DC machines: separately excited, series and shunt, motoring and generating mode of operation and their characteristics
DC machines are
electrical devices that convert electrical energy into mechanical energy or
vice versa. They operate based on the principles of electromagnetic induction
and Faraday's law. There are two types of DC machines: DC motors and DC
generators. In this answer, we will discuss the separately excited, series, and
shunt DC machines, as well as their motoring and generating modes of operation
and their characteristics.
Separately excited
DC machines: In a separately excited DC machine, the field winding is connected
to a separate DC power supply. This means that the voltage applied to the field
winding can be varied independently of the voltage applied to the armature
winding. In motoring mode, the speed of a separately excited DC motor can be
controlled by varying the armature voltage or the field voltage. In generating
mode, the output voltage of a separately excited DC generator can be controlled
by varying the field current. The characteristics of a separately excited DC
machine are as follows:
A wide range of
speed control in motoring mode
A smooth and stable
output voltage in generating mode
Good efficiency at
low to medium loads
Series DC machines:
In a series DC machine, the armature winding and the field winding are
connected in series. This means that the current flowing through the armature
winding also flows through the field winding. In motoring mode, the speed of a
series DC motor can be controlled by varying the applied voltage. In generating
mode, the output voltage of a series DC generator is proportional to the speed
of the machine. The characteristics of a series DC machine are as follows:
High starting
torque in motoring mode
High output voltage
in generating mode
Poor efficiency at
light loads
Shunt DC machines:
In a shunt DC machine, the armature winding and the field winding are connected
in parallel. This means that the current flowing through the armature winding
and the field winding is different. In motoring mode, the speed of a shunt DC
motor can be controlled by varying the field voltage. In generating mode, the
output voltage of a shunt DC generator is relatively constant. The
characteristics of a shunt DC machine are as follows:
Good efficiency
over a wide range of loads in both motoring and generating mode
A moderate starting
torque in motoring mode
A relatively
constant output voltage in generating mode
In summary, the
separately excited, series, and shunt DC machines have different modes of
operation and characteristics. The selection of a particular DC machine depends
on the specific application and the required performance characteristics.
Compound motor
A compound motor is
a type of DC motor that combines the features of both the series and shunt
motors. In a compound motor, the field windings are connected in both series
and parallel with the armature windings. There are two types of compound motors:
cumulative compound motor and differential compound motor.
Cumulative compound
motor: In a cumulative compound motor, the field windings are connected in
series with the armature winding, and the resulting magnetic field is the sum
of the series and shunt field. This type of motor has a high starting torque
and good speed regulation, making it suitable for applications that require
high starting torque, such as hoists, cranes, and elevators.
Differential
compound motor: In a differential compound motor, the field windings are
connected in opposition to the armature winding, and the resulting magnetic
field is the difference between the series and shunt field. This type of motor
has a low starting torque and poor speed regulation, making it unsuitable for
applications that require high starting torque.
Compound motors are
more complex than the other types of DC motors, but they offer a combination of
the benefits of both series and shunt motors. The selection of a particular
type of compound motor depends on the specific application and the required
performance characteristics.
Speed control of dc
motors
The speed of a DC
motor is given by the formula:
n = (60 * V) / (2 *
pi * R * K)
Where:
n: speed of the
motor in revolutions per minute (RPM)
V: applied voltage
to the armature in volts
R: armature
resistance in ohms
K: motor constant,
which is proportional to the magnetic field strength and inversely proportional
to the armature current
To control the
speed of a DC motor, there are different methods that can be used:
Armature voltage
control: By varying the voltage applied to the armature of a DC motor, the
speed of the motor can be controlled. The speed of the motor is directly
proportional to the applied voltage, so reducing the voltage will reduce the
speed, and increasing the voltage will increase the speed.
Field flux control:
By varying the magnetic field strength in a DC motor, the speed of the motor
can also be controlled. The speed of the motor is inversely proportional to the
magnetic field strength, so reducing the field flux will increase the speed,
and increasing the field flux will reduce the speed.
Armature resistance
control: By varying the armature resistance of a DC motor, the speed of the
motor can also be controlled. The speed of the motor is inversely proportional
to the armature resistance, so increasing the resistance will reduce the speed,
and reducing the resistance will increase the speed.
In summary, the
speed of a DC motor can be controlled by varying the voltage applied to the
armature, the magnetic field strength, or the armature resistance, depending on
the specific application and the required performance characteristics.
Braking of DC
machines
DC machines can be
braked in several ways, depending on the type of machine and the application.
The main methods of braking DC machines are:
Dynamic braking:
Dynamic braking is a method of braking DC motors in which the motor is
disconnected from the power supply and is connected to a resistive load. The back
EMF generated by the motor drives current through the resistive load, which
slows down the motor. The resistive load can be a bank of resistors or a grid
of high-power semiconductor devices.
Regenerative
braking: Regenerative braking is a method of braking DC motors in which the
motor is connected to a DC power supply that operates in reverse. The back EMF
generated by the motor is then used to charge the power supply, which slows
down the motor. Regenerative braking is commonly used in electric vehicles and
hybrid vehicles to recover energy that would otherwise be lost as heat.
Plugging: Plugging
is a method of braking DC motors in which the polarity of the supply voltage is
reversed. This causes the motor to run in the opposite direction, which creates
a braking torque. Plugging is a harsh method of braking and is not suitable for
all types of DC motors.
Mechanical braking:
Mechanical braking is a method of braking DC machines in which a brake is
applied to the shaft of the motor. This method is typically used in
applications where precise control of the motor speed is not required, such as
in cranes, hoists, and elevators.
In summary, the
braking of DC machines can be achieved through dynamic braking, regenerative
braking, plugging, or mechanical braking, depending on the specific application
and the required performance characteristics. Each method has its own
advantages and disadvantages, and the selection of a particular method depends
on the specific application and the required performance characteristics.
Top 10 interview questions on DC machines
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