EASA part 66 Module 3

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JAR 66 CATEGORY B1 MODULE 3 (part B) ELECTRICAL FUNDAMENTALS

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Index

1 DC GENERATION ... 1-1 1.1 SIMPLE SINGLE LOOP GENERATOR ... 1-2 1.1.1 Induced emf ... 1-2 1.1.2 Output frequency ... 1-3 1.2 COMMUTATION ... 1-3

1.3 RING WOUND GENERATOR ... 1-4 1.4 PRACTICAL DC GENERATOR ... 1-7 1.4.1 Construction ... 1-7 1.4.2 Lap wound generator ... 1-9 1.4.3 Wave wound generator ... 1-10 1.4.4 Internal resistance ... 1-11 1.4.5 Armature reaction ... 1-11 1.4.6 Reactive sparking ... 1-13 1.5 GENERATOR CLASSIFICATIONS ... 1-15 1.5.1 Series generator ... 1-15 1.5.2 Shunt generator ... 1-16 1.5.3 Self excitation ... 1-16 1.5.4 Compound generator ... 1-17 2 DC MOTORS ... 2-1

2.1 SIMPLE SINGLE LOOP MOTOR ... 2-2 2.2 COMMUTATION ... 2-2 2.3 PRACTICAL DC MOTORS ... 2-3 2.3.1 Construction ... 2-3 2.3.2 Back emf ... 2-3 2.3.3 Starting d.c. motors ... 2-3 2.3.4 Torque... 2-4 2.3.5 Armature reaction ... 2-4 2.3.6 Reactive sparking ... 2-4 2.3.7 Speed control ... 2-4 2.3.8 Changing the direction of rotation ... 2-5 2.4 MOTOR CLASSIFICATIONS ... 2-5

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4.1 PRODUCTION OF A SINEWAVE ... 4-1 4.2 THE SINEWAVE ... 4-2 4.2.1 Peak and Peak-to-Peak values ... 4-3 4.2.2 Average values ... 4-3 4.2.3 RMS values ... 4-4 4.2.4 Form Factor. ... 4-4 4.2.5 Periodic time ... 4-4 4.2.6 Frequency ... 4-4 4.2.7 Angular Velocity. ... 4-5 4.2.8 Phase Difference (Angular Difference). ... 4-5 4.3 PHASOR OR VECTOR DIAGRAMS ... 4-6 4.3.1 Addition of phasors ... 4-7 4.4 ADDITION OF AC & DC ... 4-8

4.5 MEASURING AC USING OSCILLOSCOPES ... 4-8

4.5.1 The cathode Ray oscilloscope ... 4-8 4.5.2 Types of oscilloscopes ... 4-11 4.5.3 using the oscilloscope ... 4-15 4.6 OTHER TYPES OF WAVEFORMS ... 4-27

4.6.1 Square waves ... 4-27 4.6.2 Triangular or sawtooth waves ... 4-27 4.7 AC VOLTAGE & CURRENT ... 4-28 4.7.1 Resistive loads ... 4-28 4.7.2 Capacitive loads ... 4-28 4.7.3 Inductive loads ... 4-30 4.7.4 Impedance ... 4-31 4.8 AC POWER ... 4-32 4.8.1 Resistive loads ... 4-32 4.8.2 Inductive loads ... 4-33 4.8.3 Capacitive loads ... 4-34 4.8.4 The total load on a generator ... 4-35 4.8.5 Apparent Power & actual current ... 4-35 4.8.6 True power & Real Current ... 4-36 4.8.7 Reactive power & reactive current ... 4-37 4.8.8 Power Factor ... 4-37 4.9 SERIES L/C/R CIRCUITS ... 4-38

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4.10.1 Inductance and capacitance in parallel ... 4-44 4.10.2 Parallel resonance ... 4-45 4.10.3 Impedance ... 4-46 4.10.4 Current magnification ... 4-47 4.10.5 Bandwidth ... 4-47 4.10.6 Selectivity ... 4-48 5 TRANSFORMERS ... 5-1 5.1 POWER TRANSFORMERS ... 5-1 5.2 CIRCUIT SYMBOLS & DOT CODES ... 5-2

5.3 LOSSES ... 5-4

5.3.1 Iron losses... 5-4 5.3.2 Copper losses ... 5-4 5.3.3 Flux leakage losses ... 5-5 5.3.4 Skin Effect ... 5-5 5.4 TURNS RATIO ... 5-5

5.5 POWER TRANSFERENCE ... 5-6 5.6 TRANSFORMER EFFICIENCY ... 5-6 5.7 TRANSFORMER REGULATION... 5-6

5.8 APPLYING LOADS TO A TRANSFORMER ... 5-7

5.8.1 No load conditions ... 5-7 5.8.2 Resistive loads ... 5-8 5.8.3 Inductive load ... 5-8 5.8.4 Capacitive load ... 5-9 5.8.5 Combination loads ... 5-9 5.9 REFLECTED IMPEDANCE ... 5-9 5.10 IMPEDANCE MATCHING TRANSFORMERS ... 5-10 5.11 AUTOTRANSFORMERS ... 5-11

