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II UNIT II UNIT
D.C MOTORSD.C MOTORS
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UNIT-II
D.C. MOTORS : Principle of operation of DC Motor, Types of Motors, Back EMF Equation, Characteristics of DC motor, Torque Equation, DC Motor Starter (Three Point starter), Efficiency Calculation, Swinburne's Test and speed control of armature and field.
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DC Machines
DC Motor DC Generator
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DC motor
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Force in a Conductor
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Diagram of a Simple DC Motor
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A
ZNPEb 60
and it is indicated by Eb
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a
ba R
EVI
bEV
A
ZNPEb 60
Net voltage across armature circuit =
If Ra is the armature resistance , then
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This back e.m.f is also called as counter e.m.f and indicated by Eb
Significance of Back E.M.F
The presence of back e.m.f makes the dc motor a self regulating machine i.e. it makes the motor to draw as much armature current as is sufficient to develop the torque required by the load .
a
ba R
EVI
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AAbT RIEV
EbVT
+
-
+
-
RA
VT = voltage at motor terminals
E = Back EMF induced in armature (V)
IA = armature current (A)
RA = armature resistance
Terminal Voltage Equation
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Power Equation
aaaba RIIEVI 2
AAbT RIEV
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is the electrical input to the motor
represents the copper losses in the motor
is the electrical equivalent of mechanical power developed in armature
abIE
aVI
aa RI2
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Electric Motor
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TYPES OF D.C MOTORS
(i) Shunt wound motors : in which the field winding is connected in parallel with the armature.
(ii) Series –wound Motor : in which the field winding is connected in series with the armature
(iii) Compound Wound Motor : which has two field windings ,one connected in parallel with the armature and the other in series with it
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Torque of a dc motor
• The turning or twisting moment of a force about an axis.
• It is measured as the product of force and radius at which it acts.
• T=F*r N-m
• Work done by force= F*2*3.14*r N-m
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• Power developed=F*2*3.14*r *N
• P=T*W watts (since T=F*r)
• P=(2*3.14)NT/60 watts
• P=NT/9.55 watts
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Armature Torque
• If Ta is the torque developed by armature and N is speed in rps
• P=Ta*2*3.14*N watts
• Electrical power: Eb*Ia
• Eb*Ia =Ta*(2*3.14*N)
• Ta=(Eb*Ia)/(2*3.14*N)
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• Eb=ZNP/A
• Ta= (ZP/A)*(Ia/(2*3.14))
• Ta=0.159* ZIa(P/A) N-m
• Ta is proportional to (Ia)
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SPEED CONTROL OF DC MOTORS
1. Vary the voltage supplied to the armature circuit while holding the field constant.
2. Vary the field current while holding the armature supply voltage constant.
3. Insert resistance in series with the armature circuit.
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