(a) It converts mechanical energy into electrical energy when driven by prime mover. (a) It converts electrical energy into mechanical energy, when input supply is given by any source.
(b) It works on the principle of Faraday’s Law of electro magnetic induction and its direction of induced current (e.m.f) is found by Fleming’s right hand rule. (b) It works on the principle “when a current carrying conductor placed in a magnetic field it experience or produce a mechanical force whose direction of rotation is found by Fleming’s Left hand rule.


Relays. It is an electrically controlled device that opens and closes electrical contacts to effect the operation of other devices in the same or another circuit. Relays are usually used for low current switching application. The part of relay attracted by the electromagnet to close the contact points is called armature. and the movement of the armature either closes or opens the contact points. In some cases the electromagnet operates several sets of contact points simultaneously. It can be used up to 5kw.

Solenoid. A coil of wire wound in the form of a spiral is called a solenoid. When current is passed through a solenoid, a magnetic field is produced.  The direction of the magnetic field around a current carrying solenoid can be found by using grip rule.  Grip the solenoid in the right hand, wrapping the fingers around it and the direction of fingers is, the direction in which current is flowing. Extension of thumb in the direction of the axis of coil indicates the end having north polarity.


Switches. A switch can be defined as a device for closing and opening (making or braking) an electric circuit.  It usually consists of one or more pairs of contacts made of metal or metal alloy, through which an electric current can flow when the contacts are closed. The switches can be manually operated, electrically operated or electronically operated. The manual switch is usually operated by either a lever or a push button. Electrically operated switches are called relays or solenoids.

Switches are designed with varying number of contacts to make suitable for controlling one or more electric circuit. The switch used to open and close a single circuit is called Single Pole Single Throw (SPST) switch.  A switch designed to turn two circuits on and off with a single lever is called two-pole or Double Pole Single Throw (DPST) switch.  A switch designed to route current to either of two separate circuits is called Single pole double throw (SPDT) switch. Double throw switches can be designed with or without a centre OFF position. The three-position switch (one containing a centre OFF position) could be used when it is necessary to connect a wire to a choice of two circuits or disconnect it from both is known as double pole double throw (DPDT) switches.Two-position double pole double throw switches contain no OFF position. Switches available in different configurations are as follows.

(a) Toggle Switch. These are normal on/off switches.

(b) Rotary Switch. It takes the place of several switches when the knob of the switch is rotated switch opens one circuit and closes another. E.g., Ignition switches, Voltmeter selector switches, fan regulator etc.

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 (c) Push Button Switch. It is spring loaded and designed for momentary contact, used to operate compressors, motors and aircraft fire bottle operation etc.

 (d) Micro Switch. It will open or close a circuit with a very small movement of the tripping device (1/16 inch or less). It is also known as snap action switch is a generic term used to refer to an electrical s/w that is able to be actuated by very little physical force through the use of tipping point mechanism. Micro switches are usually push button switches. They are used primarily as limit switches to provide automatic control of landing gears, actuators, motors, etc.

Protective Devices. A common cause of circuit failure is due to a short circuit.  The danger in short circuit is that an excessive amount of current may flow through limited portions of the circuit, causing wires to overheat and burn off insulation. Many fires are caused by short circuit, but this danger is largely overcome by installation of protective devices, such as fuses, circuit breakers etc.

(a) Fuses. A fuse is a strip of metal having a low melting point.  It is placed in a circuit in series with load so that load current flows through it.  When current flowing through it exceeds the capacity of fuse, the metal strip melts and breaks the circuit.

(b) Circuit Breakers. A circuit breaker serves a purpose similar to that of a fuse; however the circuit breaker can usually be reset after the circuit fault has been rectified.  There are several types of circuit breakers in general use in aircraft systems.

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