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rotating electrical Machinery is a part
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of much military equipment
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[Music]
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whether it is a simple
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[Music]
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blower or a complicated electronic
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device in a missile they all depend on
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the proper functioning of rotating
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electrical
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equipment two types of motors and
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generators are commonly in use
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alternating current or AC
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and direct current or DC motors and
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generators this film will show the
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principles governing the operation of DC
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motors and
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generators basic to the understanding of
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DC motors and generators is the simple
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generation of an electromotive Force an
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EMF mechanical energy the moving of a
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wire or conductor across a magnetic
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field by hand in this instance produces
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electrical energy
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the magnetic field is composed of lines
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of force as the conductor Cuts these
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lines an electromotive force or EMF is
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generated in the
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conductor moving the conductor down
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through the field makes the needle of a
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voltmeter deflect one way which means
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the EMF has One
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Direction moving the conductor up
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through the veh
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produces the opposite deflection of the
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needle the EMF has now changed
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Direction moving the conductor back and
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forth with the field does not make the
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needle of the voltmeter deflect there is
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no EMF because the conductor is not
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cutting the
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field to illustrate the direction of
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current flow the conventional symbols
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will be used current flowing in a
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conductor away from us is represented by
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a cross toward us by a DOT however
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moving a conductor in and out of the
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field in this straight reciprocal
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fashion is awkward and
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impractical a simple generator of EMF
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can also be made by rotating a single
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turn coil within a stationary magnetic
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field of two magnets with opposite
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polarity the Loop now in effect becomes
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two conductors because both the top and
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bottom sections cut magnetic lines
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during
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rotation since they cut lines of force
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of opposite directions as they rotate
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emfs of opposite polarity will be
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generated in the conductors in order to
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have current flow in this circuit
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polarities of the two conductors must be
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opposite
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the amount of EMF generated at any
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instant is determined by three factors
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the strength of the magnetic field that
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is the number of lines of force the
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length of the conductor cutting the
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lines of force and the velocity with
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which the conductor is turning we can
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determine the amount of instantaneous
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EMF by a simple
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formula the instantaneous EMF e
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equals B the strength of the
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Field Time L the length of the conductor
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cutting lines of force times V the
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velocity of the conductor an increase in
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the number of lines of force or the
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strength of the field increases the
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instantaneous EMF in the
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conductor increases in the length of the
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conductor cutting lines also Al
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increases the
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EMF and finally the greater the velocity
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of the conductor the greater the
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EMF this formula assumes conductor
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Motion in a straight line that is to say
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cutting the same number of lines for
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each increment of its motion but the
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conductor in an actual machine is not
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moving in a straight line but rotating
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when the conductor moves in a rotary
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path the number of lines cut varies
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depending on the position of the
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conductor at the top of the field for
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instance no lines are being cut and no
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EMF is
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generated as the conductor keeps turning
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the number of lines cut increases so
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that at a quarter turn or 90° the
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maximum number is being cut and maximum
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EMF is
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generated again at
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180° no lines are cut no
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EMF we reach a maximum again at
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270° and finally again at 360° no lines
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are cut the conductor has rotated 360
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mechanical deg which correspond in this
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instance to 360 electrical
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de therefore when the conductor moves in
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a rotary path another factor is added to
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the original formula for the
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determination of instantaneous EMF the
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formula that now applies is
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instantaneous EMF equals field strength
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time the length of the
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conductor time
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velocity multiplied by sin Theta
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Theta is the angle formed by the flux
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line and the motion of the conductor the
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number of lines cut and the amount of
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EMF generated is proportional to the
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sign of the angle formed by the magnetic
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lines with a conductor
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motion a graph of EMF versus conductor
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position during one revolution will be a
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sine wave representing alternating
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current or AC all rotary generators
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produce AC
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internally what you have seen so far is
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really the theory and operation of a
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basic AC generator but our purpose was
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to explain the principles of operation
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of a DC generator to get direct current
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we will attach each end of the the
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conductor to a segment of copper forming
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a commutator now our machine is a DC
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generator the commutator rotates with
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the
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loop stationary contacts carbon brushes
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ride on the commutator
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segments they provide a means of
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connecting a meter or any other load to
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the generator the loop of a conductor
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wound on a rotor and the commutator are
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referred to as the Armature as the loop
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revolves and the EMF in the conductor
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reverses polarity the connections to the
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load are also reversed and the current
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flow will maintain the same direction
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externally represented graphically the
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output amplitude still varies the DC is
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in the form of pulses it is a pulse
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ating direct current or
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PDC the pulsation from zero to maximum
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twice for each revolution of the loop is
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called Ripple this Ripple can be reduced
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by adding more loops and more commutator
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segments to the existing Armature two
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Loops at right angles connected to four
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commutator segments provide two outputs
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instead of
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one these out puts are 90° displaced or
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apart which combined to smooth the DC
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output however even with two loops and
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four commutator segments the rectified
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curve is still somewhat
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irregular by adding magnets we increase
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the number of fields cut by the Armature
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as we increase the number of loops and
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commutator segments the variation
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between maximum and minimum value
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decreases this in effect tends to
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flatten the DC
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output practical DC generator armatures
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have a great many Loops wound on a
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rotor the field is composed of many
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electromagnets together these factors
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tend to create an almost pure DC
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output an important problem in the
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design of generators is the prevention
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of sparking between the commutator and
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the brush assembly the prevention of
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sparking depends on the position of the
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brushes this line through points of zero
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generated EMF is called the neutral plan
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placing the brushes in this neutral
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plane reduces the tendency for sparking
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between brushes and commutator because
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during the time a brush is touching both
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commutator segments there is no
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difference in potential between these
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segments theoretically no sparking
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should occur at the commutator brushes
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when they are placed in this position
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but the current flowing in the Armature
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Loops or coils sets up a magnetic field
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of its
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own this magnetic field interacts with a
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main magnetic field and distorts it the
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Distortion causes a shift in the neutral
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plane and sparking at the brushes the
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effect is called armature reaction
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sparking may cause severe interference
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in nearby electronic
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equipment there are two ways of
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maintaining the neutral plane in its
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correct position and thus avoiding
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sparking it may be done by the
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adjustment of the brush
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position the brushes are adjusted to lie
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in the adjusted neutral plane
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the other way of maintaining the neutral
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plane is by adding interpoles to the
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generator field these interpoles are
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small magnets placed between the poles
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of the main field magnets the interpole
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fields oppose the fields created by
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armature
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reaction the neutral plane is moved back
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toward the correct position
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in addition to further counteract
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armature reaction windings called
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compensating windings are sometimes
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placed in the main pole faces the
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current in these windings is armature
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current flowing in opposite direction to
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the current in the Armature conductors
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magnetic fields in DC generators may be
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produced by electromagnets or permanent
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magnets permanent magnets are used in
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relatively small devices like a field
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telephone ringing
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generator in larger generators the field
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is created by
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electromagnets the field winding used in
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this DC generator can be represented by
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a
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symbol the symbol is that of an iron
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core
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inductor current to excite the field
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windings can be supplied from an
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external
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Source in that case the generator is
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classified as separately
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excited a small part of the generator's
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own output can also do the exciting in
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that case it will be
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a self excited generator self excited
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generators must be initially
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magnetized the residual magnetism in the
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core of a field winding provides enough
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magnetism to begin generator
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[Music]
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action the field coil winding may be
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connected in several ways this is a
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series wound generator which means the
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field Co is in series with the Armature
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because of this series Arrangement it
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has poor voltage regulation the reason
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for this can be demonstrated in the
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following manner additional load will
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cause more current to flow in the field
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coil increase in field strength
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increases voltage increase in voltage
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causes more current to flow this
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continuing action stops only when the
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core is saturated
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when the load is increased the voltage
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will
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increase when the load decreases voltage
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will decrease voltage regulation in the
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series wound generator therefore is very
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poor when instead of in series The Field
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winding is connected in parallel with
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the Armature and the load we have a
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shunt wound generat
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now the field current is independent of
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the load current therefore an increase
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in Armature current will not cause an
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increase in the voltage output voltage
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regulation here is greatly
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improved in shunt wound generators
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therefore changing load causes
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relatively small change in voltage
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output
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by changing the Armature winding a
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compound wound generator results which
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combines the best features of both types
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the series and the shunt wound generator
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when windings are arranged so that
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magnetic fields oppose each other it
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becomes in effect a series generator
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this is used only where constant current
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is the prime requirement such as in arc
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welding by changing the magnetic
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polarity of one of the fields the field
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windings Aid one another as a result
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this compound wound generator has good
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voltage and fair current
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regulation a graphic representation of
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generator output characteristics with
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terminal voltage plotted vertically and
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Armature current horizontally would look
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something like
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this as we have seen in the output of
00:16:55
the series wound generator voltage
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regulation is very poor in parallel or
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shunt wound generators the voltage
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regulation is fairly good but current
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regulation is poor compound wound
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generators offer a flat compounded
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output that is normally most desirable
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it combines the good features of both
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the shunt and series wound generators
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and provides stable voltage output under
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changing
00:17:26
loads as we have seen in our analys of
00:17:29
the DC generator its primary function is
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the conversion of mechanical energy to
00:17:34
electrical energy if we now reverse the
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procedure and connect an electrical
00:17:40
power source to the
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generator we have a DC motor instead of
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a DC
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generator motor action can be
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illustrated by attaching a power source
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to a conductor which is inside a
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magnetic field the electric current
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creates polarity in the conductor the
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South Pole of the magnet attracts the
00:18:02
North Pole of the conductor and repels
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the South Pole the North Pole of a
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magnet attracts the South Pole of the
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conductor and repels the North
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Pole this creates movement depending on
00:18:17
the direction of the steady magnetic
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field the movement also depends on the
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direction of the current flow through
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the wire
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by changing the polarity of the battery
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the conductor now moves in the opposite
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direction to see what really happens
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let's go to a drawing again here a
00:18:52
conductor is suspended in a magnetic
00:18:54
field current flow from a power source
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creates its own magnetic field in and
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around the
00:19:01
conductor this field around the
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conductor reacts with a main magnetic
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field to cause motion of the conductor
00:19:07
either out of the field or into it the
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arrow point indicates the direction of
00:19:12
the current flow in the
00:19:18
conductor in this case the flow is
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toward us the field of the conductor has
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the same direction as the main field
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above the conductor and the opposite
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direction of of the field below the
00:19:30
conductor these two magnetic forces
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added together distort the lines of the
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main field upward the field above the
00:19:38
conductor is thus made stronger and the
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field below the conductor is made
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weaker so the conductor moves
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down conversely when current flows in
00:19:54
the opposite direction that is to say
00:19:56
away from us the field of the conductor
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opposes the main field above the
00:20:03
conductor this AIDS the main field below
00:20:06
the conductor distorting the lines down
00:20:09
the field below the conductor is thus
00:20:10
made stronger while the field above the
00:20:13
conductor is made relatively weaker this
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forces the conductor to move
00:20:17
up with this basic principle of motor
00:20:20
action understood we can now examine the
00:20:22
DC
00:20:26
motor the basic DC motor like the DC
00:20:29
generator consists of a pair of magnetic
00:20:33
poles an Armature made up of a single
00:20:36
turn Loop a commutator and a brush
00:20:40
assembly as we have seen a conductor in
00:20:43
a magnetic field will move when a
00:20:44
voltage is applied to
00:20:48
it with a voltage applied and the
00:20:51
magnetic field and current flow as shown
00:20:53
the right conductor will be pushed down
00:20:56
while the left one is pushed up since
00:20:59
the forces on each conductor are now in
00:21:01
exact balance there will be no more
00:21:08
motion adding another loop and two
00:21:10
commutator segments ensures that at no
00:21:13
time will balancing forces cancel each
00:21:15
other
00:21:17
out with this setup there will be motion
00:21:20
at all
00:21:23
times as one commutator segment has
00:21:25
moved away from the brushes another now
00:21:28
takes its place and the movement
00:21:29
continues the greater the number of
00:21:31
Loops in the Armature the smoother its
00:21:35
motion for this reason rotors in
00:21:38
Practical DC motors have many
00:21:41
Loops since current in the rotor Loops
00:21:44
must reverse each half cycle two
00:21:46
commutator segments per Loop are
00:21:51
required here in the motor as in the DC
00:21:54
generator there is a neutral plane the
00:21:57
interaction of a conductor fields on the
00:21:59
main field causes this neutral plane to
00:22:01
shift and sparking to occur when a load
00:22:04
is
00:22:09
added sparking in DC motors also
00:22:12
produces burned commutators and
00:22:14
interference in nearby electronic
00:22:16
equipment this sparking can be prevented
00:22:18
in one of two
00:22:21
ways one is by the adjustment of the
00:22:23
brush
00:22:26
position the brushes are moved until
00:22:29
they lie in the adjusted neutral
00:22:35
plane in the motor as in the generator
00:22:38
small interpoles between the poles of
00:22:40
the main magnets are also used to
00:22:42
eliminate the shift of the neutral
00:22:46
plane these interpole fields tend to
00:22:48
oppose the fields created by armature
00:22:51
reaction the neutral plane is moved back
00:22:53
toward its correct position also aiding
00:22:56
our compensating windings which carry
00:22:59
Armature current in the opposite
00:23:00
direction to the current in the Armature
00:23:03
conductors the neutral plane is thus
00:23:05
maintained in its proper
00:23:13
