Q-1. What is difference between Auto-transformer and Resistive potential divider ?
Ans :
1. A resistive potential divider can not step up voltage, whereas an auto transformer can do.
2. In a Potential divider the input current is always more than its output currents but it is not so in an auto transformer. In a step down auto transformer, the output current is more than the input current.
3. In a potential divider almost entire power the load flows by conduction, whereas in auto transformer, a part of the power is conducted and the rest is transferred to the load by transformer action.
4. Thus in potential divider, there is a great loss of power in the difference resistances whereas it is not so in case of an auto transformer.
5. Voltage regulation is also poor in case of a potential divider and voltage regulation is good in case of Auto transformer.
Q-2 Why is the auto transformer not used as a distribution transformer ?
Ans:
1. There is drawback of an auto transformer over a conventional two winding distribution transformer which prohibit its use and it is due to the direct electrical connection between the primary and secondary circuits.
2. If an open circuit develop in common winding , the supply voltage would be impressed directly upon the external load circuit in series with the AC portion of the winding.
3. With a stepdown auto- transformer this could result in burned out secondary loads and/or a serious shock hazards , particularly if the voltage ratio V2/V1 is quite low.
4. Hence if an auto transformer is used to supply a low voltage system from an high voltage system , it become essential to earth the low voltage side.
5. Hence autotransformer is never used for interconnecting Hv and lv system.
Q-3 What is constant current transformer ?
Ans:
1. It will be recalled that constant direct current is obtained from dc series generator. constant alternating current ordinarily is obtained from a constant current transformer.
2. The main requirement for such transformer is that it should automatically adjust its output voltage with the change in load connected to it so that its output current remains constant.
3. If a transformer is designed with a large leakage reactance which can be automatically controlled in inverse relations to the load, it will be possible to maintain a constant output current since secondary voltage of a transformer supplying a given load depends to a large extent upon its leakage reactance.
4. In this type of transformer, the primary coil is stationary at the base while the secondary coil is free to move between vertical guides and is provided with an adjustable counter- weight, the setting of which determines the value of output current-increasing the counter weight will cause the transformer to operate at the lower output current and vice versa.
5. The stationary primary winding receives the power at constant voltage source and movable secondary winding delivers a constant current to the load circuit( lighting load)
6. Both the stationary primary and the movable secondary windings surrounded the center leg of the silicon steel laminated core; the core is quite long and operates at higher flux density.
7. The transformer is of shell type.
8. The weight of the secondary winding is always slightly greater than that of the counter weight.
9. The Secondary winding takes its lowest position at full load and have its maximum output voltage in this position.
10. In order to avoid mechanical oscillations resulting from change in load or in the input voltage to the primary oil dash pot are used.