DC Machines: DC Generators (part 3)

Series Generators

There are three basic types of DC generators: the series, the shunt, and the compound. The type is determined by the arrangement and connection of field coils. The series generator contains only a series field connected in series with the armature. A schematic diagram used to represent a series-connected DC machine. The series generator must be self-excited, which means that the pole pieces contain some amount of residual magnetism. This residual magnetism produces an initial output volt age that permits current to flow through the field if a load is connected to the generator. The amount of output voltage produced by the generator is proportional to three factors:

1. the number of turns of wire in the armature

2. the strength of the magnetic field of the pole pieces

3. the speed of the cutting action (speed of rotation)

To understand why these three factors determine the output voltage of a generator, recall that 1 volt is induced in a conductor when it cuts magnetic lines of flux at a rate of 1 weber per second (1 weber 5 100,000,000 lines of flux). When conductors are wound into a loop, each turn acts like a separate conductor. Because the turns are connected in series, the voltage induced into each conductor adds. If one conductor has an induced voltage of 0.5 volt and there are 20 turns, the total induced voltage would be 10 volts.

The second factor is the strength of the magnetic field. Flux density is a measure of the strength of a magnetic field. If the number of turns of wire in the armature remains constant and the speed remains constant, the output volt age can be controlled by the number of flux lines produced by the field poles.

Increasing the lines of flux increases the number of flux lines cut per second and therefore the output voltage. The magnetic field strength can be increased until the iron of the pole pieces reaches saturation.

Induced voltage is proportional to the number of flux lines cut per second. If the strength of the magnetic field remains constant and the number of turns of wire in the armature remains constant, the output voltage is determined by the speed at which the conductors cut the flux lines. Increasing the speed of the armature increases the speed of the cutting action, which increases the output voltage. Likewise, decreasing the speed of the armature decreases the output voltage.

--29 Characteristic curve of a series generator. Saturation of iron; Rated load; Volts

Connecting Load to the Series Generator:

When a load is connected to the output of a series generator, the initial voltage produced by the residual magnetism of the pole pieces produces a current flow through the load. Because the series field is connected in series with the armature, the current flowing through the armature and load must also flow through the series field. This causes the magnetism of the pole pieces to become stronger and produce more magnetic lines of flux. When the strength of the magnetic pole pieces increases, the output voltage increases also.

If another load is added, more current flows and the pole pieces produce more magnetic lines of flux, which again increases the output voltage. Each time a load is added to the series generator, its output voltage increases. This increase of voltage continues until the iron in the pole pieces and armature becomes saturated. At that point, an increase of load results in a decrease of output voltage.

Shunt Generators

Shunt generators contain only a shunt field winding connected in parallel with the armature ( --30). A schematic diagram used to represent a shunt connected DC machine. Shunt generators can be either self-excited or separately excited. Self-excited shunt generators are similar to self-excited series generators in that residual magnetism in the pole pieces is used to produce an initial output voltage. In the case of a shunt generator, however, the initial voltage is used to produce a current flow through the shunt field. This current increases the magnetic field strength of the pole pieces, which produces a higher output voltage. This buildup of volt age continues until a maximum value, determined by the speed of rotation, the turns of wire in the armature, and the turns of wire on the pole pieces, is reached.

Another difference between the series generator and the self-excited shunt generator is that the series generator must be connected to a load before volt age can increase. The load is required to form a complete path for current to flow through the armature and series field ( --27). In a self-excited shunt generator, the shunt field winding provides a complete circuit across the armature, permitting the full output voltage to be obtained before a load is connected to the generator.

--30 Shunt field windings are connected in parallel with the armature.

--31 Schematic drawing of a shunt generator.

--32 Residual magnetism in the pole pieces produces an initial voltage, which causes current to flow through the shunt field and the field flux to increase.

--33 Separately excited shunt generators must have an external power source to provide excitation current for the shunt field.

