DC Machines: DC Generators (part 4)

Compound Generators

Compound generators contain both series and shunt fields. Most large DC machines are compound wound. The series and shunt fields can be connected in two ways. One connection is called long shunt. The long shunt connection has the shunt field connected in parallel with both the armature and series field. This is the most used of the two connections.

The second connection is called short shunt. The short shunt connection has the shunt field connected in parallel with the armature.

The series field is connected in series with the armature. This is a very common connection for DC generators that must be operated in parallel with each other.

--39 Schematic drawing of a long shunt compound generator.

--40 Schematic drawing of a short shunt compound generator.

Compounding

The relationship of the strengths of the two fields in a generator determines the amount of compounding for the machine. A machine is overcompounded when the series field has too much control and the output voltage increases each time a load is added to the generator. Basically, the generator begins to take on the characteristics of a series generator. Overcompounding is characterized by the fact that the output voltage at full load is greater than the output voltage at no load.

When the generator is l at compounded, the output voltage is the same at full load as it’s at no load. Flat compounding is accomplished by permitting the series field to increase the output voltage by an amount that is equal to the losses of the generator.

If the series field is too weak, however, the generator becomes undercompounded. This condition is characterized by the fact that the output voltage is less at full load than it’s at no load. When a generator is undercompounded, it has characteristics similar to those of a shunt generator.

--41 Characteristic curves of compound generators. Overcompounded Flat-compounded Undercompounded Differential compound Rated load; Load amperes; Output volts

--42 The series field shunt rheostat controls the amount of compounding. Series field shunt rheostat

Controlling Compounding:

Most DC machines are constructed in such a manner that they are overcompounded if no control is used. This permits the series field strength to be weakened and thereby control the amount of compounding. The amount of compounding is controlled by connecting a low-value variable resistor in parallel with the series field ( --42). This resistor is known as the series field shunt rheostat, or the series field diverter. The rheostat permits part of the current that normally flows through the series field to flow through the resistor.

This reduces the amount of magnetic flux produced by the series field, which reduces the amount of compounding.

Cumulative and Differential Compounding:

DC generators are generally connected in such a manner that they are a cumulative compound. This means that the shunt and series fields are connected in such a manner that when current flows through them they aid each other in the production of magnetism. In the example shown, each of the field windings would produce the same magnetic polarity for the pole piece.

A differential-compound generator has its fields connected in such a manner that they oppose each other in the production of magnetism. In this example, the shunt and series fields are attempting to produce opposite magnetic polarities for the same pole piece. This results in the magnetic field becoming weaker as current flow through the series field increases. Although there are some applications for a differential-compound machine, they are very limited.

--43 In a cumulative-compound machine, the current flows in the same direction through both the series and the shunt field. Pole piece, Series field, Shunt field

--44 In a differential-compound machine, the current flows through the shunt field in a direction opposite that of the current flow through the series field. Pole piece, Series field, Shunt field

--45 A magnetic field is produced around the armature.

Countertorque

When a load is connected to the output of a generator, current flows from the armature, through the load, and back to the armature. As current flows through the armature, a magnetic field is produced around the armature. In accord with Lenz's law, the magnetic field of the armature is opposite in polarity to that of the pole pieces. Because these two magnetic fields are opposite in polarity, they are attracted to each other. This magnetic at traction causes the armature to become hard to turn. This turning resistance is called countertorque, and it must be overcome by the device used to drive the generator. This is the reason that, as load is added to the generator, more power is required to turn the armature. Because countertorque is produced by the attraction of the two magnetic fields, it’s proportional to the output or armature current if the field excitation current remains constant. Countertorque is a measure of the useful electric energy produced by the generator.

Countertorque is often used to provide a braking action in DC motors. If the field excitation current remains turned on, the motor can be converted into a generator very quickly by disconnecting the armature from its source of power and reconnecting it to a load resistance. The armature now supplies current to the load resistance. The countertorque developed by the generator action causes the armature to decrease in speed. When this type of braking action is used, it’s referred to as dynamic braking or regenerative braking.

--46 Armature reaction changes the position of the neutral plane. Normal neutral plane position Changed neutral plane position Pole piece Pole piece

Armature Reaction

Armature reaction is the twisting or bending of the magnetic lines of flux of the pole pieces. It’s caused by the magnetic field produced around the armature as it supplies current to the load. This distortion of the main magnetic field causes the position of the neutral plane to change position.

When the neutral plane changes, the brushes no longer make contact between commutator segments at a time when no voltage is induced in the armature.

This results in power loss and arcing and sparking at the brushes, which can cause overheating and damage to both the commutator and brushes. The amount of armature reaction is proportional to armature current.

