D.C.
Motors
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D.C. motors are classified according
to the method of excitation. They
may be of the shunt, series or compound
types.
Series
Motor
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The
speed of a series motor is given by
Shunt
motor
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where Ra is the armature resistance.
Since is practically constant at all loads,
speed is there almost constant.
Types
of Armature Winding
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The two main types of winding are:
1. Lap Winding. It is also known as
parallel winding or multiple winding.
In this type of winding, the numbers
of parallel paths (A) are equal to
the number of poles (P).
2. Wave Winding. In this case the
armature conductors are divided into
two parallel irrespective of the number
of poles.
Slip
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The rotor of induction motor rotates
at somewhat lesser speed than the
synchronous speed and actual speed
is known as slip.
where Nr is rotor or actual while
N is synchronous speed.
A-C
through Resistance and Inductance
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In
the resistance part of the circuit
the current is in phase with the voltage,
while in the inductive part it is
90° out of phase. Hence, to determine
the current, the effect of resistance
and reactance has to be combined which
is named as impedance:
where is
the phase angle between the voltage
and current and cos is called the power factor.
Circuit Containing Resistance, Inductance
and Capacitance
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Transformer
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Transformer
is a device for transferring energy
from one alternating current circuit
to another without any change in frequency.
It changes voltage from high to low
and low to high with a corresponding
increase or decrease in current. If
the voltage is increased, the transfer
is said to be stepped up. If it is
decreased, then it is referred as
step down.
Three
Phase Transformer
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Whenever the supply is three phase
and it is desired to transform current
at another voltage, then either a
single three phase transformer or
three separate single phase transformers
can be used. However, in practice
a single three phase transformer is
used. The three phase winding of a
transformer can be connected either
in star or in delta.
Current
and voltage in star
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The e.m.f. between any line and the
neutral gives the phase voltage while
the e.m.f. between two outer terminals
is known as line voltage.
Current in each line is the same as
phase current
i.e. Line current IL=Phase
current IP
Line
Voltage and Current in Delta
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