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.
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.
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