High voltage transformer and high frequency generator

High voltage transformer and high-frequency generator, types of transformers, the circuit of high-frequency generator and its components

1. High voltage transformer and high frequency generator Presenter :- Yashawant yadav B.Sc MIT 2 nd yr (NAMS ) Bir- Hospital

2. Outline Introduction Working principle Types of transformer Uses Summary References

3. Introduction Transformers are electrical devices consisting of two or more coils of wire used to transfer electrical energy by means of a changing magnetic field. Works only on A/C current .??? Based on the principle of mutual induction or self induction . E= N dϕ /dt

4. Physical Features: 2 coils of wire (not connected to each other) known as the primary and secondary windings, are electrically insulated from each other and wound on a laminated soft iron core. The number of primary N p  and secondary N s  turns may be varied for attaining the desired electrical power at the load.

5. Contd.. The iron core is to confine the magnetic field lines to ensure maximum magnetic flux linkage between the primary and secondary coils. (flux density ↑↑ →→ flux linkage ↑↑ →→ induced emf ↑↑) This is due to the fact that iron is easily magnetized, hence the iron core guides and direct the magnetic field lines

6. Contd.. An ideal transformer has the following additional features: Resistance of primary and secondary coils r p  and r s  are zero, so that no energy is lost in the core or in the coils. It is 100% efficient = input power equals output power No magnetic flux losses, hence both coils have the same flux through them. Note:  Transformers are assumed to be ideal in all calculations. However, we have to keep in mind that the features of the ideal transformer are only assumed.

7. Contd.. One of the main reasons that we use alternating AC voltages and currents in our homes and workplace’s is that AC supplies can be easily generated at a convenient voltage,? Transformed (hence the name transformer) into much higher voltages and then distributed around the country using a national grid of pylons and cables over very long distances.

8. Contd … Which one is more efficient for long way current transmission high voltage AC or DC ?????? =No EMF

9. Types of Transformers Step up Stepdown Step up:- I n step up transforms secondary winding in the core of iron is relatively higher in ratio than primary windings, that mean induce emf in the secondary winding is higher by the law of Faraday EMI . NS>NP Vp /Vs=Np/Ns Power=IV VpIp = VsIs

10. Contd.. Step down:- In stepdown transformer as compare to primary winding in iron core secondary windings is lower in ration , means that induced emf in the secondary windings is lower than the supply one . Np>Ns Vp /Vs=Np/Ns Power=IV VpIp = VsIs

11. Auto transformer :- It is based in the self induction phenomenon . It has a single windings around a single iron stand. In auto transformer the primary and the secondary windings are connected magnetically as well as electrically. In fact, a part of the single continuous winding is common to both primary and secondary.

12. Contd.. Advantages of Auto transformer Less costly Better regulation Low losses as compared to ordinary two winding transformer of the same rating. Disadvantages of Auto transformer The secondary winding is not insulated from the primary winding. If an auto transformer is used to supply low voltage from a high voltage and there is a break in the secondary winding, the full primary voltage comes across the secondary terminal which is dangerous to the operator and the equipment Used only in the limited places where a slight variation of the output voltage from input voltage is required.

13. Contd … occupy very important place in X-ray generator circuits. In the X-ray generator, the autotransformer is used to adjust the voltage applied to the primary of the high voltage transformer with high efficiency and convenience. Provides voltage for the primary coil of the high voltage transformer.

14. High voltage transformer The step up transformer is sometimes called high voltage transformer It has many more turns in the secondary coil than the primary coil and it increases the voltage by a factor of approximately 600 and more . The potential difference across the secondary coil may be as high as 1,50,000 V So it is immersed in oil in the transformer assembly for maximum insulation

15. Oil High voltage transformers are usually enclosed in a metal tank filled with oil. This oil penetrates into the inner spaces of the windings and increases the effectiveness of the insulation. The oil prevents the windings from dust and moisture and also acts as a cooling medium. The oil is a good insulator than air, it avoids electrical short circuiting. Oil also provides effective cooling to the transformer.

16. Single phase x ray generator A single-phase X-ray generator utilizes a single-phase AC supply as input. These generators employ full wave rectification, which utilizes the full potential of the electrical supply. the X-ray output per unit time is twice as large as that of half wave rectification The generated X-rays have 100 short pulses in one second (2 pulses/cycle, frequency = 50 cycles/sec). The exposure time for each X-ray pulse is 1/100 s = 10 ms.

17. Contd … Full wave rectification is used on systems ranging from portable, dental, mobile, and up to heavy duty fixed installations. While self rectified generators may have a maximum output of 10-15mA, full wave rectified units have been produced with up to 800mA output.

18. Three phase x ray generator /Transformer The three-phase X-ray generator uses a 3-phase AC line supply. There are three wires, each with a single phase AC sinusoidal wave. Each wave is out of phase with the other two for one-third (120°) of a cycle. A three-phase transformer is used to convert the low voltage AC to high voltage AC.

19. Contd … It has three sets of primary and secondary windings. When the voltage is rectified, the circuit produces two pulses per cycle for each line, resulting six pulses per cycle. Hence, this is named as 3-phase 6-pulse generator. Three phase generators have typical outputs of 500mA up to -1200mA.

