Week-7 Challenge: DC Motor Control

1) A)

Aim:

To explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

Explaination:

Duty cycle:

• PWM simulates different voltage outputs via a repeating series of pulses.
• These pulses occur at a steady rate, but the length of each pulse varies depending on the simulated voltage.

• The time period T consists of Ton and Toff time period.

T=Ton+Toff

• Frequency is inversely proportional to time period (f=1/T).

• By varying the duty ratio value we obtain various voltage i.e switching the devices on and off according to its switching frequency.
• These pulse signals are given to the switches such as MOSFET ,IGBT or BJT to control the speed of motor.

IGBT:

• IGBTs are the device of choice for high-voltage (400–1200 V) and medium- to high current (10–1000 A) automotive power switching applications because of their superior current conduction capability over high-voltage power MOSFETs.

• 600–900 V IGBT inverter power modules are exclusively used in electric and hybrid propulsion systems with a power rating greater than 20 kW.
• 400–600 V discrete IGBTs are also widely used as the ignition coil driver in conventional vehicles powered by internal combustion engines.
• The IGBT is a switching transistor controlled by voltage applied to the gate terminal.
• Device operation and structure are similar to that of a power MOSFET.
• The principal difference is that the IGBT relies on conductivity modulation to reduce on-state conduction losses.
• The IGBT has high input impedance and fast turn-on speed like a MOSFET, but exhibits an on-state voltage drop and current-carrying capability comparable to that of a bipolar transistor while switching much faster.

Diodes:

• Diodes are the simplest semiconductor device, comprising of a PN or Schottky junction with two external terminals.
• Diodes are used in applications that require current to flow in one direction only.
• Rectify AC current from the alternator to DC current that charges the battery.

• Allow load current freewheeling as the anti-parallel diodes for IGBTs or MOSFETs in inverter or converter applications.
• A freewheeling diode provides an alternative path for the inductive load current when the main IGBT or MOSFET turns off, and prevents high Ldi/dt voltage spikes from damaging the main switching devices.
• Suppress voltage transients when being reverse biased (Zener diodes).

Speed control of a DC motor using BJT H-bridge:

• 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.

• The above block model represents the 'Speed Control of a DC Motor Using BJT H-Bridge'.
• The supply voltage of 240 V is given to the circuit.
• Two IGBT and diode switches are placed in both the side A and B.
• The DC motor is connected with the switches to form H shape due to which it is called as H bridge circuit.
• Above circuit is functioned in such a way that the switches in side A or B are not operated simulaneously.

Input parameters:

• The input resistance of 0.001 ohms is given.
• 1v forward voltage and 1e5 snubber resistance is given to the device.
• The aboves values is maintained in all IGBT devices.

• 0.8V forward voltage and 0.001 ohms of internal resistance is given to the diode devices.

• The motor with 5HP power, 240 V , 1750 rpm speed and with field voltage of 300V is used.

• The pulse is given through the pulse generator to the IGBT to control the speed of motor.
• In this model 75 % of duty cycle is given to the circuit.
• 1 amptitude is maintained in the circuit for a time period of 2e-3 seconds.

Operation:

• The 240 V is given as the input supply to the circuit.
• The switches are arranged as shown in the diagram.
• The seperate 300V supply voltage is given to the dc motor.
• The P1,P2,P3 and P4 are the input generated pulse signal given the IGBT devices.

• This is the block in which the pulse is generated by varying the duty cycle.

• From the above diagram it is clear that during the conduction of switch Q1 and Q4 the motor operates in forward direction and Q3 and Q4 in reverse direction respectively.

• The output of the devices viewed through the scope by connecting the output current of IGBT and diode.

• The speed obtained is converted into rpm by multiplying with 30/pi.
• 6.062e-4 gain is multip,es to the speed of the motor inorder to obtain the load torque.
• These values are again fed back to the motor.

1)B)

Aim:

To comment on the armature current shoot-up from the scope results.

Explaination:

• 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.

• This the output of the IGBT and diode current.
• From this it is clear that when the IGBT is On then the diode is Off.

Phase 1:

• The value of IGBT is zero i.e off state.

• The diode is in conduction mode.

Phase 2:

• IGBT is in conduction mode.

• Diode is in off condition.

• The value of the motor speed is converted from rad/s into rpm by multiplying with 30/pi to obtain value of speed.
• The load torque is calculated by T=k*w^2 .
• Where the k represents the motor constant and it is multipled with the square of speed.

• It is observed that when the motor is operated with the duty cycle ratio of 75 percent , the speed of the motor increases gradually.
• But the armature current of the dc motor is high while starting the motor and decreases.
• Similar to the speed the load torque increases.

DC motor operating in forward direction:

• When the motor is operated in forward direction then the following speed, armature current and the load torque is obtained.

DC motor operating in reverse mode:

• When the dc motor operates in reverse mode then the above graph is obtained.
• It is found that the speed , armature current and the load torque occurs in negative cycle with the values shown below.

