Constructional Features of DC Motor
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Transcript of Constructional Features of DC Motor
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.