Motors, generators, transformers and reactors

Asynchronous motors (squirrel cage induction motors, the rotor-stator part of slipring induction motors) and the rotor-stator part of synchronous motors are within the scope of increased safety due to the omission of sparks and arcs assuming an appropriate construction of rotor and windings. Tire standards IEC 60079-7 and EN 50019 contain mechanical and thermal requirements that follow this aim. 

To ensure a certain mechanical robustness, the nominal conductor diameter of wires for windings shall be at least 0.25 mm. (Resistance thermometers and their terminals are not subject to this limitation.) 

In consequence, e motors, generators, transformers and reactors designed for the common rated supply voltages 500, 690 V and 1000 V are not suitable for very low power ratings. Smaller conductor diameters ask for another type of protection, e.g. flameproof enclosure - d -, encapsulation - m or powder filling - q -, the last mentioned restricted to transformers and reactors. 

For e-rotating electrical machines, IEC 60079-7 and EN 50019 prescribe the minimum radial air gap (at standstill) according to  

D = rotor diameter (in mm), in the formula above there are lower and upper limits with 75 =s D =s 750 

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