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

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

engineering

uk

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.

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

engineering

uk

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.

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

engineering

uk

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.

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

engineering

uk

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.

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

engineering

uk

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.

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

engineering

uk

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,

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

engineering

uk

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

engineering

uk

4

AC THEORY

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.

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

engineering

uk

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.