5.12 MUTUAL REACTORS ... 5-12

5.13 CURRENT TRANSFORMERS ... 5-13 5.14 THREE PHASE TRANSFORMERS ... 5-15 5.15 DIFFERENTIAL TRANSFORMERS ... 5-16

6 FILTERS & ATTENUATORS ... 6-1 6.1 ... 6-1

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6.2.3 Variable attenuators ... 6-9 6.2.4 ' ' type attenuators ... 6-9 6.2.5 Balanced & unbalanced networks ... 6-10 6.2.6 Attenuator symbols ... 6-10 7 AC GENERATION ... 7-1

7.1 PRINCIPLES ... 7-1 7.1.1 Output voltage ... 7-2 7.1.2 Output frequency ... 7-2 7.1.3 Effects of a resistive load ... 7-3 7.1.4 Effects of an inductive load ... 7-4 7.1.5 Effects of a capacitive load... 7-4 7.2 PRACTICAL GENERATOR CONSTRUCTION ... 7-5

7.2.1 Rotating armature type ... 7-5 7.2.2 Rotating field type ... 7-5 7.2.3 Single phase generator ... 7-6 Two phase generator ... 7-7 7.2.5 Three phase generator ... 7-7 7.3 STAR & DELTA SYSTEMS ... 7-8

7.3.1 Delta connection ... 7-9 7.3.2 Star connection ... 7-9 7.3.3 Power in ac systems ... 7-10 8 AC MOTORS ... 8-1

8.1 PRODUCTION OF A ROTATING FIELD ... 8-1

8.1.1 Single phase ... 8-1 8.1.2 Two phase... 8-2 8.1.3 Three phase ... 8-3 8.2 TYPES OF AC MOTOR ... 8-3 8.2.1 Induction motor ... 8-3 8.2.2 Synchronous motor ... 8-5 8.2.3 Shaded pole motor ... 8-6 8.2.4 Hysteresis motor ... 8-7

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1 DC GENERATION

If 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 field

direction of relative movement between the conductor and the field and can be determined by using Fleming’s right hand rule.

The size of the generated emf depends on three factors, the: strength of the magnetic field - B

effective length of the conductor in the field - l linear velocity of the conductor - v

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1.1 SIMPLE SINGLE LOOP GENERATOR

In 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.

1.1.1 INDUCED EMF

As the loop rotates an emf is induced in both sides of the conductor. Using Fleming’s 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.

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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.

1.1.2 OUTPUT FREQUENCY

As 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.

1.2 COMMUTATION

In 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.

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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 multi-segment commutator, a more constant output can be produced.

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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 emf’s 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.

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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.

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1.4 PRACTICAL DC GENERATOR 1.4.1 CONSTRUCTION

The 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.

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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.

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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.

1.4.2 LAP WOUND GENERATOR

In 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).

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1.4.3 WAVE WOUND GENERATOR

In 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.

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1.4.4 INTERNAL RESISTANCE

A d.c. machine has resistance due to the: armature windings

brushes

brush to commutator surface contact

This 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 + V

1.4.5 ARMATURE REACTION

When 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.

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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

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1.4.6 REACTIVE SPARKING

The 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.

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One 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.

1.4.6.1 EMF Commutation

Another 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.

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1.5 GENERATOR CLASSIFICATIONS

Generators 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 sub-groups; series, shunt and compound.

1.5.1 SERIES GENERATOR

The series generator has a field winding consisting of a few turns of heavy gauge wire connected in series with the armature.

On "No-load" 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.

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1.5.2 SHUNT GENERATOR

The shunt generator has a field consisting of many turns of fine wire connected in parallel with the armature.

On "No-load" 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.

1.5.3 SELF EXCITATION

For 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

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1.5.4 COMPOUND GENERATOR

Compound 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.

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2 DC MOTORS

If a current carrying conductor is placed at right angles to a magnetic field, a force will be exerted on it, causing it to move.

The direction of the force and the resultant movement depends on two factors, the :

direction of current flow in the conductor direction of the magnetic field

The direction of the force and the resultant movement can be found by using Fleming’s left hand rule as shown below:

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2.1 SIMPLE SINGLE LOOP MOTOR

The simplest form of motor consists of a single loop of wire able to rotate between the poles of a permanent magnet.

If current is applied to the loop through slip rings, a motor torque will be produced, and the loop will start to rotate. As the loop rotates past vertical, the current appears to change direction, this causes the torque to change direction, so the direction of rotation changes.

When the loop passes vertical, the current appears to change direction again, causing rotation to revert to its original direction.

If left, the loop will simply oscillate back and forth either side of the vertical position.

2.2 COMMUTATION

To make the loop rotate, the current must be made to change direction as the loop passes the vertical position, this is achieved using a commutator and brushes.

