Electrical Machines  Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.) 
5. Asynchronous motors 
5.3. Operational behaviour 

Inrush current origin
As soon as the current is switched on the rotating field rotates at full speed along the rotor bars of the squirrel cage rotor.
The equation
_{}
(Cp. Figure 52) indicates that as s = 1 (U = U), a greater inrush current is attained in the rotor which is transmitted transformerwise to the stator side.
Every asynchronous motor accepts a higher current when starting from noload position.
This inrush current, when utilising the full mains voltage, can be four to eight times as great as the rated current.
This excessive current load can lead to a disruptive voltage drop in the network. Consequently, for example, only motors with a rated performance of up to 2.2 kW may be connected directly to the 380 V network where the making current exceeds the rated current by more than seven times over. Higher powered motors require special measures for cutting back the considerable starting current.
Starting torque
Incorporating the equation
_{}
we derive for the rotor current I_{2}
_{}
The rotor note only features the small ohmic resistor R_{2} but also the inductive resistor
X_{L2} = s · w_{1} ·L_{2} = s · 2 · p · f_{1} · L_{2}
During the switching torque the resistance attains its maximum value as s = 1 and is therefore greater than the ohmic resistance.
Figure 54  Indicator diagrams of the
rotor circuit resistors
Legend as for Figure 52
The power factor cos j_{2} therefore attains a minimal value and there is similarly only a low starting torque
Despite the considerable inrush current the asynchronous motor only evidences a minimal torque when starting from noload position.
Measures to restrict the starting current
All drive operations presuppose a sound starting up, that is to say, a sufficiently high motor torque. Consequently measures must be undertaken to boost the starting torque. However, the network load which arises during start operations which may be evidenced in a voltage decline or through the inrush current, shall not exceed the prescribed values. It is therefore essential effectively to limit starting current. A simultaneous increase in starting torque is also often requested.
Starting current restriction becomes possible by
 decreasing U_{2.0}: a lesser stator voltage is fed to the motor (U_{1} ~ U_{2.0}) during starting operations. This leads to a starting procedure for which additional devices are required to connect the short circuit motor. increasing R_{2}: increasing the rotor resistance R whilst starting requires a differently constructed rotor. The shortcircuit rotor must be replaced by a differently arranged rotor featuring changeable ohmic resistance facilities.
In the equation M = C_{2} · F · I_{2} · cos j_{2} all physical values have been incorporated which might influence the torque. Such an optimal solution denotes that such values are changed which permit the starting torque to increase without increasing the starting current. This demand is only met if cos j_{2} is increased. The power factor is boosted by means of an ohmic resistor at the rotor circuit resistance. This in turn makes necessary a different rotor construction from the shortcircuit rotor.
Additional facilities make it possible to decrease the high starting current of the squirrel cage rotor motor (Cp. 5.4.1). A reduction of the starting current whilst simultaneously increasing the starting torque is only possible where differently constructed rotors are used which evidence a greater ohmic resistance during starting operations.