RAJIV GANDHI AVIATION ACADEMYAIRCRAFT MAINTENANCE ENGINEERING TRAINING INSTITUTE

MODULE 3ELECTRICAL FUNDAMENTALSTRAINING NOTES

TOPIC: D.C. GENERATIONdc generationIf a conductor is moved at right angles to a magnetic field, an emf is induced in the conductor. If an external circuit is then connected to the conductor a current will flow. The direction of the current flow depends on two factors, the:direction of the magnetic fielddirection of relative movement between the conductor and the fieldand can be determined by using Flemings right hand rule. The size of the generated emf depends on three factors, the:strength of the magnetic field Beffective length of the conductor in the field llinear velocity of the conductor vThe three are related in the formula E = B l v

simple single loop generatorIn its simplest form, a generator consists of a single loop of wire rotated between the poles of a permanent magnet. The rotating part of the machine is called the rotor or armature, it is connected to the stationary external circuit via two slip rings, thus allowing a current flow.Induced emfAs the loop rotates an emf is induced in both sides of the conductor. Using Flemings right hand rule, it can be seen that the resultant currents flow in opposite directions on each side, but in the same direction around the loop.An emf is only induced in a conductor when it is moved at right angles to the lines of flux in a magnetic field. Therefore, the loop will only have an emf induced in it when it is moving at right angles to the lines of flux, when moving parallel with the lines of flux, no emf will be induced. At any direction in between, there will be a proportion of maximum emf induced in the loop.The instantaneous value of emf induced in the loop is given by:e(instant) = E(max) sin where E(max) = lv and is the angle of the conductor with respect to the lines of flux.As the loop passes the neutral point, the conductors direction of travel through the field reverses. The conductor that was moving upwards through the field is now moving downwards, therefore, the emf's induced in the conductors must change direction, as must the resultant current flow.

Output frequencyAs the loop rotates, the emf rises to a maximum in one direction, then falls to zero and then rises to a maximum in the opposite direction, before once again falling to zero. One complete revolution is one cycle, the loop having returned to its start position.The number of cycles per second gives the frequency. The faster the loop is rotated, the more cycles per second and the higher the frequency. In this simple generator the frequency depends on the number of loop revolutions per second.The output from this generator changes polarity every time the loop rotates 180 degrees and is therefore of little use as a direct current generator.

commutationIn order to make the current flow in the same direction through the load, the connections to the external circuit must be switched every time the loop moves past its neutral position. This can be achieved using a commutator.The commutator is used in place of the slip rings and connects the rotating loop to the stationary external circuit.

A commutator has 2 functions:Firstly, to transfer current from the rotating loop to the stationary external circuit.Secondly, the periodic switching of the external circuit to keep the current flowing in the same direction through the load. Switching takes place when the loop is moving parallel to the field and has no emf induced in it.Using a single loop generator and two segment commutator, the output will be as shown above.Although current now flows in the same direction through the external circuit, it is still of little practical use, because the voltage and current fall the zero twice every cycle. Using several loops and a multisegment commutator, a more constant output can be produced.

ring wound generatorThe simple construction of the ring wound generator makes it ideal for explaining the operation of a multi-coil machine.The rotor consists of a laminated iron cylinder onto which is wound 8 equally spaced coils. The junction between each pair of coils is connected to a segment of the commutator. The number of segments equals the number of coils, this being true for all d.c. generator armature windings.The brushes are drawn inside for clarity and are positioned so that when they short circuit a coil, that coil is moving parallel to the magnetic field and has no emf induced in it.The metal used for the rotor has a very low reluctance, therefore the flux of the main field flows through it, rather than through the airgap in the centre. The parts of the coils on the inside of the rotor are therefore not cutting any flux and have no emfs induced in them. The low reluctance rotor creates a radial field in the airgap as shown above. The radial field means that the conductors are moving at right angles to the flux for a longer period of time and are therefore producing maximum emf for longer. This results in a flat top to the output waveform as shown above.The 8 coils are split into two parallel paths of four, each group of four coils being connected in series, because one set of four coils is moving up through the main field and the other set is moving down through the field, the emf's induced in each set of four coils is in the opposite direction, but it is in the same direction with respect to the brushes.

