Week-7 Challenge: DC Motor Control

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

H-Bridge

Figure 1: All the switches are OFF, causing the motor to not rotate.

In-circuit, we see that all switches are open states. No the current flowing in the
circuit, cause the DC motor can’t work.
Forwarding Mode
Then, look at Figure 2. When having the S1-switch (closed) and S3 (closed).

Figure 2: the switch-S1 and S3 are closed cause the motor rotates.
It causes the motor to get the current. As we notice that the current flowing into the
positive terminal of the motor.
Making the DC motor rotated in features of forwarding form. Or Rotate clockwise.
Reversed Mode
And, If both S4 and S2 are closed together. The Motor also get current that flow
through them. But it will don’t be the same first form. As Figure 3

Figure 3: Both switch-S2 and S4 are closed, causes the DC motor to rotate
back in the counter-clockwise direction.
Because that current will flow through the negative of motor cause current reversed
or Rotated back counter-clockwise direction.
Start to apply transistors
We will try to use all the transistors as the switch. You see in Figure 4.

Figure 4 Using the transistor as switches.
When a base of transistors gets the current electricity. It causes the transistor
running and the DC motor will rotate, too.

Bridge transistor Motor driver
As Figure 5, we use the four transistors to connected to the H-bridge circuit.

Figure 5: we use the four transistors as a switch controller.
And add a diode to protect the electricity that may flow backwards from the motor.
Making damage to the transistor.
Forward Rotate using transistors
In Figure 6 circuit, if we apply power to A-point. We will make Q1 and Q3-transistor
works. Because they get IB-current into a base.

Figure 6: apply power into A-point, Q1 and Q3 work. The motor rotates forward
directions.
So, the motor will rotate in the forward direction. Because the electrical current
flowing from Q1 into the positive of the motor. and flow through Q3 to ground
successfully.

Reverse Rotate control using transistor
Then later as in Figure 7, we change the power supply-V1 point into B-point. The Q4
and Q2 also work by they get current from the base.

Figure 7: The motor gets the current through Q4 and Q2. It rotates in the
reverse direction
It makes them have the current flow through Q4 go to the negative of motor and
through Q2-transistors to ground. Which flowing of current in this form causes the
motor to rotate in the reverse direction.

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).
Model Description
• 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.
• A Switch block at bottom of the circuit Splits the Power Supply into 4 parts namely
they are [[p) (p2) [p3) [p4]]. These inputs are in the Goto form block.
• That block is used to send a signal to "From Block" that have the specified tag. If
the tag visibility is "scoped, then a Goto tag visibility block must be used to define the
visibility of that particular tag. The block icon displays the selected tag name (local
tag are enclosed in bracket " and Scoped tag names are enclosed in braces" 0)
•Therefore these 4 types of Input are connected to transistor Ql Q2, Q3, and Q4
through "From Block"; An "From block" Receives. the Signal from the "Goto Block"
with the specified tag The block icon displays the selected tag name (local tags are
enclosed in [] brackets and Scoped tag names are enclosed in brace{} brackets).
SWITCHES
Q1 and Q2=Forward Motoring
Q2 and Q3=Reverse Motoring.
• This formation is called an H-Bridge. An H-bridge is an arrangement of transistor
that allows a circuit full control over a standard Electric DC Motor
•The DC Motor is connected to the battery and the Mechanical Input of the DC Motor
is Initiated as torque.
• Load Torque Proportional to Square of Speed
T=k* ω^2
Where,
T= Load torque
ω = Speed
• Torque for Mechanical input of DC motor also switched by Goto and From
• The Output of the DC motor is connected with the "BUS sector whereas the Speed
is plotted in the Scope. A conversion of Speed from rad/sec to rpm is held between
the BUS sectors to scope using converter block.

OUTPUT RESULTS:

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

model Description
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. 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 to obtain the electromagnetic torque needed to reach the desired
speed. The speed reference change rate follows acceleration and deceleration
ramps 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. To limit the amplitude of the current oscillations, a smoothing inductance is
placed in series with the armature circuit.
Working of Four-Quadrant Chopper DC Drive
Circuit Diagram
The Four-Quadrant chopper circuit diagram is shown in Figure 1. Here the motor
load is assumed, R. and La armature resistance and inductance of the motor
respectively. Eb is the back emf of the motor.

