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?