Chapter 6 DC Motors

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Chapter 6 DC Motors
INTRODUCTION OF
DC MOTOR
Table 6-1. DC Motor Applications
DC motor
The construction of a DC motor is virtually
the same as that of a DC generator and
many machines may be operated as either,
such as a starter-generator.
Construction of DC motor
• armature
• the yoke (or casing)
• the field coils
Construction of DC motor
• an armature mounted
in bearings
• a stationary magnetic
field
• commutator and
brushes
Direction of conductor tend to move
armature
 The armature is typically a soft iron drum
mounted on the motor shaft, with the
armature conductors set axially into the
surface of the drum. Also mounted on the
armature shaft are the commutator segments,
to which the armature conductors are
connected. The armature shaft is mounted in
ball bearings at each end, the bearings being
held in the ends of the motor casing.
Field windings
The field windings are attached to the inside
of the yoke and form two poles fitting
closely around the armature with a running
clearance of about 2.5mm.
Section through a DC Motor
DC MOTOR PRINCIPLE
OF OPERATION

Back EMF
The loop of an armature is moving through the
stationary field as the armature rotates and this
inevitably induces an EMF in the armature. This
EMF produces a current flow that opposes the
applied current from the battery and therefore
reduces the total armature current flow. The
induced voltage is known as back EMF.
net EMF
• The difference between the applied EMF and
the back EMF is known as the net EMF, and it
is this that determines the torque produced
in the armature shaft.
• In order to ensure that the net EMF is
sufficient the resistance of the armature
windings is kept as low as possible.
Armature current
The initial current flow through the armature, before
it begins to rotate, is determined by the applied
voltage and the armature resistance. If the
resistance is low the current flow will be very high.
As the motor gains speed the back EMF increases
and reduces the current flow through the armature.
Why there is a surge current when DC motor start?
Limit the excess starting current
To avoid excess starting current, some DC
motors have a resistance built in to the
armature windings, which automatically cuts
out as motor speed increases.
TYPES OF DC MOTORS
TYPES OF DC MOTORS
• series wound
• shunt wound
• compound wound
Series Wound Motor
Series Wound Motor
• The field coils are connected in series with the
armature.
• At starting, when the current flow is very high.
Consequently a characteristic of the series
wound motor is high starting torque. This is
useful in circumstances where the motor will be
required to start against a high load and where
the running load is also high.
Examples
Examples of instances where series wound
motors are used are engine starter motors,
flap operating motors and landing gear
operating motors.
Note
• Series wound motors should never be
allowed to operate without a mechanical
load applied.
• This is because they are liable to overspeed,
possibly to destruction.
Shunt Wound Motors
Shunt Wound Motors
• the field coils are connected in parallel
with the armature windings.
• The resistance of the field coils is
deliberately set to limit the field current to
that required for normal operation of the
motor, and is much higher than the
armature resistance.
Shunt Wound Motors
• On start up the current flow through the
armature is high, because of its low
resistance.
• a characteristic of the shunt wound DC
motor is low starting torque.
• As the armature speed increases, increasing back
EMF will cause the armature current to decrease.
Shunt wound motors
• Shunt wound motors are used when starting
torque is low and increases with motor
speed.
• They are particularly useful where constant
speed under varying load conditions is a
requirement.
• Typical applications in aircraft are fuel
pumps and fans.
Compound Wound Motors
Compound Wound Motors
It has two sets of field windings, one
connected in series with the armature and
the other in parallel. The low resistance
series windings are shown in heavy lining,
the higher resistance shunt windings in
lighter lining.
Compound Wound Motors
The compound wound motor combines the
characteristics of the series wound and the
shunt wound motor. It is capable of high
starting torque, but will not over-speed
under light mechanical loading and will
maintain a reasonably constant speed under
varying conditions of load.
Compound Wound Motors
• The compound wound motor is suited to
applications where loads may vary from
zero to maximum and where starting loads
may be high.
• In aircraft they are often used to drive
hydraulic pumps and used as a starter/
generator.
Reversible DC Motors
Reversible DC Motors
Reverse the rotating direction could be achieved by
means of a switching arrangement that reversed
the polarity of the DC supply to either the field or
the armature (but not both). This would reverse the
magnetic attraction and repulsion and thus reverse
the direction of rotation of the armature.
split-field motor
• In a split-field motor there are two sets of
field windings, either wound in opposite
directions on a common pole (or core) or
on alternate poles around the inside of the
motor casing.
• used to operate flaps and landing gear,
MOTOR SPEED
CONTROL
MOTOR SPEED CONTROL
METHOD
• armature control
• field control.
Armature Control
Armature Control
• This form of speed control is rarely used,
as the high armature current requires a
larger variable resistor to handle it. And it
is inefficient.
• it is just a theoretical method.
Field Control
Field Control
Variable resistance would not directly affect
the current through the armature. However,
as the magnetic field strength has increased,
so would the back EMF. The current in the
armature would reduce and motor speed
would also reduce.
Tips
To compare the magnetic field to a viscous
fluid: The thicker it gets, the harder it is for
the armature to turn. The thinner it gets, the
easier it is for the armature to turn.
AIRCRAFT DC MOTORS
ACTUATORS
• These are high-speed reversible series-wound
motors whose output is normally converted into a
driving torque via a step-down gearbox.
• Motor actuators are self-contained units, which
combine electrical and mechanical devices capable
of exerting reversible linear thrust over a short
distance or alternatively a reversible low-speed
turning effort.
Rotary Actuators
Linear Actuators
END OF CHAPTER 9

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