(introduction...)

Induction motors are mainly used to drive household and office machinery as well as smaller electrical tools. The peak power range of these motors is around 2 kW.

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 · F₁ · I₂ · cosj₂

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.

Figure 113 - Single-phase motor with auxiliary winding and (1) starting capacitor CA, (2) operating capacitor CB, (3) starting and operating capacitor

1 Starting capacitor, 2 Operating capacitor

The rotor of the single-phase asynchronous motor generally has a squirrel cage.

7.1.2. Operational behaviour

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

7.1.3. Technical data

Several examples of technical data feature in Figures 116 and 117 and in Surveys 16 and 17.

Foot induction motors

Figure 116 - Dimensional images of a foot induction motor (e.g. 65/IM 1001)

(1) Length side, (2) Drive side
1 Axial pressure, 2 Stop socket Pg9, 3 Protective conductor, 4 Air entry, 5 Minimal distance

Survey 16 - Characteristic values of foot induction motors

Design/nominal size	Rated voltage (Ws)	Rated current	Power input	Power output	Speed
-	V	A	W	W	rpm
65/IM 1001	220	0.30	60	10	1400
		0.42	80	16	2800
1)	220/380	0.38/0.22	70	16	1400
		0.49/0.28	105	25	2800
75/IM 1001	220	0.52	100	25	1400
		0.62	125	40	2800
	220/380	0.59/0.34	110	40	1400
2)		0.73/0.42	160	60	2800

1) C = 2µF 2) C = 2.5µF

Flange induction motors

Figure 117 - Dimensional images of a flange induction motor (e.g. 75/IM 3601)

(1) Length side, (2) Drive side,
1 Air entry, 2 Screw-in depth max. 12 mm

Survey 17 - Characteristic values for flange induction motors

Design/nominal size	Rated voltage (Ws)	Rated current	Power input	Power output	Speed
-	V	A	W	W	rpm
65/IM 3601	220	0.38 1)	80	16	1400
		0.50 2)	100	25	2800
	220/380	0.42/0.24	75	25	1400
		0.56/0.32	115	40	2800
75/IM 3601	220	0.58 3)	120	40	1400
		0.74 4)	150	60	2800
	220/380	0.70/0.40	150	60	1400
		1.08/0.60	200	90	2800

1) C = 2µF 2) C = 3µF 3) C = 5µF 4) C = 6µF