D.C. Machine

Power Stages in D.C. Generator

Stage A Stage B Stage C

Input (B.H.P x 735.5) – Iron & Friction loss = Electrical Power Developed in Armature (E*Ia)

Electrical Power Developed in Armature (E*Ia) – Copper Loss = Net output (Vt * IL)

Mechanical Efficiency = Stage B / Stage A = E*Ia / (B.H.P x 735.5)

Electrical Efficiency = Stage C / Stage B= Vt*IL / E*Ia

Overall Efficiency = Stage C / Stage A = Vt*IL / (B.H.P x 735.5)

Example

D.C. MotorIntroduction: Similar construction to D.C. Generator. Infect D.C. machine can be work as generator

or motor. Motor converts electrical energy in to

mechanical energy. Generator converts mechanical energy in to

electrical energy.

When a current carrying conductor is placed in a magnetic field, the conductor experiences the force.

The direction of force depends on the direction of magnetic field and also depends upon direction current in the conductor.

Fleming’s Left Hand Rule The direction of force is also decided by

applying Fleming's left hand rule. Thumb and first two fingers are use to explain

this rule.

Working Principle of D.C. Generator: It works on principle that, When current carrying

conductor is kept in magnetic field, a force is produce on the conductor in define direction.

When field winding carries the current (main flux production) similarly the armature also carry the current.

If the main flux is downwards from ‘N’ to ‘S’ pole and armature current having direction (.) in upper conductor and (+) in the bottom conductor, then applying Fleming’s left hand rule the direction of the force on the upper conductor is towards right and on bottom conductor towards right. (as seen in next Fig.)

This two equal and opposite forces produces a ’Torque’ and thus motor starts working in clockwise direction as seen in fig.

Magnitude of this force is given by,Force, F = B.I.L

Where,B = Flux density of the fieldI = Current in the conductorL = Length of the conductor

Back EMF (Eb) Also called counter EMF. The term back electromotive force, or just back-EMF,

is most commonly used to refer to the voltage that occurs in electric motors where there is relative motion between the armature of the motor and the magnetic field from the motor's field magnets, or windings.

In a motor using a rotating armature in the presence of a magnetic flux, the conductors cut the magnetic field lines as they rotate. This produces a voltage in the coil; the motor is acting like a generator (Faraday's law of induction) at the same time it is a motor. This voltage opposes the original applied voltage; therefore, it is called "back-electromotive force“.

Types of D.C. Motor D.C. Shunt Motor:

D.C. Series Motor:

D.C. Compound Motor:

Long Shunt Compound Motor:

Losses in D.C. Motor Iron Loss/ Core Loss:

1. Hysteresis Loss2. Eddy Current Loss

Copper Loss: Mechanical Loss:

1. Friction Loss2. Wind Loss

Power Stage for D.C. Motor

Example

Testing O D.C. Machine In order to check performance such as efficiency,

regulation, losses, change in speed, condition of commutation, temperature rise etc.1. Direct method of testing:

Machine directly connected with load/pulley and break arrangement.2. Indirect method of testing:

No Load test data obtain and parameters calculate.3. Regenerative method of testing: