Use of Efficient Semiconductor Converters

Use of Efficient Semiconductor Converters:

Prior to development of Use of Efficient Semiconductor Converters, inefficient power modulators were employed in several applications. While replacement of these power modulators by the semiconductor converters in appropriate applications brings about substantial amount of energy saving, the cost of replacement can be recovered in a short period ranging from one to five years, depending on the applications. Some cases are considered here.

Replacement of Resistance Controllers:

At the time of writing of this book, several electrical drives have been in use, which employ resistance controllers; as a result of which very large amount of power is being wasted. Some applications where large saving in energy can be affected are:

(i) Electric traction: The 1500 V dc traction on Bombay-Igatpuri route where both main line and suburban trains run, the 750 dc metro service in Calcutta, and 550 V dc tram service in Calcutta, use resistance controllers. Replacement of resistance controllers with choppers employing regenerative braking can save the energy from 40 to 50%. The train ratings range from 1500 to 3000 kW, and large number of trains run in 24 hrs. Therefore, the use of chopper control can provide large saving in energy.

(ii) Fans: At present resistance controllers are widely used for speed control of fans. They should be replaced by triac or thyristor voltage controllers. Unfortunately most commercially available controllers do not employ filters in order to keep the cost low. Consequently, fans controlled by them produce noise due to vibrations and they also cause radio frequency interference. Therefore, controllers with filters must be employed.

Replacement of Eddy-Current Couplings:

Several drives employ eddy-current couplings for speed control of induction motor. They control speed by dissipating power in a steel drum in the same way as resistance controllers, and therefore, they are highly inefficient. These drives can be replaced by converter-fed dc drives or variable frequency controlled induction motor drives or stator voltage controlled induction motor drives, depending on the application.

Replacement of Ward Leonard Drives:

Ward Leonard drive was very widely used in variable speed applications in the past. Even today, these are used in many applications in steel plants, paper and textile mills, mine winders, escalators, excavators, cranes and machine tools. In these drives, ac-dc conversion and variable dc voltage for de motor speed control is obtained using motor-generator set. This conversion involves two inefficient stages, the ac motor and dc generator. This conversion could be carried out more efficiently by a Use of Efficient Semiconductor Converters.

The conventional Ward Leonard drive has two distinct advantages over static Ward Leonard drive:

  • load equalisation and
  • in case of critical loads, in the event of supply failure, it provided enough time to activate standby power supply.

Except in those applications where these features (advantages) are absolutely necessary, ac motor-dc generator set can be replaced by a Use of Efficient Semiconductor Converters. Some replacement cost can be immediately recovered by selling these machines, and the remaining can be recovered in few years because of energy saving and reduced cost of maintenance; and in some cases due to the increase in production owing to fast dynamic response of the static Ward-Leonard drive.

Use of Efficient Motors:

Due to lack of awareness, several consumers buy inefficient motors because they are cheaper. This choice becomes highly uneconomical in the long run because of high charges of electricity.

Usually, the motors are inefficient due to three major factors:

  • poor quality of lamination and insulation,
  • use of less active material causing machine operation with considerable saturation and
  • poor mechanical design of bearings, clutches, gears and couplings.

This is also true of the loads (i.e. driven machinery) such as pumps, pipe fittings etc.

One example worth mentioning in detail is agriculture pumps. Presently, several agriculture pump drives available in the market are highly inefficient due to the poor design of both motor and pump. But these inefficient drives are sold in large numbers due to low price. Because of the low rate of electricity charges in rural areas, the farmers are not unduly burdened by the use of these inefficient drives and they continue to enjoy good market. But in the process, a large amount of electrical energy is being wasted. Noting that the agriculture pumps consume as much as 30% of electrical energy in several states, there is a strong need to promote the use of efficient drives and discourage the use of highly inefficient drives for agriculture pumps.

One can give several other examples. Markets in India are flooded with sub-standard motors, although there is no shortage of motors with good designs. There is need for awareness among engineers, technicians and other consumers to go for efficient motors which employ good quality lamination, use enough active material and have good mechanical design, even if they are expensive.

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