VI. Answer the following questions:

1 What is a generator used for? 2. What is a transformer used for? 3. What does a transformer consist of? 4. What kinds of cores do you now? 5. How does the electromagnetic field around the primary coil change if a varying voltage is applied to it? 6. What does the va­lue of the induced e. m. f. depend upon? 7. How are transformers classed? 8. When does the inductive reactance of the transformer increase? 9. Where must air core transformers be used?

VII. Describe the structure of a transformer.

VIII. Supplementary reading. Text 10

CURRENT TRANSFORMERS

Why Current Transformers are Used. — A current transformer is an instrument transformer for the transformation of current from one value to another, usually a lower one, or for the transformation of current at a high voltage into a proportionate current at a low volt­age with respect to earth potential. Current transformers are used in conjunction with alternating-current meters or instruments where the current to be measured is of such magnitude that the meter or instru­ment current coil cannot conveniently be made of sufficient carrying-capacity. They are also used wherever high-voltage current has to be metered, because of the difficulty of providing adequate insulation in the meter itself. In this connection supply voltages exceeding 660 volts are considered to be high voltage. In meter practice current transform­ers are used wherever the current to be metered exceeds 100 am­peres, and in some instances a lower value than this is regarded as the desirable maximum for direct measurement.

Construction of Current Transformers. — A current transformer comprises a magnetic circuit, usually in the form of iron stampings Assembled together to form a core, on which are wound two electric circuits called the primary winding and secondary winding respectively. The primary winding carries the current to be measured and is con­nected in the main circuit. The secondary winding carries a current pro­portional to the current to be measured and the secondary terminals are connected to the current winding of the meter or instrument. Both windings are insulated from the core and from each other. The secon­dary insulation is arranged to withstand a test pressure of 2,000 volts applied between the winding and the core for one minute. The insula­tion of the primary is arranged to withstand for one minute a test pressure applied between the primary and secondary windings approximately equal to four times the voltage existing under working condi­tions. During this test the core and the secondary winding are con­nected together.

The primary circuit of a current transformer may consist of a sin­gle conductor in the form of a bar or cable instead of a winding, when the current to be measured is of the order of 600 amperes or more. In low-voltage circuits the current to be measured may be so heavy that it is not convenient to provide a primary integral with the transformer (нелегко зробити так, щоби первинний ланцюг створював одне ціле із трансформатором)and the latter then consists of an iron core of appropri­ate shape with a secondary winding thereon, the whole being mounted on the busbar or cable. The nominal full-load current of a transformer termed the "rated primary current" and is the value in amperes of the primary current marked on the rating plate.

The secondary winding of a current transformer is usually constructed to deliver five amperes to the meter or instrument when rated primary current flows in the main circuit. This is referred to as the "rated secondary current" and five amperes is the standard value adopted in most countries. In power-station practice it is not unusual for the meter to be separated from its current transformers by a distance of several hundred feet. The PR loss in the connecting leads together with the loss in the meter current coils may impose a burden in excess of the transformer rating if a five-ampere secondary current is adhered to. By adopting a lower value for the rated secondary current the loss in the leads can be substantially reduced and one ampere or 0.5 ampere values are permissible alternatives. Since the loss varies as the square of the current the adoption of one of these alternatives will reduce the loss in the leads to one-twenty-fifth or one-hundredth of the original value respectively.

The magnetic and electric circuits of a current transformer can be rep­resented diagrammatically; the primary winding then is surrounding one limb of the core and the secondary winding surrounding another. In actual practice the two windings would not be sepa­rated in this manner as the primary would be superimposed on the secondary. The cores of current transformers are usually built up with lami­nations of silicon-steel but where a high degree of accuracy is de­sired a high-permeability nickel-steel may be used. Three types of magnetic circuit are in common use, namely, ring-type", "core-type" and "shell-type".