1 dc machine

7/28/2019 1 Dc Machine

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LP11 1

Basics of a Electric Motor



LP11 2

A Two Pole DC Motor



LP11 3

A Four Pole DC Motor



LP11 4

Operating Principle of a DC Machine



LP11 5

FORE FINGER = MAGNETIC FIELD

900

900

900

MIDDLE FINGER= CURRENT

FORCE = B I Al

Fleming’s Left Hand Rule Or

Motor Rule



LP11 6

FORE FINGER = MAGNETIC FIELD

900 900

900

MIDDLE FINGER = INDUCED

VOLTAGE

VOLTAGE = B l u

Fleming’s Right Hand Rule Or

Generator Rule



LP11 7

Action of a Commutator



LP11 8

Armature of a DC Motor



LP11 9

Generated Voltage in a DC Machine



LP11 10

Summary of a DC Machine

• Basically consists of

1. An electromagnetic or permanent magnetic structure called

field which is static

2. An Armature which rotates

• The Field produces a magnetic medium

• The Armature produces voltage and torque under the action

of the magnetic field



LP11 11

Voltage and Torque developed in a

DC Machine

•Induced EMF, EA = K m (volts)

•Developed Torque, Tdev = K IA (Newton-meter

or Nm)

where m is the speed of the armature in rad/sec.,

is the flux per pole in weber (Wb)

IA

is the

Armature current

K is the machine constant



LP11 12

Interaction of Prime-mover DC Generator

and Load

Prime-mover (Turbine)

DC Generator L o a

d

IA Tdev

m EA

+

-

VL

+

-T pm

EA is Generated voltage

VL is Load voltage

T pm is the Torque generated by Prime Mover

Tdev is the opposing generator torque



LP11 13

Interaction of the DC Motor

and Mechanical Load

DC Motor

Mechanical

Load(Pump,

Compressor)

Tload

mEA

+

- Tdev

EA is Back EMF

VT is Applied voltage

Tdev is the Torque developed by DC Motor

Tload is the opposing load torque

IA

VT

+

--



LP11 14

Power Developed in a DC Machine

• Input mechanical power to dc generator

= Tdevm= K IAm =EA IA

= Output electric power to load

• Input electrical power to dc motor

= EA IA= K m IA = Tdevm

= Output mechanical power to load

Neglecting Losses,



LP11 15

Equivalence of motor and generator

•In every generator there is a motor (Tdev opposes T pm)

•In every motor there is a generator (EA opposes VT)



LP11 16

Field Coil

Armature

R A

Vf

Separately Excited DC Machine

+

-



LP11 17

Shunt Field Coil Armature

R A

Shunt Excited DC Machine



LP11 18

Series Field Coil

Armature

R A

Series Excited DC Machine



LP11 19

Shunt Field Coil Armature

R A

Compound Excited DC Machine

Series Field Coil

•If the shunt and series field aid each other it is called a cumulatively

excited machine•If the shunt and series field oppose each other it is called a differentially

excited machine



LP11 20

Field Coil

Armature

R A

Vf

Separately Excited DC Motor

Torque-speed Characteristics

+

-

m

Tdev



LP11 21

Series Field Coil

Armature

R A

Series Excited DC Motor

Torque-Speed Characteristics

m

Tdev



LP11 22

Speed Control of Separately Excited

DC Motor(2)

•By Controlling Terminal Voltage V T and keeping If or constant at rated value .This method of speed control is applicable

for speeds below rated or base speed.

m

VT

Tdev1 Tdev2 Tdev3

Tdev1

<Tdev2

< Tdev3



LP11 23

Speed Control of Separately Excited

DC Motor

•By Controlling (reducing) Field Current I f or and keepingVT at rated value. This method of speed control is applicable

for speeds below rated speed.

m

Tdev1

Tdev2 Tdev3

Tdev1<Tdev2< Tdev3

1 dc machine

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