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DC Motor
&
DC Generator
DC Machines
Lab # 2
Arsalan A Rahim
Abdur Rehman Kashif
1
Magnetic Field of Current
The magnetic field lines around a long wire which carries an
electric current form concentric circles around the wire. The
direction of the magnetic field is perpendicular to the wire and is
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.
Arsalan A Rahim
Abdur Rehman Kashif
2
DC Motor
Arsalan A Rahim
Abdur Rehman Kashif
3
Magnetic Interactions with Moving
Charge
Arsalan A Rahim
Abdur Rehman Kashif
4
Magnetic Force on Moving Charge
Arsalan A Rahim
Abdur Rehman Kashif
5
Magnetic Force on a Current
Arsalan A Rahim
Abdur Rehman Kashif
6
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.
Arsalan A Rahim
Abdur Rehman Kashif
7
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
the angle < 180 degrees between the velocity and the
magnetic field. This implies that the magnetic force on
a stationary charge or a charge moving parallel to the
magnetic field is 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.
Arsalan A Rahim
Abdur Rehman Kashif
8
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
Arsalan A Rahim
Abdur Rehman Kashif
9
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.
Arsalan A Rahim
Abdur Rehman Kashif
10
Magnetic Field in DC Motor
Arsalan A Rahim
Abdur Rehman Kashif
11
Current in DC Motor
Arsalan A Rahim
Abdur Rehman Kashif
12
Motor Working
Arsalan A Rahim
Abdur Rehman Kashif
13
Force in DC Motor
Arsalan A Rahim
Abdur Rehman Kashif
14
DC Motor Operation
Arsalan A Rahim
Abdur Rehman Kashif
15
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 is used to reverse the current at that point.
The electrical contacts to the rotating ring are called
“carbon brushes"
Arsalan A Rahim
Abdur Rehman Kashif
16
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 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
Arsalan A Rahim
Abdur Rehman Kashif
17
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.
Arsalan A Rahim
Abdur Rehman Kashif
18
Voltage Generated in a Moving
Wire
Arsalan A Rahim
Abdur Rehman Kashif
19