1. Laws of electromechanical energy conversion

Electrical machines convert mechanical energy into electrical energy or vice versa. We find:

1. In DC current:

Direct current machines: as a motor or generator (dynamo).

2. In AC cuurent:

- Synchronous machines: as a motor or generator (alternator).

- Asynchronous or induction machines: as a motor or generator.

The energy conversion occurs at a lower efficiency due to losses..

Principle of the energy generator

Take the example of the following figure:

A wire conductor of length L moves at a speed v on two undefined rails placed in a uniform and invariable induction B perpendicular to conductors.

The mechanical force F_m is applied on the conductor and the circuit is supplied by a generator of an electromotive force (e.m.f.) E₀ and internal resistance R. We note I as the current with the sign conventions indicated in the previous figure.

Four laws determine the electromechanical system:

- Faraday's law: If the conductor's speed is v, an e.m.f. is created called E :

E = B . L . v

- Laplace's law: If the current in the conductor is I, there is an electromagnetic force F_e given by:

F_e = B . L . I

- Ohm's law:

E₀ = E + R.I

- The law of dynamics: if the speed v is constant:

F_m = F_e

At zero resistance (lossless circuit), we find:

* A speed v as: E₀ = E

* A current I as: F_m = F_e

Therefore, we can say that speeds are associated to the e.m.f. and the currents to the forces.

 

The machine is operating in a motor mode if F_e and v are in the same direction (see previous figure). The e.m.f. then is opposed to the current.

If the speed is in the direction of the applied force F_m, we obtain a generator operation mode. The electrical force F_e then is opposed to F_m.

We can express the power at the conductor in a mechanical form or in an electrical form as:

This is called the electromagnetic power.

It should be noted that the energy conversion is perfectly reversible.