SELECTION OF POWER RATING OF MOTORS

 SELECTION OF POWER RATING OF MOTORS

From the point of view of motor rating for various duty cycles. It can be broadly classified

as:

 Continuous duty and constant load

 Continuous duty and variable load

 Short time rating

Continuous duty and constant load

Power rating is calculated and then a motor with next higher power rating from commercially

available rating is selected.

Obviously, motor speed should also match load‟s speed requirement .It is also necessary to

check whether the motor can fulfill starting torque requirement also.

Continuous duty and variable load

 The operating temperature of a motor should never exceed the maximum permissible

temperature, because it will result in deterioration and breakdown of insulation and will

shorten the service life of motors.

 It is general practice to base the motor power ratings on a standard value of temperature, say

35  c.

 Accordingly, the power given on the name plate of a motor corresponds to the power which the

motor is capable of delivering without overheating at an ambient temperature of 35  c. the

duty cycle is closely related to temperature and is generally taken to include the environmental

factors also.

 The rating of a machine can be determined from heating considerations.

 However the motor so selected should be checked for its overload capacity and starting torque.

 This is because, the motor selected purely on the basis of heating may not be able to meet the

mechanical requirements of the basis of heating may not be able to meet the mechanical

requirements of the load to be driven by it.

 The majority of electric machines used in drives operate continuously at a constant or only

slightly variable load.

 The selection of the motor capacity for these applications is fairly simple in case the

approximate constant power input is known

 In many applications, the power input required for a motor is not known before hand and

therefore certain difficulties arise in such cases.

 For the determination of ratings of machines whose load characteristics have not been

thoroughly studied, it becomes necessary to determine the load diagram i.e., diagram shown

the variation of power output versus time.

The temperature of the motor changes continuously when the load is variable. On account of this, it

becomes difficult to select the motor rating as per heating.

 The analytical study of heating becomes highly complicated if the load diagram is irregular in

shape or when it has a large number of steps.

 Therefore it becomes extremely difficult to select the motor capacity through analysis of the

load diagram due to select the motor capacity through analysis of the load diagram due to lack

of accuracy of this method.

On the other hand it is not correct to select the motor according to the lowest or highest load because

the motor would be overloaded in the first case and under loaded in the second case. Therefore it

becomes necessary to adopt suitable methods for the determination of motor ratings.

Methods used

The four commonly used methods are:

 Methods of average losses

 Equivalent current method

 Equivalent torque method

 Equivalent power method

1. Methods of average losses

 The method consists of finding average losses Q av in the motor when it operates

according to the given load diagram.

 These losses are then compared with the Q , the losses corresponding to the continuous

duty of the machine when operated at its normal rating.

 The method of average losses presupposes that when Q av = Q nomn , the motor will

operate without temperature rise going above the maximum permissible for the particular

class of insulation.

 The figure shows a simple power load diagram and loss diagram for variable load

conditions.

 The losses of the motor are calculated for each portion of the load diagram by referring to

the efficiency curve of the motor.

 In case ,the two losses are equal or differ by a small amount ,the motor is selected .if the losses

differ considerably ,another motor is selected and the calculations repeated till a motor having

almost the same losses as the average losses is found.

 Iit should be checked that the motor selected has a sufficient overload capacity and starting

torque.

 The method of average losses dopes not take into account, the maximum temperature rise

under variable load conditions .However, this method is accurate and reliable for determining

the average temperature rise of the motor during one work cycle.

The disadvantage of this method is that it is tedious to work with and also many a times the

efficiency curve is not readily available and the efficiency has to be calculated by means of

empirical formula which may not be accurate.

2. Equivalent Current Method

The equivalent current method is based on the assumption that the actual variable current may

be replaced by an equivalent current i eq which produces the same losses in the motor as the actual

current.

Short time rating of motor

An electric motor of rated power P r subjected to its rated load continuously reaches its

permissible temperature rise after due to time. If the same motor is to be used for short time duty, it

can take up more load for a short period without increasing the maximum permissible temperature of the motor during this period.