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1) What are the types of braking in DC Motor? What are the main criteria
for regenera ve braking in a DC Motor?
Regenera ve Braking
Dynamic Braking
Plugging
Regenera ve Braking
It is a form of braking in which the kine c energy of the motor is returned to the power
supply system. This type of braking is possible when the driven load forces the motor to run
at a speed higher than its no-load speed with a constant excita on.
The motor back emf Eb is greater than the supply voltage V, which reverses the direc on of
the motor armature current. The motor begins to operate as an electric generator.
It is very interes ng to note that regenera ve braking cannot be used to stop a motor but to
control its speed above the no-load speed of the motor driving the descending loads.

Dynamic Braking
It is also known as Rheosta c braking. In this type of braking, the DC motor is disconnected
from the supply and a braking resistor Rb is immediately connected across the armature. The
motor will now work as a generator and produces the braking torque.
During electric braking when the motor works as a generator, the kine c energy stored in the
rota ng parts of the motor and a connected load is converted into electrical energy. It is
dissipated as heat in the braking resistance Rb and armature circuit resistance Ra.
Dynamic Braking is an inefficient method of braking as all the generated energy is dissipated
as heat in resistances.

Plugging
It is also known as reverse current braking. The armature terminals or supply polarity of a
separately excited DC motor or shunt DC motor when running are reversed. Therefore, the
supply voltage V and the induced voltage Eb i.e. back emf will act in the same direc on. The
effec ve voltage across the armature will be V + Eb which is almost twice the supply
voltage.Thus, the armature current is reversed and a high braking torque is produced.
Plugging is a highly inefficient method of braking because, in addi on to the power supplied
by the load, the power supplied by the source is wasted in resistances.It is used in elevators,
prin ng press etc.
These were the main three types of braking techniques preferred to stop a DC motor and
used widely in industrial applica ons.

2) Explain constant torque and constant power control of DC Motor

drives.
Constant torque and constant power control are two fundamental methods used in
controlling DC motor drives. These methods are crucial in various industrial
applications where precise control over motor speed and torque is required.

1. Constant Torque Control:

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 In constant torque control, the motor torque remains constant

regardless of the motor speed. This means that the motor will exert a
consistent level of torque irrespective of the load it is driving.
 Constant torque control is essential in applications where the load on
the motor varies, but a consistent torque output is required. For
example, in conveyor belts, elevators, and hoists where the load may
change but a constant force is needed to move the load smoothly.
 To achieve constant torque control, the motor controller adjusts the
armature voltage or current supplied to the motor based on the
desired torque and the feedback from sensors such as encoders or
tachometers. By varying the armature voltage or current, the controller
can maintain the desired torque output.

2. Constant Power Control:

 In constant power control, the motor output power remains constant

regardless of changes in speed or torque. This means that as the motor
speed increases, the torque decreases proportionally, and vice versa, to
maintain a constant power output.
 Constant power control is commonly used in applications where the
motor needs to operate over a wide range of speeds while maintaining
a constant power output. For example, in electric vehicles, drilling
machines, and milling machines.
 To achieve constant power control, the motor controller adjusts both
the armature voltage/current and the field flux of the motor. By varying
these parameters, the controller can regulate the motor speed and
torque to maintain a constant power output.

3) What are the main criteria for regenera ve braking?

The main criterion for regenera ve braking in a DC motor is that the rotor must rotate faster
than synchronous speed. When this happens, the motor acts as a generator, reversing the
direc on of the current flow and torque, and causing braking. Regenera ve braking is also
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limited by: Vehicle speed, Accelerator pedal and braking signals, Ba ery voltage, and Ba ery
state of charge (SOC).
The SOC of the ba ery should be between 10% and 90% to prevent overcharging or over-
discharging. If the SOC is too low, the ba ery's resistance will be high.
Regenera ve braking converts the kine c energy stored in the motor's rota ng parts and
connected load into electrical energy. This energy is dissipated as heat in the braking
resistance and armature circuit resistance.

