Week-7 Challenge: DC Motor Control

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. A. An H-bridge is a simple circuit that lets us to control a DC motor to go backward or forward.
In general an H-bridge is a rather simple circuit, containing four switching element, with the load at the center, in an H-like
configuration:

.The switching elements (Q1..Q4) are usually bi-polar or FET transistors, in some high-voltage applications IGBTs. Integrated solutions
also exist but whether the switching elements are integrated with their control circuits or not is not relevant for the most part for this
discussion. The diodes (D1..D4) are called catch diodes and are usually of a Schottky type.
.The top-end of the bridge is connected to a power supply (battery for example) and the bottom-end is ground.In general all four switching elements can be turned on and off independently, though there are some obvious restricons

Static Operation; The basic operating mode of an H-bridge is fairly simple: if Q1 and Q4 are turned on, the left lead of the motor will be connected to the
power supply, while the right lead is connected to ground. Current starts flowing through the motor which energizes the motor in
the forward direction and the motor shaft starts spinning

If Q2 and Q3 are turned on, the reverse will happen, the motor gets energized in the reverse direction, and the shaft will start spinning
backwards.

In a bridge, you should never ever close both Q1 and Q2 (or Q3 and Q4) at the same time. If you did that, you just have created a really
low-resistance path between power and GND, effectively short-circuiting your power supply. This condition is called ‘shoot-through’
and is an almost guaranteed way to quickly destroy your bridge, or something else in your circuit.

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

Speed control of 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).
MODEL DISCRIPTION:
1) dc motor used is model is preset model(5 HP 24V 1750 rpm) .It simulates a fan type load .
The armature mean voltage can be varied from 0 to 240 V when the duty cycle
is varied from 0 to 100%.
2)The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models)

3 )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.

thus from above graph we can see that, for 0 to 0.5 sec , the current flows through diode D3 and from 0.5 sec to 1 sec , the current flow
through Q3.
The motor starts in the 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.

the waveform for speed, armature current and load torque is shown in above graphs.
from armature current graph , we can see that ,at starting there is sudden spike increase in current at 0 sec also and at 0.5 sec also.
We can see that , as the voltage gets changed in the circuit , the speed of motor also changes.
In this transition of positive to negative direction of rotation we observe a high surge in current, this is due to the transient switching
occurring in the circuit.

from armature current graph , we can see that ,at starting there is sudden spike increase in current at 0 sec also and at 0.5 sec also
means at transition from positive to negative direction of rotation we observe a high surge in current, this is due to the transient
switching occurring in the circuit.
As there is already current flowing in the circuit & on changing of switches after 0.5 sec, the current in the circuit adds to the changed
direction of current and we observe a surge in current.
the surge in the current can be reduced by reducing the duty cycle.
in above results , the duty cycle selected as 75%.

by changing the duty cycle from 75% to 50% , the current surge can be reduced tremendously.

thus we can find that , the armature curent spike is reduced by changing the duty cycle from 75% to 50%.

. 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 choppe.

.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 DCDC converters, a lower armature current ripple .
devices are required, which increases thecomplexity of the drive system.

The machine is separately excited with a constant DC field voltage source. There is thus no field voltage control. By default, the field
current is set to its steady-state value when a simulation is started.
The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a constant DC voltage
source. Armature current oscillations are reduced by a smoothing inductance connected in series with the armature circuit.

WORKING OF FOUR QUADRANT CHOPPER:

The circuit of a four quadrant chopper or class-E chopper basically consists of four semiconductor switches CH1 to CH4 and four
diodes D1 to D4. The four diodes are connected in anti-parallel. The circuit diagram of this type of chopper

In the above circuit diagram, the choppers are numbered CH1 to CH4. For first quadrant operation CH1 is made ON, for second
quadrant operation CH2 is made ON and so on. To better understand the working of four quadrant chopper, we will discuss its
operation separately for each quadrant.

First Quadrant Operation:
For first quadrant operation, CH4 is kept ON, CH3 is kept OFF and CH1 is operated. When both CH1 & CH4 are ON simultaneously, the
load gets directly connected to the source and hence the output voltage becomes equal to the source voltage. This essentially
means that v = v . It may be noted that the load current flows from source to load as shown by the direction of i .
When CH1 is switched OFF, the load current free wheels through CH4 and D2. During this period, the load voltage and current remains
positive.
Thus, both the output voltage v and load current i are positive and hence, the operation of chopper is in first quadrant. It may be
noted that, Class-E chopper operates as a step-down chopper in this case.

