MC-B4 · DC generator characteristics

Self-Excitation and the External Characteristic of the DC Generator

A self-excited generator builds its own voltage with no external supply. See where the feedback from residual magnetism settles, and by varying the field resistance, separate for yourself when the voltage builds up and when it fails.

Where does the voltage settle, and when does it fail to build?

A small voltage from residual magnetism drives field current, which grows the flux, which grows the voltage further. This feedback settles where the magnetization curve meets the field-resistance line (V = Rf·If). Raise the field-resistance slider. Past which slope does the intersection vanish so the voltage never builds?

Field resistance RfRf = 380 Ω

Slide for continuous field resistance. The line–curve intersection is the built-up voltage.

Built-up terminal voltage

Vt = 60 V

The field-resistance line is steeper than the initial slope of the magnetization curve, so the two meet only near the residual point. The feedback cannot grow and the voltage fails to build. That slope is the critical field resistance.

No build-up

The seed is residual magnetism

The pole core keeps a residual flux, the trace of having once been magnetized. Spinning the generator makes this residual flux induce a small residual voltage in the armature. If the shunt field is wired across the armature, this voltage drives a small field current, which grows the flux and lifts the voltage further. It is a positive feedback that raises itself with no external supply. The field connection must aid the residual flux; reversed, the voltage collapses instead.

ObserveV(If=0) = Vr

The magnetization curve starts from the residual voltage.

Where the feedback stops, the critical resistance

As the voltage rises the field current rises too, but as the core saturates the magnetization curve bends and the voltage no longer climbs in proportion. The field-resistance line V = Rf·If, meanwhile, is straight. The feedback stops at their intersection. Raising the field resistance steepens the line and drops the intersection below the knee; once the line is steeper than the initial slope of the magnetization curve, the only intersection is the residual point and the voltage fails to build. That limiting slope is the critical field resistance.

ChooseV = Rf ?

The field-resistance line is a straight line in field current.

Fill inbuild-up: Rf < ?

For the voltage to build, the field resistance must be below critical.

Under load — the external characteristic by excitation

Under load the terminal voltage is Vt = E - IaRa, dropping by the armature-resistance fall and the armature reaction. A separately-excited machine, with its field apart, droops only slightly from these two. A shunt machine droops more steeply, because a falling terminal voltage also cuts its self-excited field current and weakens the flux. A series machine, whose load current is its field, rises at first and then bends over from saturation and reaction. A compound machine adds a series branch to fill the shunt droop, tuned to stay nearly flat (flat-compound) or rise slightly (over-compound) with load.

On your ownVt = E - ?

Under load the terminal voltage is the generated EMF minus the armature drop.

Back to the first screen

The voltage failed to build when the field-resistance line was steeper than the initial slope of the magnetization curve, so the two met only at the residual point. Residual magnetism seeds a positive feedback, but that feedback stops at the intersection of the saturation-bent curve and the straight resistance line. Only when the field resistance is below critical does the intersection sit high above the knee and the voltage build up. The voltage so built then changes under load as Vt = E - IaRa, slightly for separate excitation, more for shunt, and in their own shapes for series and compound. A generator’s voltage, both the building of it and the holding of it under load, is read off the magnetization curve.

The self-excited generator: residual voltage from residual magnetism seeds a positive feedback that builds the voltage and settles at the intersection of the magnetization curve and the field-resistance line (V = Rf·If). The voltage builds only if the field resistance is below the critical value (the initial slope of the curve). Under load the external characteristic is Vt = E - IaRa, with separately-excited, shunt, series and compound each drawing a different voltage-current curve.

Closing out group B

That completes the DC machine. The commutator flips AC into DC (B1), armature reaction shifts the neutral axis (B2), back-EMF sets a motor’s speed and torque (B3), and self-excitation builds a generator’s voltage (B4) — all one machine turning on a single magnetic circuit. The next group C is the induction machine, which makes the rotation itself out of a magnetic field. It begins with how three-phase current weaves a rotating magnetic field.