7.1.1. Assembly and operating principle
The torque of a three-phase asynchronous motor (Cp. Section 5.2.1.) stems from an induction voltage generated in the rotating field of the rotor windings. This induction voltage yields rotor current I2:
M = C • Φ1• I2• cosφ2
Thus, torque only arises given a relative movement between the stator field and the rotor winding. Where the lead to a winding strand in a three-phase asynchronous motor is interrupted, the said motor runs single-phased. Consequently, no torque is forthcoming as long as there is no relative movement between the stator field and the conductor arrangement in the rotor.
The single-phase driven axynchronous motor develops a torque during operation, but not whilst idling; nor can it start off its own bat.
Asynchronous motors for single-phase operation exist wherever there is no three-phase connection, and are very much desired.
However, such motors must be able to start themselves. Precondition is that a rotating field is created to replace the alternating field. This, however, is only possible if spatially positioned coils are saturated by temporally displaced currents.
Every single-phase asynchronous motor which is to start itself, must have two windings whereby the second spatially positioned winding must be saturated by a current which has been phase-displaced opposite the current of the first winding.
This second winding need only be switching on for starting and is characterised as auxiliary winding. The permanently switched on main winding covers some two thirds of the stator circumference whilst the auxiliary winding fills in the remaining part of the grooves in the lamella pack.
The single-phased asynchronous motor yields an ideal rotating field if the main and auxiliary windings are repositioned at 90 degrees and the phase displacement of the strand currents is also 90 degrees. Such operation can be virtually attained given single-phase feeding provided a capacitor is switched to the auxiliary winding. This capacitor must have a capacity in line with the rated load and desired starting behaviour.
1 Starting capacitor, 2 Operating capacitor
The rotor of the single-phase asynchronous motor generally has a squirrel cage.
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