You've stumbled upon a slightly unusual configuration, and your intuition about harmonic filtering is a good starting point. Let's break down the possibilities and likely scenarios

Possible Explanations and Connections

1. Open Delta (V-V) Connection



Description

This is the most likely explanation for using two single-phase transformers on a three-phase system. An open delta uses only two transformers to supply a three-phase load.

How it Works

Each transformer is connected between two of the three phases. The primaries would be connected as follows: Transformer 1: Primary connected between Phase A and Phase B. Transformer 2: Primary connected between Phase B and Phase C.

Capacity

An open delta bank has about 57.7% of the capacity of a full delta bank using three identical transformers. So, two 5kVA transformers would give you about 5.77 kVA of three-phase power. This could be sufficient for control power and filtering purposes.

Why 208/36 VAC? This could be a standard transformer available at the location. The 36 VAC secondaries might not be used now, but perhaps they were intended for a low-voltage control circuit or future use. It's also possible they're using the 208V side "backwards," treating the 36V winding as the primary for harmonic attenuation. (More on that later.) Harmonic Filtering Role

While not the primary purpose of an open delta, the transformers could offer some small amount of harmonic attenuation due to their impedance. However, they are not specifically designed for harmonic filtering in this setup. They are used to derive the voltage and current to drive a load connected to their secondaries.2.

Control Power



Description

The transformers might be strictly used to derive control power for the 10 HP motor controller. Motor controllers often need a clean, stable source of power for their internal logic, relays, and displays.

How it Works

As described above, the transformers are connected in open delta to get 3-phase voltage as described above. The secondaries(36V) are stepped up using additional transformer to get voltage required for internal circuits of a motor controller.

Harmonic Filtering Role

Similar to the Open Delta case above, only incidental harmonic filtering.3.

A Failed Third Transformer



Possibility

A less likely scenario is that the original installation did have three transformers in a full delta or wye-delta configuration, and one transformer failed. Rather than replace it immediately, they temporarily used the two remaining in an open delta configuration. This is generally not recommended for long-term operation if the load is substantial.



Identification

Look for evidence of a missing transformer (mounting holes, disconnected wiring).4.

Harmonic Attenuation Using the Secondaries



Description

This is more speculative, but potentially relevant. You mention triplen harmonics. Triplen harmonics (3rd, 9th, 15th, etc.) are zero-sequence harmonics, meaning they are in phase with each other in all three phases. These harmonics can cause significant neutral currents and voltage distortion.

How it Might Work

This would be highly unconventional. The idea is to use the transformers almost like reactors, but with the secondaries strategically loaded. The 36 VAC secondaries are connected in delta. A resistive load is connected across each leg of the delta-connected secondaries. Triplen harmonics in the 208V primary would induce currents in the 36V delta secondary. Because the triplen harmonics are in phase, the delta connection provides a circulating path for them, and the resistive load dissipates some of the harmonic energy as heat.

Why It's Unlikely (But Possible)

This is not a common or standard harmonic filtering technique. It requires careful calculation of the resistive load values to be effective and avoid overloading the transformers. The 2-5 kVA size seems small for significant harmonic mitigation on a 10 HP motor.

Evidence to Look For

Carefully examine the connections on the 36 VAC secondaries. Are they connected in a delta configuration? Are there resistors (or other loads) connected across each delta leg? Measuring current in the delta loop would show if there are circulating harmonic currents.

Why Two Transformers Instead of Three for Harmonic Filtering?While three transformers can be used in various ways for harmonic mitigation (e.g., zig-zag transformers), using two transformers for dedicated harmonic filtering is uncommon. The open delta is almost certainly about deriving voltage and current, rather than dedicated filtering.

How to Determine the Actual Configuration


Visual Inspection

Trace the wiring from the transformers to the 208V three-phase line. Note exactly which phases each transformer is connected to. Carefully inspect the connections on the 36 VAC secondaries. Are they used at all? Are they connected in a delta configuration? Are there any loads connected to them? Look for signs of a missing third transformer.2.

Voltage Measurements

Measure the line-to-line voltages on the three-phase side. They should be reasonably balanced around 208 VAC. If the 36 VAC secondaries are connected, measure the voltages across each leg of the delta.3.

Current Measurements (If Safe and Possible)

Carefully (using a clamp meter) measure the current in each phase of the 208V line feeding the transformers. If the 36 VAC secondaries are connected in a delta, measure the circulating current in each leg of the delta. This would indicate if harmonic currents are flowing.4.

Consult Documentation

Check the motor controller's documentation. Does it specify any external components or filtering requirements?

In Summary:The most likely scenario is that the two transformers are connected in an open delta configuration to provide control power for the motor controller. The 36 VAC secondaries may or may not be used. While they could be used in a very unconventional harmonic attenuation scheme, this is less probable.Careful inspection and measurements are needed to determine the exact configuration and purpose of the transformers. Be extremely cautious when working with electrical equipment. If you are not qualified, consult a licensed electrician. Flag for review