|Electrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)|
|7. Single-phase alternating current motors|
|7.1. Single-phase asynchronous motors (single-phase induction motors)|
The main winding of these motors is connected directly to the mains whilst the auxiliary winding is connected by means of a capacitor. The current which flows through the auxiliary winding is therefore phase-displaced with regard to the current of the main winding. The windings yield a rotating field which enables the motor to start on its own.
Rotational direction reversal, as in the case of a three-phase motor, becomes possible through a directional change of the rotating field. This ensues by altering one of the two current directions in the windings, that is to say by varying the connections of one of the two windings.
Motor with starting capacitor
Following successful starting the auxiliary winding is disconnected from the mains through a current-dependent, auxiliary contactor or by means of a centrifugal switch positioned on the motor shaft (Figure 113(1)). As a result this motor behaves no differently than a motor without auxiliary winding.
Motors with starting capacitors can develop powerful torques whereby the starting current does not exceed three to five times the rated current.
Recommended values for rating the starting capacitor for a 220 V motor are featured in Figure 114.
Figure 114 - Magnitude of the starting capacitor C related to motor power P and starting torque Ma
Motor with operating capacitor
One refers to a motor with operating capacitor (Figure 113(2)) where the capacitor and, thus, also the auxiliary winding both remain permanently switched on after starting. The capacitor has been dimensioned for rated operation; however, the motor only develops a minimal torque because of the limited capacity of this operating capacitor.
Motor with starting and operating capacitors
The most advantageous operational behaviour of a single-phase motor is given when the auxiliary winding is connected by means of two capacitors corresponding to the capacity for starting resp. for rated operation (Figure 113(3)). Both capacitors of this so-called double capacitor motor are switched on during starting and enable the motor to develop a powerful torque. Following acceleration the capacity is reduced to that of an operating capacitor. This ensues manually, through a contactor or by means of a centrifugal force switch.
The rotational torque curve during starting evidences a favourable sequence (curve of the motor with starting capacitor) and, in rated operation, switches to the curve of the motor with operating capacitor.
Figure 115 - Three-phase torque curve of a single-phase asynchronous motor
1 Without capacitor in the auxiliary winding, 2 With starting capacitor, 3 With operating capacitor, 4 With starting and operating capacitor