Challenge

Week-7 Challenge: DC Motor Control

PPiyush Mehar4 Aug 2022

1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

B. Comment on the armature current shoot-up from the scope results.

C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

Answer: -

A:-

A DC motor is used to convert the direct current (DC) electrical power into mechanical power based on the forces produced by magnetic filed(s). The output of the motor is mechanical power in terms of rotation (speed) of the shaft. According to the applications, we need to change the speed of the motor. So, deliberative change of speed is known as speed control of the motor.The term speed control is different from the speed regulation. The speed regulation means that, to maintain a speed of shaft constant against the change in load.

EMF Equation of a DC Motor

the EMF equation of a DC motor is given below:

Eb = PΦNZ / 60A

Where;

P = Number of poles
Ф = Flux per pole
N = Speed of motor (RPM)
Z = Number of conductors
A = Number of parallel paths

Once the motor is designed, the Number of poles (P), Number of conductors (Z), and Number of parallel paths (A) cannot change. So, these are fixed quantities.

Eb ∝ ΦN

Eb = kΦN

Where k = Proportionality constant

For DC motor, the EMF is also defined as;

E_b = V – I_aR_a

Where;

V = Supply voltage
I_a= Armature current
R_a= Armature resistance

Now compare both equations;

kΦN = V – I_aR_a

k = N = V – I_aR_a / kΦ

From the above equation, the speed of the motor depends on supply voltage (V), Flux (Φ), and Armature resistance (R_a).

Therefore, the speed of a DC motor can be varied, changed and controlled by changing;

Terminal voltage “V” (Applied Voltage Control Method).
External resistance with armature resistance R_a(AKA Rheostatic Control Method).
Flux per pole Φ (AKA Flux Control Method).

Here, terminal voltage and armature resistance are associated with the armature circuit and flux per pole is associated with the field circuit.

So, the speed control methods of a DC motor are classified as;

Armature Control Method
Field Control Method

In example of Speed control dc motor using BJT H bridge circuit field of dc motor is fed with constant volatge of 300 V and the armature winding is fed with controlled volatge from 0 to 240 volt as per the duty cycle..

Componenet used in circuit of Speed control of dc motor using BJT H Bridge.

1.BJT : -

Bipolar junction transitor it is an electronic power switch which ON and OFF as per signal input in its gate terminal.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). IGBT's are connected in such manner that it form H alphabate pattern and the DC motor is connected in middle arm of H hence it called as BJT H bridge.Free wheeling diode is connected to each IGBT as bypass for disspation of energy whene IGBT is in OFF modeand protect from high voltage and current.

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.

DC Motor: -

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%.

IGBTs act as switch and can be controlled by pluse generator circuit as per the switching frequency of of the switches voltage can be varry across the DC motor.

PWM technique: -

A simplest method to control the rotation speed of a DC motor is to control its driving voltage. Higher the voltage, higher is the speed the motor tries to reach. In many applications simple voltage regulation would cause lot of power loss on control circuit, so a pulse width modulation method (PWM) is used in many DC motor controlling applications.

As per the width of pulse and ON OFF time(DUTY CYCLE), variable avergae voltage is applied on DC motor.

As per the given model dutly cycle is 75% means .75*240=180V is applied to DC motor armature winding.

for controlling switching of IGBT signal obtained in between 0 to 1 using PWM generator,and as per threshold value of aramture voltage.in given circuit threshold value is 0.5.

the PWM signal for different IGBT are shown in below graph.

hence when from above graph it is clear that from 0 to 0.5 second Q1 and Q4 is active and Q2 and Q3 is active from 0.5 to 1 second .

the results are below as per the duty cycle of 75%.

Speed +1000rpm -1000 rpm.
Armature current +10A -10 A.
Torque +6 -6 NM.

Conclusion: -

When Q1 and Q4 is active and flyback diode D2 and D3 is active,at that time motor run in forward direction(speed is positive ) and from 0.5 to 1 seconds IGBT Q2 and Q3 active and Diode D1 and D4 is active At that time motor run in reverse direction.(speed is neagtive).
Speed is directly proporational to the armature voltage.
load torque is directly proporational to square of speed.

