Week-7 Challenge: DC Motor Control

 

 AIM: 

• Modify the model of Speed control of a DC motor using BJT H-bridge such that armature current doesn’t shoot up when motor changes direction from forward to reverse using MATLAB/Simulink.
• Refer and compare the model of The Four-Quadrant Chopper DC Drive (DC7) block with H-bridge model.
• Create a suitable EV model using DC7 Block and explain in brief.

 SOLUTIONS: 

• Study on the Speed control model of a DC motor using BJT H-bridge and experiment with the model to minimize the motor's armature current.

The example shows the simulation of an H-bridge used to generate a chopped voltage and to control the speed of a DC motor.

                                

                                 fig : Simulink model of 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 the square of speed). The armature means 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.

 Motor Specifications : 

• Power = 5 HP
• Rated voltage = 240V
• Rated Speed = 1750rpm

Simulation:

At the time of simulation, the motor starts in a positive direction with a duty cycle of 75% (mean DC voltage of 180V). At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.

                           

                                  fig :The output graph of the IGBT current & Diode current

Observation:

(1) Pulse width (percentage of period) = 75%

                            

               fig : Output graph of Speed, Armature current and Load Torque for 75% duty cycle

Comments:

• Speed is proportional to the applied load.
• The initial spike of current is observed in the armature is very high for this duty cycle.
• The torque of 6N-m is applied to the motor initially and at time =0.5 sec, the direction of rotation is reversed and a negative torque of the same magnitude can be observed.
•  Pulse width (percentage of period) = 50%

                           

                         fig : Output graph of Speed, Armature current and Load Torque for 50% duty cycle

Observation :

• By reducing the duty cycle, the shoot up in the armature current is reduced with the decrease in the torque also.
• As we know that the armature resistance of a motor is very small, generally less than one ohm, therefore the starting current would be very large.
• As the motor speed increases, the back emf increases and the difference in supply voltage & back emf also goes on decreasing. This results in a decrease of armature current until the motor attains its stable rated speed. Under this condition, the armature current reaches its desired rated value.
• To optimized the armature current, we need to control the duty cycle for each period of time.

 

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•  Comparing the model of "The four-quadrant chopper DC drive (DC7)" with the H-bridge model.

The example shows the DC7 four-quadrant chopper DC drive during speed regulation.

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.

                           

                                         fig: Simulink model of an example of 4 Quadrant chopper DC7 drive

Simulation: 

• Before starting the simulation, set the initial bus voltage to 515 V via the GUI block ('Initial States Setting' button and 'Cbus' variable).
• Start the simulation. You can observe the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope. The current and speed references are also shown.
• The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.
• The speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.
• The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.
• At t = 2 s, speed reference drops to -1184 rpm. The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of the load torque.
• At t = 2.2 s, the current reverses in order to produce a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase.
• At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.

Ouput graph:

                              

                      fig : Output graph of the duty cycles of the switches, armature voltage, armature current and the motor speed

Chopper circuit:

Chopper circuits are known as DC to DC converters. Similar to the transformers of the AC circuit, choppers are used to step up and step down the DC power. They change the fixed DC power to variable DC power. Using these, DC power supplied to the devices can be adjusted to the required amount.They can convert the steady constant DC voltage to a higher value or lower value based on their type.DC choppers are more efficient, speed and optimized devices. These can be incorporated on electronic chips. They provide smooth control over the DC voltage.

There are 4 switches with diodes along with the supply voltage, resistor, inductor and the back emf in series. The operation happens in 4 modes by closing the 2 diagonally opposite switches at a time. 

Modes:

1. Forward Motoring mode.
2. Forward Regenerative braking mode.
3. Reverse Motoring mode.
4. Reverse Regenerative braking mode. 

 

                                                         

                                     fig : Circuit diagram of chopper DC drive

 

 Working principle : 

(1) 1st Quadrant 

• When S1 is in operating condition, the current flows from supply voltage through S1, armature resistance, & inductor to the output load.
                                                                            $V_s=V_o$
• When S1 is OPEN, the load current freewheels through S4 and D2.
• In this quadrant, induced current & output voltage remains positive.
• It may be noted that it operates as a step-down chopper in this case.