Quadrant I operation when Switch S1 turned on
The Four-Quadrant chopper equivalent circuit diagram for Quadrant I is shown in
Figure 3. Here switch S1, operated, Switches S1, and S4 conduct, output voltage Vo
and the output current to both are positives, power flows from source to load and
inductor stores energy, the motor rotates in the forward direction hence called
forward motoring

The Four-Quadrant chopper equivalent circuit diagram for Quadrant is shown in
Figure 4. Switch S1, turned off but switch S4, and diode D2, conducts, the output
current is i 0 positive and the output voltage Vo becomes zero, inductor release
energy and free-wheeling and action using diode D2 takes place, the motor rotates
in the forward direction hence called forward motoring

The Four-Quadrant chopper equivalent circuit diagram for Quadrant II is shown in
Figure 5 Let us assume that the motor is running in the forward direction. Here
switch S2, operated Switch S2, and diode D4, conducts, the output voltage Vo is
zero and Eb, is responsible for the negative output current (machine behave as gen
generator and inductor stores energy)

The Four-Quadrant chopper equivalent circuit diagram for Quadrant II is shown in
Figure 6. Switch S2, turned off, diode D1, and diode D4 conducts, output voltage Vo,
becomes positive and the output current I o is negative, inductor release energy using

diodes D1 and D4, power flows from load to source and hence called as reverse
braking

The Four-Quadrant chopper equivalent circuit diagram for Quadrant III is shown in
Figure 7. The polarity of back emf Eb must be reversed. Here switch S3, operated,
Switches S3, and S2, conducts, the output voltage Vo and the output current i o both
are negatives. Power flows from source to load and the inductor stores the motor
rotates in the reverse direction hence called reverse motoring.

Quadrant III operation when Switch S3, turned off

The Four-Quadrant chopper equivalent circuit diagram for Quadrant III is shown in
Figure 8. The polarity of back emf Eb must be reversed Switch S3, turned off but
switch S2, and diode D4 conducts, output current lo is negative and the output
voltage Vo becomes zero, inductor release energy and free-wheeling action using
diode D4, takes place, the motor rotates in the reverse direction hence called
Reverse motoring

The Four-Quadrant chopper equivalent circuit diagram for Quadrant IV is shown in
Figure 9. The polarity of back emf Eb must be reversed. Let us assume that the
motor is running in the reverse direction. Here switch S4 operated, Switches S4, and
diode D2 conducts, the output voltage Vo is zero and Eb is responsible for the
positive output current io machine behave as generator and inductor stores energy

The Four-Quadrant chopper equivalent circuit diagram for Quadrant is shown in
Figure 10. The polarity of hack emf Eb must be reversed Switch S4 turned off, diode
D2, and diode D3 conducts output voltage Vo becomes negative and output current
io positive, inductor release energy using diodes D2, and D3, power flows from load
to source and hence called a reverse braking

1) The 4 quadrant chopper works in all four modes like
• forward motoring
• forward breaking
• reverse motoring
• reverse breaking
but the BJT h bndge model
•forward motoring
• reverse motoring
2)To change the direction of rotation of motor from forward to reverse and vice
versa, the current signals get transited from one to another switch while in h bridge
model, the direction of rotation of the motor is changed by changing the polarity of
the current.
3)The 4 quadrant chopper have a high switching frequency while the h bridge model
has a low switching frequency

4)The armature current in the 4 quadrant chopper has a little spike at the transition
from one to another switch because of high switching frequency while the h bridge
model has a high spike of current.
5)The 4 quadrant chopper is generally used in an industrial application while the h
bridge model is used in robotics.

2. Develop a 2-quadrant chopper using Simulink & explain the working of
the same with the relevant results. (Refer to an article - Multiquadrant
operation of the motor )
A 2-quadrant Chopper Explanation
A 2-quadrant Chopper is a category of the chopper that can operate in the first as
well as a 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 the Two Quadrant Class-A chopper. In this article, we will discuss A 2-quadrant
chopper, its working principle with the help of a circuit diagram and its application.
Operation / Working of A 2-quadrant Chopper:
As we know that, a Class-A and Class-B chopper operates in the first and second
quadrants respectively. Therefore, if we connect both these types of the chopper in
parallel then it is possible to have chopper operation in the first as well as a second
quadrant.
In fact, A 2-quadrant chopper is obtained by the parallel connection of Class-A and
Class-B chopper. The figure below shows the circuit diagram of this type of chopper.

Carefully observe the circuit diagram. You will 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 the first quadrant operation we should switch ON chopper CH1
and forgetting the 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 the circuit diagram. Thus, the average output voltage
V o and current I o are positive and hence the operation of the chopper is in the first
quadrant. In fact, this is the Class-B mode of operation.
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 the 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 the OFF time of chopper CH2. Thus, the

load current is always negative i.e. operation of the chopper is within the second
quadrant. In fact, this is the Class-B mode of operation.
From the above two cases, we can conclude the following points:
 The output voltage V o is zero when chopper CH2 is ON or free-wheeling diode
FD conducts.
 The output voltage V o is equal to source voltage V s when chopper CH1 is ON or
diode D2 conducts.
 The load current flows in the direction shown in the circuit diagram i.e.i o is
positive when CH1 is ON or FD conducts.
 The load current flows opposite to the direction shown in the circuit diagram
i.e.i o is negative when CH2 is ON or D2 conducts.
The average load voltage is always positive but the average load current may be
positive or negative. Therefore, power flow may be from the source to load (first
quadrant operation) or load to the source (second quadrant operation). The
operating region of A 2-quadrant chopper is shown below by hatched area.