4) Explain about chopper fed DC Drives

The variable voltage to the armature of a dc motor for speed control can be obtained from a
dc chopper which is a single stage dc to dc conversion device. The voltage varia on at the
load terminals can be obtained by using either current limit control or me ra o control. In
the former, as has already been discussed, the chopper is controlled such that the load
current has a varia on between two limits. When the current reaches the upper limit the
chopper is turned off to disconnect the motor from the supply. The load current freewheels
through freewheeling diode and decays. When it falls to the lower limit the chopper is
turned on, connec ng the motor to the supply. An average current is always maintained.
When the chopper is controlled by TRC the ra o of TON/TOFF of the chopper is changed. In
this case the opera on is at fixed frequency if (TON+TOFF) is kept constant. TON only is
varied to obtain voltage control. The opera on will be at variable frequency with TON kept
constant and (TON+TOFF) varied. But owing to several advantages of simplicity, a fixed
frequency TRC is normally used. Chopper circuits are used to control both separately excited
and series motors.
1. Chopper Circuit: The heart of a chopper-fed DC drive is the chopper circuit. It typically
consists of semiconductor switching devices such as power transistors or thyristors, along
with diodes, capacitors, and resistors. These components are arranged to switch the DC
voltage supplied to the motor on and off rapidly.
2. Pulse Width Modula on (PWM): Chopper circuits u lize pulse width modula on (PWM)
techniques to control the effec ve voltage applied to the motor. By adjus ng the width of
the pulses (on- me vs. off- me), the average voltage seen by the motor can be varied,
effec vely controlling its speed.
3. Speed Control: The speed of the DC motor is directly propor onal to the voltage applied to
its armature. By varying the duty cycle of the PWM signal generated by the chopper circuit,
the effec ve voltage applied to the motor can be controlled, thus regula ng its speed.
4. Direc on Control: Chopper-fed DC drives can also control the direc on of rota on of the
motor. This is achieved by reversing the polarity of the voltage applied to the motor
armature. By controlling the switching sequence of the chopper circuit, the direc on of
current flow through the motor can be changed, resul ng in a change in direc on of
rota on.
5. Regenera ve Braking: One of the advantages of chopper-fed DC drives is their ability to
implement regenera ve braking. During braking, instead of dissipa ng the kine c energy of
the motor as heat, the chopper circuit can reverse the polarity of the motor voltage,
effec vely turning it into a generator and returning energy to the power supply.
6. Efficiency and Control: Chopper-fed DC drives offer high efficiency and precise control over
motor speed and torque. They are commonly used in applica ons requiring variable speed
control, such as industrial drives, electric trac on systems, and renewable energy systems.
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7. Protec on and Safety: Chopper-fed DC drives typically include protec on features such as
overcurrent protec on, overvoltage protec on, and thermal protec on to safeguard the
motor and the drive system from damage due to abnormal opera ng condi ons.
8. Current Limiting: To protect the motor and the drive system from damage due to
excessive current, chopper-fed DC drives often incorporate current-limiting features.
These features monitor the motor current and limit it to a safe level by adjusting the
duty cycle of the PWM signal. If the motor current exceeds the set limit, the PWM
signal is adjusted to reduce the effective voltage applied to the motor, thereby
limiting the current.

5) Explain the discon nuous mode of opera on of single phase fully

controlled converter drives.
In a single-phase fully controlled converter drive, discon nuous conduc on mode occurs
when the load current becomes zero and all thyristors remain off. In this mode, the inductor
current is interrupted, and the load current ripple content increases.
Discon nuous conduc on mode is usually found in converters with single-quadrant switches,
but it can also occur in converters with two-quadrant switches. It's not suitable for DC motor
speed control.
This situa on will typically occur when the load current becomes zero in between the firings
of T1T2 and T3T4. Once the load current becomes zero all thyristors remain off. In this mode
the load current remains zero. Consequently the converter is said to be opera ng in the
discon nuous conduc on mode.

6) What is the purpose of dual converters? Explain the quadrant of

opera on of dual converter with neat diagram
A dual converter is a power electronics control system that uses two converters to get either
polarity DC from AC rec fica on. The two converters are connected back to back, with one
converter ac ng as a rec fier and the other as an inverter. The dual converter has two
func onal modes:
Non-circula ng current mode: Only one converter performs at a me.
Circula ng current mode: Both converters are on at the same me.
The dual converter operates in four quadrants:
First quadrant: Voltage and current are both posi ve.
Second quadrant: Voltage is posi ve and current is nega ve.
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Third quadrant: Voltage and current are both nega ve.

Fourth quadrant: Voltage is nega ve and current is posi ve.
The first converter operates in two quadrants, depending on the firing angle α. When α is
less than 90°, the converter acts as a rec fier, producing a posi ve average load voltage and
load current. When α is more than 90°, the converter acts as an inverter.
The circula ng current in a dual converter allows for smooth reversal of load current and
improved speed of response. It also helps maintain con nuous conduc on of both
converters, regardless of load.

7) Regenera ve braking is not possible in DC Series motor. Why?

Regenera ve braking is not possible in DC series motors because they lack a permanent
magnet, which is required to produce a magne c field when the motor is used as a
generator. When regenera ve braking occurs, the motor acts as a generator, conver ng the
motor's kine c energy into electrical energy that is returned to the supply. This process can
save energy.

8) Explain closed loop speed control of electric drive with diagram.

In closed loop system, the output of the system is feedback to the input. The closed loop
system controls the electrical drive, and the system is self-adjusted. Feedback loops in an
electrical drive may be provided to sa sfy the following requirements.
Enhancement of speed of torque
To improve steady-state accuracy.
Protec on
The main parts of the closed-loop system are the controller, converter, current limiter,
current sensor, etc. The converter converts the variable frequency into fixed frequency and
vice-versa. The current limiter limits the current to rise above the maximum set value.
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Closed-Loop Speed Control

CLOSED LOOP SPEED CONTROL
The block diagram of the closed loop speed control system is shown in the figure below. This
system used an inner control loop within an outer speed loop. The inner control loop
controls the motor current and motor torque below a safe limit.
Consider a reference speed ω*m which produces a posi ve error Δ ω*m. The speed error is
operated through a speed controller and applied to a current limiter which is overloaded
even for a small speed error. The current limiter set current for the inner current control
loop. Then, the drive accelerates, and when the speed of the drive is equal to the desired
speed, then the motor torque is equal to the load torque. This, decrease the reference speed
and produces a nega ve speed error. When the current limiter saturates, then the drive
becomes de-accelerate in a braking mode. When the current limiter becomes desaturated,
then the drive is transferred from braking to motoring.

9) What are the types of speed control method used in DC Motor?

1) FLUX CONTROL METHOD
It is already explained above that the speed of a dc motor is inversely propor onal to
the flux per pole. Thus by decreasing the flux, speed can be increased and vice versa.
To control the flux, a rheostat is added in series with the field winding, as shown in
the circuit diagram. Adding more resistance in series with the field winding will
increase the speed as it decreases the flux. In shunt motors, as field current is
rela vely very small, Ish2R loss is small. Therefore, this method is quite efficient.
Though speed can be increased above the rated value by reducing flux with this
method, it puts a limit to maximum speed as weakening of field flux beyond a limit
will adversely affect the commuta on.

2) ARMATURE CONTROL METHOD

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Speed of a dc motor is directly propor onal to the back emf Eb and Eb = V - IaRa.
That means, when supply voltage V and the armature resistance Ra are kept
constant, then the speed is directly propor onal to armature current Ia. Thus, if we
add resistance in series with the armature, Ia decreases and, hence, the speed also
decreases. Greater the resistance in series with the armature, greater the decrease
in speed.

3) VOLTAGE CONTROL METHOD

) Mul ple voltage control:

In this method, the shunt field is connected to a fixed exci ng voltage and armature
is supplied with different voltages. Voltage across armature is changed with the help
of suitable switchgear. The speed is approximately propor onal to the voltage across
the armature.

b) Ward-Leonard System:
ward leonard system speed control of dc motorThis system is used where very
sensi ve speed control of motor is required (e.g electric excavators, elevators etc.).
The arrangement of this system is as shown in the figure at right.
M2 is the motor to which speed control is required.
M1 may be any AC motor or DC motor with constant speed.
G is a generator directly coupled to M1.
In this method, the output from generator G is fed to the armature of the motor M2
whose speed is to be controlled. The output voltage of generator G can be varied
from zero to its maximum value by means of its field regulator and, hence, the
armature voltage of the motor M2 is varied very smoothly. Hence, very smooth
speed control of the dc motor can be obtained by this method.
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10) Advantages of Regenera ve braking

Fuel efficiency
Regenera ve braking improves fuel efficiency by genera ng electricity while braking, which
reduces the amount of work the engine needs to do.
Energy recovery
Regenera ve braking recovers energy from the vehicle's mo on and stores it in ba eries,
ultracapacitors, or flywheels. This improves the vehicle's range and lowers opera ng costs.
Brake pad life
Regenera ve braking reduces wear and tear on brake pads because the electric motor does
most of the braking.
Low heat genera on
Regenera ve braking generates li le heat, so there's no brake fade or overhea ng on long
downhill stretches.
Environmentally friendly
Electric motors don't produce exhaust emissions, which helps reduce air pollu on and
greenhouse gas emissions.