him machine lab 2

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University of Duhok

Faculty of Engineering&Applied Sceince

School of Engineering

Electrical &Computer dep.

Electronic Labortaryndstage

Experiement Name:BJT Transtistor as an AmplifierExperiment No: 3Student Name : Himdad Z. MiroDate of work: 1-11-2011

Broup

Groups Name1.Himdad Z.Miro

2.Diana H.

3.Noor H.

4.HudaN.


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OPERATION OF D.C MOTORS

RUNING & REVERSING

HEORY:For the D.c motors the armature current (Ia) is

Ia=(v-Eb)/Ra.(1)

& Eb=KN

Then:-

N=(V-Ia Ra)/K.(2)

Where:

V=Voltage across the armature

Ra=Armature resistance

N=speed

=Magnetic flux

K=Constant

Equation (2)shows that

1-The speed of the motor is proportional to the voltage across the armature.

2-The speed of the motor inversely proportional with excitation current.


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3-We Switched on the motor at aconstant armature voltage andthe motor that be on.4- By changing the field current by varying Rf.ext the variation ofthe motor speed if the value of If is decrease the value of Rf willbe increase and the speed of motor will be incease and so on.5-When field current at aconstant and changing voltage across thearmature if the value of voltage is increase by this the speed of themotor will be decrease ..6-We stopped the motor by switching off the main supply Changing the direction of rotation :We changed the direction of rotation by:-1-Change polarity of armature circuit applied voltage-We chage the polarity of the armature by putting A1 istead of A2and A2 istead A1 then we on the motor by this the diriction of themotor will be change in clockwise to anticlockwise.

2-Change polarity of field circuit applied voltage(change Of the field current direction)


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-In first the direction of rotation is clock wise by changing thepolarity of field circuit E1,E2 the direction will be change it toanticlockwise..

QUSTIONS:Note:if we change both field and armature applied voltagepolarity,the direction of rotation will not change.

1-How is the torque developed by a D.C motor ?On which factordoes it depend?

The torque developed by D.C motor is directly propotional to theflux and armature current it depends on the following factors thatshown in this eq.E=NZ/60(P/A)2- What is the back E.M.F of a D.C motor ?Is it more or less thanthe voltage applied to the armature?As the motor armature rotates ,the system of conductor comeacross alternate north and south pole magnatic fields cause in


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e.m.f induced is in the direction opposes the flow of current inmotor operation it is called back e.m.f .Back e.m.f is less than the voltage applied to the armature.Eb=V-Ia Ra

3-How can the direction of rotation change?if the line terminal arereversed ,will the direction of rotation change ?why?-We can change the direction by changing the polarity of A1 A2and we can by change the polarity of E1 EIf the line terminals are reversed the direction of rotation will notchange.4-Discuss the results and observation notice.In the result we know that:1.By increasing the Rf external the fail current decrease and speedof motor will increase.2.By decreasing Ra external Ia will increase and speed of motorincease.3. y decreasing the supply volt the rpm decrease .4.The direction of rotation for above stepsfor clock wise.


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him machine lab 2

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