What are alternating and direct currents 1

This motor is constructed like a direct current series motor and can in principle also be operated with direct current. When connected to alternating current, both the excitation field and the armature field reverse direction at the same time with each half-wave. This means that the direction of rotation is retained. The excitation winding is divided into two parts and thus, in addition to generating the excitation field, also serves to reduce the electromagnetic interference that occurs on the commutator due to the brush fire. Suppression capacitors are also required.

The mechanical structure

In principle, the armature of the all-current motor is the same as that of the DC motor. It has a commutator (either a disc collector or a drum collector) and is contacted by means of carbon and thus reverses its polarity.

The big difference is that it does not have a permanent magnet, but also electrical coils on the stator, which generate the excitation field. Since permanent magnets have a finite coercive field strength and are therefore out of the question for any scalable task, permanent magnets (i.e. DC motors) are only suitable for smaller torques and designs.

Historically, there was the all-current motor first, as it did not depend on direct current. It can be operated with direct current, but also with alternating voltage or any mixed signal of direct and alternating current. This can only work because the magnetic field always changes simultaneously in both the stator and the rotor due to the applied current signal. This means that the direction of rotation always remains unchanged when both polarity reversals occur in phase.

So there is a superimposed polarity reversal. On the one hand, the polarity of the current is reversed on both sides due to the alternating field. Since the polarity of the entire magnetic field changes, together with the direction of the current in the coils, the direction of the Lorentz force does not change. The motor can continue to drive unabated.

Superimposed on this polarity reversal is the desired, angle-of-rotation-dependent polarity reversal in the rotor coils, which ensures that a rotor tooth is either magnetically north or south to the outside, depending on the angle of rotation, in order to be attracted or repelled by the corresponding stator magnet.

On the right of this picture you can see an all-current motor that can cope with either AC or DC voltage. The rotor is three-pole, the stator two-pole. A field coil generates the primary field of the motor.

On the left in the figure you can see a DC motor. Here the field coil has been replaced by a permanent magnet. It can now only be driven with a DC voltage. The rotor is 5-pole, the stator 2-pole. Overall, the current consumption is reduced when using a permanent magnet, since the additional current for applying the primary field in the coil is no longer necessary.

The universal motor in practice

AC motors require any voltage source to operate, regardless of whether it generates an alternating voltage or direct voltage. It works with the mains voltage from the socket (at the appropriate voltage level, of course) and is therefore often used for corded power tools. It only has 2 connection terminals for +/- and ground. The direction of rotation must be reversed within the motor, i.e. the polarity of the stator must be changed relative to the rotor. As with all electric motors, the torque is proportional to the current that runs through the windings.

With a given load, the speed can be determined by the level of the applied voltage. It is very robust and insensitive to temperature. It can also cope with bumps without any problems, as no sensitive permanent magnet is integrated.

Permanent magnets lose their magnetic force over time in the event of strong vibrations or magnetic overload. In the case of magnetic materials made of rare earths (neodymium), operation above 120 ° is already harmful to the magnetization. Not so with universal motors, whose magnetic field is generated LIVE on site and can be rebuilt as often as required ... with the disadvantage of the additional current required for the excitation field.

The universal motor loses speed under load, but is very powerful when switched on.

The area of ​​application

The universal motor is the most widely used small motor. It is used in many hand-held electrical devices, e.g. hand drills or circular saws.