1. What is Eddy current?
We have seen that whenever the flux, linking with a closed electric circuit changes, an EMF is induced in the circuit and a current flows, the value of which depends on the emf around the circuit and the resistance of the circuit. It is not necessary that the circuit be a wire and that the flux passes entirely through it. If a solid block of metal is traversed by varying flux, metallic circuits in the block itself, which are linked by the flux, will carry current. If the magnetic circuit is made up of iron and if the flux in the circuit is variable currents will be induced by induction in the iron circuit itself. All such currents are known as Eddy Currents.
Eddy currents result in loss of power, with consequent heating of the material. The magnitude of power loss due to eddy currents is often a matter of considerable importance in electrical engineering. Electrical machinery usually involves varying fluxes. In DC as well as AC machines, we have an armature built of iron. In a permanent magnetic field or through which a revolving magnetic field passes.
2. What do you mean by creeping?
In energy meters sometimes the disc of energy metres makes slow but continues rotations at no load i.e. when potential coil is excited but with no current flowing in the load. This is called creeping. This error may be caused due to over compensation for friction, excessive supply voltage, vibration, stray magnetic field etc.
In order to prevent this creeping, two diagrammatically opposite holes are drilled in the disc. This causes sufficient distortions of the field. The result is that the disc tends to remain stationary when one of the whole holes comes under one of the poles of the shunt magnet.
3. What is armature reaction?
When the generator is supplying no load and the field winding is energized, there exist in it only the mmf of the main poles which creates the main flux. When the generator is loaded, current flows through the armature winding and magnetising effect or mmf that acts at right angles to the main field flux, is set up. The effect of magnetic field set up by the armature current on the distribution of flux under main poles is known as armature reaction.
The effect of armature reaction is to weaken the field strength in the gaps under the leading pole tips and strengthen under trailing pole tips. The magnetic field of the machine is distorted and the physical neutral line is shifted in the direction of rotation.
The armature mmf can be resolved into two components with one of the components in phase opposition to the main field mmf and is called the demagnetizing component and the other component is at right angles to the main field mmf and is called the cross magnetizing component.
4. What are the reasons for the use of three phase system?
A three phase system is an arrangement of three winding producing three single phase voltages of the same magnitude and frequency but displaced 120° from one another. The three winding suitably connected so that the three voltages act simultaneously in the circuit, each supplying power at different interval of time to a common load. The following are the advantages of three phase system over single phase system.
Constant power: In single phase circuit the instantaneous power various simultaneously from zero to pick value at twice the supply frequency. This pulsating nature of power is objectionable for many application.
Greater output: The output of a three phase machine is greater than that of a single phase machine for a given volume and weight of machine.
Cheaper: The three phase motors are much smaller and less expensive than single phase motors because less materials is required.
Power transmission economics: Transmission of power by three phase system is cheaper than that of single phase system.
Three phase rectifier services: Rectified three phase voltage is smoother than rectified single phase voltage.
5. What are type tests which are conducted on circuit breaker?
Circuit breaker is a device design to close or open contact members, thus closing or opening and electrical circuit under normal or abnormal conditions.
Types tests are conducted on first few prototype circuit breakers of each types for the purpose of providing the capabilities and confirming the rated characteristics of the circuit breaker of that design. Such tests are conducted in specially built testing laboratories. Type tests are performed as per the Recommendations of standards (IEC) or (IS).
Type tests can be broadly classified as
1. Mechanical performance tests
2. Thermal test
3. Dielectric or insulation tests and
4. short circuit test in order to check the making capacity, breaking capacity, short time rating current and operating duty.
6. What are moving iron instruments?
These instruments are widely used in laboratories and switch boards at commercial frequencies because these are cheaper in cost, robust in constructions and can be manufactured with required accuracy.
There are two general types of such instruments i.e. attraction type and the repulsion type.
The attraction type instruments operate on the principle of attraction of a single piece of soft iron into a magnetic field and repulsion type instruments operates on the principle of repulsion of two adjacent iron pieces magnetized of the same magnetic field. Repulsion type instruments are more sensitive as in these instruments large operating torque is developed by having iron elements positioned closed together inside the field coil where the magnetizing effect is maximum.