# Summary of direct current generators

SUMMARY

This chapter introduced you to the basic principles concerning direct current generators. The different types of dc generators and their characteristics were covered. The following information provides a summary of the major subjects of the chapter for your review.

MAGNETIC INDUCTION takes place when a conductor is moved in a magnetic field in such a way that it cuts flux lines, and a voltage (emf) is induced in the conductor.

THE LEFT-HAND RULE FOR GENERATORS states that when the thumb, forefinger, and middle finger of the left hand are extended at right angles to each other so that the thumb indicates the direction of movement of the conductor in the magnetic field, and the forefinger points in the direction of the flux lines (north to south), the middle finger shows the direction of induced emf in the conductor.

AN ELEMENTARY GENERATOR consists of a single coil rotated in a magnetic field.

It produces an ac voltage.

A BASIC DC GENERATOR results when you replace the slip rings of an elementary generator with a two-piece commutator, changing the output voltage to pulsating dc.

A MULTIPLE COIL ARMATURE (adding coils to the armature) decreases the ripple voltage in the output of a dc generator, and increases the output voltage.

A MULTIPOLE GENERATOR is the result of adding more field poles to a dc generator. They have much the same effect as adding coils to the armature. In practical generators, the poles are electromagnets.

COMMUTATION is the process used to get direct current from a generator. The coil connections to the load must be reversed as the coil passes through the neutral plane. The brushes must be positioned so that commutation is accomplished without brush sparking.

ARMATURE REACTION takes place when armature current causes the armature to become an electromagnet. The armature field disturbs the field from the pole pieces. This results in a shift of the neutral plane in the direction of rotation.

COMPENSATING WINDINGS AND INTERPOLES are used to counteract the effects of armature reaction. They are supplied by armature current and shift the neutral plane back to its original position.

MOTOR REACTION is caused by the magnetic field that is set up in the armature. It tends to oppose the rotation of the armature, due to the attraction and repulsion forces between the armature field and the main field.

ARMATURE LOSSES in dc generator armatures affect the outputs. These losses are as follows:
1. Copper losses are simply 12R (heat) losses caused by current flowing through the resistance of the armature windings.
2. Eddy currents are induced in core material and cause heat.
3. Hysteresis losses occur due to the rapidly changing magnetic fields in the armature, resulting in heat.

ARMATURE TYPES used in dc generators are the Gramme-ring (seldom used) and the drum-type, used in most applications.

FIELD EXCITATION is the voltage applied to the main field windings. The current in the field coils determines the strength and the direction of the magnetic field.
SEPARATELY EXCITED GENERATORS receive current for field coils from an outside source such, as a battery or another dc generator.
SELF-EXCITED GENERATORS use their own output voltage to energize field coils.
SERIES-WOUND DC GENERATORS have field windings and armature windings connected in series. Outputs vary directly with load currents. Series-wound generators have few practical applications.

SHUNT-WOUND DC GENERATORS have field windings and armature windings connected in parallel (shunt). The output varies inversely with load current.

COMPOUND-WOUND DC GENERATORS have both series field windings and shunt field windings. These generators combine the characteristics of series and shunt generators. The output voltage remains relatively constant for all values of load current within the design of the generator. Compound generators are used in many applications because of the relatively constant voltage.

AMPLIDYNES are dc generators that are designed to act as high-gain amplifiers. By short-circuiting the brushes in a normal dc generator and adding another set of brushes perpendicular to the original ones, an amplidyne is formed. Its power output may be up to 10,000 times larger than the power input to its control windings.

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