Comparison between zener breakdown and avalanche breakdown

S.NO	Zener breakdown	Avalanche breakdown
1.	Breakdown is due to strong electric field developed across the narrow depletion region.	Breakdown is due to charge multiplication
2.	Observed if Vz > 6 volt	Observed if Vz < 6 volt
3.	The VI characteristics is very sharp with zener breakdown	The VI characteristics is not very sharp with avalanche effect
4.	The breakdown voltage decreases as junction temperature increases	The breakdown voltage increases as junction temperature increases
5.	It occurs in heavly doped diodes	It occurs in normally doped diodes

What is the difference between fuse and circuit breaker?

(i) Fuse performs both detection and interruption functions. Circuit breaker performs interruption function only. The detection of fault is made by relay system.

(ii) The breaking capacity of fuse is very small compare to that of circuit breaker.

(iii) The operating time of the fuse is smaller than operating time of CB.

(iv) After every operation, replacement is required in the fuse. In case of circuit breaker, replacement is not required.

Comparison of PN junction diode and zener diode

Comparison of PN junction diode and zener diode

s.no	PN junction diode	zener diode
1	Symbol	Symbol
2.	Operated in forward biased condition and never operated in reverse biased condition	Operated in reverse breakdown region
3	Characteristics lies in 1st   quadrant	Characteristics lies in 3rd  quadrant
4	Applications: Rectifiers, clippers, clampers, voltage multipliers	Applications: Regulator, zener limiters
5	Low power dissipation capability	High power dissipation capability

construction details of a dc machine

Construction details of a dc machine

Parts of dc machine

•       Yoke

•       Pole core and pole shoe

•       field winding

•       Armature core

•       Armature winding

•       Commutator

•       Brushes and bearings

Yoke:

–      It is the outermost cover of the M/C, provides mechanical Support for poles

–      It also provide protection to whole machine from dust, moisture etc.

–      It also carries magnetic flux produced by the poles

–      Yoke is also called as frame.

–      For small M/C yoke is made of cast iron.

–      For large M/C it is made of cast steel or rolled steel.

Pole core and Pole shoe:

•       Pole of a generator is an electromagnet.

•       Each pole is divided into two parts

      (i)            Pole core

    (ii)            Pole shoe

Pole core: carries field winding which is necessary to produce flux.

Pole shoe: it is a extended part of pole core, it spreads the flux in air gap.

Materials used:

–      Pole core is made of cast iron or cast steel.

–      The laminations is done to reduce eddy current losses.

Field winding:

•       It is wound around pole core and called as field coil

•       it is connected in series to from field winding

•       When Current is passed through field winding it electro magnetize the poles which produce necessary flux.

•       Made up of aluminium or copper.

Armature:

•       Consists of two parts

      (i)            Armature core

    (ii)            Armature winding

Armature core:

•        It is cylindrical in shape mounted on the shaft and consists of slots on its periphery and air ducts to permit air flow through armature which serves cooling purpose.

•       It provides house for armature winding.

Materials used:

•       Made up of cast iron or cast steel.

•       It has laminated construction to reduce eddy current losses as low as possible.

Armature winding:

•       It is placed in armature slots and generation of e.m.f takes place in armature winding in case of generator.

Two types (i) Lap winding  (ii) wave winding.

Materials used:

•       Made up of conducting materials like copper or aluminium.

Brushes and Bearings:

Brushes:

•       Brushes collect the current from commutator and apply it to external load.

•       They are rectangular in shape and rest ion the commutator segments against spring tension.

Materials used:

•       Brushes are made of carbon or graphite

Commutator:

·         It is cylindrical in shape, made of copper and it has several segments.

·         It converts the induced ac emf into dc emf.

Bearings:

•       Bearings are fitted inside the cover to provide free and smooth rotation to the armature.

•       The types of bearings are ball bearing and roller bearing.

What is the necessity of starter in DC motor ?

What is the necessity of starter in DC motor ?

The voltage equation of dc motor is

V = Eb + IaRa

at start Eb = 0, V= IaRa

Ia = V / Ra.  (Ra alone limits the Ia)

generally motor is switched on with normal voltage and Ra is very small, the Ia at start is very high.This will blow out the fuses and may cause permanent mechanical damage to the motor.To limit this starting current to a safe value a variable resistance is connected in series with armature.This resistance is called as starter.

Simple DC motor

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