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To improve the torque and produce smoother running, more loops or coils are added to the armature, each having its own commutator segment. The

construction is as described earlier in d.c. generators. 2.3 PRACTICAL DC MOTORS

2.3.1 CONSTRUCTION

Direct current generators are constructed in the same manner as d.c. generators, therefore further description is unnecessary. The similarities are such that one machine can be operated as the other with only minimal adjustment. In the case of starter generators, the only adjustment necessary is achieved electrically. Most motors have some form of rating, this being a limit on their performance. Ratings take various forms depending on the type, size and use of the motor, but are generally based on a limit on the speed, duration or altitude of operation. As with generators, the limit on a motors performance depends very much on the ability of the machine to dissipate heat. Cooling may be natural, by convection and radiation, or assisted by rotor mounted fans, blast air or slipstream.

2.3.2 BACK EMF

When a conductor moves in a field, an emf is induced in the conductor.

The armature coils of the motor are moving in a magnetic field and

therefore must have an emf induced in them, this emf acts against the applied voltage and is called back emf.

The resultant of the two voltages is called the effective voltage. The armature current is due to the effective voltage, not the applied voltage.

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2.3.4 TORQUE

The torque produced by a d.c. motor is directly proportional to the armature current and the magnetic field strength.

T = IARMATURE

Some torque is lost within the motor, especially if a fan is fitted to the rotor shaft. The torque lost is not constant, usually increasing with an increase in speed.

2.3.5 ARMATURE REACTION

The overall field of a d.c. motor consists of the armature field and the stator field. The two fields react, as in the d.c. generator, producing armature reaction. Armature reaction causes the magnetic neutral axis of the motor to be moved around in the opposite direction to that of the generator, against the direction of rotation. The problem can be overcome as in d.c. generators, by fitting

compensating windings.

2.3.6 REACTIVE SPARKING

d.c. motors also suffer from reactive sparking. For fixed load motors, the problem is overcome simply by moving the brushes onto the magnetic neutral axis. For variable load motors, interpoles are used as in d.c. generators.

2.3.7 SPEED CONTROL

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2.3.7.1 Field control

With field control, a decrease in field current causes an increase in motor speed; main field decreases

back emf across armature decreases effective voltage increases

armature current increases

motor torque increases over load torque motor speed increases

This occurs because a small change in the main field strength causes a large change in the armature current. Of course, this cannot continue uncontrolled because eventually the field will be lost. Field control is generally used for speed control of normal running speed and upwards.

2.3.7.2 Armature control

With armature control, an increase in armature current causes an increase in motor torque over load torque and an increase in motor speed. A decrease in armature current causes a decrease in motor speed. Armature control is generally used for control of normal running speed and downwards.

2.3.8 CHANGING THE DIRECTION OF ROTATION

To change the direction of rotation it is only necessary to change the direction of the main field or the armature current. If both are changed, the motor will rotate in the same direction.

In the majority of cases where a bi-directional d.c. motor is required on an aircraft, a split field motor is used. This motor will be examined in more detail later in the notes, suffice to say it has two fields windings, one for clockwise rotation, the other for anti-clockwise rotation.

2.4 MOTOR CLASSIFICATIONS

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2.4.1 SERIES MOTOR

A series motor has a low resistance, heavy gauge field winding in series with the armature winding. On light loads its speed is high, the armature current is low and the field is weak. On heavy loads. speed is low, the armature current is large and the field is strong. Series motors have a wide speed variation with load.

The armature torque is proportional to the field strength and armature current. In series motors the field strength depends on the armature current, so the torque produced is approximately proportional to the square of the armature current. In practice it is slightly less (particularly on heavy loads) due to armature reaction and saturation of the magnetic circuit.

As speed increases, the torque decreases, until the load torque and motor torque balance. If the load of a series motor is removed, the speed may become

dangerously high. It is not normal practice to run series motors off-load .

When starting a series motor, it is normally connected straight to the supply, the initial current being limited by the combined resistance of the field and armature windings and by the inductance of field winding. The field strength builds up quickly, giving a high starting torque, a fast acceleration and a rapid back-emf build up. There is a short period of high current drain on the supply.

Where a large change in operating speed is required, as in turbine engine starting, a starter resistor is initially connected in series with the motor and

removed when the motor is required to increase speed. The starter resistor must be able to withstand the large initial current. Applications include starter motors, winches and aircraft actuators.

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2.4.2 SHUNT MOTOR

Shunt wound motors have a high resistance field winding connected in parallel with the armature. The field current will be constant if the input voltage is constant and no field control resistor is used.

When the load torque is increased, the motor slows down. The decrease in speed, causes a fall in the back-emf and an increase in armature current which produces more motor torque. When the motor torque and load torque are again balanced, the speed becomes constant.

Small decreases in speed cause relatively large increases in armature current. Between no-load and full-load, the variation in speed of a d.c. shunt motor with a low resistance armature is small enough for it to be considered a constant speed motor. With a high resistance armature, there is a more noticeable variation in speed with load.

When a shunt motor has a constant input voltage:

on light loads, the magnetic field is constant and the torque is directly proportional to the armature current.

on heavy loads the magnetic field is reduced by armature reaction and the torque does not rise in direct proportion to the armature current.

If a shunt motor does not increase speed when connected to the supply, then no back-emf is produced. This results in a very high armature current, a large armature reaction and a reduced torque and the motor will not start.

