Constant-Current Transformer

The constant-current transformer, usually called a regulator, has a movable secondary winding that automatically changes position to provide constant current for any load within its full-load rating. The balance point between coil weight and magnetic force may be adjusted to provide the desired output current.

A moving-coil regulator is recommended because of the close regulation required for streetlighting work. It consists of a fixed primary coil and a movable secondary coil on a laminated core (fig. 1-6). Voltage applied to the primary winding causes voltage to be induced in the secondary winding. When the secondary circuit is closed, the magnetic field in the secondary reacts with the primary-coil field to push the movable coil up. The balance point between coil weight and magnetic force is designed to provide the desired secondary current (usually 6.6 amperes).

Since as little as 1-percent overcurrent reduces lamp life by 20 to 25 percent, close adjustment is necessary. This is obtained by a movable weight on the balancing lever. The primary current remains constant for all loads, but the power factor varies with the load, thereby changing the primary input. Since the secondary voltage increases with the load, regulator size should be restricted to keep the secondary voltage within desired limits.

Figure 1-6.\Moving-coil constant-current regulator.

On most existing installations, the constantcurrent regulator is of the outdoor type (fig. 1-7). Three main types of installation are used for these regulators: two-pole platform, timber or steel construction single-pole platform, and pole mounted. Any regulator larger than 20 kilovolt amperes should be mounted on a platform.

Constant-current regulators should be loaded as near 100 percent as possible since both efficiency and power factor are best at this load. Specifications of the American Institute of Electrical Engineers (AIEE) require constantcurrent transformers to deliver the rated secondary current at 10-percent overload. A larger size regulator should not be installed before this lo-percent overload is reached. When larger regulators must be installed and are not readily available, a booster transformer may be used with its secondaries connected into the series street circuit and its primaries connected to the primary feeder supplying the regulator (fig. 1-8). Since transformers used for this purpose should have secondary bushings insulated for the high voltage of the series street circuit, a special booster transformer is preferred to an ordinary distribution transformer for use with constant-current regulators of 10 kilowatts and larger. In using a booster transformer, the primary coil must be

Figure 1-7.\Pole-mounted regulator.

Figure 1-8.\Method of relieving slightly overloaded regulators with a distribution or booster transformer.

isolated from the secondary coil, which necessitates removing any internal lead connecting the two coils. The additional load handled by this device equals the product of the street-circuit current and the secondary voltage of the transformer. Thus, if a 2,400/240-volt transformer is used, the additional load which the street circuit can carry is 240 volts 6.6 amperes or 1.584 kilowatts.

Table 1-1 shows the maximum number of series lamps in the various sizes that may be used for full-load rating on a regulator. The average number of watts of energy consumption for each size lamp may be computed since the regulator ratings are based on their output. In this manner, the load of a circuit consisting of different size lamps may be computed.

Example: What size regulator would be required to supply the following lamps?

25 1,000-lumen, 6.6-ampere, straight-series lamps.

50 2,500-lumen, 6.6-ampere, straight-series lamps.

10 6,000-lumen, 20-ampere lamps with isolating transformer.

Solution: Table 1-1 shows that the average energy consumption of a 1,000-lumen, 6.6-ampere, straight-series lamp with film cutout is 69 watts per lamp. In a similar manner, the average energy consumption of a 2,500-lumen lamp is 167 watts, and a 6,000-lumen, 20-ampere lamp with isolating transformer is 405 watts. Totaling the combined load shows the following:

Therefore, a 15-kilowatt regulator would be required.

NOTE: The table makes allowances for line losses in the average series street circuits.