A general-purpose motor is a series-excited motor-which […]
A general-purpose motor is a series-excited motor-which means that the magnetic field and the armature winding are connected in series-and this uses carbon brushes and a commutator for mechanical commutation. Although its structure is very similar to a series-excited DC motor, a general-purpose motor contains basic modifications that allow it to run on AC or DC power sources.
When the motor is running on an AC power source, this alternating magnetic flux causes a reactance voltage, which limits the current below the value produced by a DC voltage. In order to limit the influence of the armature reaction, the general-purpose motor uses a compensation winding, which is installed in the stator slot, and the main field winding is connected in series with the armature and field winding at 90 degrees. (This arrangement is called "conduction compensation")
The current flowing in each coil of the compensation winding is opposite to the current in the corresponding armature circuit near it. Coils of equal length in the opposite direction produce a cancellation effect, which reduces inductance and therefore reactance.
AC power also generates more eddy currents, compared to motors running on DC power. In order to reduce eddy current losses, general motors use laminated iron cores (instead of solid iron), which will increase resistance and reduce eddy currents.
The armature reaction is due to the main magnetic flux being disturbed by the armature magnetic flux. The distortion and weakening of the main magnetic flux change the position of the magnetic neutral plane, which is the axis where the carbon brush should be placed.
This is because the armature and the field winding are connected in series. A general-purpose motor either runs on a DC power supply or an AC power supply. Series connection means that both windings are supplied by the same power source, so if the voltage changes polarity, the same is true for the AC power source. The armature and field currents also change polarity, and the direction of the torque does not change. (The direction of rotation is opposite, by reversing the current of the magnetic field circuit).
One of the main advantages of general-purpose motors is that they can obtain unlimited speeds, which can reach 20,000 rpm in real applications. The speed-torque curve is essentially a straight line between the locked-rotor torque (zero speed) and the no-load speed (zero torque). This means that as the load (torque) increases, the speed will decrease. In fact, like a series-excited DC motor, running a general-purpose motor without load (zero torque) will result in an out-of-control situation. The speed keeps increasing until the motor begins to disintegrate. The reversal of this phenomenon is that general-purpose motors produce high starting torque (low speed and high torque).
The ability to run at high speeds means that general-purpose motors are suitable for parts that involve rapid rotation, such as fans, hair dryers, and vacuum cleaners. The ability to generate high torque at low speeds makes them suitable for mixers and portable drills. Because they are mechanically commutated, general-purpose motors cannot be suitable for continuous-running applications due to the maintenance and wear of carbon brushes. Carbon brushes make them relatively noisy, because their main application scenarios are small appliances.