20. Contd.. It is also possible, to produce 12-pulse per cycle, by using different configurations of transformers and rectifiers. With the twelve-pulse generator, one winding is con- figured delta, and the other star. The voltage peaks between these two windings have a 30 degree phase- shift.

21. Contd …. Since voltage supplied to the X-ray tube never falls to zero, the ripple factor is very low (13.5%) 3 phase 6 pulse and 3.5% for 3 phase 12 pulse generator The single phase X-ray generator ripple factor is 100,

22. High frequency x ray generator Inside the high-frequency generator, the AC mains power is rectified, and smoothed by a large value capacitor, to become a DC voltage supply. The 'inverter' converts the DC voltage back into a high-frequency AC voltage. High-frequency generators have many advantages over conventional generators, operating at 50 or 60Hz power main frequency. The high-tension transformer now uses ferrite(a ceramic compound consisting of a mixed oxide of iron and one or more other metals which has ferrimagnetic properties) instead of an iron core, with an increase in efficiency.

23. Contd … A high-frequency generator waveform has less ripple, in many cases less than 2%. However, final ripple depends on other design considerations. The high frequency generator transformers are efficient, compact and less costly. For many high-frequency generators however, the inherent capacitance of the HT cables provides the required smoothing, without added capacitors.

24. Contd..

25. Contd … This measurement is compared to a reference voltage equivalent to that for the required kV. If there is any difference, the inverter control circuit changes the pulse rate to correct the error. This is called 'closed loop' or 'feedback' regulation.

26. Disadvantages of HF generator Skin effect Proximity effect Skin effect :- When an Alternating Current flows through a  conductor , it is not distributed uniformly throughout the conductor cross-section. AC current has a tendency to concentrate near the surface of the conductor. This phenomenon in alternating currents is called as the  skin effect .

27. Contd.. Skin effect increases with increase in the frequency. Consequently, the effective resistance of the conductor is slightly increased. The skin effect depends upon the following factors: Conductor material: Better conductors and ferromagnetic materials experience higher skin effect Frequency: increases with increase in the frequency Shape of the conductor: skin effect is lesser for stranded conductors than solid conductors

28. Proximity effect When two or more conductors carrying alternating current are close to each other, then distribution of current in each conductor is affected due to the varying magnetic field of each other. The varying magnetic field produced by alternating current induces eddy currents in the adjacent conductors. Due to this, when the nearby conductors carrying current in the same direction, the current is concentrated at the farthest side of the conductors. When the nearby conductors are carrying current in opposite direction to each other, the current is concentrated at the nearest parts of the conductors.

29. Contd … The proximity effect also increases with increase in the frequency. Effective resistance of the conductor is increased due to the proximity effect.

30. Transformer rating The transformer rating refers the maximum safe output that can be taken from the secondary winding. The ratings are specified in three ways as follows: The highest voltage which the transformer can provide, The maximum current which the transformer can give on continuous running phase The maximum current which the transformer can give for a period not exceeding one second.

31. Contd.. If the rating is exceeded, the transformer may overheat and burn out its insulation and winding. For three-phase generators ratings are calculated by the formula, kW = (kV × mA) ÷ 1000 For example, a three-phase generator operating at 150 kV and 500 mA, will have a rating as follows: Rating = (150 × 500) = 75 kW 1000 For single-phase generators, the formula is, kW = (kV × mA × 0.7) ÷ 1000.

32. Efficiency of transformer The efficiency of the transformer is the ratio between the output power and input power. output power i.e. efficiency = output power input power In actual transformer, the output power is always lesser than the input power due to some energy losses. Hence, the efficiency is always less than <100%. *100

33. Losses in Transformers copper losses eddy current losses hysteresis and flux leakage losses. Copper losses:- Whenever a current I flows through a resistance R, an amount of power equal to × R × t watt is converted into heat.  

34. Eddy Current Losses:- The iron core consists of concentric layers of iron, each acts as a circuited single turn coil. Whenever the magnetic field changes, an emf will be induced in the core. The current produced by the induced emf in the core is called eddy current, which will give rise to Rt heat losses. These eddy currents can be eliminated by making the iron core in the form of thin sheet of metal, and each sheet is insulated from its neighbor by a thin layer of paper. This type of core is known as laminated core  

35. Hysteresis Losses The transformer core is a magnetic material. When the direction of AC changes, the magnetization is also gets reversed. During this reversal, some energy is lost due to the molecular friction and the energy appears as heat. The loss of energy by molecular friction is called hysteresis loss. This can be reduced in practice by choosing a suitable magnetic material, such as mu-metal, which has low hysteresis loss. Mu-metal is a ferromagnetic alloy containing 78% nickel, 17% iron, and 5% copper. It has high permeability.

36. Flux Leakage All the magnetic flux linked with the primary is not linked with the secondary coil. This is said to be flux leakage, which results in loss of energy. This can be minimized by using good core design like shell type of core

37. References https://www.allaboutcircuits.com/search/more?q=transformers&start=10 Bushong 20 th edit THE PHYSICS OF RADIOLOGY AND IMAGING ( K.Thalayan ) Google