constructional features of dc m

8/13/2019 Constructional Features of DC M

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Constructional Features of DC M/C


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Essential Parts of DC M/C

1. Field System2. Armature

3. Commutator4. Brushes5. Bearings & Shaft


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Field System Purpose To create a uniform magnetic field. Permanent Magnet/ Electromagnet Field system consists of 4 parts

i. Yoke or Frame Used as frame of the machine and

carries the flux produced by poles. Not laminated.Made of cast steel / cast iron.

ii. Pole cores Used to carry the coils of insulatedwires carrying the field/exiting current. Usually of

circular cross section. Made of cast steel.iii. Pole shoes Support for the field coils. To spread

out the flux more uniformly. To reduce reluctance.iv. Magnetizing coils


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Armature Rotating part of the dc m/c Purpose Rotate the conductors in the

Magnetic field Made of highly permeable silicon-steel. Laminated. There are slots over the periphery of the

armature Slot Used to house the armature windings


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Commutator

Important part of DC M/C Functions:

Provides electrical connection b/w rotating

armature and stationary electric circuit. Performs mechanical rectification. Keeps rotor magnetic axis stationary in space.

Cylindrical Structure Made of Cu. Commutator segments are insulated from

each other by mica


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Brushes

Purpose Collect current from thecommutator and supply it to external electriccircuit or vice versa.

Brushes are rectangular in shape and rest onthe commutator.

Made of carbon graphite / metal graphite /Cu. Wear out rapidly. Hence constant lubrication is required.


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Bearings & Shaft

Bearings are used to reduce friction. Ball bearings/ Pedestal bearings/ Roller

bearings are generally used. Shaft is used to transfer mech. Power from or

to m/c. The shaft is made of mild steel.


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DC M/C Armature Winding


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Armature Winding

It is an arrangement of conductors to developa desired emf.

Improve the quality of voltage (similar tobattery voltage).

Depending upon the rating of the m/carmature winding are of different types.

Two common types of Armature winding: Lap winding Wave winging


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Terminology Conductor- Each individual length of wire lying

in MF. Coil Conductors with one or more turns and

with two terminals. Coil side- Each coil has two coil sides. Overhang


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Winding Arrangement of coils is calledwinding.

Slot (S) Coil pitch/ coil span Distance between the

coil side of a coil in terms of number of slots is

called coil pitch. Pole pitch Number of slots per pole. Commutator segment Commutator Pitch Separation of coil sides of

coil in terms of in terms of commutatorsegments is called commutator pitch

i di


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Lap winding : Y c = 1 Wave winding : Y c= 2* Yp Single layer winding : Only one coil side is

place per slot.Therefore total no. of coils = S/2

Double layer winding : Each slot will housetwo coil sides. Physically one coil side is placedin the lower portion of slot while other placedabove it.

Therefore total no. of coils = S In nth slot coil side placed in upper deck is

numbered as n and coil side placed in lowerdeck is numbered as n


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General Procedure

Given : Slots and Poles. Calculate coil span ( = S/P) Calculate commutator pitch Y c. (Decide lap or

wave winding) Complete the winding and connect the coil

side terminals to commutator segment. Finally decide and place the stationary

brushes on the correct commutator segments.


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Developed Diagram


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Lap Winding

Yc = 1 If Yc = +1 => Progressive lap winding If Yc = -1 => Retrogressive lap winding


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Lap winding in practical dc m/c Given : S = 16 & P = 4


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For LAP Winding: No. of commutator segments = S. No. of parallel paths = P. No. of Brushes = P.

Lap winding is adopted for low voltages, high

current DC machines

1. Slots = 8 and P = 4. Draw the complete lap

winding diagram.

constructional features of dc m

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