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5.7 Induction (asynchronous) motors
5.7.1
General characteristics
The function of the motor is to rotate the drum at different speeds:
high speed for the spin phases
low speed for the wash phases
The primary characteristics of these motors is that they do not require a start-up current. Instead, the
current in the rotor builds up by induction, hence the name "induction" motor. These motors are also
termed "asynchronous" because the speed of rotation is not the same as the synchronization speed.
5.7.2
Operating principle
The three-phase asynchronous motors consist of a stator on which the solenoids are winded (windings
impregnated with resins which guarantee an optimal water protection) which are the polar expansions.
These are always a three-multiple number.
Inside the stator, there is an integral rotor to the shaft made up of a group of magnetic blades including also
(normally die-cast aluminium) a circuit called squirrel cage, because it consists of a series of bars forming a
cylinder between two rings.
When the stator generates a rotating magnetic field, electrical currents are inducted into the cage. These,
opposing to the generator field, produce a twisting movement to the rotor.
The maximum rotation per minute of an asynchronous motor depends on the power frequency and on the
number of polar couples.
The three-phase motors are highly efficient due to the absence of brushes. They can be powered also with
inverter circuits, starting from a direct current power source.
The stator windings can be connected by star or by triangle.
5.7.3
Checking the efficiency
It is possible to have an indication of the efficiency of the
motor by measuring the windings resistances:
∼
Winding y ohm 5,6
±7% (contacts 2-3)
∼
Winding x ohm 5,6
±7% (contacts 2-1)
∼
Winding z ohm 5,6
±7% (contacts 3-1)
Winding T (tachometric) ohm 120
2005.11 dmm
∼
±7% (contacts 4-5)
40/60
599 37 38-46
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