Caution:
Choppers CH1 and CH2 should not be ON simultaneously as this would lead to a
direct short circuit on the supply lines.
Developing a 2-quadrant chopper using the Simulink model:

let us take two MOSFET blocks and diodes blocks shown in the circuit diagram
the dc voltage source is connected as shown. let the voltage dc voltage source be 48
volts.

let us consider a series RLC branch block and select inductor L. thus block works as
a load to consider LE load le motor load, and de voltage source of 24 volts is also
connected in series with an 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 the use of not gate, when one
MOSFET is ON the other MOSFET will get off and when another MOSFET is ON the
first MOSFET will get off
The amplitude of pulse generator is kept 12 so that we can clearly see the pulses at
the scope and let period be 0.02 seconds and let pulse width be 50%.

SCOPE RESULT:

the voltage waveform is not going less than zero the minimum value of voltage is
zero and the maximum value is always the positive direction.
however, in the current waveform, the current increases and decreases the current
may be positive as well as negative and the pulse amplitude is 12 thus from the
pulse waveform we can see that the pulse amplitude is 12.

3. Explain in a brief the operation of the BLDC motor.
What is a Brushless DC motor (BLDC)?
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 an
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 mechanical commutation system. BLDC motors are
also referred to as trapezoidal permanent magnet motors.

Unlike conventional brushed type DC motor, wherein the brushes make the
mechanical contact with a commutator on the rotor to form an electric path between
a DC electric source and rotor armature windings, BLDC motor employs electrical
commutation with a 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 the 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 per cent, whereas
brushed type DC motors are 75 to 80 per cent 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 Brushless DC motor
In this motor, the permanent magnets are attached to the rotor. The current-carrying
conductors are located on the stator. They use electrical commutation to convert
electrical energy into mechanical energy.
The main design difference between brushed and brushless motors is the
replacement of a 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.

The Brushless motor does not have any current-carrying commutator 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
“Outrunner” style or “Inrunner” style.
 Outrunner – The field magnet is a drum rotor that 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 outrunner style but is capable
of spinning at very high rpm.

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 rotors
are most commonly used.

The construction of this motor has many similarities to the three-phase induction
motor as well as the conventional DC motor. This motor has stator and rotor parts as
like all other motors.

The stator of a BLDC motor is made up of stacked steel laminations to carry the
windings. These windings are placed in slots that 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 a three-phase star connected stator.Each winding is constructed with numerous interconnected coils, where one or more
coils are placed in each slot. To form an even number of poles, each of these
windings is distributed over the stator periphery.

The stator must be chosen with the correct rating of the voltage depending on the
power supply capability. For robotics, automotive and small actuating applications,
48 V or less voltage BLDC motors are preferred. For industrial applications and
automation systems,100 V or higher rating motors are used.
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. 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 the 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 a 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 the BLDC motor is controlled electronically,
the stator windings should be energized in sequence to rotate the motor. Before
energizing a particular stator winding, acknowledgement of the rotor position is
necessary. So the Hall Effect sensor embedded in the 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 responses.
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 is
placed in a magnetic field it experiences a force. As a consequence of the reaction
force, the magnet will experience an equal and opposite force. In the case of, 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 an
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 a
trapezoidal shape.
Due to the force of interaction between the electromagnet stator and the permanent
magnet rotor.
Consider the figure below in which the 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 the motor to rotate.
Observe that the 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). In this way motor moves in a clockwise
direction.

Here, one might get a question that how we know which stator coil should be
energized and when to do. This is because; the motor continuous rotation depends
on the switching sequence around the coils. As discussed above that Hall sensors
give shaft position feedback to the electronic controller unit.

Based on this signal from the 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 transistors or other solid-state switches to rotate the motor
continuously. The figure below shows the simple BLDC motor drive circuit which
consists of a MOSFET bridge (also called an 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.

In addition to the switching for a rated speed of the motor, additional electronic
circuitry changes the motor speed based on the 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 Brushless DC 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
Limitations of Brushless DC motor
 These motors are costly
 The electronic controller required to control this motor is expensive
 Requires complex drive circuitry
 Need of additional sensors
Applications of Brushless DC 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 equipment 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