Unit – 1 IntroductionPart – 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.

TemperatureAn 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 ratingsThe 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 ratings1. S1 = Continuous duty.

2. S2 = Short time duty.

3. S3 = Intermittent periodic duty. 3

4. S4 = Intermittent periodic duty with

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

At any time t from start, heat energy developed in the body during an infinitely small time dt

Heating

= heat energy developed/see x dt = Q dt

In these period dt the temperature of the body rises by d, theheat 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/m2-0C.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