DC GENERATOR

A DC generator is a rotating machine which converts mechanical energy (or power) into electrical energy (or power) delivering direct current.

Principle: a DC generator works on the principle of Faraday’s laws of electromagnetic induction.

Working principle: The working principle is that when magnetic flux is cut by a conductor or a number of conductors, e.m.f. is induced in the conductor or conductors and the mechanical energy supplied by a prime-mover is converted into electrical energy.

Difference between DC generator and an Alternator:

i)                    In a DC generator field is static and armature is rotating, but in an alternator either field or armature can be made rotating.

ii)                  In DC generator generated alternating e.m.f.  is rectified to DC by the arrangements of commutator but alternator requires no such commutator.

iii)                Most DC generators are self excited but the field of an alternator is excited from a separate exciter which is practically a self excited DG generator mounted on the same shaft of the alternator.

iv)                In DC generator a closed type of winding is arranged on the armature but in alternator coils are not arranged in closed fashion.

Function of Yoke in DC generator: Yoke provides a mechanical protection to the internal system of the machine and magnetic path of the pole flux. The yoke carries 50% of total flux per pole.

Factors on which the actual flux distribution depends: The actual flux distribution depends upon:

i)                    Size of air gap,

ii)                  Shape of pole shoes

iii)                Clearance between the tips of the adjacent pole shoes.

Function of armature core: The armature core houses the armature windings and provides a low reluctance path for the magnetic lines of force. Armature core is made of laminated silicon steel. It is laminated to reduce the eddy current losses. The holes are provided in the armature core to reduce the weight of the core as well as to provide air circulation for cooling the core. The armature core is not made of wood because of its high reluctance and non-capability of bearing high rotational torque. Super enameled copper wire or strips are used in the armature core with lap and wave windings.

Commutator: It is a cylindrical structure attached to the armature and mounted on the same shaft, the segments of which area separated from one another by mica insulation and connecte4d to armature coils through risers.

The armature windings are palced on the rotor because of the necessity for commutation.

Dummy coil: Dummy coils are inactive filler coils placed in the excess slots not required by the armature winding which are not connected to the rest of the winding and commutator segments but taped.

Dummy coils are used in the armature to bring out the mechanical balance of the armature.

Equalizer ring:  An equalizer ring is a balancing ring made of copper conductors fitted at the driving end of the armature or it may be a separate winding behind the commutator of a DC machine connected to the two or more points of the armature winding providing a low resistance path at the same potential. Equalizer rings are used in lap winding of a DC machine to avoid unequal distribution of current at the brushes as well as to get sparkless commutation.

                                              SINGLE LINE ANSWERS

Dielectric Strength: The point on which voltage can stand.

1.      Material thickness, 2. Temperature, 3. Size of electrode, moisture absorbent value.

1.      Air – 3.2 Kv/mm

2.             – 12 to 20Kv/mm

3.      Mica_ 20 to 60 Kv/mm

4.      Porcelain – 15 to 60 Kv/mm

5.      Trx oil – 20 kv/mm – 50 kv/2.5mm (IS 335)

Ø  Tungsten is used as a filament of lamp because the temperature coefficient of tungsten is high and melting point is also high.

Ø  Vacuum is a media in the lamp because filament should not burn.

Ø  Gas is filled in the lamp to increase the capacity of the lamp. Gas: ozone, Helium  etc(inert gasses)

Type of lamps and advantages:

1.      Fluorescent lamp: Household use & internal light of the bldg.

2.      Incandescent lamp: Household use & internal light of the bldg.

3.      Mercury vapor Lamp: Road & ground

4.      Sodium vapor lamp:  Road & ground

5.      Neon lamp: Advertisement

6.      Arc lamp: Search light & cinema projector

ü  To remove motor bearing permanently – use puller

ü  To fix motor bearing: Bearing to be heated up to 80⁰C & then it can be fixed easily

ü  Roller bearing is mostly provided to driving end side.

