DC Motor
&
DC Generator
Electrical Machine
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Magnetic Field of Current
The magnetic field lines around a long wire which carries an
electric current form concentric circles around the wire. The
di ti off th
direction
the magnetic
ti fi
field
ld iis perpendicular
di l tto th
the wire
i and
d iis
in the direction the fingers of your right hand would curl if you
wrapped them around the wire with your thumb in the direction of
the current
current.
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DC M
Motor
t
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Magnetic
g
Interactions with Moving
g
Charge
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Magnetic Force on Moving Charge
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Magnetic Force on a Current
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Fleming’s Left Hand Rule
Fleming's left hand rule (for electric motors) shows the
direction of the thrust on a conductor carrying a current in a
magnetic field.
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Magnetic Force
The characteristics of this equation are as follows
1. The force is perpendicular to both the velocity v of the
charge q and the magnetic field B.
2. The magnitude of the force is F = qvB sinθ where θ is
th angle
the
l < 180 d
degrees b
between
t
th
the velocity
l it and
d th
the
magnetic field. This implies that the magnetic force on
a stationary charge or a charge moving parallel to the
magnetic field is zero
zero.
3. The direction of the force is given by the right hand
rule. The force relationship above is in the form of a
vector product.
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Units of Magnetic Field, B
From the force relationship above it can be deduced that the
units of magnetic field are
Newton seconds /(Coulomb meter)
or
Newton per Ampere meter.
This unit is named the Tesla. It is a large unit, and the smaller
unit Gauss is used for small fields like the Earth's magnetic
field.
1 Tesla = 10,000 Gauss
The Earth's magnetic field is of the order of half a Gauss
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Lorentz Force Law
The electric force is straightforward, being in the direction
of the electric field if the charge q is positive,
The direction of the magnetic part of the force is given by
the right hand rule.
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Magnetic Field in DC Motor
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Current in DC Motor
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Motor Working
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Force in DC Motor
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DC Motor Operation
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Commutator and Brushes on
DC Motor
To keep the torque on a DC motor from reversing every
time the coil moves through the plane perpendicular to
the magnetic field, a split-ring device called a
commutator
t t iis used
d tto reverse th
the currentt att th
thatt point.
i t
The electrical contacts to the rotating ring are called
“carbon brushes"
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DC Generator
DC Generator work on the same principle as that of the
DC Motor but in the reverse direction, i.e. the prime mover
is attached to the Shaft, thus the voltage
g g
generated is
available at the commutator.
Fleming’s Right Hand Rule is used to find out the
direction of the current in the moving conductor
placed in a magnetic field
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Fleming's Right Hand Rule
Fleming's right hand rule (for generators) shows the
direction of induced current flow when a conductor
moves in a magnetic field.
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Voltage Generated in a Moving
Wire
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DC Machine T
Types
pes
DC Machines
Series
Machine
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Separately
Excited
Machine
Compound
Machine
Shunt
Machine
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AC Machine T
Types
pes
AC
Machines
1-Phase
((Only
y
Motors)
Pole Shaded
Motor
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Capacitor
Start Motor
3 –Phase
Machines
Induction
Machine
Synchronous
Machine
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