Electrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.) 7. Single-phase alternating current motors (introduction...) 7.1. Single-phase asynchronous motors (single-phase induction motors) (introduction...) 7.1.1. Assembly and operating principle 7.1.2. Operational behaviour 7.1.3. Technical data 7.2. Three-phase asynchronous motor in single-phase operation (capacitor motor) 7.2.1. Assembly and operating principle 7.2.2. Operational behaviour 7.3. Split pole motors 7.4. Single-phase commutator motors (universal motors) 7.4.1. Assembly 7.4.2. Operating principles 7.4.3. Operational behaviour 7.4.4. Technical data

Electrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)

7. Single-phase alternating current motors

Single-phase alternating current motors have assumed importance these days in particular as electrification in the home and at work continues apace. Indeed, such motors are especially suitable for automation techniques by means of economical, labour and time-saving equipment. Generally speaking low-powered motors suffice to drive such devices and, consequently, such motors can be connected to the single-phase network.

7.1. Single-phase asynchronous motors (single-phase induction motors)

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

7.2. Three-phase asynchronous motor in single-phase operation (capacitor motor)

7.2.1. Assembly and operating principle

A three-phase asynchronous motor whose stator winding has been designed for 220/380 V may be selectively driven through either a single-phase of three-phase mains. The winding is star-connected for 380 V voltage. The winding is delta-connected for single-phase operation. In addition, an operating capacitor is parallel switched to each strand.

A capacitor provides phase-displaced current for the third strand winding this yielding a rotating field. The rotational direction is altered by repoling the capacitor connection.

Figure 118 - Three-phase asynchronous motor as capacitor motor

1 Anti-clockwise, 2 Clockwise

7.2.2. Operational behaviour

Initial torque is only some 30 per cent of rated torque. Consequently, in some cases it becomes necessary to parallel switch a starting capacitor of approx. 150 F/kW to the operating capacitor. However, because of its excessive current acceptance after starting, this operating capacitor should be switched off, for example by means of a centrifugal switch.

The magnitude of the operating capacitor must be selected in accordance with a motor power of some 70 F/kW and in line with voltage level.

7.3. Split pole motors

Such a motor has pronounced poles with exciter winding in the stator in a similar manner to the direct current machine. Part of the main pole surface has been separated by a split in the pole and enclosed by a copper ring. The rotor features a squirrel cage of aluminium.

Figure 119 - Assembly of a split-pole motor

1 Exciter winding, 2 Short-circuit ring, 3 Squirrel cage rotor, 4 Main pole, 5 Split pole

In principle the split pole motor is a single-phase motor with permanently switched on auxiliary winding (short-circuit ring). The exciter winding establishes an alternating field which also extends to the short-circuit ring. Thereby a voltage is induced in the short-circuit ring capable of driving a powerful current into the ring. This yields an alternating field in the split pole which has not only been spatially displaced against the alternating field of the main pole, but also has a delayed action effect, that is to say is temporally shifted. The preconditions for a rotating field have been met: Interacting with the rotor induction currents, a torque is yielded which is sufficient for motor self-starting. The alternating field of the split pole interacts temporally displaced as compared to the alternating field of the main pole; this yields the rotational field direction from the main to the split pole. The field direction of rotation is thus constructionally conditioned. A directional change in the rotating field and, thereby, rotational direction reversal of the rotor is not possible with split pole motors. In view of the substantial copper loss in the squirrel ring, the efficiency of these motors is extremely limited (20 to 40%). Consequently, the motors can only operate economically up to a power of approx. 2 kW. Their starting current seldom exceeds twofold rated current.

7.4. Single-phase commutator motors (universal motors)

7.4.1. Assembly

A universal motor can be driven both by single-phase and direct current voltage. Both assembly and circuitry correspond to direct current series motors.

Figure 120 - Circuitry of a universal motor with interference suppression capacitor

1 Direct current
2 Alternating current
3 Terminal designations

Because of the low power and subsequent minimal incidence of commutator sparking, universal motor stators dispense with interpole and compensation windings.

Figure 121 shows the lamella section of a universal motor. The exciter windings, also known as pole or field coils, have been positioned on the pole core.

Figure 121 - Lamella form of a universal motor

7.4.2. Operating principles

The rotational direction of a direct current motor changes either when the rotor current I₂ changes its direction (-M = C · F · (-I)) or the exciter current alters in the exciter winding (-M = C · (-F₁) · I₂).