The emf induced in four coils is as shown below. The emf in the other four coils is in the opposite direction, but in the same direction with respect to the brushes. It can be seen that the emf no longer falls to zero and only has a small ripple on it.The ring wound generator is no longer used. Although simple in construction, there are difficulties in winding the coils through the rotor, also, half of each coil is wasted because it has no emf induced in it.

practical dc generatorConstructionThe size and weight of generators vary considerably, but all are constructed in a manner similar to that shown above.The field assembly consists of a cylindrical frame, or yoke, onto which the pole pieces are bolted. Generators generally have at least four pole pieces, although small machines may have only two. Wound around each pole piece is a field coil. The yoke has a low reluctance and provides a path for the main field of the machine. To reduce eddy currents the yoke is usually laminated.The armature core also provides a path for the main field and is therefore also of low reluctance and laminated.

The armature windings are located in slots cut in the core, being wedged in with insulation to prevent them being thrown out by centrifugal forces. The coils are normally wound so they return along a slot in the rotor that is one pole pitch away (see diagram below).Pole pitch is a term used to describe the angle between one main pole and the next main pole of the opposite polarity.The emf induced in each side of the coil is again in opposite directions, but assisting around the coil. This type of winding is called a drum winding and has the advantage that the coils can be wound and insulated before being fitted into the rotor. There are two types of drum winding, Lap wound and wave wound.The armature windings are connected to risers attached to the commutator. The commutator consisting of copper segments separated by mica insulation.The brush gear assembly consists of a holder and rocker. The holder allows the brushes to slide up a down, whilst preventing them from moving laterally. The rocker allows the brushes to be rotated around the commutator so they can be positioned on the magnetic neutral axis.It should be noted that the output power from a d.c. generator is governed primarily by its ability to dissipate heat. Methods of cooling vary, a large, low power generator would normally be cooled naturally by convection and radiation. Smaller, higher power generators will need some form of cooling system that blows or draws air through the generator. The cooling system may use ram air from a propeller slipstream or from movement of the aircraft through the air, or more commonly, a fan attached to the rotor shaft of the generator.Lap wound generatorIn a lap wound generator, the end of each coil is bent back to the start of the next coil, the two ends of any one coil being connected to adjacent segments of the commutator (see diagram above). This form of construction is used on large heavy current machines. The number of parallel paths for current always equals the number of brushes and the number of field poles (see diagram).

Wave wound generatorIn a wave wound generator, the end of each coil is bent forward and connected to the start of another coil located in a similar position under the next pair of main poles (see diagram above). The two ends of one coil are connected to segments two pole pitches away. This type of machine has two parallel paths and uses only two brushes irrespective of the number of poles (see diagram).This type of winding is used in smaller machines and is therefore more common on aircraft generators.

Internal resistanceA d.c. machine has resistance due to the: armature windings brushes brush to commutator surface contactThis is called internal resistance and can be measured across the terminals of the generator. For the purposes of calculation, the internal resistance is represented as a single value in series with the generated emf.Internal resistance causes the generators terminal voltage to vary with changes in the load current. As the load current increases, the voltage dropped across the internal resistance increases and the terminal voltage decreases.The generated emf E = Ir + VArmature reactionWhen armature current is flowing, a field is produced around the armature conductors. The overall field of the machine is then produced by interaction between the main field and the armature field.The armature field is at 90 degrees to the main field of the machine and therefore distorts it as shown below.This distortion of the field is called armature reaction and has the effect of weakening the field at points A and strengthening the field at points B.The machine is working near to saturation and therefore the overall effect is a weakening of the field and a reduction in the generators output voltage.Distortion of the field also means that the magnetic, or electric neutral axis is moved around in the direction of rotation, away from the machines geometric neutral axis. When the brushes now short an armature coil, it is no longer at the point where zero emf is induced in it, therefore the brushes must be moved. The position they are moved to depends on the size of the armature current, the greater the current, the further the brushes must be advanced.Armature reaction can be reduced by fitting compensating windings. Compensating windings are small windings wound in series with the armature and fitted into slots cut in the pole faces of the main fields.When armature current flows, current flows in the compensating windings and produces a magnetic field that cancels the armature field.With careful design, correction is applied for all values of armature current, bringing the magnetic neutral axis back onto the geometric neutral axis and restoring the overall strength of the machines field.

