Direct Current Machine: Generalities

Direct Current Machine: Generalities

A direct current rotating machine can operate indifferently as a generator or as a motor.

In the first case it is called dyinamo, in the second one direct current motor.

Its prevalent application is as a motor: there are applications of a medium-big power, with rated voltages of the order of the thousand of volt; there are also several applications of small motors.

CAs it happens in the synchronous machine, the d.c. machines have an inductor supplied with direct current and an inductor whose conductors are the seats of e.m.f. and alternating currents.

In d.c. machines the inductor is set in the stator and the armature in the rotor.

Stator

The stator is made up of the frame on whose internal surface the main poles are mounted equipped with pole shoes, facing the rotor. Being the seat of constant magnetic induction, the frame and the polar cores are carried out in massive iron; on the contrary the pole shoes can be massive or with bars, because their induction suffers fluctuations.

Around each polar core there are identical coils that altogether make up the armature winding (or excitation winding); they are connected in such a way that, when they are flown by the excitation current, the m.m.f. of two consecutive poles have the same modules and opposite directions (a centrifugal one and a centripetal one).

The simplest connection has the coils of two consecutive poles antiseries connected.

Except in smaller machines, in correspondence of the wheelbase between the main poles there are smaller salient poles, called commutating poles or auxiliary poles, equipped with windings, whose function will be explained later.

In bigger machines the main pole shoes are equipped with longitudinal slots housing the conductors of the balancing windings.

Fig. 1 - Stator

A = commutating pole F = armature conductors

B = commutating winding G = frame

C = inductor coil H = bar collector

D = polar core I = brushes and brush holder

E = pole shoe L = balancing conductors

Rotor and collector

The rotor is always carried out in bar iron, because it is seat of magnetic induction alternated in time.

It is equipped with longitudinal slots, normally of open type, housing the armature conductors; qthese are connected to the heads so to build up windings similar to the armature ones of the synchronous ones; the windings are in turn interconnected so to build up one or more closed windings, as we will show later.

The collector is a characteristic member of the d.c. machines allowing to convert the alternating voltages and currents of the armature conductors in the direct currents and voltages present at the power terminals of the machine.

It is made up of copper bars occupying the different azimuthal positions, isolated with mica or plastic reinforced by fiber glass from 0,5 to 1,5 mm thick and connected to the windings of the armature winding.

The bars are also connected to the two power terminals of the machine through creeping contacts with the fixed brushes as to the stator. These are present in pairs, symmetrically set along the azimuthal, development of the collector. Rather than using brushes of big section we prefer using more than one brush set in rows, to obtain a better contant with the bars.

Fig. 2 - Collector

A = rotor

B = bar collector

C = brushes and brush holder

D = shaft.

The following figure (Fig. 3) shows the internal components of a direct current machine.

Fig. 3 - Fundamental parts of a direct current machine

1 = bearings 7 = collector

2 = lifting eyebolt 8 = stator

3 = bearings 9 = brushes and brush holder

4 = shaft support flange 10= collector support flange

5 = ventilation turbine 11= cover

6 = armature

The section plane of a direct current machine is shown in the side figure (Fig. 4). Fig. 4

Rated values

The main rated values of the direct current machine are listed in the Table.

Pn Rated