HIGH-TENSION TRANSFORMER

The high voltage transformer is used to transfer low voltage into high voltages required to operate X-ray tubes. This is known as high tension generator, which provide voltage from 20–150 kV and current up to 1000 mA for the X-ray tubes. It is a step up transformer with two windings and a shell type core. The number of turns in the secondary is higher than that of primary, and it is decided by the voltage ratio. If 400 volt is to be transformed into 80,000 volts, then the voltage ratio is 80,000/400, requiring 200 turns in secondary per primary turn. Thus primary winding consists of a few hundred turns of thick copper wire, which is well insulated and wound on a cylinder. A thin copper sheet is fitted over the primary winding and it is earthed. This is known as stress shield, which protects the primary circuit during breakdown of the secondary insulation . The secondary winding consists of about 100,000 or more turns of thin copper wire coated with insulated varnish. This is wound in an insulating cylinder, which is placed over the primary winding. The layers are separated from each other by thin paper prepared with wax for insulation. The voltage difference between any two layis about only 200–300 volts. This method of design reduces the risk due insulation breakdowns. The core of the high tension transformer is rectangular in shape, which is earthed and well laminated. Usually, the secondary is wound in two equal multiple parts and the center of the winding is earthed through the core. This means that instead of running from 0 to + 150 kV, the two secondary cables run from – 75 kV to + 75 kV. This is to reduce the insulation, size and cost. Since the current measurement in the primary coil is not an accurate representation of the current in the secondary coil, the current must be measured on the secondary side.

                             A milliameter (mA meter) is connected between the inner ends of the two secondary windings at which the transformer isgrounded, which is also the center of the coil. This minimizes the risk of electrical shock to the operator. Though the mA meter is connected at this point, it is placed at remote distance at the control console. The entire unit is immersed in an earthed metal tank, which is filled with oil. The metal tank is closed with a tight lid. In the case of dental and mobile X-rays, the heat production is very low and hence, oil is not used. Instead of oil, plastic is used as an insulator. The transformer is immersed in plastic when it is in fluid state. Later on, the plastic solidifies and acts like a solid insulatorers