Power factor
All electrical installations will have a power-factor, a relationship between the power (Kw) and the apparent power (Kva). This relationship will either have a lagging power factor, unity power factor or a leading power factor. Except in very specific instances the power factor will not be leading.
A lagging power factor.
This is where the current in a circuit reaches the maximum and minimum values later than the applied voltage has reached its maximum and minimum values. this circumstances is brought about by the presents of an inductance in the circuit. In industrial and commercial applications the most common inductive device is the AC motor. This is because by its construction it is built up of coils, it retards the growth and decay of current in the circuit, which causes a current to be out of phase with that applied voltage.
Power factor correction
Power factor is the ratio of watts to voltage amperes in a circuit, or power to apparent power.
Power Factor = Watt / Volts x Amperes = Cos q
Every load will require a specified power input and the general aim is to supply this while keeping the volt-ampere demand to a minimum, i.e. the power factor to a maximum of as near unity as possible.
The main cause of a low power factor in industrial plant is the induction motor which is the most widely used industrial drive. Particular note should be taken for the values at reduced load. For this reason as well as others, motor should not be oversized on installation, and motors under loaded due to cyclic loads will have a similar effect. The reason for this is that the cause of lagging power, is the magnetizing current drawn by the motor, which is constant, irrespective of load. On light load, therefore, the proportion of lagging current to the total current is increased with a consequent reduction in power factor.
Methods of Correction
Compensation may be effected by either rotary or static equipment and the following is a list of the general methods available. However, for the vast majority of installations static capacitors would be the automatic choice. For that reason while giving the full list the discussion is confined to static capacitors.
Static capacitors,
Synchronous induction motors,
Salient pole synchronous induction motors,
Induction motors with phase advancers
Synchronous condensers.
Showing posts with label wind generation. Show all posts
Showing posts with label wind generation. Show all posts
Monday, March 16, 2009
Starting of asynchronous motors
Starting of asynchronous motors
Star delta starters are specially suited to machines which do not present a high load torque at start up or which normally start off-loads. The relatively high peak current during the star to delta transition, which is characteristic of these starters, means that where large motors above a certain rating are to be used, some form of current limiting techniques may be necessary.
Introducing a delay of one to two seconds during the star to delta transition. This will effectively reduce the level of peak transient current. It should be noted however, that this solution may only be applied to low inertia loads, to avoid too large a drop in speed during the transition.
Three stage starting, star – delta plus resistance stage in star. The transient peak current still exits but its value is limited by the resistance being left in circuit for about three seconds after the star delta transition.
Star-delta starting plus resistance stage and a closed transition. The resistance bank is put into the circuit just before the star contactor opens. In this way, the motor current is never actuarially broken and so the transient peak is eliminated. The adoption of the above techniques will obviously mean that more components are needed and so for a given starter, the cost could increase quite considerably.
For difficult applications the use of a suitable electronic “soft start” such as the telemecanique “altistart” for example, could well be the best solution.
Star delta starters are specially suited to machines which do not present a high load torque at start up or which normally start off-loads. The relatively high peak current during the star to delta transition, which is characteristic of these starters, means that where large motors above a certain rating are to be used, some form of current limiting techniques may be necessary.
Introducing a delay of one to two seconds during the star to delta transition. This will effectively reduce the level of peak transient current. It should be noted however, that this solution may only be applied to low inertia loads, to avoid too large a drop in speed during the transition.
Three stage starting, star – delta plus resistance stage in star. The transient peak current still exits but its value is limited by the resistance being left in circuit for about three seconds after the star delta transition.
Star-delta starting plus resistance stage and a closed transition. The resistance bank is put into the circuit just before the star contactor opens. In this way, the motor current is never actuarially broken and so the transient peak is eliminated. The adoption of the above techniques will obviously mean that more components are needed and so for a given starter, the cost could increase quite considerably.
For difficult applications the use of a suitable electronic “soft start” such as the telemecanique “altistart” for example, could well be the best solution.
Sunday, March 15, 2009
Wind Farm Prospects
Profitability of a wind farm depends on the wind speed in that location, cost and performance of a wind turbine installation, and energy prices.
Large turbines selling power to the grid in recent times have found to be financially viable where the average wind speed is more than about 7 m/s and are compeditive against coal and gas. They are likely to become attractive to more businesses in future, as technology continues to improve and the deregulated energy market develops throughout the UK and Ireland.
Small turbines and wind pumps may be viable with average wind speeds as low as 5 m/s, if the only alternative is a more expensive power source such as a diesel generator, but this is only suited to very remote locations.
De-regulation of the electricity supply industry in the UK has changed the situation, and allowed more traders in energy to enter the marketplace and compete against traditional methods of electricity generation.
A wind turbine owner now has the prospect of supplying power to consumers anywhere in the country, as well as to his or her own business. Needless to say this is subject to a number of constraints such as licence and metering requirements and charges for use of the grid system - cables, poles, pylons, transformers, etc. - but there are also exemptions which can help to make this attractive.
The average power requirement on most farms is quite small in comparison to the output of a large wind turbine, but may be similar to the output of a smaller machine. The attractiveness of wind farm generation in locations that previously were deemed to expensive because of the terrain or the low average wind speeds is increasing due to the increase in fossil fuel costs. Wind power is the future, a future that is fast approaching.
Large turbines selling power to the grid in recent times have found to be financially viable where the average wind speed is more than about 7 m/s and are compeditive against coal and gas. They are likely to become attractive to more businesses in future, as technology continues to improve and the deregulated energy market develops throughout the UK and Ireland.
Small turbines and wind pumps may be viable with average wind speeds as low as 5 m/s, if the only alternative is a more expensive power source such as a diesel generator, but this is only suited to very remote locations.
De-regulation of the electricity supply industry in the UK has changed the situation, and allowed more traders in energy to enter the marketplace and compete against traditional methods of electricity generation.
A wind turbine owner now has the prospect of supplying power to consumers anywhere in the country, as well as to his or her own business. Needless to say this is subject to a number of constraints such as licence and metering requirements and charges for use of the grid system - cables, poles, pylons, transformers, etc. - but there are also exemptions which can help to make this attractive.
The average power requirement on most farms is quite small in comparison to the output of a large wind turbine, but may be similar to the output of a smaller machine. The attractiveness of wind farm generation in locations that previously were deemed to expensive because of the terrain or the low average wind speeds is increasing due to the increase in fossil fuel costs. Wind power is the future, a future that is fast approaching.
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