B. Comment on the armature current shoot-up from the scope results.

At starting of motor back EMF is very very small hence armature resistance is also small hence aramutre current is on higher side.

E_b = V – I_aR_{a ;--------------------------------------}_{IaRa=V------------------------------------ (Eb =0 at starting of motor)}

Hence to reduce current spike at starting we have to reduce the voltage level to reduce the voltage we have to reduce the duty cycle.

below results are of duty cycle 40%.

Speed +300 -300 RPM
Armature current =6 ,-6 A
Torque =0.6, -0.6 NM

C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge 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
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.

Results Explanation: -

Waveform of IGBT 1&4 and IGBT 2&3 is exactly opposite.
Armature current follows refrence value of current.
Initial refrence value of speed is 500 RPM this value is achieved after 1.5 seconds.
At 2 seconds refrence value of speed become -1184 RPM and actual value become -1184RPM at time 6 seconds.
When refrence value of speed become -1184 RPM armature current start incresing in negative directions.

Comparision with H bridge model

Four-Quadrant Chopper can operates in all four modes (Forward Motoring,Forward Braking, Reverse Motoring and Reverse Braking), but H-bridge can operate only in two modes (Forward and Reverse).
Regenerative Braking is possible in four quadrant chopper, but in H-bridge Regenerative Braking is not possible.
In Four-Quadrant Chopper Two PI regulator (Speed Regulator and Current Regulator) is used to obtain electromagnetic torque needed to reach the desired speed and control the armature current and stabilize the system, but in H-bridge model PI regulator is not used.
Four-Quadrant Chopper model is more efficient as it operate in all four quadrant.

Q 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 )

Circuit dia of 2 quadrant chopper is

same circuit is implemented in matlab and simulink with help of following blocks

https://1drv.ms/u/s!AqbpDVcFOhawvi9VuxLWsYJb7tEk?e=WPcTHU

Mosfet
DC voltage source
Diode
Inductor
Scope
Voltage and current measurnments

In this type of Chopper, the average voltage will be always positive but the average load current might be positive or negative. The two Switches CH1 and CH2 should not be turned on simultaneously as the combined action may cause a short circuit in supply lines.
For regenerative braking and motoring these type of chopper configuration is used.
We will always get a positive output voltage V₀ as the freewheeling diode FD is present across the load. When the chopper CH1 is on the current flows from source to load. The inductor stores energy. When Ch1 turn OFF the freewheeling diode starts conducting and the output voltage v₀ will be equal to V_s. The direction of the load current i₀will be reversed. The current i₀ will be flowing towards the source. The load current will be negative when the CH2 is on. The current completes its path through diode D2.

From above graph it is clear that current flow in negative direction means from load to source i.e fwd braking mode.

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

Brushless DC motors (BLDC) have been a much focused area for numerous motor manufacturers as these motors are increasingly the preferred choice in many applications, especially in the field of motor control technology. BLDC motors are superior to brushed DC motors in many ways, such as ability to operate at high speeds, high efficiency, and better heat dissipation.
They are an indispensable part of modern drive technology, most commonly employed for actuating drives, machine tools, electric propulsion, robotics, computer peripherals and also for electrical power generation. With the development of sensorless technology besides digital control, these motors become so effective in terms of total system cost, size and reliability.
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 silicon controlled rectifiers 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.

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.

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.

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 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 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.

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.

Brushless DC Motor Drive

As described above that the electronic controller circuit energizes appropriate motor winding by turning transistor or other solid state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of MOSFET bridge (also called as 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.
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 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 BLDC Motor

BLDC motor has several advantages over conventional DC motors and 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 statorLess electromagnetic interference
Quite operation (or low noise) due to absence of brushes

Disadvantages of Brushless 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 motorsRequires complex drive circuitry
Need of additional sensors

Applications of Brushless DC Motors (BLDC)

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