Electrical Machines

1. Efficiency of a dc machine and condition
for maximum efficiency
By: Muhammad Fazil Memon
ID: 2852-2016
Electrical Machines

2. Table Of Contents
 Introduction
 DC Machines Fundamentals
 Basic Structure of DC Machines
 Efficiency of a Dc Machine
 Condition for Maximum Efficiency in DC Machine
 Power Flow and Losses in DC Machines
 References

3. Direct Current (DC) Machines Fundamentals
 Generator action: An emf (voltage) is induced in a conductor if it moves
through a magnetic field.
 Motor action: A force is induced in a conductor that has a current going
through it and placed in a magnetic field.
 Any DC machine can act either as a generator or as a motor.

4. Basic Structure of DC Machines
 The rotating electrical or DC machine has mainly two parts; one is Stator, and
another one is Rotar. The stator and rotor are separated from each other by
an air gap. The stator is the outer frame of the machine and is immovable.
The rotor is free to move and is the inner part of the machine

5. Efficiency of a Dc Machine
 Electric motor efficiency is the ratio between power output (mechanical) and
power input (electrical). Mechanical power output is calculated based on the
torque and speed required (i.e. power required to move the object attached to
the motor), and electrical power input is calculated based on voltage and
current supplied to the motor. Mechanical power output is always lower than
the electrical power input, as energy is lost during conversion (electrical to
mechanical) in various forms, such as heat and friction. Design of an electric
motor aims to minimize these losses to improve efficiency.
 For a d.c. machine, its overall efficiency is given by,

6. Continue
Efficiency:
Efficiency = (output / input )*100
Input to the Motor (Total power) = VI
Motor efficiency
Ƞm = 1 – losses / input

7. Condition for Maximum Efficiency in DC
Machine
 It varies from motor to motor, but typically the maximum efficiency is when
running at about 80 percent of maximum speed, driving a load that is about
20 percent of the maximum stall torque. Exact details should be available
from the manufacturer.

8. Power Flow and Losses in DC Machines
 Electrical or copper losses (I2 R losses)
 Brush losses
 Core losses
 Mechanical losses
 Stray load losses
Armature loss:
Field loss:
Copper losses
Brush losses
Core losses
the hysteresis losses and eddy
current losses occurring in the
metal of the motor. These losses
vary as B2 and, for the rotor, as
the (n1.5)

9. References:
1. Dr. Bhag S.Guru, “Electric Machinery and transformers”, oxford university press, 3rd edition.
2. D.R. Kohli-S.K Jain, “A laboratory course in ELECTRICAL MACHINES” Nem Chand & Bros., 2000