|Electrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)|
|5. Asynchronous motors|
The magnetic flow change required in the rotor winding is generated by the stator by means of a circulating field (rotating field).
In this manner the stator aligns itself to a marked extent to the synchronous machine and, like this machine, also features a three phase winding (Cp. Figure 37) able to generate a rotating field. Machines with an operating voltage of 660 V have all winding origins and ends connected to the terminal board. Consequently, the strands can be interlinked in a star-delta arrangement (Cp. Figure 48). In the case of higher operating voltages interlinkage often ensues inside the machine.
Figure 48 - Circuits of the strand windings at the terminal board
U; V; W winding designations
The asynchronous machine is the sole rotating electric machine in which the transmission of electric energy to the rotor does not ensue electromechanically but inductively. Both stator and rotor of the asynchronous machine may be compared to the parts of a transformer whose secondary section has been arranged to rotate.
Two different rotor types of asynchronous motors are customary.
This rotor has a squirrel cage armature and rods of copper, bronze or aluminium have been positioned in the grooves of the cylindrical rotor lamella pack and short-circuited to each other through end windings.
Figure 49 - Squirrel cage rotor
1 (aluminium, bronze, copper)
Slip ring rotor
The slip ring rotor has a three-phase winding similar to the one of the stator winding.
Figure 50 - Slip-ring rotor
The three spatially positioned coils within 120 degrees are star connected and the origins connected to three slip rings. In operation these slip rings are short-circuited. Thereby there is no difference in the mode of operation of the slip ring rotor and the squirrel cage rotor. Figure 51 shows a sectional presentation of the overall construction of a squirrel cage motor.
Figure 51 - Squirrel cage motor, protection degree IP34 and design IM 1001
1 Stator, 2 Rotor, 3 End shield to the A-side, 4 End shield of the B-side, 5 Ventilator, 6 Ventilator hood, 7 Foot, 8 Bearing lid (external), 9 Bearing lid (internal), 10 Slip-ring capsule, 11 Counterbalance disk, 12 Slip-ring body, 13 Brush holder, 14 Cover sheet, 15...17 Stator terminal boxes