Design and operating conditions

The stator of an asynchronous machine, i.e. the stationary part, is in principle the same as a synchronous machine: three spatially distributed windings generate the rotating field in the stator.

However, the rotor of an asynchronous machine is totally different from that of a synchronous machine. The rotor of an asynchronous machine has neither permanent magnets nor DC windings. There are two different types of rotor for asynchronous machines: the squirrel-cage, or cage rotor, and the slipring rotor.

The three-phase windings of a cage rotor are made of bars that are inside slots in the rotor. Short-circuiting rings connect the ends of the bars.

The ends of the windings of a slipring rotor are only connected internally on one side. The beginnings of the windings are connected via sliprings and graphite brushes to the outside of the machine. There, they can be short-circuited over rotor resistances. This can improve the behaviour of the asynchronous machine during start-up.

The most important advantage of the asynchronous machine is its simple and robust construction since a cage rotor has no sliprings, which could be mechanically strained.

The synchronous speed nS of the rotating field in the stator can again be calculated from the mains frequency f and the pole pair number p:

When the asynchronous machine is at a standstill, the rotating field of the stator passes the standing rotor. This induces a voltage in the conductors of the rotor windings. Due to this voltage, bar-type currents emerge in the bars of the winding in the rotor. The magnetic field of these currents creates a tangential force and the rotor starts to move. In motor mode, the rotor speed n is always lower than the synchronous speed nS of the stator since a speed difference is needed to induce voltages in the rotor. The relatively small difference between rotor speed n and synchronous speed nS is called slip:

If an asynchronous machine works as a generator, the rotor moves faster than the rotating field of the stator. The machine generates active power and feeds it via the stator to the mains. Table 5.7 summarizes different operating conditions of an asynchronous machine.

In contrast to the synchronous generator, the asynchronous generator always needs reactive currents as shown in the next section. An overexcited synchronous machine or power electronics can generate the necessary reactive

Table 5.7 Speed and Slip at Different Operating Conditions for an Asynchronous Machine

Operating condition

Speed

Slip

Standstill (short circuit) Motor mode

Synchronous speed (no load) Generator mode Brake mode n = 0, nS > 0 0 < n < n n = nS n> n

currents. If the asynchronous generator works in an island grid, a capacitor bank can generate reactive currents and provide the required reactive power.

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