Reactive sparkingThe diagrams above represent the movement of the commutator under the brush. Prior to being shorted by the brush, current in coil A is at a maximum value left to right. After leaving the brush, current will be flowing at maximum value in the opposite direction through the coil, as shown in coil B. Whilst the coil is shorted by the brush, the current must drop to zero ready for it to go to maximum value in the opposite direction when it comes off the brush.Unfortunately, the coil has inductance, when shorted, a back emf is produced that tries to maintain current flow. When the coil comes off the brush, the current has not reduced to zero, resulting in an excess of current that jumps as a spark from the commutator to the brush. The sparking produced is called reactive sparking. Not all sparking at the commutator is reactive sparking, sparks may also be caused by:worn or sticking brushesincorrect spring tensioncommutator flatsproud micaOne way of overcoming the problem is to increase the resistance of the brushes, this reduces the time constant of the inductive circuit and enables the current to collapse to zero during commutation. However, increasing the resistance of the brushes produces a power loss and increases the overall resistance of the machine. The increase in internal resistance causes greater fluctuations in output voltage with changes in load current.EMF CommutationAnother way of overcoming reactive sparking is to use emf commutation. The purpose of emf commutation is to neutralise the reactance voltages that lead to reactive sparking. One way of achieving this is to advance the brushes beyond the magnetic neutral axis, this means the coils are under the influence of the next main pole before being shorted and will therefore have an emf induced in them.The induced emf will be of opposite polarity to the reactance voltage and will reduce it, reducing the reactance voltage reduces the current in the coil and allows time for it to drop to zero whilst the coil is shorted.Unfortunately, advancing the brushes is only good for one value of armature current, if the current increases, the brushes must be advanced further. Advancing the brushes also increases the demagnetising effects of armature reaction.A better way of applying emf commutation is to fit commutating or interpoles between the main poles of the machine. Interpoles have the same polarity as the next main pole and are connected in series with the armature.The interpoles induce emfs in the short circuited coils that exactly cancels the back emf, thus allowing the current to fall to zero instantly. Being in series with the armature means that the reactance voltage is always eliminated irrespective of the value of armature current.By careful design, the interpoles can also be used to eliminate armature reaction in the interpole region.generator classificationsGenerators are usually classified by the method of excitation used. There are three classifications; permanent magnet, separately excited and self excited.A permanent magnet generator has a limited output power and an output voltage that is directly proportional to speed. A separately excited generator has its field supplied from an external source. The output voltage being controlled by varying the field current.Self excited generators supply their own field current from the generator output, again the output voltage is controlled by varying the field current. This group may be subdivided into three subgroups; series, shunt and compound. Series generatorThe series generator has a field winding consisting of a few turns of heavy gauge wire connected in series with the armature.On "Noload" there is no armature current and therefore no field current. The only voltage generated is due to residual magnetism within the fields.As the load current increases, the field current increases and the terminal voltage rises, the increase in voltage more than compensating for the loss due to armature reactance and internal resistance. The voltage continues to rise until saturation of the field occurs.A series generator therefore has a rising characteristic and is generally only used as a line booster.

Shunt generatorThe shunt generator has a field consisting of many turns of fine wire connected in parallel with the armature.On "Noload" the terminal voltage is a maximum. As the load current increases, the terminal voltage decreases due to the resistance of the armature and armature reactance.The shunt generator has a falling characteristic and is used for d.c. generation on aircraft.Self excitationFor a d.c. generator to self excite, certain conditions must be met:The generator must have residual magnetism.The excitation field, when formed, must assist the residual magnetism.For shunt generators, additional criteria need to be met:The field resistance must be below a critical value.The load resistance must not be too low.Due to the first two points above, the only way to reverse the output voltage of a d.c. generator is to reverse the polarity of the residual magnetism. If the supply to the field winding, or the drive direction is reversed, the excitation will oppose the residual magnetism and the field will be lost.

Compound generatorCompound generators have both series and shunt field windings and fall into one of two categories:differential compound generators, in which the two fields are wound so as to oppose each other.cumulative compound generators, in which the fields are wound so as to assist each other.Differential compound generators are generally used where a high initial voltage is required, but only a low running voltage. Devices such as arc welders or arc lighting may use this form of generator.Cumulative compound machines can be wound to produce over, level or under compounding. Under compounding is more common in aircraft generators, the output voltage falling as the load current is increased.