Constructional Features of DC M/C

Essential Parts of DC M/C

1. Field System2. Armature3. Commutator4. Brushes5. Bearings & Shaft

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. Made of cast steel / cast iron.

ii. Pole cores – Used to carry the coils of insulated wires 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

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

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• Number of segment is equal to no of armature

coils.

Brushes• Purpose – Collect current from the

commutator and supply it to external electric circuit or vice versa.

• Brushes are rectangular in shape and rest on the commutator.

• Made of carbon graphite / metal graphite /Cu.

• Wear out rapidly.• Hence constant lubrication is required.

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.

DC M/C Armature Winding

Armature Winding

• It is an arrangement of conductors to develop a desired emf.

• Improve the quality of voltage (similar to battery voltage).

• Depending upon the rating of the m/c armature winding are of different types.

• Two common types of Armature winding:– Lap winding –Wave winging

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

• Winding – Arrangement of coils is called winding.

• 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.

• If the coil-pitch is equal to the pole-pitch, then

winding is called full-pitched.

• It means that coil span is 180 electrical degrees. In

this case, the coil sides lie under opposite poles,

hence the induced e.m.fs in them are additive.

•If the coil-pitch is less than the pole-pitch,then the winding is fractional-pitched.In tis case,there is a phase difference between the e.m.fs in the two sides of the coil

•Commutator segment

•Commutator Pitch – Separation of coil sides of coil in terms of in terms of commutator segments is called commutator pitch

•Lap winding : Yc = 1

•Wave winding : Yc= 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 house two coil

sides. Physically one coil side is placed in the

lower portion of slot while other placed above 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 lower deck

is numbered as n’

• Alternate numbering : In nth slot, coil side placed

in upper deck is numbered with odd number and

coil side placed in lower deck is numbered with

even number

• Pitch of a Winding (Y or Yr): It is defined as distance b/w two coil sides of same coil or consecutive coils in terms of coil side number.

• Back Pitch (YB): Distance between coil sides of same coil in terms of coil side number is called back pitch.

• Front pitch (YF): Distance between coil side2 of coil x and coil side1 of coil x+1 in terms of coil side number.

General Procedure• Given : Slots and Poles.• Calculate coil span ( = S/P)• Calculate commutator pitch Yc.

(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.

Developed Diagram

Lap Winding • Yc = ± 1• If Yc = +1 => Progressive lap winding• If Yc = -1 => Retrogressive lap winding

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

• 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.