(2) 2nd Quadrant 

• For this quadrant, S2 is operated while keeping the S1, S3 & S4 OPEN. When S2 is CLOSE, the DC source in the load drives current through S2, D4, E and L. Inductor L stores energy during the on the period of S2.
• When S2 is turned CLOSE, the current is fed back to the source through D1 & D4. It should be noted at this point that (E+Ldi/dt) is more than the source voltage Vs. As load voltage Vo is positive and Io is negative, it is the second quadrant operation of the chopper. Since the current is fed back to the source, this simply means that load is transferring power to the source.
• In this quadrant, configuration operates as a step-up chopper.

(3) 3rd Quadrant 

• For this quadrant, S1 is kept OPEN, S2 is kept CLOSE and S3 is operated. When S3 is CLOSE, the load gets connected to the source and hence load voltage is equal to source voltage. But carefully observe that the polarity of load voltage Vo is opposite to what shown in the circuit diagram. Hence, Vo is assumed negative and it may be seen that Io is flowing in the direction opposite to shown in the circuit diagram and hence negative.
• Now, when S3 is CLOSE, the negative load current freewheels through the S2 and D4. In this manner, Vo and Io both are negative.
• Hence, the chopper operates in the third quadrant with both the power voltage and current are negative.

(4) 4th Quadrant 

• To obtain this quadrant operation, S4 is operated while keeping S1, S2 and S3 OPEN. The polarity of load emf E needs to be reversed in this case too like third quadrant operation.
• When S4 is turned CLOSE, positive current flows through S4, D2, L and E. Inductance L stores energy during the time S4 is CLOSE. When S4 is made OPEN, the current is fed back to the source through diodes D2 & D3.
• Here the load voltage is negative, but the load current is always positive. This leads to chopper operation in the fourth quadrant.
• Here, power is fed back to the source from the load and the chopper acts as a step-up chopper.

                                                                        

                                                fig : Four Quadrant representation of the Chopper circuit

Comparison:

Parameters	4 quadrant chopper DC drive (DC7)	H-bridge drive
Braking	Regenerative barking is possible	Dynamic braking can be achieved
Current flow	The current values are chopped but not continuous	Current values are continuous
To change the direction of motor	Current signals are interrupted with one another	The polarity of the current is changed
Application	Automobiles to control motor speed and direction	Robotics & Electro-Mechanical devices to control DC motor speed and direction

 

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• Simulink model of EV using DC7 block.

For this problem, we need to change a few parameters from the example of 4 Quadrant chopper DC7 drive. We replace the following things:

• DC  battery source.
• Drive Cycle (WOT).
• Current measurement block.
• Vehicle load characteristics base on the torque equation.

$T=24.7+0.0051{\omega }^2$

                               

                                                        fig : Simulink model of the EV using DC7 block

                                

                               fig : Simulink block diagram of Vehicle load characteristics using the toque equation 

Results:

                                

                                                          fig(xi) The output graph w.r.t. the default values of WOT drive cycle.

                              

                                                fig (xii) The output graph w.r.t. the change in initial speed in the WOT drive cycle.

 

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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 )

Type C chopper or Two Quadrant class A chopper:

Class-C or Type-C Chopper is a category of chopper which can operate in first as well as second quadrant. This basically means that, the power can either flow from source to load or load to source in this chopper. This kind of chopper is also known as Two Quadrant Class-A chopper.

Operation or Working of Class-C or type C:

As we know that, a Class A and Class B chopper operates in first and second quadrant respectively. Therefore, if we connect both these types of chopper in parallel then it is possible to have chopper operation in first as well as second quadrant. In fact, Class-C or Type-C chopper is obtained by the parallel connection of Class-A and Class-B chopper. Figure below shows the circuit diagram of this type of chopper.

We can notice that chopper CH1, free-wheeling diode (FD) and load are forming Class-A Chopper whereas chopper CH2, D2 and Load are forming Class-B chopper. Both these choppers are connected in parallel. To obtain first quadrant operation we should switch ON chopper CH1 and for getting second quadrant operation we should switch ON chopper CH2. Let us discuss this in detail.

Case 1: When CH1 is switched ON/OFF

When chopper CH1 is switched ON, source V_s directly gets connected to the load and hence, load voltage V_o is equal to source voltage. The direction of load current is from source to load as shown in the circuit diagram which is assumed positive.

When CH1 is switched OFF, the free-wheeling diode FD comes into the circuit as it gets forward biased and hence shorts the load. Therefore, the output voltage V_o becomes zero. However, the i_o continues to die down through the FD and L in the same direction as shown in circuit diagram. Thus, the average output voltage V_o and current I_o are positive and hence operation of chopper is in first quadrant. 

