Unit – 1 Introduction
Part – 4
Rating of Machines &
Temperature Rise
N.Pandiarajan
Associate Professor/EEE
SSNCE
Rating of Machines


Rating of machines refers to the whole of the
numerical
values
of
electrical
and
mechanical quantities with their durations
and sequences assigned to the machines by
the manufacturer.
(stated on the name plate, the machine
complying with the specified temperature
conditions.)
Rating of a motor

Power output or the designated operating
power limit based upon certain definite
conditions assigned to it by the
manufacturer.
Temperature
 An
electric motor is normally
rated upon thermal basis of
temperature rise i.e., maximum
possible temperature at which
the insulating materials may be
operated without deterioration
Types of duties and ratings
The
types of ratings are
defined for electric motors
depending upon the load and
duration. The following are the
types of duty.
Types of duties and ratings
1.
2.
3.
4.
= Continuous duty.
S2 = Short time duty.
S3 = Intermittent periodic duty.
S4 = Intermittent periodic duty with
S1
3
starting.
Types of duties and ratings
S5 = Intermittent periodic duty with
starting and braking.
S6 = Continuous duty with intermittent
periodic loading.
S7 = Continuous duty with starting and
braking.
S8 = Continuous duty with periodic speed
changes.
Continuous Duty (S1)
(Continuous Rating)
•
Duration of load is for a sufficiently
long time such that all the parts of the
motor attain thermal equilibrium i.e
the motor attains its maximum final
steady temperature rise.
Examples: running fans, pumps.
Continuous Duty (S1)
Short Time Duty (S2)
Definition
The motor operates at a constant load for
some specified time which is then followed
by a period of rest.
 Motor
cannot reach its thermal
equilibrium.
Examples-Railway turntable, navigation
lock gates.

Short Time Duty (S2)
Intermittent Periodic Duty (S3)
 On
intermittent duty the periods of
constant load and rest with machine
de-energized are alternate.
 Load periods are too short and periods
of rest also too small.
Example: cranes, lifts and certain metal
cutting machine tool drives.
Intermittent Periodic Duty (S3)
 Duty
factor is defined as the ratio
of the heating period to the period
of whole cycle.
 Duty factor is used for evaluation
of intensity of heating.
Intermittent Periodic Duty
Intermittent Periodic Duty
with Starting

This type of duty consists of a sequence of
identical duty cycles each consisting of a
period of starting, a period of operation at
constant load and a rest period.
Example: Metal cutting, drilling tool
drives and mine hoist.
Intermittent Periodic" Duty with
Starting and Braking (S5)

This type of duty consists of a sequence of
identical duty cycles each consisting of a
period of starting, a period of operation at
constant load, a period of braking and a
rest period.
 Example:Billet
mill drive, manipulator
drive, machine tool drives, drive for
electric train.
Continuous Duty with
Intermittent Periodic Loading
 This
type of duty consists of a
sequence of identical duty cycles each
consisting of a period of operation at
constant load and period of operation
at no load.
 Example: Pressing, cutting, shearing
and drilling machine.
Continuous Duty with
Starting and Braking

This type of duty consists of a sequence of
identical duty cycles each having a period of
starting, a period of operation at constant load
and a period of electric braking.
There is no rest or de-energized period.
Duty factor for this duty cycle is 1

Example: Main drive of a blooming mill.


Continuous Duty
with Periodic Speed Changes

Consists of a sequence of identical duty
cycles each consisting of a period of
operation at constant load corresponding to
a determined speed of rotation, followed
immediately by a period of operation at
another load corresponding to a different
speed of operation.
Methods used for determination of
motor rating for variable load drives
i) Method of average loss
ii) Equivalent current method.
iii) Equivalent torque method
iv) Equivalent power method.
Method of average losses
Standard Specifications



The specifications are guidelines for the
manufacturers to produce economic products
without compromising quality.
The manufacturers who are compiling with the
standards will be issued a certification for their
products.
The quality of the certified products will be
periodically monitored by the standard
organisation.
Standard Specifications of
Electrical machines
1. Standard ratings of machines.
2. Types of enclosure.
3. Standard dimensions of conductors to be
used.
4. Method of marking ratings and name
plate details.
Standards Organisation



In India, the International Organisation for
Standards (ISO) has laid down their
specifications (ISI) for various products.
Recently they have released revised standards
ISO 9002, to comply with international
standards.
Example: IS 1180 – 1964 - Specifications for
outdoor 3-phase distribution transformer up to
100 kVA.
Temperature Rise
in a machine


The temperature of a machine rises when it
is run under steady load conditions starting
from cold conditions.
As the temperature rises, the active parts of
the machine dissipate heat partly by
conduction, partly by radiation, and in most
cases, largely by means of air cooling.
Derivation of Heating and Cooling
Time Constants
Heating
At any time t from start, heat energy
developed in the body during
an infinitely small time dt
= heat energy developed/see x dt = Q dt
In these period dt the temperature of
the body rises by d, the
heat energy stored in the body is
givenby
= weight of body x specific heat x
difference in temperature
Heat energy dissipated by the body
=Specific heat dissipation x Surface
x Temperature rise x time
Heat energy developed is equal to heat energy
stored plus heat energy dissipated by the body
The machine reaches a final steady state
temperature rise em when t =infinity. Under
this condition rates of heat production and
dissipation are equal when the machine
attains final steady temperature rise
Heating time constant
 Heating
time constant is defined as the
time taken by the machine to attain
0.632 of its final 'steady temperature
rise.
Cooling Curve
Cooling time constant
 Cooling
time constant is defined as the
time taken by the machine for its
temperature rise to fall to 0.368 of its
initial value.
Problem
A
field coil has a heat dissipating
surface of 0.15 m2 and a length of
mean turn of 1m. It dissipates loss of
150 W, the emissivity being 34 W/m20C.Estimate
the
final
steady
temperature rise of the coil and its
time constant if the cross section of
Problem
of the coil is 100 x 50 mm2. Specific
heat of copper is 390 J/Kg -oC. The
space factor is 0.56. Copper weighs
8900 kg/m3