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A low resistance shunt motor is normally started with a variable resistor, set to maximum resistance, placed in series with the armature. This reduces the armature current and armature reaction, thereby increasing the starting torque. As the speed increases, the back emf increases and armature current decreases. As the speed builds, the resistance is gradually decreased until at normal running speed it is totally removed from the circuit.

An automatic method used to insert a resistor is series with the armature for starting, and to remove it once the back-emf has been developed is referred to as a 'T’ Start circuit.

At the instant the motor is switched ‘on’, the armature is stationary and producing no back-emf, therefore the voltage at A is almost zero and the relay is

de-energized. The resistance is in circuit limiting the current.

As the rotor starts to turn and the back-emf increases, the potential at point A starts to increase.

At a pre-determined speed the potential at point A and the current through the relay coil will be sufficient to cause the relay to energize, removing the resistor from the armature circuit.

Speed control - The speed of a shunt motor is normally controlled by a variable resistor placed in series with the field winding. When the resistance is increased, the field current is reduced, the back-emf decreases and the effective voltage increases. The increase in effective voltage produces an increase in armature current and an increase in speed. When required to reduce the speed of the motor, the field resistance is decreased.

Separately excited shunt motors - Separately excited d.c. shunt motors have the same operating characteristics as self excited shunt motors and therefore require no additional consideration.

Applications - Shunt motors are used where a constant speed is required and will be found in inverter drives and windscreen wipers.

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2.4.3 COMPOUND MOTOR

These are used to meet specific requirements, we may require a motor: that has a high starting torque, but will not race off-load.

to increase, decrease or maintain speed as the load on it varies.

These requirements can be met with suitable compounding. As with generators, there are two forms of compound motor.

Differential compound - fields connected to oppose each other Cumulative compound - fields connected to assist each other

2.4.4 SPLIT FIELD MOTOR

In certain applications it is necessary to change the direction of rotation of a motor. Typical examples would be in valves and actuators. We have already seen that this can be achieved by reversing the direction of the armature or field current, however, there is also a special form of reversible series motor known as a split field motor.

A split field motor is simply a series motor with two field windings. The fields are wound in opposite directions, with one being used for each direction of rotation. The direction is usually controlled by a single pole, double throw switch as shown

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When it is required that the actuator drive to position 2, the selector switch is moved to position 2. Current flows through the field winding, brake solenoid and armature winding. The brake is released and the motor starts to turn. As soon as the motor moves, it is no longer in position 1, so switch A moves across. This allows the direction to be reversed (by returning the selector switch to position 1) should the need dictate. When the motor reaches the limit of travel at position 2, switch B moves across, removing the motor power supply. The brake solenoid, field winding and armature de-energise, the brake is applied and the motor stops. If the selector switch is now moved to position 1, the upper field winding, brake solenoid and armature are energised. The brake is released and the motor runs in the opposite direction towards position 1. Again as soon as the motor turns, it is no longer at position 2 so the lower switch moves over to contact the field winding.

2.5 RATING

Most motors have a rating - a limit on performance or operation. Ratings take various forms - output, time, speed, altitude. As with generators, the output depends very much on the machines ability to dissipate heat. All machines require some form of cooling. Low output motors, or those that are not used for continuous operation may be cooled naturally. Others may be fitted with

centrifugal or straight fans to drive air through machine, this being usual on small machines. Others use air ducted from slipstream.

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3 STARTER GENERATORS

Many gas turbine aircraft are equipped with starter-generator systems. These starting systems use a combination starter-generator which operates as a starter motor to drive the engine during starting, and after the engine has reached a self-sustaining speed, operates as a generator to supply the electrical system power. The starter-generator unit shown below left, is basically a shunt generator with an additional heavy series winding. This series winding is electrically connected to produce a strong field and a resulting high torque for starting.

Starter-generator units are desirable from an economical standpoint, since one unit performs the functions of both starter and generator. Additionally, the total weight of starting system components is reduced, and fewer spare parts are required.

The starter-generator shown below right has four windings; (1) series field, (2) shunt field, (3) compensating, and (4) interpole. During starting, the series, compensating, and interpole windings are used. The unit is operating in a similar manner to a direct-cranking starter, since all the of the windings used during starting are in series with the source. While acting as a starter, the unit makes no practical use of its shunt field. A source of 24 volts and 1,500 amperes is usually required for starting.

When operating as a generator, the shunt, compensating and interpole windings are used. The output voltage is controlled in the conventional manner, by

connecting the shunt field in the voltage regulator circuit. The compensating and interpole windings provide almost sparkless commutation from no-load to full-load.

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The following diagram illustrates the external circuit of a starter-generator with an undercurrent controller. This unit controls the starter-generator when it is used as a starter. Its purpose is to ensure positive action of the starter and to keep it operating until the engine is rotating fast enough to sustain combustion. The control block of the undercurrent controller contains two relays; one is the motor relay which controls the input to the starter, the other, the undercurrent relay, controls the operation of the motor relay.

To start an engine equipped with an undercurrent relay, it is first necessary to close the engine master switch. This completes the circuit from the aircraft's bus to the start switch, the fuel valves, and the throttle relay. Energising the throttle relay starts the fuel pumps, and completing the fuel valve circuit provides the necessary fuel pressure for starting the engine.

