/(Ton + Toff ) = Pulse on time/ Pulse period
Period =Ton + Toff
Duty cycle is the main paramter of PWM technique.
Types of PWM techniques:
There are 3 types of PWM techniques.
- Lead edge modulation
- Trail edge modulation
- Pluse center two edge modulation/Phase correct PWM
Lead edge modulation: In this technique, the lead edge of the signal is fixed and the trail edge is modulated.
Trail edge modulation: In this technique, the lead edge of the signal is modulated keeping the trail edge fixed.
Pluse center two edge modulation/Phase correct PWM: In this method, the pulse center is fixed and both edges of the pulse is modulated.
Applications:
- PWM Techniques are used in Telecommunications for encoding purposes.
- Pulse Width Modulation helps in voltage regulation and thus finds its use in controlling Brightness in Smart Lighting Systems and also controls the speed of motors.
- Computer Motherboard requires PWM Signals that controls the heat generated in the board. 4 Pin PWM header is embedded in the fan that helps to dissipate the heat from the motherboard.
- It is also used in Audio/Video Amplifiers.
H-Bridge:
- An H-Bridge is an electronic circuit that switches the polarity of a voltage applied to a load.
- These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards.
- Most DC-to-AC converters (power converters), most AC/AC converters, the DC-to-DC push–pull converter, isolated DC-to-DC converter most motor controllers, and many other kinds of power electronics use H bridges.
Operation:
- The H-bridge arrangement is generally used to reverse the polarity/direction of the motor, but can also be used to 'brake' the motor, where the motor comes to a sudden stop, as the motor's terminals are shorted, or to let the motor 'free run' to a stop, as the motor is effectively disconnected from the circuit.
- The following table summarises operation, with S1-S4 corresponding to the diagram above.
The two basic states of H-bridge are:
Main 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 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.
Outputs:
1) Currents:
2) Speed, Armature current, Load torque:
3) Pulse generator:
Conclusion:
- Therefore, the simulink model " Speed control of DC motor using BJT H-bridge" is explained.
1. B. Comment on the armature current shoot-up from the scope results.
Answer:
The armature current graph is shown in below:
- The armature current value is goes up to 40A and then comedown to a constant value when Q1 and Q4 is on.
- Similarly the same graph is appeared in negative direction when Q2 and Q3 is on.
1. C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
Answer:
Main model:
- The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source.
- The armature voltage is provided by an IGBT converter controlled by two PI 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.
- Actually DC-7 is fed from AC-DC converter (which is fed from 3-phase AC supply), but to model for electric vehicle they are replaced by simple DC source.
- And all the remaining kept same and simulated.
Output:
Comparision of BJT H-bridge model and four-quadrant chopper DC drive (DC-7) block:
-
DC-7 chopper H-Bridege model There are 4 modes of operation There are only two modes of operation They are forward motoring, forward regenerative breaking, reverse motoring and reverse braking They are forward motoring and reverse motoring High efficiency than H-bridge model Low efficiency Regenerative braking is possible Regenerative breaking is not possible. To control the speed, we need to change pulse genrator To control the speed, we need to change the polarity.
Conclusion:
- The comparision of DC-7 chopper and H-bridge model is discussed.
2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.
Answer:
Chopper:
- A DC to DC converter is known as chopper, is a static device which is used to obtain a variable DC voltage from constant DC voltage source.
- Choppers are widely used in trolley cars, battery-operated vehicles, traction motor control, control of large motor dc motors, regualtors etc.
- There are two types of choppers. 1) Step-down chopper 2) Step-up chopper.
- In step-down chopper, the output voltage will be less than the input voltage, where as in step-up chopper, the output voltage will be greater than the input voltage.
2-Quadrant chopper:
Two qudrant-chopper is operated in first and second quadrants of output volatge and current characteristics.
Main model:
- The main model for the two-quadrant chopper model is modelled as above.
- The not gate is used to avoid triggering of MOSFETs simultaneously. So if NOT gate is used, one MOSFET is triggered and another is not triggered.
i) L = 1 mH:
Output:
Torque vs Speed:
ii) L=10 μH:
Output:
Torque vs Speed:
Conclusion:
- The two-qudrant chopper is developed and explained.
3. Explain in a brief about operation of BLDC motor.
Answer:
- A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electricity.
- It accomplishes electronically controlled commutation system instead of a mechanically commutation system.
- Commutation is the process of producing rotational torque in the motor by changing phase currents through it at appropriate times.
- BLDC motors are also referred as trapezoidal permanent magnet motors.
- 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.
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.
a) Stator:
- 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 automation systems, 100 V or higher rating motors are used.
b) 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.
c) Hall sensors:
- The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor’s response.
- Hall sensors provide the information to synchronize stator armature excitation with rotor position.
- Since the combination 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, acknowledgement 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 genarates 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 and Operation of BLDC motor:
- BLDC motor works on the principle similar to that of a conventional DC motor.
- That is 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.
- 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 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.
Advantages of BLDC motor:
BLDC motors has several advantages over conventional DC motors.Some of these are,
- 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 BLDC 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 BLDC motor:
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
Conclusion:
Refernces: