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The construction of a DC motor is virtuallythe same as that of a DC generator and

many machines may be operated as either,

such as a starter-generator.

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� armature�

the yoke (or casing)� the field coils

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� an a r a t r nt i n a rings

� a st a ti na r agne ti

fie l� t a t r and r s e s

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The armature is typically a soft iron drum mountedon the motor shaft, with the armature conductorsset axially into the surface of the drum. Alsomounted on the armature shaft are the commutatorsegments, to which the armature conductors areconnected. The armature shaft is mounted in ballbearings at each end, the bearings being held in theends of the motor casing.

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The field windings are attached to the insideof the yoke and form two poles fittingclosely around the armature with a running

clearance of about 2.5mm.

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The loop of an armature is moving through thestationary field as the armature rotates and thisinevitably induces an EMF in the armature. This

EMF produces a current flow that opposes theapplied current from the battery and thereforereduces the total armature current flow. Theinduced voltage is known as back EMF.

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� The difference between the applied EMF and theback EMF is known as the net EMF, and it is this thatdetermines the torque producedin the armature shaft.

� In order to ensure that the net EMF issufficient the resistance of the armaturewindings is kept as low as possible.

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The initial current flow through the armature, beforeit 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 increasesand reduces the current flow through the armature.

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To avoid excess starting current, some DCmotors have a resistance built in to thearmature windings, which automatically cuts

out as motor speed increases.

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� series wound� shunt wound� compound wound

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� The field coils are connected in series with thearmature.

� At starting, when the current flow is very high.Consequently a characteristic of the serieswound motor is high starting torque. This isuseful in circumstances where the motor will berequired to start against a high load and wherethe running load is also high.

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Examples of instances where series woundmotors are used are engine starter motors,flap operating motors and landing gear

operating motors.

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� the field coils are connected in parallelwith the armature windings.

� The resistance of the field coils is

deliberately set to limit the field current tothat required for normal operation of themotor, and is much higher than thearmature resistance.

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� O n start up the current flow through thearmature is high, because of its lowresistance.

� a characteristic of the shunt wound DCmotor is low starting torque.

� As the armature speed increases, increasing backEMF will cause the armature current to decrease.

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� Shunt wound motors are used when starting torqueis low and increases with motorspeed.

� They are particularly useful where constantspeed under varying load conditions is arequirement.

� Typical applications in aircraft are fuel

pumps and fans.

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It has two sets of field windings, oneconnected in series with the armature andthe other in parallel. The low resistance

series windings are shown in heavy lining,the higher resistance shunt windings inlighter lining.

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The compound wound motor combines thecharacteristics of the series wound and theshunt wound motor. It is capable of high

starting torque, but will not over-speedunder light mechanical loading and willmaintain a reasonably constant speed undervarying conditions of load.

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� The compound wound motor is suited toapplications where loads may vary from

zero to maximum and where starting loads may behigh.� In aircraft they are often used to drive

hydraulic pumps and used as a starter/

generator.

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Reverse the rotating direction could be achieved bymeans 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 themagnetic attraction and repulsion and thus reversethe direction of rotation of the armature.

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� In a split-field motor there are two sets offield windings, either wound in oppositedirections on a common pole (or core) or

on alternate poles around the inside of themotor casing.

� used to operate flaps and landing gear,

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� armature control� field control.

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� This form of speed control is rarely used, as thehigh armature current requires alarger variable resistor to handle it. And it is

inefficient.� it is just a theoretical method.

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� These are high-speed reversible series-woundmotors whose output is normally converted into adriving torque via a step-down gearbox.

� Motor actuators are self-contained units, whichcombine electrical and mechanical devices capableof exerting reversible linear thrust over a shortdistance or alternatively a reversible low-speed

turning effort.

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