Upload
bala-sankar
View
218
Download
1
Embed Size (px)
DESCRIPTION
This ppt explains the constructional features of DC Motor. This will be useful for the one who's interested with the motors.
Citation preview
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.