When the motor rotates in the state shown in Figure 2.33 (a), try reversing the voltage polarity applied to the motor. If, as in this figure (b), the polarity of the magnet does not change, the current flowing to the rotor will reverse and the rotor will reverse rotation.
However, in the case of a series-wound motor, as shown in Figure (c), the polarity of the magnet will also reverse. Therefore, the rotation direction will not change.
That is, this kind of motor has the feature of continuing to rotate under AC power supply.
However, if AC is applied to a series-excited motor designed for DC use, a kind of failure will occur. That is, in motors where the stator core uses cast iron and mild steel, currents caused by electromagnetic induction (called eddy currents) will occur inside the core and generate a lot of heat. Therefore, in commutator motors using AC, like induction motors, the core adopts an insulated laminated structure using silicon steel sheets.
This kind of motor can rotate using both alternating current and direct current. Therefore, it is also called a universal motor. However, in practical use, it is usually operated using alternating current. Therefore, it is called an AC commutator motor.
Characteristics and deficiencies
The AC commutator motor has the following characteristics.
・It can use the household AC100V power supply
・It adopts an induction motor and can rotate at high speed.
・When the load increases, the rotational speed decreases and the torque increases.
・The starting torque is relatively large.
Due to the above characteristics, therefore, it is generally used in home appliances such as vacuum cleaners and electric drills (Figure 2.34), which require light weight and high output, or in tools such as juice extractors and coffee bean grinders that need induction motors for high-speed rotation.
On the other hand, AC commutator motors also have the following disadvantages:
・Generate noise
・Considering the service life of the brushes, it is not suitable for continuous operation, etc.