Second Quadrant Operation:
To obtain second quadrant operation, CH2 is operated while keeping the CH1, CH3 & CH4 OFF. When CH2 is ON, the DC source in the
load drives current through CH2, D4, E and L. Inductor L stores energy during the On period of CH2.
When CH2 is turned OFF, 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 second quadrant operation of chopper. Since, the current
is fed back to the source, this simply means that load is transferring power to the source. Kindly read step up chopper for detailed
analysis and better understanding.
For second quadrant operation, load must contain emf E as shown in the circuit diagram. In second quadrant, configuration operates
as a step-up chopper.
Third Quadrant operation;

To obtain third quadrant operation, both the load voltage and load current should be negative. The current and voltage are assumed
positive if their direction matches with what shown in the circuit diagram. If the direction is opposite to what shown in the circuit
diagram, it is considered negative. One important thing to notice is that the polarity of emf E in load must be reversed to have third
quadrant operation. Circuit diagram of Class-E chopper for third quadrant operation is shown below.

or third quadrant operation, CH1 is kept off, CH2 is kept ON and CH3 is operated. When CH3 is ON, load gets connected to 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, v is assumed negative. Let us now see what is the status of load current i . It may be seen that i is
flowing in the direction opposite to shown in the circuit diagram and hence negative.
Now, when CH3 is turned OFF, the negative load current free wheels through the CH2 and D4. In this manner, v and i both are
negative. Hence, the chopper operates in third quadrant.

Fourth Quadrant Operation:
To obtain fourth quadrant operation, CH4 is operated while keeping CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be
reversed in this case too like third quadrant operation.
When CH4 is turned ON, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is ON. When
CH4 is made OFF, current is fed back to the source through diodes D2, D3. Here load voltage is negative but the load current is always
positive. This leads to chopper operation in fourth quadrant. Here, power is fed back to the source from load and chopper acts as a
step-up chopper.
The operation of a four quadrant chopper or Class-E chopper is summarized in the figure below.

COMPARISON OF 4 QUADRANT CHOPPER AND BJT H BRIDGE:
1) the 4 quadrant chopper works in all four modes like
forward motoring
forward breaking
reverse motoring
reverse breaking
but the BJT h bridge model works on only two modes
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
other switch while in h bridge model, the direction of rotation of motor is changed by changing the polarity of current.
3)the 4 quadrant chopper have high switching frequency

while h bridge model have low switching frequency.

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 DIAGRAM FOR TWO QUADRANT CHOPPER:

two quadrant chopper is also called as type c chopper. Type C chopper is obtained by connecting type –A and type –B choppers in
parallel. We will always get a positive output voltage V as the freewheeling diode FD is present across the load. When the chopper is
on the freewheeling diode starts conducting and the output voltage v will be equal to V . The direction of the load current i will be
reversed. The current i 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.

let us take two mosfet blocks and diodes blocks and connect it as shown in circuit diagram.
the dc voltage source is connected as shown.
let the voltage of dc voltage source be 24 volts.

let us consider an series RLC branch block and select inductor L . thus block works as a load.
to consider LE load ie motor load , and dc voltage source of 12 volt is also connected in series with inductor.

the pulse generator block is used to create gate pulse.
the not gate is used in the model so that the two mosfet will not get closed at the same time otherwise short circuit will occur .due to
use of not gate , when one mosfet is ON the other mosfet will get off and when other mosfet is ON the first mosfet will get off .
The amplitude of pulse generator is kept 10 so that we can clearly see the pulses at scope and let period be 0.02 seconds and let
pulse weidth be 50%

the voltage waveform is not going less than zero . the minimum value of voltage is zero and maximum value is always in the positive
direction.
however in current waveform , the current increases and decreases. the current may be positive as well as negative.
and the pulse amplitude is 10 thus from pulse waveform we can see that the pulse amplitude is 10.

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

A. Brushless DC electric motors also known as electronically commutated motors .A brushless DC motor (known as BLDC) is a
permanent magnet synchronous electric motor which is driven by direct current (DC) electricity and it accomplishes electronically
controlled commutation system (commutation is the process of producing rotational torque in the motor by changing phase
currents through it at appropriate times) instead of a mechanically commutation system. BLDC motors are also referred as
trapezoidal permanent magnet motors.
Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with commutator on the rotor so as
to form an electric path between a DC electric source and rotor armature windings, BLDC motor employs electrical commutation with
permanent magnet rotor and a stator with a sequence of coils. In this motor, permanent magnet (or field poles) rotates and current
carrying conductors are fixed.

