Week-7 Challenge: DC Motor Control

QUESTION -1

Objective

> Run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’. Modify the model such that armature current doesn’t shoot up when motor changes direction from forward to reverse.

> Comparison of 'The Four-Quadrant Chopper DC Drive (DC7) block’ with 'H-bridge model'.

> Make a suitable EV model using DC7 block.

Speed control of a DC motor using BJT H-bridge

At first, to run the speed control model of a DC motor using BJT H-bridge in matlab, the command - (power_Hbridge) is run in the command window of MATLAB.

We get a simulink window of the model as shown below

From the above image it is seen that the model consists a motor(5HP 240V 1750rpm field:300V), which is connected to a 240V supply with four bipolar junction transistors(BJT) and diodes, which acts as full bridge rectfiers. The model is know as H-bridge because the connection of the BJT's and diodes are in such a way that switches the polarity of the voltage applied to the motor.

The structure of a simplified H-bridge circuit is given below

The two basic states of the h-bridge is show below

The two

From the above circuits, it shows that voltage can be applied in both the direction when using a H-bridge structure.

The following table summarises the operations corresponding to the switches S1, S2, S3 and S4.

The above table can be similarly compared with the speed control model of the DC motor.

>When Q1, Q4 are connected the motor moves right.

>When Q2, Q3 are connected the motor moves left.

The frequence of the switching is controled by the pulse generator block of the model. The pulse generator consists of inputs such as amplitude, period, pulse width (or) duty cycle and phase delay. The outputs of the pulse generator block are P1, P2, P3 and P4 which are connected to the BJT's. When the pulse width is changed, the results of the pulse generator changes which in turn control the output direction and speed of the motor.

Simulation of the model with no modifications in the pulse generator block.

The default value of pulse width is 75. The pulse generator block is given below.

Result of simulation.

The scope consisting the speed of the motor, armature current and load torque of the motor is given below.

From the result it is seen that the armature current shoots up when the motor is made to change direction at 0.5s of the simulation.

Simulation of the model with change in the pulse generator block.

The default value of pulse width 75 is reduced to 50 and then the model is simulated.

Result of simulation.

The scope consisting the speed of the motor, armature current and load torque of the motor is given below.

From the result we can see that @ T=0.5 o

direction and the current shoot is reduced accordingly. Due to the reduced pulse width, the speed of the motor and the load torque is also reduced.

Simulation of the model with change in the pulse generator block.

The pulse width is again reduced from 50 to 25 percent and then simulated.

Result of the simulation

The scope consisting the speed of the motor, armature current and load torque of the motor is given below.

From the result it is see that, the current does not shoot up when the motor changes direction @ T=0.5s of the simulation. The speed of the motor and the load torque in of the motor are low.

Conclusion

From all the simulations it is concluded that, when the duty cycle of the pulse generator block is low then the speed, current and load torque is also low accordingly.

The Four-Quadrant Chopper DC Drive (DC7) block

The Dc7-four quadrent chopper 200 HP DC drive model is opened by running the command (dc7_example) using the command window of MATLAB.

The simulation window of the model is given below.

The model consists of a 380V 50Hz three phase AC supply with phase angle 0 which is connected to a converter block. In the converter block it has a rectifier bridge and a braking chopper. The output of the rectfier bridge is connected to the braking chopper. The braking chopper is used to limit the bus voltage during braking mode with a frequency of 4KHz.

A DC7 block situated near the converter block takes inputs such as speed reference, voltage input from AC to DC converter block and load torque value or speed of motor or mechanical rotation port from the motor.

With the above inputs and the standard inputs of the DC machine, controller and converter the outputs are processed in the four quadrent chopper DC drive block. The controller has a schematics by which the speed and current are controlled. The controller schematics are given below.

The Four-Quadrant Chopper DC Drive is given below

The reference speed is given to the speed regulator and then to the current regulator. The electromagnetic torque required to reach the desired speed is obtained. The IGBT's 1 and 4 are opposite to the IGBT's 2 and 3. They generate the average armature voltage needed to obtain the required armature current. A smoothing inductance is placed in series with the armature current to limit the amplitude of the current oscillation.

Then the signal of motor, controller and converter are given to a subsystem block using a bus operator.

Outputs of armature current, armature voltage and speed are given by the motor and the outputs of current, reference speed, IGBT 1&4 and IGBT 2&3 are given by controller. The converter signal is terminated.

By using the Mux operator the two signals of current from motor and controller are made into one common signal and the speed of the motor and reference speed from motor and controller respectively are made into one common signal.

Now, The duty cycle of IGBT 1&4, duty cycle of 2&3, Armature voltage, aramture current and speed are plotted using a scope.

By simulating the model the results plot from the scope is given below.

Comparison of The Four-Quadrant Chopper DC Drive (DC7) block with H-bridge

>The simulation is continuous in H-bridge whereas the simulation is discrete using a four quadrent chopper DC drive.

>Since H-bridge uses continuous simulation the simulation time is less when compared to DC7 block.

>Direction of the motor can be changed by changing the polarity in a H-bridge structured circuit and, the direction of the motor can be changed in a four quadrant chopper DC drive by changing the input signal to the DC7 block.

