TCBA - Tesla Coil Builders Association

Devoted to the construction, operation and theoretical analysis of the Tesla coil

TCBA Volume 10 - Issue 1

Page 16 of 18

The controls for the unit are fairly straight forward. All switches are low voltage which operate relays for ease of wiring and safety. The switches are all lighted so that their status can be obtained easily. There is a master power switch which controls a large contactor in the base of the control unit. This supplies power from the incoming lines to all of the other systems. There are switches which turn on lights for the controls and at the tank circuit. There is a switch for the gap motor and a switch for the cooling fan for the gap. There is a safety key switch which must be turned in order to enable the operation switch. Once the key is turned, and the gap motor is on, the operate relay is enabled and the switch is illuminated, indicating a ready to fire state. The operate switch is momentary and cannot be locked on.

The other controls are three variable autotransformers or variacs. The small one controls gap motor speed and is mounted on the switch and instrument panel. The other two variacs are mounted beneath the main panel in two separate panels. The upper unit controls input voltage to the power transformer from 120 to 240 volts. The second is used as a sliding inductor for inserting reactance for current control and power factor correction. Both units are fused and are capable of handling forty-five amperes. They weigh about seventy-five pounds each and have large wheels to turn them.

The power control unit has filtering and safety circuits incorporated into it as well. The incoming line is filtered by a commercial forty-amp line filter which uses LC circuits to cut down on noise travelling into or out of the unit. There are large G.E. PA series varistors installed where the power lines go to the power transformer as well. These protect the console from transient voltages coming back from the tank. The relays used for the low voltage switch circuits help isolate the operator from line current and all metal pieces and assemblies are hooked to a heavy safety ground buss that runs through the cabinet.

Operation of the coil has proved to be spectacular. At full power, arcs in excess of eight feet in length have been seen. Continuous six feet arcs are easily obtainable. Theory says, however, that longer arcs are possible. Currently, the meters read four kilowatts input and six thousand volt-amps for operation at full power. This indicates a power factor problem, particularly that the unit is running too inductive. There are several plans in the works to correct this, most of which involve changing the primary capacitor to a larger unit. Hopefully, arcs as large as fifteen feet can be obtained with five kilowatts input.