SM mechanical characteristics should consider for two modes

The 1^st mode: Operation mode, when the rotor is in synchronism and loaded by resistance moment.

The 2^nd mode: Since SM doesn`t have own starting moment its starting makes certain problem particularly when stator supplying without frequency regulating.

Mechanical static SM characteristic is at operation mode, fragment a presents the straight line by itself parallel to abscissa axis, i.e. this characteristic is absolutely rigid, speed under load changing is still constant. Independently from any factors except frequency the only condition of this characteristic existing is condition to operate in overload capacity limits; otherwise SM is not in synchronism.

а – operation mode of already synchronous motor;

б – starting mode at asynchronous starting.

Figure 3.50 – Mechanical characteristics.

Mechanical characteristics of starting mode, fragment of a figure b are constructedin that case when the starting windings are used for realizing of starting. SM in this case has a special starting winding in a form of short-circuited cage, and DM at starting operates as ordinary AM. SMs have two types of starting cages.

The 1^st type. Characteristic 1 corresponds to it. Cage has low active resistance, therefore starting moment is insignificant, though sliding value at speed (and at rated resistance moment) is small, i.e. rated speed is close to synchronous speed (speed of coming into synchronism). Thereby the 1^st type provides the moment of coming into synchronism, but synchronous moment is small.

Such starting cages are used in that drives where starting conditions are simplified. Firstly it is mechanisms with fan characteristic (resistance moment begin to increase from unoperating run with the speed increasing: fans, air blowers, centrifugal pumps, propellers and etc.).

The 2^nd type. Characteristic 1 corresponds to it. Cage has raised active resistance, therefore starting moment is significant, though sliding value at speed (and at rated resistance moment) is significant, i.e. speed of coming into synchronism significantly differs from synchronous, that eventually complicates the process of coming into synchronism.

Such starting cages are used in that drives where the static resistance moment at starting time is high, for example, unregulative rolling mills.

Starting cages of synchronous motors is rated for short-time mode (only for starting moment) with duration up to 30 s. Prolonged starting time will cause the cage overheating.

Дата добавления: 2014-01-05