LOADING CONDITIONS AND CLASSES OF DUTY

TYPES OF LOAD:

            1. Continues duty load

            2. Short time duty load

            3. Intermittent periodic duty load

1. Continues duty load:

            On this duty the duration of load is for 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. Drives continious duty are continiously running for long time. The continious rating of a motor may be defined as the load that may be carried by the machine for an indefinite time without the temperature rise of any part exceeding the maximum permissible value.

Example:

Ø  Fans

Ø  Pumps 

2. Short time duty load:

            The motor operates at a constant load for some specified time which is then followed by a period of rest. The period for load is so that the machine cannot reach its thermal equlibirium i.e. steady temperature rise while the period of rest is too long that the motor temperature drops to the ambient temperature. Standard short time ratings are: 10, 30, 60, and 90 minutes.

Example:

Ø  Railway turntable

Ø  Navigation lock gates

3. Intermittent periodic duty load:

            On intermittent duty the periods of constant load and rest with machine de-energised alternate. The load periods are too short to allow the motor to reach its final steady state value while the period of rest are also too small to allow the motor to cool down to the ambient temperature. The load period is about 10 minutes long.

Example:

Ø  Cranes

Ø  Lifts

Ø  Metal cutting machine


CLASSES OF DUTY 

                                  1. Continuous duty

                                  2. Intermittent duty

                                  3. Short time duty

1. Continuous duty:

                        Continuous duty is the term adopted to describe the service condition where in the running period is of sufficient duration for the temperature rise to attain its steady state value. It is not necessary that the load be constant throughout. Several types of equipments run continuously with a constant load. Examples are pumps, fans, compressors and conveyors. The load curve of this service condition is shown in figure.

Continuous duty with constant load:

Continuous duty with variable load:

2. Intermittent duty:

                        For intermittent duty the running periods and pauses are not long enough to enable the temperature to rise to steady state during the former period and the motor cool down during the later period. Several kinds of hoists, machine tools and forging equipments are subjected to intermittent duty. Special motors are manufactured for these service conditions. They are given an equivalent continuous load name plate rating for a certain duty factor. The load curve of this service condition is shown in figure.

3. Short time duty:

                        The machine on short time duty the temperature rise does not reach its steady state value during the working period. Moreover the idle period between consecutive working periods is of sufficient duration for complete cooling to take place. Auxiliaries for several kinds of machine tools and also for cranes, hoists, etc. remain idle or keep running at no load for a long time after each working cycle. The load curve for this service condition is shown in figure.

 


 


 


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