The armature coils are switched electronically by transistors or SCR at the correct rotor position in such a way that armature field is in
space quadrature with the rotor field poles. Hence the force acting on the rotor causes it to rotate. Hall sensors or rotary encoders are
most commonly used to sense the position of the rotor and are positioned around the stator. The rotor position feedback from the
sensor helps to determine when to switch the armature current.
This electronic commutation arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more
reliable and less noisy operation is achieved. Due to the absence of brushes BLDC motors are capable to run at high speeds. The
efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient. There are
wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large
power ranges.

this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature winding are located on the
stator. They use electrical commutation to convert electrical energy into mechanical energy.
The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric
switch circuit. A BLDC Motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor
revolve at the same frequency.

Brushless motor does not have any current carrying commutators .The field inside a brushless motor is switched through an amplifier
which is triggered by the commutating device like an optical encoder.

BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as
single-phase, two-phase, or three-phase motors. However, three-phase BLDC motors with permanent magnet rotor are most
commonly used.

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

The layout of a DC brushless motor can vary depending on whether it is in “Out runner” style or “Inrunner” style.
Outrunner – The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require
high torque and where high rpm isn’t a requirement.
In runner – The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the
out runner style, but is capable of spinning at very high rpm.

Stator;
Stator of a BLDC motor made up of stacked steel laminations to carry the windings. These windings are placed in slots which are
axially cut along the inner periphery of the stator. These windings can be arranged in either star or delta. However, most BLDC motors
have three phase star connected stator.
Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. In order to form an
even number of poles, each of these windings is distributed over the stator periphery.
The stator must be chosen with the correct rating of the voltage depending on the power supply capability. For robotics, automotive
and small actuating applications, 48 V or less voltage BLDC motors are preferred.

Motor;
BLDC motor incorporates a permanent magnet in the rotor. The number of poles in the rotor can vary from 2 to 8 pole pairs with
alternate south and north poles depending on the application requirement. In order to achieve maximum torque in the motor, the flux
density of the material should be high. A proper magnetic material for the rotor is needed to produce required magnetic field density.
ferrite magnets are inexpensive, however they have a low flux density for a given volume. Rare earth alloy magnets are commonly
used for new designs. Some of these alloys are Samarium Cobalt (SmCo), Neodymium (Nd), and Ferrite and Boron (NdFeB). The rotor
can be constructed with different core configurations such as the circular core with permanent magnet on the periphery, circular
core with rectangular magnets, etc.

Hall Sensors ;
Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the commutation of BLDC
motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor. Before energizing
a particular stator winding, acknowledgment of rotor position is necessary. So the Hall Effect sensor embedded in stator senses the
rotor position.
Most BLDC motors incorporate three Hall sensors which are embedded into the stator. Each sensor generates Low and High signals
whenever the rotor poles pass near to it. The exact commutation sequence to the stator winding can be determined based on the
combination of these three sensor’s response.

WORKING OF BLDC MOTOR:
The underlying principles for the working of a BLDC motor are the same as for a brushed DC motor; i.e., internal shaft position
feedback. In case of a brushed DC motor, feedback is implemented using a mechanical commutator and brushes. With a in BLDC
motor, it is achieved using multiple feedback sensors. The most commonly used sensors are hall sensors and optical encoders. Hall
sensors work on the hall-effect principle that when a current-carrying conductor is exposed to the magnetic field, charge carriers
experience a force based on the voltage developed across the two sides of the conductor.
If the direction of the magnetic field is reversed, the voltage developed will reverse as well. For Hall-effect sensors used in BLDC
motors, whenever rotor magnetic poles (N or S) pass near the hall sensor, they generate a HIGH or LOW level signal, which can be
used to determine the position of the shaft.

In a commutation system – one that is based on the position of the motor identified using feedback sensors – two of the three
electrical windings are energized at a time as shown in figure 4.
In figure 4 (A), the GREEN winding labeled “001” is energized as the NORTH pole and the BLUE winding labeled as “010” is energized as
the SOUTH pole. Because of this excitation, the SOUTH pole of the rotor aligns with the GREEN winding and the NORTH pole aligns with
the RED winding labeled “100”. In order to move the rotor, the “RED” and “BLUE” windings are energized in the direction.

This torque is at its maximum when the rotor starts to move, but it reduces as the two fields align to each other. Thus, to preserve the
torque or to build up the rotation, the magnetic field generated by stator should keep switching. To catch up with the field generated
by the stator, the rotor will keep rotating. Since the magnetic field of the stator and rotor both rotate at the same frequency, they
come under the category of synchronous motor.
This switching of the stator to build up the rotation is known as commutation. For 3-phase windings, there are 6 steps in the
commutation; i.e., 6 unique combinations in which motor windings will be energized.