I've spent a good deal of time in the field recently trouble shooting motor problems and observed the following. Often the bus voltages run high. When the voltage is high the magnetizing requirement of induction motors is also higher. Often the assumption is that if the voltage is higher the current will be lower without considering motor efficiency or power factor. Motors are designed to perform best at their rated nameplate voltage. First make sure that your bus voltages are where they should be. In North America the voltage rating is typically 460 volts 60 Hz. The Motor Control 480 volts allowing for some voltage drop in the cabling. To keep the Power Factor of the system close to unity we used synchronous motors. Shunt Capacitors are also used but I choose to detune them. Detuning capacitors protects them from system harmonics. When using either synchronous motors or fixed shunt capacitors the bus voltage may also rise. This should be considered when you set the taps on the xfrms.
Whilst power factor can be derived to a degree during the design stage, for the purposes of equipment selection, the power factor correction criteria should be flexible enough to allow adjustment and measured after the plant/ load is running at near normal load. By definition the power factor is defined at full load. Plenty of times clients asked to do an upgrade to plant, and often the existing distribution structure can be enhanced with power factor correction if the additional load is marginal to equipment ratings, saving large costs and downtime.
Its economical, reasons: 1. Improving the power factor reduces the VA which means Equipment rated in VA (Transformers, Synchronous motors etc.) will be smaller and hence cheaper. 2. The conductor size on low power factor is increased during transmission and distribution of the same amount of power at constant voltage. 3. Low power factor results in poor regulations in the sense that high current causes increased voltage drop in transformers, alternators, transmission lines and distributors thus reduces voltages at the supply end impairing the performance of the device. Now in order to maintain receiving end voltage within the prescribed limits, extra equipment (Voltage regulations) are need which makes it very expensive.
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