Duty cycle :45%

• The above graph represents the speed , armature current and torque.

• IGBT and diode current graph is shown above.

Duty cycle 75% and 45%:

• From the above graph it is clear that by varying the duty cycle the motor speed is varied.
• As the duty cycle increases, the speed of the motor also increases rapidly.

1)C)

Aim:

To refer ‘The Four-Quadrant Chopper DC Drive (DC7) block’and to compare it with the H-bridge model.

Principle:

• The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors.
• This drive features closed-loop speed control with four-quadrant operation.
• The speed control loop outputs the reference armature current of the machine.
• Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived.
• This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.
• The main advantage of this drive, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration).
• In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained.
• However, four switching devices are required, which increases the complexity of the drive system.

• The Three-Phase Source block implements a balanced three-phase voltage source with an internal R-L impedance.
• The three voltage sources are connected in Y with a neutral connection that can be internally grounded or made accessible.

• We can specify the source internal resistance and inductance either directly by entering R and L values or indirectly by specifying the source inductive short-circuit level and X/R ratio.
• 380V of phase to phase voltage is given with 50 Hz frequency.
• R=0.01 ohms and L=0.1e-3 henry is given as input.
• It is connected to AC to DC rectifier.
• The rectifier bridge is connected to the braking chopper with the following inputs as shown below.

• The output obtained from the rectifier is DC which is connected to the four quadrant chopper.

• The Vcc and Gnd terminals are connected to the rectifier output port .

DC machine:

• Mutual inductance: 3.320 H
• Armature resistance:0.076 ohms
• Armature inductance:0.00157 H
• Field resistance: 310 ohms
• Field inductance:232.25 H

Converter:

• Smoothing inductance:1e-3
• Field DC source: 310 V

Controller:

• Speed: 1184 rpm
• Low pass filter cut off frequency:40 Hz
• Sampling time:100e-6

Reference speed:

• Step time:2 sec
• Initial value: 500
• Final value:1184

• The obtained speed is converted into rpm and fed back to the mechanical input load torque Tm.

• The values of armature voltage, armature current, speed and operation of power IGBT devices are connected to the outport and scope to view the results.

Output:

• The operations performed by the circuit are forward motoring, reverse regeneration, reverse motoring, and forward regeneration.
• Graph 1:Dutycycle of IGBT 1 and 4 , the motor rotates in forward direction.
• The obtained duty cycle is more than 50%
• Graph 2:Dutycycle of IGBT 2 and 3, the motor rotates in reverse direction.
• The obtained duty cycle is less than 50%.
• Graph 3:Armature voltage, we can see the variation in the voltage from 500 V to -500V.
• Graph 4:Armature current, there is a sudden increase in the current during forward motoring operations and sudden decrease in the current negatively during the reverse operations of the motor.
• Graph 5:Motor speed varying from 0 to 500 rpm speed and suddenly decreasing due to reverse operations.

Converter signals:

• The required measurements can be choosen from the available signals module and the output can be viewed.

• This is the ouput obtained in the converter signal Uab and Udc inputs.

Comparision of Speed control of a DC motor using BJT H-bridge vs The Four-Quadrant Chopper DC Drive (DC7) block:

Speed control of a DC motor using BJT H-bridge	The Four-Quadrant Chopper DC Drive (DC7) block
It is not a closed loop speed control operation	Closed-loop speed control with four-quadrant operation
The switch is controlled by a Simulink® signal (1/0).	Speed control loop outputs the reference armature current of the machine.
The armature mean voltage can be varied from 0 to 240 V when the duty cycle is varied from 0 to 100% through pulse generator.	Duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.
It operates in two cycles positive cycle as forward motoring and negative cycle as reverse braking.	It can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration)
It is not as low when compared to DC7 block.	Lower armature current ripple due to high switching of devices.
It is not a complex drive system.	Increases the complexity of the drive system

2)

Aim:

To develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

Principle:

• A chopper is basically a dc to dc converter whose main function/usage is to create adjustable dc voltage from fixed dc voltage sources through the use of semiconductors.

Type -C chopper or Two-quadrant type-A 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.

Type D Chopper or Two-Quadrant Type –B Chopper:

• The circuit diagram of the type D chopper is shown in the above figure.
• When the two choppers are on the output voltage v₀ will be equal to V_s .
• When v₀ = – V_s the two choppers will be off but both the diodes D1 and D2 will start conducting.
• V₀ the average output voltage will be positive when the choppers turn-on the time T_on will be more than the turn off time T_off its shown in the wave form below.
• As the diodes and choppers conduct current only in one direction the direction of load current will be always positive.
• The power flows from source to load as the average values of both v₀ and i₀ is positive.
• From the wave form it is seen that the average value of V₀ is positive thus the forth quadrant operation of type D chopper is obtained.
• From the wave forms the Average value of output voltage is given by

V0= (V_s T_on-V_sT_off)/T = V_s.(T_on-T_off)/T

Simulation:

Type -C chopper or Two-quadrant type-A Chopper:

• The Step block provides a step between two definable levels at a specified time.

