A universal motor is a special type of motor designed to run on single-phase DC or AC power. These motors are usually wound in series (inductor and field winding are in series) and therefore produce high starting torque (see characteristics of DC motors here). That's why universal motors are often built into the device they are supposed to drive. Most universal motors are designed to operate at higher speeds, in excess of 3500 RPM. They run at a slower speed on AC power than on DC power of the same voltage due to the reactance voltage drop which is present in AC and not DC.
There are two basic types of universal motor: (i) compensated type and (ii) uncompensated type
Universal engine construction
Building a universal motor is very similar to building a DC machine. It consists of a stator on which the field poles are mounted. Field coils are wound on the field poles.
However, the entire magnetic path (stator field circuit and also armature) is laminated. Lamination is necessary to minimize the eddy currents they induce when operating on AC.
The rotating armature is of the wound type with straight or inclined slots and commutator with brushes resting on it. Switching on AC is poorer than on DC. due to the current induced in the armature coils. That's why the brushes used are of high resistance.
A universal motor runs on single-phase DC or AC power. When the universal motor is supplied with DC power, it works as a series DC motor. (see here the operation of a DC series motor). When current flows in the field winding, it produces an electromagnetic field. The same current also flows from the armature conductors. When a current conductor is placed in an electromagnetic field, it experiences a mechanical force. Due to this mechanical force, or torque, the rotor starts to rotate. The direction of this force is given by Fleming's left-hand rule.
When powered with AC power, it still produces unidirectional torque. As the armature winding and field winding are connected in series, they are in the same phase. Therefore, as the AC polarity periodically changes, the direction of current in the armature and field winding reverses at the same time.
Thus, the direction of the magnetic field and the direction of the armature current are reversed such that the direction of force experienced by the armature conductors remains the same. So, regardless of AC or DC power, the universal motor works on the same principle as the DC series motor.
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9 окт 2022
