Asynchronous machine: Generalities

The asynchronous, or induction, machines are rotating machines used both as generators and as motors: this second application is far the most important and diffused (asynchronous motor).
The asynchronous machines can be divided in:

Three-phase: for mean or big powers, until the megawatt quantity order.
Single-phase: widely used for modest powers (from few watts to hundreds of watt.

The asynchronous machines are basically composed of a fixed part, the stator and a moving one, the rotor.

Stator
The stator, set in the frame, is similar to the armature of the synchronous machine: it is rolled and it shows, along the internal surface, the slots housing the stator windings. The periodicity of these ones determines p polar couples (and 2p poles). Unlike what happens in the synchronous ones, in the asynchronous machines the stator winding generally make up the inductor.

The laminations have to consider the moving parts and therefore the percentage of silicon won't be greater than 1% with a greater thickness to resist to the mechanical characteristics.


S = Stator,  R = Rotor, 1 = stator conductors, 2 = rotor conductors, 3 = air gap.


A=shaft, C=frame, R=rotor, S=stator, T=air gap, Cr=armature conductors, Cs=inductor conductors, Ms=inductor terminals, Mr=armature terminals, sp=brushes, an=ring.

Fig. 1
Rotor
The rotor is laminated too, for big machines, and it shows along the external surface the slots housing the rotor windings. The air gap has a thickness changing from some tenths of millimeters, for small motors, until some millimeters for the ones having a high power. In connection with the typology of the rotor winding, the asynchronous machines can be wound rotor or squirrel cage machines.

Among the different manufacturing typologies of the asynchronous machines we find the following ones:

Wound rotor (or ring) machines
In this type of machine the rotor has windings similar to the stator ones, with the same number of phases and poles.
In the three-phase case they are star connected and the three free terminals are connected to three metallic rings, keyed on the shaft to a rotor end (Figure 1), that, through three brushes, carry out sliding contacts, with which the rotor windings are connected to the starting rheostat
  
Squirrel cage (or short circuited) machines
In the use as motors of small and mean power, the squirrel cage machines are often preferred to the wound rotor ones for their manufacturing simplicity and solidity, together with a lower operation flexibility.
Single cage machines
In this type of machine each rotor slot houses a single conductor or bar, made of copper, bronze or aluminum. All the bars are connected among them in correspondence of each head through two welded rings, by short circuiting them (Figures 2, 3).


Fig. 3: Single cage rotor


Fig. 2
Double cage machines
In this type of machine each slot houses two bars, an external one with a small section and an internal one having a much bigger section, by carrying out in this way two coaxial cages, each one similar to the one of Figure 2. In this case too, all the bars are connected among them in correspondence of each head through welded rings that short circuitthem permanently.


Fig. 4: Double cage rotor

 This solution is used in the motors that have to start under load, because the static torque increases when the static current decreases. The difficulty is born in the scaling of the air gaps that limit their optimum operation.

The advantage is that it can be started with no-load reduced voltage.
In this case it is not necessary to isolate the rotor conductors from the iron.

Rated values

In an asynchronous machine too the rated values locate the entities of the main quantities with which we obtain the optimum operation of the machine.
The main rated values of the asynchronous machine are listed in the Table.

Pn Rated power [W]
Un Rated voltage [V]
In Rated current [A]
cosjn Rated power factor  
fn Rated frequency [Hz]
nn Rotation rated speed  [rpm]
Cn Rated torque at the shaft [Nm]
sn Rated sliding [%]

Rated voltage and current have the meaning of effective values; the rated power has the meaning of converted power (it is therefore measured in W). 
For the single-phase and three-phase machines respectively the following relationships are valid:

Sliding is a characteristic quantity of the asynchronous machine, defining the relative variation of the rotor speed as to the "synchronism" one.