--47 In a generator, the brushes are rotated in the direction of armature rotation to correct armature reaction.

--48 Interpoles are connected in series with the armature. Interpoles or commutating winding

--49 Interpoles must have the same polarity as the pole piece directly ahead of them.

Correcting Armature Reaction:

Armature reaction can be corrected in several ways. One method is to rotate the brushes an equal amount to the shift of the neutral plane. This method is only satisfactory, however, if the generator delivers a constant current. Because the distortion of the main magnetic field is proportional to armature current, the brushes have to be adjusted each time the load current changes. In the case of a generator, the brushes are rotated in the direction of rotation of the armature. In the case of a motor, the brushes are rotated in a direction opposite that of armature rotation.

Another method that is used often is to insert small pole pieces, called interpoles or commutating poles, between the main field poles. The interpoles are sometimes referred to as the commutating winding because they are wound with a few turns of large wire similar to the series field winding. The interpoles are connected in series with the armature, which permits their strength to increase with an increase of armature current. Interpole connections are often made inside the housing of the machine. When the interpole connection is made internally, the A1 lead is actually connected to one end of the interpole winding. When the interpole windings are brought out of the machine separately, they are generally labeled C1 and C2, which stands for commutating field. It’s not unusual, however, to find them labeled S3 and S4.

In a generator, the magnetic field of the armature tends to bend the main magnetic field upward. In a motor, the armature field bends the main field downward. The function of the interpoles is to restore the field to its normal condition. When a DC machine is used as a generator, the interpoles have the same polarity as the main field pole directly ahead of them (ahead in the sense of the direction of rotation of the armature). When a DC machine is used as a motor, the interpoles have the same polarity as the pole piece behind them in the sense of direction of rotation of the armature.

Interpoles do have one disadvantage. They restore the field only in their immediate area and are not able to overcome all the field distortion. Large DC generators use another set of windings called compensating windings to help restore the main magnetic field. Compensating windings are made by placing a few large wires in the face of the pole piece parallel to the armature windings. The compensating winding is connected in series with the armature so that its strength increases with an increase of output current.

--50 Compensating winding helps correct armature reaction. Compensating winding -- Pole piece

Setting the Neutral Plane

Most DC machines are designed in such a manner that the position of the brushes on the commutator can be set or adjusted. An exposed view of the brushes and brush yoke of a DC machine.

The simplest method of setting the brushes to the neutral plane position is to connect an AC voltmeter across the shunt field leads. Low-voltage AC is then applied to the armature ( --52). The armature acts like the primary of a transformer, and the shunt field acts like the secondary. If the brushes are not set at the neutral plane position, the changing magnetic field of the armature induces a voltage into the shunt field. The brush position can be set by observing the action of the AC voltmeter. If the brush yoke is loosened to permit the brushes to be moved back and forth on the commutator, the voltmeter pointer moves up and down the scale. The brushes are set to the neutral plane position when the voltmeter is at its lowest possible reading.

--51 Cut-away of a DC machine.

--52 Setting the brushes at the neutral plane. Low-voltage AC supply

--53 The equalizer connection is used to connect the series fields in parallel with each other. Equalizer connection

--54 One generator takes all the load, and the other becomes a motor. Motorized generator -- Overexcited generator

Paralleling Generators

There may be occasions when one DC generator cannot supply enough current to operate the connected load. In such a case, another generator is connected in parallel with the first. DC generators should never be connected in parallel without an equalizer connection. The equalizer connection is used to connect the series fields of the two machines in parallel with each other. This arrangement prevents one machine from taking the other over as a motor.

Assume that two generators are to be connected in parallel and the equalizing connection has not been made. Unless both machines are operating with identical field excitation when they are connected in parallel, the machine with the greatest excitation takes the entire load and begins operating the other machine as a motor. The series field of the machine that accepts the load is strengthened, and the series field of the machine that gives up the load is weakened. The machine with the stronger series field takes even greater load, and the machine with the weaker series field reduces load even further.

The generator that begins motoring has the current flow through its series field reversed, which causes it to operate as a differential-compound motor. If the motoring generator is not removed from the line, the magnetic field strength of the series field will become greater than the field strength of the shunt field. This causes the polarity of the residual magnetism in the pole pieces to reverse. This is often referred to as flashing the field. Flashing the field results in the polarity of the output voltage being reversed when the machine is restarted as a generator. The equalizer connection prevents field reversal even if the generator becomes a motor.

The resistance of the equalizer cable should not exceed 20% of the resistance of the series field winding of the smallest paralleled generator. This ensures that the current flow provided to the series fields will divide in the approximate inverse ratio of the respective series field winding.