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When the battery and start switches are turned on, three relays close. They are the motor relay, ignition relay and battery cut-out relay. The motor relay closes the circuit from the power source to the starter motor; the ignition relay closes the circuit to the ignition units; and the battery cut-out relay disconnects the battery. On this particular aircraft opening the battery circuit is necessary because the heavy drain of the starter motor would damage the battery, this is not the general case. The majority of aircraft are designed to be started using the battery so as to make the aircraft independent of ground resources, the battery will however be disconnected from the bus when ground power is connected and care must be taken to ensure the ground power unit is capable of supplying the current required by the starter motor.

Closing the motor relay allows a very high current to flow to the motor. Since this current flows through the coil of the undercurrent relay, it closes. Closing the undercurrent relay completes a circuit from the positive bus to the motor relay coil, ignition relay coil, and battery cut-out relay coil. The start switch is allowed to return to its normal "off" position and all units continue to operate.

As the motor builds up speed, the current draw by the motor begins to decrease, as it decreases to less than 200 amps, the undercurrent relay opens. This action breaks the circuit from the positive bus to the coils of the motor, ignition and battery cut-out relays. The de-energising of these relay coils halts the start operation.

After the procedures described are completed, the engine should be operating efficiently and ignition should be self-sustaining. If however, the engine fails to reach sufficient speed, the stop switch may be used to break the circuit from the positive bus to the main contacts of the undercurrent relay, thereby halting the start operation.

On a typical aircraft installation, one starter-generator is mounted on each engine gearbox. During starting, the starter-generator unit functions as a d.c. starter motor until the engine has reached a predetermined self-sustaining speed. Aircraft equipped with two 24 volt batteries can supply the electrical load required for starting by operating the batteries in a series configuration.

The following description of the starting procedure used on a four-engine turbojet aircraft equipped with starter-generator units is typical of most starter-generator starting systems.

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Starting power, which can be applied to only one starter-generator at a time, is connected to a terminal of the selected starter-generator through a corresponding starter relay. Engine starting is controlled from an engine start panel. A typical start panel (see diagram below) contains an air start switch and a normal start switch.

The engine selector switch shown has five positions ('1, 2, 3, 4, and off'), and is turned to the position corresponding to the engine to be started. The power selector switch is used to select the electrical circuit applicable to the power source being used (ground power unit or battery). The air-start switch, when placed in the "normal" position, arms the ground starting circuit. When placed in the "air-start" position, the igniters can be energised independently of the throttle ignition switch. The start switch, when in the "start" position, completes the circuit to the starter-generator of the engine selected, and causes the engine to rotate. The engine start panel shown above also includes a battery switch. When an engine is selected with the engine selector switch, and the start switch is held in the "start" position, the starter relay corresponding to the selected engine is energised and connects that engine's starter-generator to the starter bus. When the start switch is placed in the "start" position, a start lock-in relay is also energised. Once energised, the start lock-in relay provides its own holding circuit and remains energised providing closed circuits for various start functions.

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On some aircraft a battery lockout switch is installed in the external power receptacle compartment. When the door is closed, activating the switch, the ground starting control circuits function for battery starting only. When the door is open, only external power ground starts can be accomplished.

A battery series relay is also necessary in this starting system. When energised, the battery is connected in series to the starter bus, providing an initial starting voltage of 48 volts. The large voltage drop which occurs in delivering the current needed for starting, reduces the voltage to approximately 20 volts at the instant of starting. The voltage gradually increases as the starter current decreases with engine acceleration and the voltage on the starter bus eventually approaches its original maximum of 48 volts.

Some multi-engine aircraft equipped with starter-generators include a parallel start relay in their starting system. After the first two engines of a four-engine aircraft are started, current for starting each of the last two engines passes through a parallel start relay. When starting the first two engines, the starting power requirement necessitates connecting the batteries in series. After two or more generators are providing power, the combined power of the batteries in series is not required. Thus, the battery circuit is shifted from series to parallel when the parallel start relay is energised.

To start an engine with the aircraft batteries, the start switch is placed in the "start" position. This completes a circuit through a circuit breaker, the throttle ignition switch and the engine selector switch to energise the start lock-in relay. Power then has a path from the start switch through the "bat start" position of the power selector, to energise the battery series relay, which connects the aircraft batteries in series to the starter bus.

Energising the No 1 engine's starter relay directs power from the starter bus to the No. 1 starter-generator, which then cranks the engine.

At the time the batteries are connected to the starter bus, power is also routed to the appropriate bus for the throttle ignition switch. The ignition system is

connected to the starter bus through an overvoltage relay, which does not become energised until the engine begins accelerating and the starter bus voltage reaches about 30 volts.

As the engine is turned by the starter to approximately 10% r.p.m. the throttle is advanced to the "idle" position. This action actuates the throttle ignition switch,

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4 AC THEORY

4.1 PRODUCTION OF A SINEWAVE

The only practical way of generating an electromotive force (emf) by mechanical means is to rotate a conductor in a magnetic field. As the conductor rotates in the magnetic field, its direction of motion relative to the magnetic field is

continually changing, therefore, the emf induced in the conductor is continuously changing. The emf will start at zero when the conductor is moving parallel with the lines of flux, it will rise to a maximum value when the conductor is moving at 90° to the lines of flux, before decaying back to zero rising to a maximum value in the opposite direction. In this way, an alternating emf is produced which, when connected to a circuit, produces an alternating current flow.

