Rotating Electrical Machines

Chapter 5

Direct Current Machines

5.1. Introduction

The so-called “direct current machines” are electromechanical energy converters in which the electrical energy is exchanged with their environment (supply or load) under the form of direct voltages and currents. This is possible due to the brush-commutator systems which plays the part of a “mechanical rectifier”. That is the reason why these machines are sometimes called “commutator machines”.

Like all rotating machines, DC machines are reversible and can operate as a motor or as a generator (they are sometimes called “dynamos” in this case). However it can be noted that the development of power electronics, and particularly of diode and thyristor rectifiers, from the 1960s onwards, has gradually marginalized the use of DC generators. Today the generator function of these machines is restricted to the recovery of kinetic energy during braking and slowing down, we shall therefore focus this chapter on the motor mode.

5.2. Main notations

In the remainder of this chapter, we shall allot the index “1” to the field system, and the index “2” to the armature. We shall also set down:

– e: air-gap thickness;

– E: electromotive force;

– J: moment of inertia of all of the rotating parts;

– L_i: self inductance of the winding i;

– M: mutual inductance between the field system and the armature;

– R_i: resistance of the winding i;

– U: supply voltage;

– μ₀: free space permeability (μ₀ = 4π 10^–7 H/m);

– Ω: angular speed of the rotor;

– Γ: machine ...