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close this bookElectrical Machines - Basic Vocational Knowledge (IBE - Deutschland; 144 pages)
View the documentIntroduction
Open this folder and view contents1. General information about electrical machines
Open this folder and view contents2. Basic principles
close this folder3. Execution of rotating electrical machines
View the document3.1. Size
Open this folder and view contents3.2. Designs
Open this folder and view contents3.3. Degree of protection
Open this folder and view contents3.4. Cooling
close this folder3.5. Mode of operation
View the document3.5.1. Definition
View the document3.5.2. Operational mode designation
View the document3.5.3. Frequent nominal cycle ratings
View the document3.6. Heat resistance categories
Open this folder and view contents3.7. Connection designations of electrical machines
Open this folder and view contents3.8. Rotating electrical machines in rotational sense
View the document3.9. Rating plate
Open this folder and view contents4. Synchronous machines
Open this folder and view contents5. Asynchronous motors
Open this folder and view contents6. Direct current machines
Open this folder and view contents7. Single-phase alternating current motors
Open this folder and view contents8. Transformer

3.5.1. Definition

Operating an electric motor always gives rise to undesirable energy conversion. This in turn leads to heating up which, above all, strains the winding insulations. The service life of a machine is decisively influenced by its inside temperature. Thermal overloading can engender operational disturbances. Estimates indicate that a temperature increase of 8K reduces machine life by 50 per cent. Heating up results first and foremost through energy passage in the windings. The designation W = I2• R • t shows that the conversion into heat and the related temperature rise are determined by the current flow and its duration. Temporary overloading is permissible as, due to thermal inertia, the temperature increase remains insignificant.

A torque increase for the work unit, respectively a mass inertia when starting or braking give rise to greater losses in the motor through the flow of higher starting or braking currents. Load starting and braking thus exert an influence on the degree of heating up. Consequently, for reasons related to thermal load, electric motors must be aligned to the load rhythm of the work unit.

Mode of operation relates to the nature and sequence pattern, equally the duration of standstill and idling times, also to the nominal load of electrical machines.
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