By making the conductor in the form of a loop, we have the basis of the simple ac generator.

All generators, both dc and ac, have this basic design. In a dc machine the output to the load is continually switched by the commutator, so that the load current always flows in one direction. In an ac machine the output to the load is continually reversing it direction.

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If the generated emf of the loop is measured and plotted as the loop rotates, the result will be as shown in the diagram below.

It can be seen that when the conductors are moving parallel to the lines of flux, and not cutting them, the induced emf is zero. When the conductors are cutting the lines of flux at right angles, maximum emf is induced in them. By convention, the part of the waveform above the zero line is labelled positive and the part below the line is labelled negative.

4.2 THE SINEWAVE

If the conductor is rotated at uniform speed in a uniform magnetic field, the output waveform is said to be ‘sinusoidal’ and we refer to this type of waveform as a sine wave. There are many other wave shapes that can be generated or

developed, but it is the sine wave that is used for main power supply systems. It is therefore necessary for the engineer to be very familiar with this particular

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4.2.1 PEAK AND PEAK-TO-PEAK VALUES

Amplitude values and their calculation apply equally to current and voltage measurement.

The Peak or Maximum Value. The maximum value attained by the wave in either direction is called the maximum value, or more usually, the peak value.

The Peak-to-Peak Value. The maximum value in one direction, to the maximum in the other direction is called the Peak-to-Peak value. It must not be confused with peak value, which is measured in one direction only. Peak-to-peak values are often used on oscilloscopes because it is easier to measure from top to bottom of the waveform, but the majority of calculations require the use of the peak value. It must be remembered to divide the peak-to-peak value by two in order to obtain the peak value for calculations.

The Instantaneous Value. As previously stated, the value at any instant can be calculated by multiplying the peak value by the sine of the angle (from 0º) through which the conductor has rotated.

4.2.2 AVERAGE VALUES

The amplitude of an ac waveform may be defined in terms of its average values. Over one complete cycle, this would mathematically be zero (the wave goes as far positive as it does negative) If the pulses of voltage or current are always in one direction, the average value can be calculated from:

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4.2.3 RMS VALUES

Whilst the Peak and Average values of ac have their place and uses, they are not a lot of use for everyday work on ac. What is required is a value of ac which relates to an equivalent value of dc. Suppose an electric fire is operating with 5 amperes of d.c. current flowing through it and it is giving out a certain amount of heat. We want to know the value of a.c. which will produce the same amount of heat. Such a value is given by the Root Mean Square (rms) value of an a.c. current.

For a sinusoidal waveform, the rms value = peak value × 0.707.

In other words, a sine wave of peak value ‘y’ produces a certain amount of heat when passed through a given resistor. To produce the same heating effect, in the same resistor using d.c., would require a d.c. with a steady current of only 0.707 of ‘y’.

By convention, it is not necessary to add ‘rms’ to a voltage or current value but, if peak or average values are being referred to, then the word ‘peak’ (Pk) or

‘average’ (Av) must be added after the value.

4.2.4 FORM FACTOR.

The form factor of a waveform is a number which indicates its shape: Form Factor = rms value

average value

For a sine waveform, this works out at 0.707 / 0.637 = 1.11. For any other waveform, the values will be different and so the Form Factor will be a different number. (This is given in these notes for information only as the aircraft engineer should not have to concern himself with the form factor).

4.2.5 PERIODIC TIME

The time taken to complete one cycle is called the ‘periodic time’ (t). It is measured in seconds or fractions of a second.

4.2.6 FREQUENCY

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Periodic time and frequency are related. T = 1/f and f = 1/T

4.2.7 ANGULAR VELOCITY.

The velocity at which a phasor rotates is very important and can be calculated from:

Speed =Distance_Time

Distance (one revolution) = 2 radians. Time (periodic time) = 1/f.

Angular Velocity ( ) (omega) = 2_1/f radians per second = 2 f radians per second.

(A proper understanding of this formula is essential as it is used in other formulae).

Referring back to our simple loop it can be seen that, if the loop was rotating at 120 revolutions per second, the output frequency would be 120 Hz. It therefore follows, that the frequency of the output of an ac generator is directly proportional to its speed of rotation.

4.2.8 PHASE DIFFERENCE (ANGULAR DIFFERENCE).

If two conductors are caused to rotate at the same angular velocity, then two waves would be generated. Any angle between them is said to be their phase difference. In the following diagram, the phase difference is 90º. As the

conductors rotate in an anti-clockwise direction, the dotted wave is said to lead the solid wave by 90º.

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4.3 PHASOR OR VECTOR DIAGRAMS

Waveform diagrams are difficult to visualise and engineers have devised a diagrammatic method known as a phasor or vector diagram to simplify the problem.