ü  Ball bearing can be provided to both the ends or non-driving end.

ü  Fuse is used in place of neutral link: If there is unbalance load, current will flow through fuse and it will blow and series circuit will form. Then low voltage will flow from excess load side & high voltage from lesser load side. 2 phase supply will flow if it is single phase and equipment will burn.

ü  3 ph loads should be balanced because in the balance load, current is same in each phase. If there is unbalance load, current will also unbalance. So there is a change to damage the equipment. Sometimes in the balance load, current flowing through neutral is zero. In the unbalance load, current flows through neutral & the phase on which load is excess, voltage will low and equipment will not work properly. If load of the phase is less, voltage will more and the equipments can get damaged. So balancing of load is necessary.

ü  Auto transformer starter is used in place of star delta starter to increase Starting torque as in starting, 80% voltage is getting to winding

ü  Star delta starter gives less starting torque because in starting 58% voltage is getting to winding.

ü  If motor trips while starting by star-delta starter, timer setting may low. Timer to be set on 80% speed of the motor.

ü  For heavy torque, DOL starter is used. 100% voltage is getting to the winding.

ü  Inductance: If supply is given to a coil, which opposes to the voltage is inductance.

ü  Impedance: Ckt consist of resistance, Inductance & capacitance, the total resistance is called impedance. Its unit is Ωmho. Z= √R²+(XL-XC)².

ü  Resistance = V x I

ü  Inductance =        L = X_L/2πF

ü  Capacitance = 1/2πFX_C

ü  Capacitive reactance   X_c = 1/2πFX_C

ü  Inductive reactance     X_L = 2πFX_L

ü  Capacitor50 mfd & 100 mfd =100 mfd Capacitor will take more current

ü  PF depend upon inductive reactance & capacitive reactance.

ü  PF on load: Resistive ckt – Unity

                   : Inductive ckt – Lagging

                   : Capacitive ckt- Leading

ü  No volt coil is connected in the series of field coil.

ü  Over load coil is connected in the series of supply and load

ü  3ph, 10HP motor installation: It will take mostly 15 amps current.

i.                    3½ core, 2.5sq mm Al conductor, PVC insulated armored cable

ii.                  32 Amps switch-fuse unit or isolator

iii.                16 swg copper conductor for earthing

iv.                 Over load relay: 10 – 25Amps.

v.                   Back up fuse: 35Amps HRC (2.5 times of full load current)

vi.                 Indication lamps On Off, 3 ph, trip : 6 Nos.

vii.               Ammeter & voltmeter selector switches

viii.             Motor insulation resistance

ix.                 Cable insulation resistance

                        Output 

ü  Efficiency of motor= ---------  x 100%

                                      Input

                                     Out put KWH

ü  All day efficiency  = ------------------- X 100%

                                    Input KWH

                                        No load Voltage – Load Voltage

ü  Voltage Regulation = -------------------------------------------

                                                    No load Voltage

ü  If the Gap between Stator & Rotor of induction motor is minimum, magnetizing current will less and power factor will increase.

Torque = Ir x Cos Φ = Rotor current x Rotor pf.

ü  Fault level : Switch yard – 1000 MVA

                      33 Kv bus bar- 750 MVA

                      3.3 Kv bus bar- 175 MVA

                      MV bus bar     - 35 MVA

ü  Neutral displacement: On Earth fault, lightening, voltage increases. This increased voltage is controlled.

ü  Lightening arrestor: On system voltage it works as a insulator & on high voltage it works as conductor.

ü  Direct stroke: By direct earth wire, in earth or ground – 80% negative, 20% positive charge.

ü  Non direct stroke: Drop in overhead line & observe arrestor & goes to ground: -ve + positive.

ü  If DC supply is given to transformer, it will burn because there will no induction and it will become a resistive ckt and will take high current and will burnt transformer winding.

ü  DG AVR Fails: It means failure of excitation, hence DG will continue running but no EMF will be generator.