DC Generator Systems Introduction
VARIABLE RESISTOR
4) EPICYCLIC REDUCTION GEARING
5) B R A LIMIT SWITCHES
As soon as the motor starts to rotate ‘Limit Switch A’ breaks the circuit to the ‘Closed’ indicator and causes the light to go out. When the valve is in its fully open position the limit switches are arranged, so that ‘Limit Switch A’ completes the circuit to the ‘Close Field Winding’, whilst ‘Limit Switch B’ breaks the circuit to the ‘Open Field Winding’, and operates the ‘Open’ indicator. If the switch is then placed in the ‘Close’ position, current will flow through the ‘Close Field Winding’, and then through the armature. The two fields will interact, and the motor will rotate in the opposite direction, thus closing the valve.
This occurs because the polarity of the field windings reverse, but the direction of the current through the armature remains the same, thus the resultant interaction of the fields will cause the armature to run in the reverse direction. When the valve is fully closed, the position of the limit switches will reverse, thus completing the circuit to the ‘Open Field Winding’, and operating the ‘Closed’ indicator.
Electromagnetic Brakes
Most actuators are fitted with electromagnetic brakes, as shown below, which are designed to prevent over-travel when the motor is switched off. The design of the brake system varies with the type and size of the actuator, but in all cases the brakes are spring-loaded to the
‘ON’ condition whenever the motor is de-energised, thus preventing the actuator over-
Conversely the brakes are immediately withdrawn whenever power is applied to the appropriate field winding, since the brake solenoid is connected in series with the armature.
Clutches
Friction clutches, that are usually of the single-plate type or multi-plate type, which depends on the size of the actuator, are also incorporated in the transmission systems to protect them against the effects of mechanical over-loading
.
Instrument Motors
DC motors are not widely used in aeroplane instruments, but form the gyroscopic element in one or two types of turn-and-bank indicator. The motor armature together with a concentrically mounted outer rim forms the gyroscope rotor. The purpose of the rim is to increase the rotor mass and radius of gyration, thus increasing its rigidity. The armature rotates inside a cylindrical two-pole permanent magnet stator, which is secured to the gimbal ring. Current is fed to the brushes and commutator via flexible springs to permit gimbal ring movement.
The rotor speed is kept constant by a centrifugal cut-out type governor, which consists of a fixed contact and a movable contact, that are normally held in the closed position by an adjusting spring. The contacts are fitted in series with the armature winding, and a resistor is connected in parallel with the contacts. When the maximum speed is attained, the centrifugal force acting on the movable contact overcomes the spring restraint, and causes the contacts to open. Current to the armature then passes through the resistor and reduces the rotor speed, until it resumes its nominal value.
Architecture of a Starter/Generator System
Some types of turbo-propeller aeroplanes utilise a single unit for starting the engine and supplying the aeroplane's DC power. This unit is called a ‘Starter/Generator’, and a typical system is shown below.
It is basically a compound machine, which is coupled to the engine by way of a drive shaft and gear train.
Operation of a Starter/Generator System
¾ The starter relay energises and the two batteries are connected in parallel supplying 24 volts to the starter motor. This reduces the initial starting current and torque, thus extending the life of the starter motor.
¾ When the engine reaches 10% RPM a speed sensor energises the paralleling relay (A), which causes the batteries to be momentarily connected in series, as shown below, thus supplying the starter motor with 48 volts.
¾ At 60% engine RPM the starter and paralleling relay are de-energised, which removes the power from the starter motor, and reconnects the batteries in parallel.
¾ When the engine is running correctly the generator control switch is operated, and the DC generator feeds the busbar.
Inverters
Certain electrical systems on aeroplanes require AC at a constant frequency, thus it is necessary to provide a means of producing a constant frequency supply on DC, and AC frequency wild powered aeroplanes. This is achieved via an inverter, of which the following basic types exist:-
Rotary Inverter. This type is basically a DC motor, which drives an AC generator on a
common shaft, as shown below.
The motor drives the AC generator at a constant speed to give a constant frequency output, which is achieved by adjusting the field excitation of the DC motor, and the output voltage of from the AC generator is maintained by similarly adjusting its field excitation. This type of inverter has a DC input of 28 volts and produces a 3 Phase AC output of 115V at 400HZ. Most rotary inverters are only 50% efficient, and typically a 100 Watts DC input will produce a 50 VA AC Output.
Static Inverter. This type differs from the rotary type in that it is constructed using solid-
state transistorised circuitry. It is also more robust, more reliable, and requires less servicing. Static inverters cannot match the power output of rotary inverters, although most have an efficiency of approximately 70%.
Multiple Inverter Installations
A typical multiple inverter system is shown below, and is the type, which is commonly fitted on twin-engine turbo-propeller aeroplanes.
This system consists of three inverters, of which the No.1 and No.2 inverters, are of the same type, and supply normal constant frequency AC power, whilst the No.3 inverter is a smaller type, and is used to supply the essential AC loads in an emergency.
Inverters cannot be operated in parallel, so it is thus necessary to devise a method by which each of the main inverters, No.1 and No.2, receive approximately the same running time. This is achieved in some airlines by using the No.1 inverter as the main one, and the No.2 inverter as the standby one on the outgoing journey, and vice versa on the return journey. Many inverters also have their outputs monitored for correct voltage and phase rotation. If either of these factors is incorrect the inverter will be automatically removed from the loads.