Case 2: When CH2 is switched ON/OFF

When chopper CH2 is switched ON, load DC source E drives current through CH2 and load. The direction of this current i_o will be opposite to that shown in circuit diagram and hence is assumed negative. Output voltage V_o is zero during this time. When CH2 is made OFF, diode D2 gets forward biased and hence the current into the source from the load. The output voltage is V_s in this time as the load is connected to the source through D2 during OFF time of chopper CH2. Thus, the load current is always negative i.e. operation of chopper is within second quadrant.

The average load voltage is always positive but the average load current may be positive or negative. Therefore, power flow may be from source to load (first quadrant operation) or load to source (second quadrant operation). The operating region of Class-C or Type-C chopper is shown below by hatched area.

CAUTION: Choppers CH1 and CH2 should not be ON simultaneously as this would lead to direct short circuit on the supply lines.

APPLICATION: Class-C or Type-C choppers are used for motoring and regenerative breaking of DC Motors.

Realization using Simulink:

Hence, the simulink model corresponding to the circuit diagram is built using basic simulink blocks. The pulse generator is used with the NOT gate to prevent misfiring of the MOSFETs so that they do not conduct at the same time which results in the short circuit.

Pulse generator Parameters:

An amplitude of 20 is chosen so that we could see the pulses clearly.

DC Voltage Source Parameters:

Source side: 24V

Load side: 12V

the voltage is always positive but the current becomes both positive and negative. Hence, the Type C chopper is simulated and verified.

 

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 3. Explain in a brief about operation of BLDC motor. 

 Definition of Brushless DC motor :

A brushless DC motor consists of a rotor in the form of a permanent magnet and stator in the form of polyphase armature windings. It differs from the conventional dc motor in such that it doesn’t contain brushes and the commutation is done using electrically, using an electronic drive to feed the stator windings.

Basically a BLDC motor can be constructed in two ways- by placing the rotor outside the core and the windings in the core and another by placing the windings outside the core. In the former arrangement, the rotor magnets act as an insulator and reduce the rate of heat dissipation from the motor and operate at low current. It is typically used in fans. In the latter arrangement, the motor dissipates more heat, thus causing an increase in its torque. It is used in hard disk drives.

4 Pole 2 Phase Motor Operation

The brushless DC motor is driven by an electronic drive that switches the supply voltage between the stator windings as the rotor turns. The rotor position is monitored by the transducer (optical or magnetic) which supplies information to the electronic controller and based on this position, the stator winding to be energized is determined.  This electronic drive consists of transistors (2 for each phase) which are operated via a microprocessor.

The magnetic field generated by the permanent magnets interacts with the field induced by the current in the stator windings, creating a mechanical torque. The electronic switching circuit or the drive switches the supply current to the stator so as to maintain a constant angle of 0 to 90 degrees between the interacting fields.  Hall Sensors are mostly mounted on the stator or on the rotor. When the rotor passes through the hall sensor, based on the North or South Pole, it generates a high or low signal. Based on the combination of these signals, the winding to be energized is defined. In order to keep the motor running, the magnetic field produced by the windings should shift position, as the rotor moves to catch up with the stator field.

In a 4 pole, 2 phase brushless dc motor, a single hall sensor is used, which is embedded on the stator. As the rotor rotates, the hall sensor senses the position and develops a high or low signal, depending on the pole of the magnet (North or South). The hall sensor is connected via a resistor to the transistors. When a high voltage signal occurs at the output of the sensor, the transistor connected to coil A starts conducting, providing the path for the current to flow and thus energizing coil A. The capacitor starts charging to the full supply voltage. When the hall sensor detects a change in polarity of the rotor, it develops a low voltage signal at its output and since the transistor 1 doesn’t get any supply, it is in cutoff condition. The voltage developed around the capacitor is Vcc, which is the supply voltage to the 2^nd transistor, and coil B is now energized, as current passes through it.

BLDC motors have fixed permanent magnets, which rotate and a fixed armature, eliminating the problems of connecting current to the moving armature. And possibly more poles on the rotor than the stator or reluctance motors. The latter may be without permanent magnets, just poles that are induced on the rotor then pulled into an arrangement by timed stator windings. An electronic controller replaces the brush/commutator assembly of the brushed DC motor, which continually switches the phase to the windings to keep the motor turning. The controller performs comparative timed power distribution by using a solid-state circuit instead of the brush/commutator system.