The terms vector and phasor are interchangeable, however, the term vector is more general, being used to denote any quantity that has both magnitude and direction, whereas the term phasor, tends to be associated with electrical engineering. To avoid repetition, the word phasor will be used in these notes. Imagine a phasor of length of Vm rotating in an anticlockwise direction, rather like the conductor rotating in the magnetic field. If you plot the vertical displacement of the tip of the line at various angular intervals, the curve traced out is a

sinewave.

When the line is horizontal, the vertical displacement of the tip of the line is zero, corresponding to the start of the sinewave at point A. After the line has rotated 90 in an anti-clockwise direction, the line points vertically upwards, point B on the diagram. After 180 of rotation the line points to the left of the page, and the vertical displacement is again zero. Rotation through a further 180 returns the line to its start point.

A phasor is a line representing the rotating line Vm, frozen at some point in time. Although line Vm was drawn to represent the maximum values, a phasor is normally scaled to represent r.m.s. values, and can be used to represent voltage current, power or indeed flux. One rotation of the phasor produces one cycle of the waveform, therefore the number of rotations completed per second gives the frequency.

The 3 'o-clock position on a phasor diagram is considered to be the reference point of the diagram. Whether the current, voltage, mmf or flux is drawn pointing

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4.3.1 ADDITION OF PHASORS

The addition of sine waves is greatly simplified by the use of phasor addition, however it should be remembered that, phasors can only be used to add sinewaves of the same frequency.

To add two phasors, a parallelogram is produced, the two extra sides being drawn parallel to the phasor already present.

Each extra side should start at the end of each phasor as shown. Once the parallelogram has been produced, the resultant voltage is represented by a line from the origin to the intersection of the two new lines. The length of this new phasor represents the magnitude of the new voltage and the angle between it and the other phasor is the phase angle between them. When adding more than two phasors, it is simply a matter of reducing pairs to a single phasor, as

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4.4 ADDITION OF AC & DC

It is possible for both ac and dc to exist in the same circuit or conductor. In such cases the ac is said to be superimposed on the dc, or the dc has an ac ripple. The resultant waveform depends on the relative values of ac and dc, as shown in the diagrams above.

4.5 MEASURING AC USING OSCILLOSCOPES

4.5.1 THE CATHODE RAY OSCILLOSCOPE

Cathode ray oscilloscopes are analogue-graphical instruments which enable electrical waveforms to be displayed for analysis and measurement purposes. A typical instrument is represented in the diagram below.

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With reference to the above diagram, the grids g1, g2 and g3 of the cathode ray tube (CRT) form an electron gun which projects a stream of electrons between deflecting plates onto the screen. The screen is coated with a phosphorescent material so that a luminous spot is produced on the screen. A property of the screen coating material allows the spot to persist for a period of time when the stream of electrons is moved or interrupted. The amount of illumination depends on the quantity of electrons in the stream and their velocity on impact with the screen.

The potential at grid g1, which is negative with respect to the cathode, controls the quantity of electrons emitted from the cathode. Adjusting R1 varies the potential at g1, hence R1 controls the brightness of the illuminated spot. Positive potentials at g2 and g3 accelerate the electrons towards the screen. The potential difference between g2 and g3, varied by adjusting R2, sets up an electrostatic field which enables the electron stream to be focused at the screen.

The position of the spot on the screen is determined by the simultaneous effect of voltages applied to the X and Y deflecting plates. A potential difference between the X deflecting plates causes the spot to move across the screen in the

horizontal direction, through a distance proportional to the potential difference. A potential difference between the Y deflecting plates exerts a similar control over the vertical movement of the spot.

The outputs of the X and Y amplifiers establish the potential differences between corresponding pairs of deflecting plates. If these voltages vary in magnitude the spot moves over the screen to produce a continuous trace. Since one voltage controls horizontal deflection and the other controls vertical deflection, the trace forms a graphical representation of one voltage as a function of the other.

4.5.1.1 The Time Base

Most applications require that a signal waveform is displayed as a function of time. To meet this requirement a time base circuit supplies a voltage which varies linearly with time, usually, to the

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The period t1, is the sweep, that is the time the spot takes to move linearly from left to right across the screen. During the much shorter period t2, called the flyback time, the spot returns rapidly to the left of the screen to start a new cycle. During flyback the screen may blacked out by a negative pulse generated by the time base circuit and applied to g1, the control grid.

If the sweep period (T) of the time base is equal to, or is a multiple of, the periodic time of the signal applied to the Y deflecting plates, a stationary display of the signal voltage variations with time will be obtained. In the diagram above, the sweep period (T) equals the periodic time 1_f of the signal waveform. In practice the time base is adjusted so that signals over a wide frequency range may be displayed against a convenient time scale.

4.5.1.2 Synchronisation

The time base and the displayed waveform may be synchronised by employing a trigger circuit actuated by the signal itself, that is, by using the output of the Y amplifier. Alternatively, an external signal source or the mains supply may be used for this purpose.

The trigger circuit generates a pulse to initiate one sweep of the time base when the voltage applied to the circuit reaches a predetermined value. The circuit is adjustable so that a particular trigger point on either the positive or negative half cycle of the displayed waveform may be selected.

