II UNIT II UNIT

D.C MOTORSD.C MOTORS

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.

DC Machines

DC Motor DC Generator

DC motor

Force in a Conductor

Diagram of a Simple DC Motor

A

ZNPEb 60

and it is indicated by Eb

a

ba R

EVI

bEV

A

ZNPEb 60

Net voltage across armature circuit =

If Ra is the armature resistance , then

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

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

Power Equation

aaaba RIIEVI 2

AAbT RIEV

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

Electric Motor

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

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

• 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

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)

• Eb=ZNP/A

• Ta= (ZP/A)*(Ia/(2*3.14))

• Ta=0.159* ZIa(P/A) N-m

• Ta is proportional to (Ia)

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.