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Another factor that affects the heat capacity of the focal spot track is the waveform of the KV. Single-phase power delivers energy to the anode in pulses, as shown below. Three-phase and constant potential generators deliver the heat at an essentially constant rate, as indicated. The figure below compares the temperatures produced by a single-phase and a constant potential machine delivering the same total heat. Because of the pulsating nature of single-phase power, some points on the anode surface are raised to higher temperatures than others. These hot spots exceed the temperature produced by an equal amount of three-phase energy. When an x-ray tube is operated from a single-phase power supply, the maximum power must be less than for constant potential operation to keep the hot spots from exceeding the critical temperature. In other words, constant potential operation increases the effective focal spot track heat capacity and rating of an x-ray tube.
The effect of KV waveform on tube rating should not be confused with the effect of waveform on heat production, which was discussed earlier. However, both factors should be considered to determine if there is any advantage, from the standpoint of tube heating, to using three-phase or constant potential power. In comparing three-phase or constant potential and single-phase operation, three factors should be considered:
The real advantage of constant potential operation is related to the first two factors. Because of the increased efficiency of x-ray production, and the increased penetrating ability of the radiation, a lower KV or MAS value is required to produce a given film exposure. This more than compensates for the increased heat production associated with constant potential operation. The increased rating, or maximum permissible power, associated with the constant potential waveform also adds to the advantage. An x-ray tube can generally be operated at a higher power level when the power is supplied from a three-phase or constant potential power supply, and it will also produce radiation more efficiently. A rating chart for an x-ray tube operated at different waveforms and rotation speeds is shown in the figure titled, Rating Curves for an X-Ray Tube Operated under Different Conditions. The highest power capacity is obtained by using three-phase power and high-speed rotation; notice that the real advantage occurs at relatively short exposure times. As exposure time is increased, overlapping of the focal spot track and the diffusion of heat make the difference in power capacity much less significant. The actual rating charts supplied by an x-ray tube manufacturer are shown in the figure referred to in the previous paragraph. It is common practice for each of the four operating conditions (wave-form and speed) to be on a separate chart. Each chart contains a number of different curves, each representing a different MA value. The vertical scale on such a rating chart is KV. A chart of this type is still a power rating chart. Each combination of KV and MA represents a constant power value. Such a chart is easier to use, since it is not necessary to calculate the power. The rating chart is used by the operator to determine if the technical factors, KV, MA, and exposure time, for a given exposure will exceed the tube's rated capacity. Most rotating anode tubes contain two focal spots. As mentioned previously, the size of the focal spot significantly affects the heat capacity. Remember that a given x-ray tube has a number of different rating values, depending on focal spot size, rotation speed, and waveforms. Some typical values are shown in the table below. Heat Rating (in Joules) for Typical X-Ray Tube
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