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prepared by:
Hatem Al-Ghannam

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Introduction

-The stator of the dc motor has poles, which are excited by dc current to produce magnetic fields.
-In the neutral zone, in the middle between the poles, commutating poles are placed to reduce sparking of the commutator. The commutating poles are supplied by dc current.
-Compensating windings are mounted on the main poles. These short-circuited windings damp rotor oscillations
-The poles are mounted on an iron core that provides a closed magnetic circuit.
-The motor housing supports the iron core, the brushes and the bearings.
-The rotor has a ring-shaped laminated iron core with slots.
-Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees.
-The coils are connected in series through the commutator segments.
The ends of each coil are connected to a commutator segment.
-The commutator consists of insulated copper segments mounted on an insulated tube.
-Two brushes are pressed to the commutator to permit current flow.
-The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing.
-The rotor has a ring-shaped laminated iron core with slots.
-The commutator consists of insulated copper segments mounted on an insulated tube.
-Two brushes are pressed to the commutator to permit current flow.
-The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing.
-The commutator switches the current from one rotor coil to the adjacent coil,
-The switching requires the interruption of the coil current.
-The sudden interruption of an inductive current generates high voltages .
-The high voltage produces flashover and arcing between the commutator segment and the brush.

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MMME2104
Design & Selection of Mining Equipment
Electrical Component

DC Machines

Lecture Outline
• DC Generators
– Operating principle
– Separately excited generator
– Shunt generator
– Compound generator
• DC Motors
– Shunt motor
– Series motor
– Compound motor
– Starting and braking
– Basics of speed control

DC Generators: Operating Principle

The difference between AC and DC generators:
• AC generators use slip rings
• DC generators use commutators
Otherwise, the machine constructions are
essentially the same.

Induced voltage in a DC generator: E = B L v (Faraday’s Law)
For a DC generator, this equation can be manipulated to give:
Eo = C n Φ / 60
Eo = voltage between the brushes (V)
N = speed of rotation (rpm)
Φ = flux per pole (Wb)
C = total number of conductors on the armature*
*The number of conductors equals the number of slots (coils) times the
number of turns per coil times two

Neutral zones:
Neutral zones are those places on
the surface of the armature
where the flux density is zero.
When a generator operates at noload,
the neutral zones are
located exactly between the
poles.
No voltage is induced in a coil that
cuts through the neutral zone.
We always try to set the brushes
so they are in contact with coils
that are momentarily in a
neutral zone.