Where both values change, which corresponds to exchanging the conductor mains of a direct current series motor, then the motor retains its rotational direction:

M = C · (-F₁) · (-I₂)

Therefore a single-phase commutator motor also operates in case of alternating voltage.

7.4.3. Operational behaviour

The value of the yielded torque is also determined in universal motors by means of the general motor equation. As in the case of the direct current series motor, a considerable torque is developed at low speed. Figure 122 depicts the speed-torque curve.

Figure 122 - Speed-torque curve of the universal motor

As universal motors may be driven by either direct or alternating voltage, it is necessary to heed that the inductive resistance is absent during direct voltage connection. Given alternating voltage connection there is rather more brush sparking because of commutator current change and alternating voltage current direction change. Pole gaps remain small in the rotor field and brush sparking is within acceptable limits. The disruptive effect of brush sparking on radio reception can be eliminated by switching on capacitors (Figure 120).

The circuitry also indicates that, when direct voltage is connected, the number of turns at like voltage and speed have to be increased as compared to alternating voltage feeding. The greater number of turns compensates for the lacking resistance. Although inrush current is greater than rated current there is no likelihood that small motor power might be impaired through disruptive mains overloading. A rotational direction change can be attained in universal motors by switching over the winding at the terminal board. However, where field and armature windings have been soundly connected in series, rotational direction change is not possible. Universal motors are especially suitable for electrical small tools, household equipment and office machinery. Such motors also figure in hoovers, coffee machines and drills.

7.4.4. Technical data

Foot commutator motors

Figure 123 - Dimensional images of a foot commutator motor (e.g. 70/IM 1001)

(1) Length side, (2) Drive side, 1;2 Variable

Survey 18 - Characteristic values of foot commutator motors

Design/nominal size	Rated current	Rated speed	Power input	Power output
	A	rpm	W	W
70/IM 1001	0.2	3000	30	12
	0.15	3000	27.5	16
	0.11	5000	25	12
	0.27	5000	48	25
87/IM 1001	0.26	3000	57	25
	0.48	3000	85	40
	0.55	3000	92	50
	0.45	5000	95	40
	0.36	5000	140	80
	0.78	8000	165	100
	1.1	8000	210	125
119/IM 1001	1.2	3000	180	125
	1.7	5000	300	200
	2.2	8000	450	320

Flange commutator motors

Figure 124 - Dimensional images of a flange commutator motor (e.g. 70/IM 3001)

(1) Length side, (2) Drive side, 1;2 Variable, (3) Earthing screw

Survey 19 - Characteristic values of flange commutator motors

Design/nominal size	Rated current	Rated speed	Power input	Power output
	A	rpm	W	W
70/IM 3001	0.2	3000	30	12
	0.15	3000	27.5	16
	0.11	5000	25	12
	0.27	5000	48	25
87/IM 3001	0.26	3000	57	25
	0.48	3000	85	40
without terminal boxes	0.45	5000	95	40
	0.78	8000	165	100
87/IM 3001	0.55	3000	92	50
with terminal boxes	0.63	5000	140	80
	1.1	8000	210	125
119/IM 3001	1.2	3000	180	125
	1.7	5000	300	200
	2.2	8000	450	320

Built-in commutator motors

Survey 20 - Characteristic values of built-in commutator motors

Design/nominal size	Rated voltage (Ws)	Rated current	Power input	Power output	Speed
	V	A	W	W	rpm
Nominal sizes	220	0.6	105	50	3000
		0.72	125	63	3000
		0.86	165	80	3000
87/IM 5001		0.85	150	80	5000
		1.0	175	100	5000
		1.2	200	125	5000
		1.2	235	125	8000
		1.3	260	160	8000
		1.65	315	200	8000
Nominal sizes	220	0.13	24	6	3000
		0.17	28	8	3000
		0.20	31	10	3000
52/IM 5001		0.18	37	10	5000
		0.18	39	12	5000
		0.25	47	16	5000
		0.24	49	16	8000
		0.30	55	20	8000
		0.29	58	25	8000

Questions for repetition and control

1. How does a single-phase induction motor generate its rotating field?

2. Explain the operation of a three-phase asynchronous motor through a single-phase mains.

3. Explain the efficiency principles of the universal motor torque.

4. What must be heeded if a motor which had been connected to the alternating voltage mains, is to be driven by direct current?