Written by VR Saturday, 02 April 2011 19:54
Cycloconverter control has the advantages of smooth low speed operation, four –quadrant operation with regenerative braking and good dynamic response. But it has low speed range and because it uses large number of thyristors it becomes economically acceptable only when the drive rating is high. A synchronous motor without the damper winding is used because the damper winding reduces the inductance of the machine and therefore its ability to filter out harmonics in the output voltage of cycloconverter.Since the drive operates in self-controlled mode the damper winding is not needed for its conventional roles.The drive is employed in low speed gearless drives for ball mills in cement plants, mine hoists reversing rolling mills requiring fast dynamic response and in ships equipped with diesel generator fed cycloconverter controlled synchronous motor drives. These drives have power ratings in the mega watt range. At such high power levels, considerable saving in cycloconverter cost is obtained by operating the motor at unity power factor by adjusting the field current. A typical rating of a synchronous motor for a ball mill in a cement plant is 8750 hp unity power factor 14.5 rpm,4.84 Hz,1900V and 40 poles. A cycloconverter is ideally suitable for such a low frequency supply. Earlier gears were employed to get low speed operation. Absence of gear in this drive reduces the cost and maintenance requirements. Because of similarity with an ac commutator motor the drive is also known as ac commutatories motor.
Written by VR Saturday, 02 April 2011 19:52
- Examine earthing devices periodically and always prior to their use.
- Use only earthing switches or any other special apparatus when provided for earthing.
- Verify that the circuit is dead by means of discharging rod or potential indicator.
- Earthing should be done in such a manner that the person doing this job are protected by earth connections on both sides of their working zone.
- All the three phases should be effectively earthed and short-circuited may be proceeding on one phase only.
Written by VR Saturday, 02 April 2011 19:50
- Protection against direct contact. This can be achieved by either preventing the current from passing through the body of person or livestock or by limiting the value of the current lower than the shock current.
- Protection against Indirect contact: Danger may enter from contact with exposed conductive parts. The protection against thus can be achieved either by preventing a fault current from passing through the body or livestock or limiting the value of fault lower than the shock current. Automatic disconnection of the supply on the occurrence of fault likely to cause a current to flow through a body in contact with exposed conductive parts.
- Protection against thermal effect in normal service: The electrical installation can be so arranged that there is no risk of ignition of flammable materials due to high temperature of electric arc. Also during normal operation of the electrical equipment there shall be no risk of person or livestock suffering burns.
- Protection against fault current: For this particular attention should be given the earth fault currents. Conductors other than live conductors and any other part intended to carry a fault current shall be capable of carrying that current without assuming excessive temperature.
- Protection against Over voltage: Persons or livestock shall be protected against injury and property shall be protected against any harmful effects of a fault between live parts of the circuits supplied at different voltage and also from atmospheric phenomena and switching voltages.
Written by VR Saturday, 02 April 2011 19:49
For street lighting we have to consider the width of the road level of lighting. The level of lighting should be adequate to give visibility which will guarantee the user man safety and visual comfort. The following 4 factors contribute to the fundamental criteria of quality of public lighting:
- The level of illuminance
- Uniformity of illuminance :This is required to provide comfort for drivers and motorist. The minimum illuminance should be not less than 0.4
- Glare: The glare affecting can either reduce the ability to see or it can cause discomfort to eyes. Owing to low illumination the eye is in its most sensitive state and therefore glare must be avoided
- A good optical guidance is required in long switches of light roads and even in the intersection of roads.
Written by VR Saturday, 02 April 2011 19:48
Factors to be considered for electrical installation in a high rise building:
- A high building which is more than 15m high shall be categorized as H.R building.
- Outdoor transformer installation is not permitted at the premises of H.R.B.
- For total connected load of 50 K.V.A and above separate transformer shall be installed by the owner.
- For 150 K.V.A and above separate transformer with H T shall be installed by the consumer.
- Generator of adequate rating shall be installed to feed essential loads.
- Generator capacity shall be minimum 20% of total connected load.
- Floor wise single point supply isolation facility shall be provided. Maximum 2 floors can be combined.
- Cables shall be taken to different loads through cable ducts.
- Maximum number of 3d outlets from a panel shall be limited to 8 and the 1d outlets can be limited to 12.
- For calculation of total connected load minimum 1.2 Kw shall be taken for 1d connection and 5Kw for 3d.
- Generator capacity shall be adequate to operate lift.
- Oil filled breakers and SFO are not used in HRB.
- Lightning protection shall be provided for HRB as per IS 2309.
- Diversity factor shall be maximum 2.
- Transformers and Generators shall be installed near the building having easy access and ventilation.
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