--34 Characteristic curves of self- and separately excited shunt generators. Separately excited generator Self-excited generator Rated load Output volts Load amperes

--35 The shunt field rheostat is used to control the output voltage. Shunt field rheostat

Separately excited generators have their fields connected to an external source of DC. The advantages of the separately excited ma chine are that it gives better control of the output voltage and that its voltage drop is less when load is added. The characteristic curves of both self-excited and separately excited shunt generators are shown.

The self-excited generator exhibits a greater drop in voltage when load is added because the armature voltage is used to produce the current flow in the shunt field. Each time the voltage decreases, the current flow through the field decreases, causing a decrease in the amount of magnetic flux lines in the pole pieces. This decrease of flux in the pole pieces causes a further de crease of output voltage. The separately excited machine does not have this problem because the field flux is held constant by the external power source.

Field Excitation Current:

Regardless of which type of shunt generator is used, the amount of output voltage is generally controlled by the amount of field excitation current.

Field excitation current is the DC that flows through the shunt field winding.

This current is used to turn the iron pole pieces into electromagnets.

Because one of the factors that determines the output voltage of a DC genera tor is the strength of the magnetic field, the output voltage can be controlled by the amount of current flow through the field coils. A simple method of controlling the output voltage is by the use of a shunt field rheostat. The shunt field rheostat is connected in series with the shunt field winding. By adding or removing resistance connected in series with the shunt field winding, the amount of current flow through the field can be controlled. This in turn controls the strength of the magnetic field of the pole pieces.

When it’s important that the output voltage remain constant regardless of load, an electronic voltage regulator can be used to adjust the shunt field current. The voltage regulator connects in series with the shunt field in a similar manner as the shunt field rheostat. The regulator, however, senses the amount of voltage across the load. If the output voltage drops, the regulator permits more current to flow through the shunt field. If the output voltage becomes too high, the regulator decreases the current flow through the shunt field.

--36 The voltage regulator controls the amount of shunt field current. Voltage regulator

--37 Armature resistance causes a drop in output voltage.

Generator Losses:

When load is added to the shunt generator, the output voltage drops. This voltage drop is due to losses that are inherent to the generator. The largest of these losses is generally caused by the resistance of the armature. It’s assumed that the armature has a wire resistance of 10 ohms. When a load is connected to the output of the generator, current flows from the armature, through the load, and back to the armature. As cur rent flows through the armature, the resistance of the wire causes a voltage drop. Assume that the armature has a current flow of 2 amperes. If the resistance of the armature is 10 ohms, it will require 20 volts to push the current through the resistance of the armature.

Now assume that the armature has a resistance of 2 ohms. The same 2 amperes of current flow require only 4 volts to push the current through the armature resistance. A low-resistance armature is generally a very desirable characteristic for DC machines. In the case of a generator, the voltage regulation is determined by the resistance of the armature. Voltage regulation is measured by the amount that output voltage drops as load is added.

A generator with good voltage regulation has a small amount of voltage drop as load is added.

Some other losses are I ^2R losses, eddy current losses, and hysteresis losses.

Recall that I^2 R is one of the formulas for finding power, or watts. In the case of a DC machine, I^2 R describes the power loss associated with heat due to the resistance of the wire in both the armature and field windings.

Eddy currents are currents that are induced into the metal core material by the changing magnetic field as the armature spins through the flux lines of the pole pieces. Eddy currents are so named because they circulate around inside the metal in a manner similar to the swirling eddies in a river. These swirling currents produce heat that heats the surrounding metal and causes a power loss. Many machines are constructed with laminated pole pieces and armature cores to help reduce eddy currents. The surface of each lamination forms a layer of iron oxide, which acts as an insulator to help prevent the formation of eddy currents.

Hysteresis losses are losses due to molecular friction. As discussed previously, AC is produced inside the armature. This reversal of the direction of current flow causes the molecules of iron in the core to realign themselves each time the current changes direction. The molecules of iron are continually rubbing against each other as they realign magnetically. The friction of the molecules rubbing together causes heat, which is a power loss. Hysteresis loss is proportional to the speed of rotation of the armature. The faster the armature rotates, the more current reversals there are per second and the more heat is produced because of friction.

--38 Eddy currents heat the metal and cause power loss. Eddy currents