TRICOR's Model 401 Variable Air Core Inductor

Overview

Electromechanical switches are used in a wide variety of modern day applications. Applications range from simple DC circuits to large high-power AC implementations controlling such devices as motors, solenoids and relays. Verifying that the switching devices satisfies the specification requirements for these varied applications require accurate, traceable test verification. Specific hardware is required to establish an acceptable test platform. For DC applications, simple power supplies or current sources can be used to satisfy specification criteria. However, AC applications require a more elaborate test setup. The reason for this is AC loads are typically inductive in nature; that is, motors, solenoids and relays do not act as pure resistive devices.

Therefore, to verify the actuation devices pass the overload and endurance tests for electrical safety agency approvals instrumentation must be employed that allows tailoring of the current waveform (power factor) at the specified operating voltage. Inductors are used as the primary component for providing this control.

An equivalent test circuit is comprised of a primary power source, series impedance, device under test and inductive component . The inductor is either an iron core or air core device. Iron core inductors are commonly used for experimental testing of switches due to their smaller size and lower cost. The major disadvantage of using an iron core reactor or an electrical motor for an AC electrical certification test is the variance in electromagnetic characteristics of iron core devices due to hysteresis, eddy current losses and the operation point within the B/H curve of the core material. As the field increases in an iron core device, the permeability varies based on the flux density change. The most extreme change occurs at saturation of the core material. At that point, inductance decreases dramatically. Although the iron core inductor does provide a possible solution, significant documentation and testing must be maintained to verify linear operation. For official agency approval (UL, UDE, IEC, etc.) it is more prudent to use an air core inductor for repeatability and accuracy of tests. The Model 401A air core inductor exhibits linear inductance regardless of field density. The air core inductor is traditionally a large coil of wire wound on a bobbin type form without any internal core material. The air core inductor has no core loss and includes a parallel shunting resistor to divert a certain percentage of the test current.

With the Model 401A, the required inductance is selected thru a two step process.

A series of taps are provided across the stationery and rotatable coil sections for gross inductance selection;
The movable coil section is rotated within the stationery section to achieve the fine inductance tuning.

Table 1 shows the 12 taps located on the front panel of the unit. By connecting to the various taps and adjusting the coils in a clockwise or counterclockwise direction, a range of 1.6 millihenries to approximately 140 millihenries can be achieved.

Table 1
Inductor Taps	Min. Inductance	Max. Inductance
L1-L7	1.6 mhy	3.8 mhy
L1-L8	3.3	7.8
L2-L8	6.5	15.5
L3-L9	14.5	35.0
L4-L10	25.5	62.5
L5-L11	39.4	98.1
L6-L12	56.1	141.8

Maximum Applied Voltage 480 vac
Maximum Current 100 amps (50% Duty Cycle)

The Model 401A incorporates two independent variable air core inductors (Figure 1). This allows the end user flexibility for a variety of test setups. Additional features of the unit are an AC ampere meter used to measure the shunt current across the reactor and various test terminals for calibration checks, and a mechanical brake device employed to permanently hold the rotatable reactor at its final adjustment point. The unit is 29"W by 32"D by 66"H in size and weighs approximately 550 pounds.

If accurate, certifiable test results must be achieved for your product verification, the TRICOR Model 401A Variable Air Core Inductor provides the minimum of setup and test time.

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