• 0.2 sec step time with 50 as initial value is given as input.

• The Rate Transition block transfers data from the output of a block operating at one rate to the input of a block operating at a different rate.
• The step input value is given to the inport 1 rate transistion block with sampling time 0.001 sec.
• The inport 2 contains the data from current sensor.
• Using summing block both the obtained values are given to the PI controller.

• From PI controller with proportional gain 0.4 and gain interval 7 of the signal is given to the pulse with modulation generator.

• Hence the obtained pulse is given to the semiconductor module of two quadrant chopper.

Two quadrant chopper:

• The two quadrant chopper inbuilt switch is available in the semiconductor library.
• The MOSFET switch is choosen with the parameters shown above.
• 15 V power supply is given to the chopper circuit through the voltage source.
• The current sensor is connected in series and the voltage sensor in parallel.
• The inductance load with value of E as 5V is given in load side.

Output:

• The above image represents the output of the two quadrant chopper.
• Graph 1 represents the output current: We can find both positive and negative current as it is two quadrant operation of I and II.
• Graph 2 represents the duty cycle : We can find the switching of the MOSFET.
• Graph 3 represents the output voltage: The voltage is positive in this two quadrant operation.

Type D Chopper or Two-Quadrant Type –B Chopper:

• The same procedure followed for type C chopper shall be followed.
• The input constant value 30 is given.
• The gate pulse is given to the MOSFET device.
• For two quadrant type D chopper the Model configuration is changed by Right clicking-Simscape-Model change -First and fourth quadrant operation.
• The voltage sensor is connected parallel and the current sensor in series.
• In the load side the the inductance and voltage source with step input is given.
• The output is viewed through the scope.

Output:

• The output of the current ,voltage and duty cycle is shown in the above image.

Result:

The 2-quadrant chopper using simulink is developed and explained with the relevant results.

3)

Aim:

To explain in a brief about operation of BLDC motor.

Explaination:

• A brushless DC motor is a rotating self-synchronous machine with a permanent magnet rotor and with known rotor shaft positions for electronic commutation.

• Several types of electrical machines and drives have been referred to as BLDC,
  including:
  1. Permanent magnet synchronous machines (PMSMs)
  2. BLDC trapezoidal back-electromotive force (back-EMF) surface-mounted magnet machines,
  3. BLDC sinusoidal back-EMF surface-mounted magnet machines
  4. BLDC interior-magnet machines
  5. The BLDC motor and drive combination
  6. Axial-flux BLDC

Construction:

• In this motor, the permanent magnets attach to the rotor.
• The current-carrying conductors or armature windings 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.

• 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 is not a requirement.
• Inrunner: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.
• BLDC motor works on the principle similar to that of a brushed DC motor.
• 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 the BLDC motor, the current carrying conductor is stationary and the permanent magnet is moving.
• When the stator coils get a supply from 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.
• 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 which causes the motor to rotate.

Operations:

• The following setup of three windings in the stator designated A, B and C.

• We know that when a current is applied through a coil, a magnetic field is created and the poles of the generated magnet will depend on the direction of the current flowing through the coil.
• Using this principle, if we supply current to the coil A so that it will generate a magnetic field and attract the rotor magnet.
• The position of the rotor magnet will shift slightly clockwise and will align with A.
• If we now pass current through coils B and C one after the other then the rotor magnet will rotate in clock wise direction.

• To increase efficiency, we can wind the opposite coils using a single coil so that we get double attraction.
• Further increasing the efficiency, we can energize two coils at the same time so that one coil will attract the magnet and the other coil will repel it.
• During this time, the third will be idle.
• For a complete 360⁰ rotation of the rotor magnet, six possible combinations of the coils A, B and C are applicable and are shown in the following timing diagram.

• Base on the above diagram, we can confirm that at any time, one phase is positive, one phase is negative and the third phase is idle.
• So, based on the inputs from the Hall Sensors, we have two switch the phases as per the above diagram.

Driving Brushless DC Motors

• If stator and rotor are essential parts of a BLDC Motor that are integral to it, then the driving electronics are equally essential.
• Block diagram of a typical Brushless DC Motor control or drive system is shown in the following image.

• This drive circuitry is often known as Electronic Speed Controller System or simply an ESC.
• One common setup is called the Full Bridge Drive Circuit.
• It consists of an MCU with PWM outputs, six MOSFETs for the three phases of the stator windings, feedback from the Hall sensors and some power supply related components.

• The MCU can be programmed to appropriately switch the MOSFETS based on the data from the Hall Sensors.

Advantages:

• 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 stato.r
• Less electromagnetic interference.
• Low noise due to absence of brushes.

Limitations:

• These motors are costly.
• Electronic controller required control this motor is expensive.
• Requires complex drive circuitry.
• Need of additional sensors.

Applications:

• 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.

Attachments:

https://drive.google.com/file/d/1jadgcHNarz3UDxqNT694HT3NwWF3V180/view?usp=sharing