>Use of snubber in H-bridge circuit is not required but it is required for the DC7 block. This is due to the rapid raise in the voltage when a change in the input signal occurs in the DC7 block.

>Speed of the motor can be varied by variying the duty cycle of the motor in H-bridge circuit. In the DC drive with DC7 block speed of the motor can be varied by the input signal or speed reference.

>Regenerative braking can be done in H-bridge circuit but cannot be done in the four quadrant chopper DC drive.

Suitable EV model using DC7 block

The Four quadrent DC drive model is modified to make a suitable EV model using DC7 block, it requires a DC battery input, drive cycle data for the vehicle and torque load characteristic of the vehicle.

The three phase AC supply, speed reference and the AC to DC converter blocks are removed from the four quadrent DC drive model

For the battery input, a DC voltage block is added to the system with amplitude voltage 515V. The positive of the DC supply block is connected to a current measurement block by which the current output is measured and ploted using a scope. The positive and negative terminals are connected with the Vcc and Gnd of the DC7 block respectively.

The speed reference block of the four quadrent DC drive model is replaced with a drive cycle block. The drive cycle data

Torque load characteristic of the vehicle is said to be,

T = 24.7+0.0051(w^2)

A subsystem for calculating the torque (Tm) with speed of the motor (w) given as input created as shown below.

The simulink window after changing the four quadrent DC drive model accordingly is show below.

The results of simulating the above EV model using the DC7 block

Drive cycle plot

Current output plot from DC voltage supply

Resultant plot of IGBT 1&4, IGBT 2&3, current, voltage and speed of motor

Duty cycle IGBT 1&4 plot

It is seen that duty cycle increases as the vehicle accelerates and decreases when the vehicle decelerates.

Duty cycle IGBT 2&3 plot

The duty cycle decreases as the vehicle accelerates and increases during deceleration of the vehicle.

Armature voltage

The armature voltage of the motor is approximately constant throughout the simulation

Armature current

The current increases as the vehicleaccelerates and decreases when the vehicle decelerates.

accelerates and decreases when the vehicle decelerates.

At T=5s, the speed of the vehicle reduces and the plot shows that the current at that point goes negative. The negative current shows that the motor tends to rotate in the opposite direction in order to decrease the speed of the vehicle according to the drive cycle input.

Motor speed

From the plot it is clearly seen that the motor speed matches the drive cycle input.

Conclusion

Thus a suitable EV model is made using the DC7 block and simulated in which the vehicle speed matches the drive cycle data given to the model successfully.

QUESTION-2

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 V₀ as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage v₀ will be equal to V_s . The direction of the load current i₀ will be reversed. The current i₀ 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.

TWO QUADRANT CHOPPER MODEL USING SIMULINK:

let us take two mosfet blocks and diodes blocks and connect it as shown in circuit diagram.

the dc voltage source is connected as shown.

let the voltage of dc voltage source be 24 volts.

let us consider an series RLC branch block and select inductor L . thus block works as a load.

to consider LE load ie motor load , and dc voltage source of 12 volt 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 mosfet is ON the first mosfet will get off .

The amplitude of pulse generator is kept 10 so that we can clearly see the pulses at scope and let period be 0.02 seconds and let pulse weidth be 50%

SCOPE RESULT:

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.

QUESTION-3

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.

  WORKING OF BLDC MOTOR:

  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.
  
  In a commutation system – one that is based on the position of the motor identified using feedback sensors – two of the three electrical windings are energized at a time as shown in figure 4.

  In figure 4 (A), the GREEN winding labeled “001” is energized as the NORTH pole and the BLUE winding labeled as “010” is energized as the SOUTH pole. Because of this excitation, the SOUTH pole of the rotor aligns with the GREEN winding and the NORTH pole aligns with the RED winding labeled “100”. In order to move the rotor, the “RED” and “BLUE” windings are energized in the direction shown in figure 4(B). This causes the RED winding to become the NORTH pole and the BLUE winding to become the SOUTH pole. This shifting of the magnetic field in the stator produces torque because of the development of repulsion (Red winding – NORTH-NORTH alignment) and attraction forces (BLUE winding – NORTH-SOUTH alignment), which moves the rotor in the clockwise direction.

  

  

  

  This torque is at its maximum when the rotor starts to move, but it reduces as the two fields align to each other. Thus, to preserve the torque or to build up the rotation, the magnetic field generated by stator should keep switching. To catch up with the field generated by the stator, the rotor will keep rotating. Since the magnetic field of the stator and rotor both rotate at the same frequency, they come under the category of

  the stator and rotor both rotate at the same frequency, they come under the category of synchronous motor.

  This switching of the stator to build up the rotation is known as commutation. For 3-phase windings, there are 6 steps in the commutation; i.e., 6 unique combinations in which motor windings will be energized.

  ADVANTAGES OF BLDC 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
    
    
    DISADVANTAGES OF BLDC MOTOR
    
    
  • These motors are costly
  • Electronic controller required control this motor is expensive
  • Requires complex drive circuitry
  • Need of additional sensors
    
    
    APPLICATION OF BLDC 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 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 obatined is studied.
    
    3)operation of BLDC motor is studied