position DC motors operate most
00:23:16
efficiently when sparking is
00:23:22
eliminated we saw earlier that when a
00:23:24
conductor is moved by mechanical energy
00:23:26
in a magnetic field and EMF is generated
00:23:30
this is generator
00:23:35
action in the DC motor when rotation is
00:23:38
desired it is necessary to apply an EMF
00:23:42
to the conductor however when used as a
00:23:44
motor an opposing EMF is also generated
00:23:47
in the
00:23:51
conductor this is called the counter
00:23:54
electromotive force or
00:23:56
cemf by lenses Lo
00:23:59
the generated cemf must oppose the
00:24:02
applied
00:24:05
EMF the amount of CF depends on the
00:24:08
speed of rotation this is of practical
00:24:11
importance in large
00:24:15
Motors when starting large Motors the
00:24:18
problem exists of limiting current
00:24:20
through the rotor windings until a CF
00:24:23
can be built up if the full current is
00:24:25
applied before the CF develops it may
00:24:28
burn out the rotor
00:24:32
windings starting boxes are used with DC
00:24:35
motors in order to avoid this
00:24:37
application of current before the cemf
00:24:40
is built
00:24:48
up here is a basic shunt motor with its
00:24:51
starting
00:24:55
box in the starting position the circuit
00:24:58
to the rotor windings is closed through
00:25:00
a series of large resistance
00:25:04
coils as the lever of the switch is
00:25:06
moved rotor speed and CF build up
00:25:09
gradually and the resistance coils are
00:25:12
subsequently cut out until running speed
00:25:14
has been reached the lever is held in
00:25:16
the fully open position by an
00:25:19
electromagnet if for any reason the
00:25:21
power should fail or the field coil open
00:25:24
the electromagnet becomes deenergized
00:25:26
and the lever is returned to the
00:25:28
starting position by Spring
00:25:35
action just as in DC generators DC
00:25:39
motors seldom use permanent magnets for
00:25:41
the field instead electromagnets are
00:25:44
used like with a DC generator field
00:25:47
windings are constructed in several ways
00:25:50
each type of winding has special
00:25:52
characteristics special values and
00:25:54
specific uses
00:26:00
the series wound motor has good starting
00:26:02
torque or turning Force torque depends
00:26:05
on Armature current and on field
00:26:12
strength since field strength is
00:26:14
proportional to current the high
00:26:17
starting current before CF is developed
00:26:20
affects torque as the square of the
00:26:23
current the motor begins to turn
00:26:26
attempting to develop enough CF to
00:26:29
completely oppose the applied
00:26:33
EMF the load prevents this acting to
00:26:37
control the speed of the motor but if
00:26:39
the load is suddenly removed like in the
00:26:42
case of a broken belt the motor will
00:26:44
build up speed trying to develop more CF
00:26:48
until it destroys
00:26:55
itself the shunt wound motor has less
00:26:58
starting torque but it is less dependent
00:27:00
on load for speed
00:27:04
control in the shunt wound motor the
00:27:07
field coils are connected in parallel
00:27:09
directly across the DC input
00:27:14
terminals the starting torque is not as
00:27:17
great as in the series motor since field
00:27:19
strength is not affected by Armature
00:27:25
current the speed of a shunt motor isir
00:27:28
L constant under conditions of changing
00:27:30
load as more load is applied the speed
00:27:33
of the Armature decreases this decreases
00:27:36
the CF and increases the current
00:27:44
input the increase in current input
00:27:48
boosts the coupling between the Field
00:27:49
and Armature and increases the torque
00:27:52
causing the motor to resume approximate
00:27:54
running speed
00:27:58
a sudden reduction in load will not
00:28:00
damage the motor because the field
00:28:02
current is independent of rotor current
00:28:04
in the shunt wound
00:28:08
motor the desirable characteristics of
00:28:11
both the series and shant wild Motors
00:28:13
can be achieved in the compound wound
00:28:15
motor in order to obtain good starting
00:28:18
torque the series field is used when
00:28:20
running speed has been attained a
00:28:22
centrifugal switch cuts out the series
00:28:25
field and cuts in the shunt field it is
00:28:28
now a shunt motor and the speed
00:28:30
regulation is
00:28:36
good compounding provides good starting
00:28:39
torque and good speed regulation this
00:28:42
allows for efficient operation and
00:28:44
minimizes the possibility of damage to
00:28:46
the
00:28:50
motor now for a quick summary the
00:28:53
operation of all rotating electrical
00:28:55
Machinery is based on one simple
00:28:57
principle the generation of an
00:29:01
EMF the principle is used in the
00:29:03
construction of a simple AC generator
00:29:06
the generator output e or instantaneous
00:29:11
EMF equals B strength of
00:29:14
Field Time L length of the
00:29:19
conductor time V velocity of the
00:29:24
conductor but because the movement of
00:29:26
the conductor in the field is actually
00:29:28
circular we must also multiply with a
00:29:31
sign of the angle formed by the lines of
00:29:33
force and motion of the conductor in
00:29:36
order to arrive at the
00:29:43
EMF all generators are basically
00:29:46
alternating current generators and
00:29:48
produce AC
00:29:55
internally the basic AC generator
00:29:58
becomes a DC generator when a commutator
00:30:01
is attached to the
00:30:06
conductor each commutator segment
00:30:09
rotates with its respective conductor
00:30:11
producing a direct
00:30:17
current this current which is a
00:30:19
pulsating current is made Smoother by
00:30:22
the addition of more magnets and more
00:30:24
loops
00:30:33
sparking in a generator is sometimes
00:30:35
caused by a shift in the neutral plane
00:30:38
this can be corrected by adjusting the
00:30:40
brush
00:30:45
position or by the use of interpoles and
00:30:48
compensating
00:30:55
windings current for field Landings may
00:30:58
be supplied from an outside Source in
00:31:01
which case the generator will be
00:31:02
separately excited or the current may be
00:31:05
a part of the generator's own output in
00:31:08
which event it is called self
00:31:14
excited generator field windings are
00:31:16
constructed in three ways series W where
00:31:20
the field windings and the Armature are
00:31:21
in
00:31:24
series shunt wound in which the field
00:31:27
winding is in parallel with the Armature
00:31:29
and the
00:31:33
load and compound wound in which the
00:31:36
best features of both the shunt and
00:31:38
series wild generators are
00:31:43
combined voltage regulation in the
00:31:45
series wild generator is poor in the
00:31:48
shunt wild generator voltage regulation
00:31:50
is fairly good but current regulation is
00:31:53
poor compound wound generators provide
00:31:56
stable voltage under changing load
00:31:58
this output is normally most
00:32:05
desirable motor action is the opposite
00:32:08
of generator procedure in a DC motor
00:32:11
voltage is applied to two or more Loops
00:32:13
in a magnetic field this causes polarity
00:32:16
in the loops the interaction of this
00:32:19
polarity with the polarity of the field
00:32:21
makes the loops
00:32:24
rotate this is the basic DC motor when
00:32:28
the conductor of the motor is rotated by
00:32:30
an applied EMF it also generates a
00:32:33
counter electromotive Force according to
00:32:35
lens's law this generated cemf must
00:32:39
oppose the applied
00:32:43
EMF as in generators field windings and
00:32:46
DC motors are of three types the series
00:32:49
wound motor has good starting torque but
00:32:52
since its only governing factor is the
00:32:54
load the speed regulation is poor
00:33:01
if the load is disengaged suddenly the
00:33:03
motor will race to
00:33:07
destruction in shun wound motors with
00:33:10
the field coils connected in parallel
00:33:12
across the DC input terminals the
00:33:14
starting torque is not too good but
00:33:17
since the field current is independent
00:33:19
of the rotor current the speed
00:33:20
regulation is quite good
00:33:28
the compound wound motor combines the
00:33:30
best features of both types it uses a
00:33:32
series section for good starting
00:33:36
torque then it switches to a shunt
00:33:39
arrangement for good speed
00:33:48
regulation DC electrical motors and
00:33:50
generators are at the heart of much
00:33:53
military equipment a proper
00:33:55
understanding of them is therefore
00:33:57
important
