UNIT I DC MACHINES 9 hours Three phase circuits, a review. Construction of DC machines Theory of operation of DC
generators Characteristics of DC generatorsOperating principle of DC motors Types of DC motors and their characteristics Speed control of DC motors- Applications. Unit 1 DC MACHINES
Three phase circuits Three phase circuits is a polyphase system when 3 phases are used together from the generator to the load Each phase are having a phase difference of
120 If the load is single phase, then one phase can be taken from the three phase circuit and the neutral can be used as ground to complete the circuit.
Waveform Why three phase preferred over single phase? Single phase system The conductor needed for a
single phase circuit is very much less. The instantaneous power falls to zero when the single phase supply is disconnected.
Three phase system The conductor needed in three phase circuit is 75% that of conductor needed in single phase circuit.
The instantaneous power exists even after one phase is disconnected as the net power from all the phases gives a continuous power to the load.
Types of connections Star Connection Delta Connection Star Connection
Vph = VL / Iph = IL 3 phase wires and 1 neutral wire from the star point. Preferred for long
distances. Delta Connection Vph = VL Iph = IL/ 3 wires from the phases
are used. No Neutral wire Preferred for short distances. Types of currents
Balanced current Equal current flows through all the 3 phases. Unbalanced current - Unequal current flowing through all the 3 phases. Y- arrangement arrangement
Delta-Delta connection Star Delta connection Delta Star connection Simple loop generator
Principle of Operation Faradays Law of Electromagnetic induction:When a current carrying conductor is placed in a magnetic field an EMF is induced. Generator :- Flemings Right hand rule Motor :- Flemings Left hand rule
DC Machines DC machines convert electrical energy to mechanical energy and vice versa. This process of conversion is called as electromechanical energy conversion. If the conversion is from mechanical to electrical
energy, the machine is said to act as a generator. If the conversion is from electrical to mechanical energy, the machine is said to act as a motor. DC machines
Construction Types Theory of operation Characteristics Speed control of DC motor Applications
DC machine Construction DC machine Construction DC machine Construction
DC machine Construction DC machine Construction Armature of a DC machine
Armature of a DC machine Armature of a DC machine Dismantled view of a DC machine
Dismantled view of a DC machine Dismantled view of a DC machine Parts of a DC machine
Yoke Pole core & Pole shoes Pole coils Armature core
Purpose: Provides mechanical support Carries the magnetic flux produced by the poles. Materials used: Small machines Made of Cast Iron Larger machines Made of Cast steel or rolled
steel Pole cores & Pole shoes Spreads out the flux in the air gap Reduces the reluctance
of the magnetic path Supports the exciting coils or field coils Pole coil or field coil or exciting coil Copper wires or strips wound around the pole
core. When current is passed thorugh these coils they produce magnetic flux surrounding the armature conductors. Materials used: Made up of copper
Armature core Armature windings Usually former wound in the form of flat rectangular coils.
Various conductors of the coil are insulated from each other Conductors are placed in the armature slots Slot insulation is folded over the armature conductors. Materials used:
Made up of copper Commutator Facilitates collection of current from the armature conductors The commutator & brushes arrangement converts AC to DC It is of cylindrical structure and built up of wedge-shaped segments of high-conductivity
hard-drawn or drop forged copper. These segments are insulated from each other by thin layers of mica. The number of segments is equal to the number of armature coils. Each commutator segment is connected to the armature conductor by means of a copper lug or riser.
Brushes & bearings Collects current from the commutator They are housed in a brush holder. The brush holder is mounted on a spindle and can slide in the rectangular box open at both
ends. The number of brushes per spindle depends on the magnitude of the current to be collected from the commutator. Pole Pitch
Distance between 2 adjacent poles. It is equal to the number of armature conductors (or armature slots) per pole. If there are 48 conductors and 4 poles, the pole pitch is 48/4 = 12.
Conductor The length of a wire lying in the magnetic field and in which an emf is induced, is called a conductor (or inductor) as, for example, length AB or CD in the following figure. Coil & Winding element
The two conductors AB and CD along with their end connections constitute one coil of the armature winding. The side of the coil is called the winding element. No. of winding elements = 2 x No. of Coils The coil may be single turn coil or multi-turn coil. Multi-turn coil may have many conductors per coil side.
The group of wires or conductors constituting a coil side of a multi-turn coil is wrapped with a tape as a unit and is placed in the armature slot. Coil span or Coil pitch (YS) Coil span is the distance between 2 sides of the
coil. Full pitched winding If coil span or coil pitch = pole pitch, the winding is called full pitched wdg.
Full pitch means coil span is 180 electrical degrees. In full pitch, coil sides lie under opposite poles and their induced emf is scalar sum.
So maximum emf is induced in full pitch winding Chorded wdg or Short pitched wdg or Fractional pitched wdg The pitch of the winding is
less than pole pitch Single Layer Winding : Winding in which one conductor or one coil side is placed in each armature slot Double Layer Winding : Two conductors or coil sides per slot is placed in each armature slot
Types of windings Lap winding Wave winding Lap winding
Laps back with its succeeding coils. For Simplex lap winding, No.of parallel paths = No. of poles For duplex winding, No.
of parallel paths = 2x No. of poles Lap Winding Different pitches for Lap winding
Back Pitch (YB) No of coil sides or slots spanned by the back end connections. (Z/P) Front Pitch (YF) - No of coil sides or slots spanned by the front end connections.
Resultant Pitch (YR) Distance between the beginning of one coil to the beginning of next coil to which it is connected. Commutator Pitch (YC) Distance between the segments to which the
two ends of the coil are connected. For Lap, (YC = YB- YF) & for Wave, (YC = YB+YF) Equalizer rings or Equalizer connections A thick copper conductor
connecting the equipotential points of lap winding for equalizing the potential of different parallel paths. Avoids unequal
distribution of current at the brushes thereby heling to get sparkless commutation. One Equalizer ring for a pole pair is used.
Interpoles or Compoles Small poles placed in between the main poles. As their polarity is same as that of main poles, they induce emf in
the coil which helps in current reversal. The induced emf is called commutating emf or reversing emf which opposes the reactance emf thereby making commutation sparkless.
Cross magnetising effect due to armature reaction is also neutralised. Design of Lap Winding
Lap Winding Advantages: 1. This winding is necessarily required for large current application because it has more parallel paths. 2. 2. It is suitable for low voltage and high current generators. Disadvantages:
3. It gives less emf compared to wave winding. This winding is required more no. of conductors for giving the same emf, it results high winding cost. 2. It has less efficient utilization of space in the armature slots. Wave winding
The end of one coil is not connected to the beginning of the same coil but is connected to the beginning of another coil of the same polarity
as that of the first coil Simplex wave winding Circular form LAP & WAVE WINDING
Cross- arrangementsectional view of Armature Lap & Wave windings with pitches Types of DC machines DC Generator
DC motor DC generator Losses
Separately excited DC generator Self excited DC generator Compound generator
Characteristics of DC generator Open circuit characteristics or Magnetization characteristics or No load characteristics Load Characteristics Internal or Total characteristics External characteristics
Open Circuit Characteristics Eo Vs If The data is obtained by operating the generator at no load and keeping
speed constant Field current is varied and the corresponding terminal voltage is recorded
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