 Advantages of Brushless DC Motors

• Better speed versus torque characteristics
• High dynamic response
• High efficiency
• Long operating life due to a lack of electrical and friction losses
• Noiseless operation
• Higher speed ranges

Applications:

The cost of the Brushless DC Motor has declined since its presentation, because of progressions in materials and design. This decrease in cost, coupled with the numerous focal points it has over the Brush DC Motor, makes the Brushless DC Motor a popular component in numerous distinctive applications. Applications that use the BLDC Motor include, yet are not constrained to:

• Consumer electronics
• Transport
• Heating and ventilation
• Industrial engineering
• Model engineering

Principle of Working

The principles for the working of BLDC motors are the same as for a brushed DC motor, i.e., the internal shaft position feedback. In the case of a brushed DC motor, feedback is implemented using a mechanical commutator and brushes. Within BLDC motor, it is achieved using multiple feedback sensors. In BLDC motors we mostly use a Hall-effect sensor, whenever rotor magnetic poles pass near the hall sensor, they generate a HIGH or LOW-level signal, which can be used to determine the position of the shaft. If the direction of the magnetic field is reversed, the voltage developed will reverse too.

Controlling a BLDC Motor

The Control unit is implemented by microelectronic has several high-tech choices. This may be implemented using a micro-controller, a dedicated micro-controller, a hard-wired microelectronic unit, a PLC, or similar another unit.

The Analog controller is still using, but the can not process feedback messages and control accordingly. With this type of control circuits, it is possible to implement high-performance control algorithms, such as vector control, field-oriented control, high-speed control all of which are related to electromagnetic state of the motor. Furthermore outer loop control for various dynamics requirements such as sliding motor controls, adaptive control, predictive control…etc are also implemented conventionally.

Besides all these, we find high-performance PIC (Power Integrated Circuit), ASIC (Application Specific Integrated Circuits) …etc. that can greatly simplify the construction of the control and the power electronic unit both. For example, today we have complete PWM (Pulse Width Modulation) regulator in a single IC that can replace the entire control unit in some systems. Compound driver IC can provide the complete solution of driving all six power switches in a three-phase converter. There are numerous similar integrated circuits with more and more adding day by day. At the end of the day, system assembly will possibly involve only a piece of control software with all hardware coming to the right shape and form.

PWM (Pulse Width Modulation) wave can be used to control the speed of the motor. Here the average voltage is given or the average current flowing through the motor will change depending on the ON and OFF time of the pulses controlling the speed of the motor i.e. The duty cycle of the wave controls its speed. On changing the duty cycle (ON time), we can change the speed. By interchanging output ports, it will effectively change the direction of the motor.

Speed Control

Speed control of the BLDC motor is essential for making the motor work at the desired rate. The speed of a brushless dc motor can be controlled by controlling the input dc voltage. The higher the voltage, the more is the speed. When motor works in normal mode or runs below rated speed, the input voltage of armature is changed through the PWM model. When a motor is operated above rated speed, the flux is weakened by means of advancing the exiting current.

The speed control can be closed-loop or open-loop speed control.

Open Loop Speed Control – It involves simply controlling the dc voltage applied to motor terminals by chopping the dc voltage. However, this results in some form of current limiting.

Closed Loop Speed control – It involves controlling the input supply voltage through the speed feedback from the motor. Thus the supply voltage is controlled depending on the error signal.

The closed-loop speed control consists of three basic components.

1. A PWM circuit to generate the required PWM pulses. It can be either a microcontroller or a timer IC.
2. A sensing device to sense the actual motor speed. It can be a hall effect sensor, an infrared sensor, or an optical encoder.
3. A motor drive to control the motor operation.

This technique of changing the supply voltage based on the error signal can be either through the pid controlling technique or using fuzzy logic.

Application to Speed Control of Brushless DC Motor

The motor operation is controlled using an optocoupler and MOSFET arrangement, where input DC power is controlled through the PWM technique from the microcontroller. As the motor rotates, the infrared led present at its shaft gets illuminated with white light due to the presence of a white spot on its shaft and reflects the infrared light. The photodiode receives this infrared light and undergoes a change in its resistance, thus causing a change in supply voltage to the connected transistor and a pulse is given to the microcontroller to generate the number of rotations per minute. This speed is displayed on the LCD.

The required speed is entered in the keypad interfaced to the Microcontroller. The difference between the sensed speed and the desired speed is the error signal and the microcontroller generates the PWM signal as per the error signal, based on the fuzzy logic to give the dc power input to the motor.

Thus using closed-loop control, the speed of the brushless dc motor can be controlled and it can be made to rotate at any desired speed.