Okay, you've provided a good amount of detail. Let's break this down and try to diagnose why you're repeatedly burning out your 240V starter coils on your air compressor.

Understanding Your Power System (240V Delta with a High Leg) You have a 240V Delta electrical service. This means the voltage between any two of the three phases is 240V. You also have a High Leg Delta (also known as a "wild leg" or "stinger leg"). This is the key to your 208V measurement. One of the phases is center-tapped, and that center tap is grounded (becoming your neutral). The voltage from one phase (the "high leg") to neutral is higher than the voltage from the other two phases to neutral. In your case, that high leg voltage is 208V to neutral, but it is still 240V line to line. Importantly: You do NOT treat the high leg as a neutral. It is a phase. Use it as such.

Why a 208V Coil is (Probably) Incorrect

Google and AI are getting confused between your 240V line-to-line voltage and the 208V high leg to neutral. Your 240V line-to-line voltage is what matters for your motor starter coil. If you use a 208V coil on 240V, you're likely over-volting the coil and shortening its lifespan.

Possible Causes for Your Coil Failures

Given that your voltage measurements seem correct and the power company says your service is good, we need to look closer at the following:1. Coil Voltage Mismatch (Even with the Same Part Number)



Quality Control/Counterfeits

Are you absolutely certain the coils are genuine Allen-Bradley and of the correct voltage rating (240V)? There are counterfeit electrical parts in the market, and the quality can vary even with legitimate parts. Could you be getting batches with slight manufacturing defects? Buy from a reputable supplier.

Over-Voltage

While your line-to-line is 240v. Is it consistently 240v? Use a recording meter to monitor the voltage over a period of time (a few days). Voltage spikes or surges can damage coils. The power company says the service is good, but it's worth verifying yourself, especially when other equipment in your shop is running.

Underrated Coil

Even with the "correct" part number, double-check the coil's specifications. Allen-Bradley has different coil voltage ranges. Make absolutely sure it's designed for a range that includes 240V. It may also be possible that the "240 volt" coil you are buying is a 220-240v and could be damaged by slightly higher voltages.2.

Mechanical Issues with the Starter



Sticking Contacts

If the starter's contacts are sticking or not making good contact, the coil will remain energized longer than necessary, leading to overheating. Inspect the contacts for pitting, corrosion, or excessive wear. Clean or replace the contacts if needed.

Mechanical Binding

Ensure that the armature (the moving part of the starter) moves freely and isn't binding. Any resistance to movement puts extra stress on the coil to pull in and hold the armature.

Incorrect Spring Tension

The return spring on the armature needs to be strong enough to reliably open the contacts when the coil is de-energized, but not so strong that it makes the coil work harder to pull in the armature. Make sure the spring isn't damaged or weakened.3.

Overheating



Ambient Temperature

If the starter enclosure is in a very hot environment, it can contribute to coil overheating.

Ventilation

Make sure the starter enclosure has adequate ventilation.

Over-Cycling

Are you starting and stopping the compressor excessively? Frequent starts and stops put extra strain on the coil.4.

Wiring and Connections



Loose Connections

Check all connections in the starter circuit (including the coil connections). Loose connections can cause voltage drops and heat, which can damage the coil. Use a torque screwdriver to ensure proper tightness of screw terminals.

Incorrect Wiring

Double-check that the control wiring (to the coil) is correct.5.

Motor Load/Problems



Overload

Although you said the motor runs fine, an overloaded motor will draw excessive current, which can stress the entire system, including the starter. Verify that the motor isn't running overloaded (e.g., compressor not building pressure properly).

Motor Winding Fault

A developing fault in the motor windings could be drawing excessive current or creating voltage imbalances, which could stress the starter coil. Have the motor insulation resistance tested (Megger test).

Single-Phasing

While less likely with a contactor, if one phase were to drop out momentarily, the motor would try to run on two phases (single-phasing), drawing significantly more current and potentially damaging the starter.

Troubleshooting Steps


Visual Inspection

Thoroughly inspect the starter, wiring, and motor connections. Look for signs of overheating, corrosion, or damage.2.

Voltage Measurements (Under Load)

Measure the voltage at the starter coil terminals while the compressor is running. See if there are any voltage fluctuations.3.

Current Measurements

Measure the current draw of the motor on each phase while it's running. Compare these readings to the motor's nameplate amperage. Imbalanced currents can indicate a problem.4.

Temperature Measurement

Use an infrared thermometer to check the temperature of the starter coil and other components while the compressor is running. Excessive heat is a red flag.5.

Monitor Voltage Over Time

As mentioned before, use a recording meter to monitor the voltage over a few days to detect voltage spikes or sags.6.

Consider a Surge Suppressor

Install a surge suppressor in the control circuit feeding the coil. This can help protect the coil from voltage spikes.

Additional Tips



Overload Protection

Ensure that the overload relays in your starter are properly sized for the motor's nameplate current and are functioning correctly.

Consider a Soft Starter

A soft starter gradually increases the voltage to the motor during startup, reducing inrush current and mechanical stress. This can prolong the life of both the motor and the starter. However, this is a significant upgrade.

Consult a Qualified Electrician: If you're still having trouble, it's best to consult a qualified industrial electrician. They have the experience and tools to diagnose and repair complex electrical systems. They can also do more in-depth motor testing.By systematically investigating these areas, you should be able to pinpoint the cause of your coil failures and implement a solution. Good luck! Flag for review