M4-DC Machines unit 1 .pdf

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M4‐ DC Machines

Unit 1

Dr. Prabodh Bajpai

Associate Professor

Indian Institute of Technology Kharagpur

• ‐

– Constructional Features

– Lap

winding

– ave w n ng

U1‐ Constructional features of DC

machines

DC machines

• First rotating electrical machines introduced to drive

locomotives in 1839

• Electromechanical energy conversion device – Operate as generator converting mechanical energy from

pr me mover to e ectr ca energy

– Also operate as motor converting electrical energy into

• Various combination of field winding provide wide

ran e of out ut V‐I and s eed‐tor ue characteristics

• Easy speed control, simpler and flexible drive system

makes wide use in Industry, electric traction, cranes

Construction of DC Machines

• Stator constitute of field poles, yoke and field

• Rotor constitute of armature core and armature

w n ng

• Commutator segments attached to the shaft of the rotor

• A pair of suitably placed stationary carbon

brushes touching the commutator segments

Stator

• Stator has projected poles with coils wound over it.

• The field winding are wound around pole core such

that they set up alternate north and south poles.• Pole shoe is curved and wider that pole core in order

to spread the flux more uniformly in the air gap.

• Field winding consist of few turns for series field and large no. of turns for shunt field

• Field coil act as electromagnets that produce flux in

t e a r gap nee e to generate an n uce em

• Yoke is part of frame and provide return path for flux

rom one po e o ano er.

• Double layer lap or wave windings are generally used

for armature.

• All the armature coils are connected in series forming a closed armature circuit.

• As the coils are distributed, the resultant voltage

acting in the closed path is zero thereby ensuring no c rcu a ng curren n e arma ure.

• The junctions of two consecutive coils are terminated

. • Stationary carbon brushes are placed physically under

commutator segments.

DC Machines

• The rotor has a ring‐shaped laminated iron core with slots.

• The commutator consists of insulated copper segments

• Two brushes

pressed

commutator

permit

current

• The brushes are placed in the neutral zone, where the

magnetic field is close to zero, to reduce arcing.

adjacent coil,

• The switching requires the interruption of the coil current.

• T e su en

nterrupt on

n uct ve

current

generates

voltages .

• The high voltage produces flashover and arcing between the

commutator segment and the brush.

DC Machine Construction

9Sectional diagram of a DC machine

How a D.C voltage is obtained across brushes

• The generated voltage in a coil when rotated

alternating in nature.

• To convert t is A.C. vo tage into D.C., a num er

commutator

segments attached to the shaft an a pa r o su ta y p ace stat onary car on

brushes touching the commutator segments.

• They are called mechanical rectifier due to

necessar rectification from A.C to D.C.

Action of commutator segments and brushes

• A simple d.c machine working as generator with

armature occu in various ositions.

• Armature has got a single rectangular coil with sides 1 and 2 whose terminals firmly joined to commutator

segments C1 and C2 respectively.

• Commutator segments made of copper are insulated by mica insulation and rotate along with the armature.

• Magnitude and polarity of the voltages in armature

conductors in different position may be different

• Brushes are suitably placed for obtaining maximum

vo tage, t e magn tu e o t e vo tage across t e

brushes will remain constant

• B1 and B2 are stationary carbon brushes placed over the

rotatin commutator in such a wa that the alwa s

make electrical contact with the commutator segments.

• It is from the two brushes, two terminals are taken out

and called the armature terminals.

• Brushes are kept in brush holders with a spring

arrangement.

• Spring tension is so optimally adjusted that brushes

make good contact with C1 and C2 and at the same time

allows the rotor to move freely.

• Fixed brushes B1 and B2 make periodically contact with

both C1 and C2 as rotor rotates.

• Let the armature be driven at a constant angular speed

of ω in the ccw direction.

• In position(i), ωt = 0°, – plane of the coil is vertical i.e., along the reference line

– No induced voltage across B1 and B2 as no flux density

component

perpendicular

tangential

velocity

• In position(ii), ωt = 45°,

it will be . Similarly conductor 1 being under S pole, polarity

of the induced voltage in it will be .

– Therefore across B1 and B2 we will get a voltage with B1 being

+ve and B2 being ‐ve.

• In position (iii) & (iv), The polarity of the voltage in

conductors 2 and 1 does not change because 2 remains

remains

• In position (v) to (viii), conductor 2 comes under S pole

and conductor 1 under N pole.

• Therefore polarity of voltage in conductor 1 is while polarity of voltage in conductor 2 is .

• B1 now makes contact with C1 and B2 makes contact

• Thus polarity of B1 remains +ve as before and that of B2

rema ns –ve una ere .

• Polarity of voltage across C1 and C2 will periodically

. C1C2 .

• Polarity of voltage across B1 and B2 will not change with

V B1B2 always remains unidirectional .

• Brushes are not associated with a fixed conductor but

they make contact with different conductors when they

come at some fixed position in space.

• Value of B is not constant under a pole, it is sinusoidally

distributed therefore magnitude of V C1C2 and V B1B2 does

not remain constant, since e = Blv

Armature winding terminology

• A coil has two coil sides occupying two distinct specified

slots and spacing between them is Coil span, defined as

ration

P• Double layer winding house two coil sides in each slot

belonging to two different coils

• For a double layer winding total number of coils must be equal to the total number of slots.

• Coil sides of a coil are numbered depending on the slot

num ers n w c t ese are p ace

• Number of commutator segments must be equal to S,

num er o s o s

Armature winding terminology

• Separation of coil sides of a coil in terms of number of

commutator se ments is commutator itch .

• For lap winding y c = ±1 and for wave winding PS yc

• Complete windings diagram shows

– Positions of coil sides in slots,

– interconnection of the coils through commutator segments

using appropriate numbering of slots,

– commutator segments

Armature winding types

• Practical D.C. machine armature have large number of

slots housing many coils along with a large number of

commu a or segmen s.

• Each coil ends are terminated on two commutator

• Armature windings may be of different types (lap/ wave)

segments, lap connected armature winding is obtained.

– when ends of a coil are terminated on segments apart by

approx. two pole pitch, a wave connected winding results.

• Number of parallel paths in armature,

– a = P for LAP winding.

– a = 2 for WAVE winding

Lap type Armature winding

• Second coil 2 ‐ 6' is in the lap of the first coil 1 ‐ 5', hence

the winding is called lap winding.

• Winding proceeding from left to right (assuming y c =+1) called progressive simplex lap winding.

• For y c = ‐1, winding would proceeded from right to left

• A winding table have information of number of coil sides

where the free ends of the coil sides will be terminated

• Number of brushes must be equal to number of poles.

The spatial distance between two coil sides of a coil should be

one pole pitch apart

equal to the number poles P.

Wave type Armature winding

• In this winding coil sides of a coil is not terminated in

adjacent commutator segments, i.e., y c ≠1.

• Instead y c is selected closely equal to two pole pitch in terms of commutator segments. Mathematically y c ≈

• Windin ro resses like a wave hence called wave

winding

multiple of P. It will be progressive wave winding if +ve

si n is taken and retro ressive wave windin if –ve si n

Simplex wave winding

For a P polar simplex wave winding, between any two

consecutive commutator segments P/2 coils will be present

Comparison of Lap and wave winding

• For a given current density and conductor cross section

– Total armature current obtained in lap winding is P/2 times

a o a ne n wave w n ng

• For given number of armature conductors

– No. of conductors connected in series per path in wave

winding is P/2 times that in lap winding

brushes in wave winding is P/2 times that in lap winding

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