Okay, let's analyze your proposed setup using a 3-way switch and a DPDT relay to control a mechanically held contactor. It's a clever approach, but we need to consider some potential pitfalls.

Here's a breakdown of the concerns and considerations


Functionality Analysis



3-Way Switch Positions

When the 3-way switch is in one position, it will energize one coil of the DPDT relay, switching the NC contacts. This will momentarily energize either the "Latch" or "Unlatch" coil of the mechanically held contactor. When the 3-way switch is in the other position, it will energize the other coil of the DPDT relay, switching the NC contacts again and energizing the opposite coil of the contactor.

Momentary vs. Continuous

Mechanically held contactors need a momentary pulse to latch or unlatch. The DPDT relay, wired as you described, should deliver that momentary pulse. The key is that the DPDT relay coils are not continuously energized after the initial switch position is selected.2.

Potential Problems & Concerns



Simultaneous Coil Energization

This is the

biggest risk. Depending on the exact timings of the 3-way switch and the DPDT relay, there's a chance that both the Latch and Unlatch coils of the mechanically held contactor could be energized simultaneously for a very brief period. This could cause the contactor to chatter, fail to latch properly, or even be damaged over time. Relay Bounce

Relays, including the DPDT relay, can experience contact bounce when they switch. This means the contacts make and break rapidly for a few milliseconds before settling. This could create multiple short pulses to the contactor coils, which could also lead to chattering or unreliable operation.

Relay Holding

You need to ensure that the DPDT relay is not a "latching" relay itself. A standard DPDT relay should de-energize when the voltage is removed from the coil. If it is a latching relay, then it will not work.

3-Way Switch Reliability

Depending on the quality of your 3-way switch, you might also see bouncing when flipping from one traveler to the other.3.

Improvements & Alternatives



RC Snubber Circuits

A common solution for relay bounce and arcing is to add an RC snubber circuit across the coils of the mechanically held contactor. This would consist of a resistor and a capacitor in series, connected across each coil. The values of the resistor and capacitor need to be chosen based on the coil voltage and current. This will help protect against arcs that may occur when the coil is de-energized.

Diode Protection

You could also add diodes in reverse bias across each coil of the mechanically held contactor. This is a standard practice when driving inductive loads with relays and is intended to protect the switching circuits from the back EMF generated when the coil is de-energized.

Momentary Switch Configuration

As you mentioned, using a double-throw center-off momentary switch is the cleanest and most reliable solution. This eliminates the potential for simultaneous coil energization and relay bounce issues.

PLC or Dedicated Controller

For more complex control scenarios or where reliability is paramount, consider using a small PLC (Programmable Logic Controller) or a dedicated controller designed for mechanically held contactors.4.

Important Safety Considerations



Proper Enclosure

All components must be housed in a suitable electrical enclosure, following all applicable codes and regulations.

Wiring Standards

Ensure all wiring is done correctly and neatly, with proper wire gauges and terminations.

Testing

Thoroughly test the circuit after installation to ensure it functions correctly and safely. Pay close attention to the behavior of the contactor when switching between positions.

In Summary

While your proposed setup might work, it has potential reliability issues due to simultaneous coil energization and relay bounce. It's not the ideal* solution. The double-throw center-off momentary switch is a much better option. If you must use the 3-way switch and relay configuration, carefully consider adding RC snubber circuits and diodes across the contactor coils to mitigate the risks. And always prioritize safety and code compliance.

Before implementing this, I strongly recommend testing it on a bench setup with low voltage DC to see how the DPDT relay and contactor interact. This can help you identify potential problems before connecting it to mains voltage. Flag for review