Where the signal to be observed is non-periodic, or when the signal appears

infrequently, the time base is triggered by the signal, performs one sweep and then waits for the next signal to appear. In order that the beginning of a non-periodic signal can also be examined, the vertical deflecting voltage is delayed relative to the trigger pulse so that the time base is started before the signal to be observed appears on the screen. The time relationship is shown in the diagram.

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4.5.1.4 Amplifiers & Attenuators

The X and Y amplifiers and attenuators provide the voltage scaling required to ensure that the instrument and the measured signal are compatible. Since the oscilloscope is required to display complex voltage waveforms, it is essential that fundamental and harmonic frequencies must undergo the same amplification or attenuation, and that the time relationships between different frequencies must be maintained. It therefore follows that both the amplifier and the attenuators, must have flat amplitude against frequency and transit time against frequency,

characteristics.

4.5.2 TYPES OF OSCILLOSCOPES

4.5.2.1 Sampling Oscilloscopes

At very high frequencies, say above 300MHz, it is not possible using existing techniques to produce a continuous display on an oscilloscope. To obtain a satisfactory display a sampling technique must be used.

As shown in the diagram below, in a sampling oscilloscope the time base circuit produces a stepped voltage waveform to deflect the electron beam in the

horizontal direction. Prior to each step, a pulse is generated which initiates the sampling process.

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4.5.2.2 Multiple Trace Display

Oscilloscopes equipped with multiple trace facilities enable two or more signals to be displayed simultaneously. Essential features of these instruments are a

separate input channel for each signal and a means of separating the electron beams for display. The most widely used instruments enable two signals to be compared, although four beam instruments are quite common.

Cathode ray tubes equipped with two electron guns and two sets of deflecting plates, so that each channel is completely independent, are employed in instruments known as Dual Beam Oscilloscopes. Alternatively, a single gun may be used to produce two traces by switching the Y deflecting plates from one input signal to the other for alternate sweeps of the screen. Although the signals are sampled, the display appears to the eye as a continuous, simultaneous, display of both signals. Oscilloscopes employing this techniques, which is called the alternate mode, can only be used as single channel instruments to

investigate transient waveforms.

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4.5.2.3.2 Chopped Mode

The electronic switch is free running at 100 - 500KHz and is independent of the frequency of the sweep generator. The switch successively connects small segments of the 1 and 2 waveforms to the vertical amplifier. If the chopping rate is much faster than the horizontal sweep rate, the individual little segments fed to the vertical amplifier reconstitute the original 1 and 2 waveforms on the screen, without visible interruptions in the two images.

4.5.2.4 Delayed Sweep

Both time bases in operation.

A - delaying sweep B - delayed sweep

Either or both (alternate) signals can be fed to X plates. This allows a closer examination of part of the waveform. CRO contains two linear calibrated sweeps, a main sweep and a delayed sweep. The main sweep is initiated by its trigger pulse at time t0. The delayed sweep will be triggered at time t1, intensifying the original display.

If the CRO sweep control is now set to delay position, the intensified portion will be shown expanded on the screen.

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4.5.2.5 Direct Viewing Storage C.R.T.

The dielectric storage sheet consists of a layer of scattered phosphor particles capable of having any portion of its surface area written to. This dielectric sheet is deposited on a conductive coated glass faceplate called the "storage target backplate".

The flood electrons are distributed evenly over the entire surface area of the storage target.

After the write gun has written a charge image on the storage target, the flood guns will store the image. The written portions of the target are bombarded by flood electrons that transfer energy to the phosphor layer in the form of visible light. This light pattern can be viewed through the glass faceplate.

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The digital storage CRO stores the data representing the waveforms in a digital memory. The input signal is "digitised", i.e. it is sampled and then converted into binary numbers by the A/D converter.

The resolution of the system depends on the number of bits used by the converter. Converters are said to have a resolution of 1 part in 2 or 'n bit

resolution' where n is the number of bits, i.e. 10 bit resolution would digitise to 210 (1024) discrete levels: the resolution would be 1 part in 1024 or 0.098%.

This digitised input is then converted back to an analogue signal for display by the D/A converter.

(1-2MHz which may be extended to 200MHz using sampling techniques).

4.5.3 USING THE OSCILLOSCOPE

An oscilloscope is an extremely comprehensive and versatile item of test equipment which can be used in a variety of measuring applications, the most important of which is the display of time related voltage waveforms. Such an item probably represents the single most costly item in the average service shop and it is therefore important that full benefit is derived from it.

The oscilloscope display is provided by a cathode ray tube (CRT) which has a typical screen area of 8cm 10cm. The CRT is fitted with a graticule which may be an integral part of the tube face or on a separate translucent sheet. The graticule is usually ruled with a 1cm grid to which further bold lines may be added to mark the major axes on the central viewing area. Accurate voltage and time measurements may be made with reference to the graticule, applying a scale factor derived from the appropriate range switch.

A word of caution is appropriate at this stage. Before taking meaningful

measurements from the CRT screen it is absolutely essential to ensure that the front panel variable controls are set in the calibrate (CAL) position. Results will almost certainly be inaccurate if this is not the case!