Monday, January 12, 2015

Special Transformer Applications : Constant-current transformers .

CONSTANT-CURRENT TRANSFORMERS

Street lamp circuits may be connected in series or in parallel. The method used depends on the policies of the local utility company. If the lamps are connected in series, some method must be provided to short out any defective lamps. Remember that if one lamp in a series circuit burns out, all of the lamps in the circuit will go out. Shunting in a series circuit is accomplished by placing a small cutout device in the lamp socket. The device consists of two metal contacts that are separated by a film of insulating material. If one lamp filament burns out,

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the entire voltage of the series circuit is applied across the cutout device. This momentary voltage punctures the insulation between the metal contacts and shorts out the lamp.

Figure 15–16 is a drawing of a typical lamp socket used on a series street lighting circuit. As the lamps burn out, the total impedance of the series circuit decreases. If a constant voltage were maintained, the current would increase and cause other lamps to burn out.

Most series street lighting circuits require a current of 6.6 A. The voltage of the circuit will depend on the number of lamps in series. A constant 6.6 A is maintained in the circuit by a constant-current transformer. This type of transformer takes power into its primary winding at a constant voltage and a variable current. It delivers power from the secondary winding at a constant current and a variable voltage.

Functioning of the Constant-Current Transformer

The primary and secondary coils of the constant-current transformer are both placed on a silicon steel laminated core. The primary winding is stationary. Its input comes from a constant-voltage source. The secondary winding moves on the center leg of the core. Its movement is balanced by a counterweight–dashpot arrangement. The output of the secondary winding is connected to the series street lighting circuit.

A constant-current transformer is shown in Figure 15–17. The primary winding is energized from a constant-voltage source. The position of the movable secondary winding is adjusted by a counterweight until the desired value of secondary current is obtained. If there are a number of lamp failures in the series circuit, the total impedance of the series circuit decreases. At the same time, the current in the series circuit, the current in the movable secondary coil, and the current in the stationary primary coil will increase.

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The force of repulsion between the coils will increase (as described by Lenz’s law). This force causes the secondary coil to move away from the primary coil. As a result, there is less primary flux linking the turns of the secondary winding, and the induced voltage in the secondary will decrease. The secondary coil continues to move away from the primary coil until the secondary current returns to its normal value of 6.6 A. When the current reaches this value, the repulsion force between the coils is balanced by the counterweight system.

Energizing the Primary

The primary of the constant-current transformer may be energized from an oil switch. The transformer is operated by a phototube and an amplifier circuit combined with a relay system. For street lighting applications, this type of control operates as follows. When the normal light intensity decreases below a set level, the phototube causes the amplifier circuit to operate the relay system and energize the transformer primary. The secondary coil moves so that the required 6.6 A is maintained in the circuit. When the light intensity increases above the set level, the phototube and the amplifier circuit operate the relays to open the coil switch. Thus, both the transformer and the street lighting circuit are deenergized.