Electric motor and self-induction

Breadboard construction

A current measuring device is installed in the circuit of a DC electric motor with permanent magnets as stators (see Fig. 1.1). This clearly measures the current flowing through the rotor. A measurement data acquisition system can also be used for more accurate measurement, but the relatively high inrush currents must be taken into account.

For the second part of the experiment, a zero-point-center scale is set on the current meter and the circuit is extended with an interchangeable switch. This allows the source to be disconnected from the motor and a short circuit of the two coil connections can be made instead. The current meter with the middle position in turn shows the current flow through the rotor.

Tip: If no current meter with a suitable zero-point center scale is available, one can be improvised with the help of the zero-point adjustment (see Fig. 1.2).

Observation on power-up

When the electric motor is switched on, the current flow increases 𝐼 by the anchor. But then, when the motor turns quickly, the current decreases a bit.

Explanation

If the motor or rotor is at rest, only the voltage plays a role in the resulting current 𝑈0 of the power supply and the resistance 𝑅 of the motor plays a role in the current flow. The current results from 𝐼=𝑈0𝑅.

As soon as the motor turns, the rotor also represents a rotating coil in the magnetic field, into which a voltage is 𝑈ind is induced. So the rotating coil also acts as a generator. The induced voltage is greater the faster the coil rotates. In this case, the voltage induced in the moving armature is opposed to the external voltage of the current source, which is the cause of the rotation, due to Lenz's rule.

The total voltage responsible for the current flow is therefore calculated as follows: 𝑈ges=𝑈0−𝑈ind. For the resulting current through the armature, the following therefore applies:𝐼=𝑈ges𝑅=𝑈0−𝑈ind𝑅

Observation with braked engine

Fig. 2 Increase in current when operating an electric motor under load

If you brake the motor without changing the voltage at the power source by loading it by friction on the axle, the current increases immediately.

Explanation

The reason for this is that the induction voltage 𝑈ind, the external tension 𝑈0 and thus reduces the current flow, decreases when the motor is braked, i.e. turns more slowly. As a result, the total voltage is greater and a larger current flows.

Illustrating recuperation

Fig. 3 Recovery of the kinetic energy of a running electric motor into electrical energy with the help of the generator principle (recuperation)

Now install a changeover switch in the experiment, which detaches the power source from the motor and at the same time keeps the circuit closed (see Fig. 3).

At the beginning, the changeover switch closes the circuit with the power supply. If the engine is turning quickly, open the switch (don't put it in the second position yet!) and observe that it takes quite a while for the engine to turn off and come to rest again.

Now close the changeover switch again so that the engine can start again. When the motor turns quickly, you now press the changeover switch so that a closed circuit is created with only a rotating coil and ammeter.

You can observe that a (weaker) current continues to flow through the rotor that continues to run after the switchover, but now in the opposite direction. In other words, a voltage is induced in the rotating conductors that is the cause of this current flow. The arrangement works as a generator and part of the kinetic energy can be recovered in this way (technical language: recuperation). The generator voltage is directed against the voltage of the external power source in the same direction of rotation of the rotor.

When the circuit is closed by the rotor, the rotor also comes to a standstill faster than without a closed circuit. In technology, this is used, for example, to recharge the battery of a vehicle when braking.