dc machines

DC Machine

Electromechanical energy conversion device that works as generator or motor

DC machine two modes

Generator mode and Motor mode

Motor converts

Electrical energy into mechanical energy

Generator converts

Mechanical energy into electrical energy

Exam trap

Same DC machine construction can work as motor or generator depending on energy flow

⭐ Principle of DC machine

Faraday’s law + Fleming’s left hand rule + Fleming’s right hand rule

Faraday’s law use in DC machine

EMF induced when conductor cuts magnetic flux

Fleming’s right hand rule used for

Generator direction of induced EMF/current

Fleming’s left hand rule used for

Motor direction of force/motion

⭐ Basic parts of DC machine

Yoke, pole, field winding, armature, commutator, base

Yoke

Outer frame of machine providing mechanical support and magnetic path

Pole

Carries field winding and produces magnetic poles N and S

Field winding

Coil wound on pole core carrying current to create electromagnet

Armature

Rotating part containing armature conductors/winding where EMF induced

Commutator

Mechanical rectifier/current reversing device connected to armature

Base

Mechanical support of machine

Stator in DC machine

Stationary part of machine

Rotor in DC machine

Rotating armature part

What is mounted in bearings

Armature

⭐ How EMF is induced

Whenever a conductor cuts magnetic field, EMF is induced

Basic requirements for generating EMF

Magnetic field, conductor, relative motion between them

Exam trap

No relative motion or changing flux means no induced EMF

Types of induced EMF

Dynamically induced EMF and statically induced EMF

Dynamically induced EMF

EMF induced when conductor moves in stationary magnetic field or flux cutting occurs

Statically induced EMF

EMF induced in stationary conductor due to changing flux linkage

DC generator based on

Conductors moved through stationary magnetic field

AC generator based on PPT note

Conductors stationary and field moving

⭐ Fleming’s Right Hand Rule

Thumb = motion, Forefinger = field, Middle finger = induced EMF/current

Memory hook right hand

Motion-Field-Current for generator

⭐ DC machine stator contains

Yoke, poles, field windings

Why field winding used

Current through it creates magnetic field/electromagnet

Armature core

Laminated iron or steel cylinder with teeth slots

Why laminated armature core

Reduces eddy current losses

Why teeth cut in core

To house armature windings

Armature winding

Single-loop or multi-loop conductor system on armature

Types of armature winding

Wave winding and Lap winding

⭐ Wave winding

Has two parallel paths irrespective of number of poles

Wave winding output

High voltage, low current

Use wave winding

When high voltage required

⭐ Lap winding

Number of parallel paths equals number of poles

Lap winding output

High current, low voltage

Use lap winding

When high current required

Wave vs Lap winding

Wave = 2 paths always; Lap = paths = poles

Wave vs Lap output

Wave high V low I; Lap high I low V

⭐ Types of DC generators

Separately excited and self excited (implied by methods of excitation/topic)

Methods of excitation DC machine

Field winding energized separately or from machine itself

⭐ Principle of simple DC motor

Current carrying conductor placed in magnetic field experiences force

Force direction in motor found by

Fleming’s left hand rule

Motor coil action

Opposite forces on coil sides produce torque and rotation

Why coil rotates continuously

Current reverses every half turn through commutator

What happens at 90° position

Brushes contact different commutator halves reversing current direction

If current not reversed

Coil would reverse torque after half turn and stop oscillating/no continuous rotation

Direction mentioned in PPT

Anti-clockwise rotation for shown current direction

⭐ Why commutator essential in motor

Ensures unidirectional rotation by reversing armature current each half turn

Why commutator essential in generator

Gives unidirectional DC EMF at terminals

Exam trap

Commutator in generator converts internally induced AC in armature to DC output

⭐ Back EMF

EMF induced in rotating armature of DC motor opposing supply voltage

Law explaining back EMF

Lenz’s law

Back EMF symbol

E

Supply voltage symbol

V

Armature current symbol

Ia

Armature resistance symbol

Ra

⭐ Motor voltage equation

V = E + IaRa

Rearranged motor equation

E = V − IaRa

Meaning of IaRa

Voltage drop in armature winding resistance

Why back EMF important

Limits armature current during running condition

Starting condition of motor

At start E≈0 so armature current very high if not controlled