Okay, let's break down the pros and cons of mechanical contactors vs. solid-state relays (SSRs) in your specific context, and then address the mounting location considerations and the multi-motor SSR idea.

Mechanical Contactors vs. Solid State Relays (24V Control, 208V 3-Phase, 5HP or Less)Mechanical ContactorsPros



Cost

Generally less expensive, especially in lower volumes.

High Inrush Handling

Can handle motor starting inrush currents very well. This is a key advantage for motor control.

Simplicity

Easier to troubleshoot and understand. Most electricians are very familiar with them.

Availability

Readily available from numerous suppliers.

Isolation

Naturally provide excellent electrical isolation between control and load circuits.

Ruggedness

Typically more robust in harsh environments (temperature, vibration) if properly selected.

Serviceable: Contacts can be replaced on many modelsCons



Limited Lifespan (Wear)

Mechanical wear on contacts leads to eventual failure. Lifespan is based on the number of switching cycles.

Slower Switching Speed

Slower response time compared to SSRs. This is generally not an issue for motor control, but can be in other applications.

Audible Noise

Clicking sound when switching. Can be an issue in noise-sensitive environments.

Arcing/Contact Bounce

Can generate electrical noise and cause contact wear.

Larger Size

Generally physically larger than SSRs for the same power rating.

Solid State Relays (SSRs)Pros



Long Lifespan

No moving parts, so lifespan is theoretically much longer (limited by component aging and thermal stress).

Faster Switching Speed

Much faster switching times than contactors. Again, usually not critical for motor control but can be in other applications.

Silent Operation

No clicking or buzzing.

Less Electrical Noise

Solid-state switching minimizes electrical noise.

Smaller Size

Can be more compact than contactors.

Vibration Resistant

Less sensitive to vibration due to lack of moving parts.

Cons



Cost

Generally more expensive than mechanical contactors.

Heat Dissipation

Generate heat that must be managed with heat sinks. Proper heat sinking is critical for reliable operation.

Voltage Drop

SSRs typically have a small voltage drop across them when conducting.

Susceptibility to Overvoltage/Overcurrent

More sensitive to voltage spikes and overcurrent conditions. Require proper protection (fuses, MOVs, etc.).

Leakage Current

A small amount of current can leak through the SSR even when it's supposed to be "off." This can be problematic for some loads.

Inrush Current Handling

Typically not as good as mechanical contactors for handling motor inrush currents. Requires careful selection and potentially oversizing the SSR. This is a major disadvantage for motor control.

More Complex Troubleshooting

Can be more difficult to diagnose problems with SSRs compared to contactors.

Recommendation for Your Application (5HP or Less Motor Control)

For general motor control (on/off), mechanical contactors are usually the better choice due to their cost-effectiveness, robust inrush current handling, and ease of maintenance. The longer lifespan and silent operation of SSRs are usually not worth the added cost and complexity for basic motor control applications, especially given the inrush current issue. If you have extremely high switching frequency or a very specific need for silent operation, and you can address the inrush current issue and heat dissipation effectively, then SSRs might be considered.

Mounting Location: Near PLC vs. Near Motor (100ft Max)

This is a classic trade-off.

Mounting Near the PLC



Pros



Centralized Control

Easier wiring and troubleshooting from a central location.

Environmental Protection

PLCs and control panels are often in cleaner, temperature-controlled environments.

Easier Maintenance

All control components are in one place for easier access.

Potentially Lower Initial Cost

May reduce the number of enclosures required.

Cons



Longer Power Cable Runs

Requires running heavier gauge power cables from the control panel to each motor (especially for 3-phase). More expensive cable and potentially higher voltage drop over the distance.

More Complex and Expensive Conduit Runs

Longer power cable runs may require more conduit and labor.

Potential for Noise Issues

Longer control signal runs (24V) can be susceptible to electrical noise if not properly shielded. Voltage drop in the control signal can also be an issue over longer distances, especially with multiple loads on the same output.

Safety Considerations

High voltage power cables are run over a longer distance increasing potential hazards if not installed properly.

Mounting Near the Motor



Pros



Shorter Power Cable Runs

Reduces the length and cost of heavy gauge power cables. Minimizes voltage drop.

Simplified Conduit Runs

Shorter power cable runs mean simpler and less expensive conduit runs.

Reduced Noise Susceptibility

Shorter control signal runs are less susceptible to noise.

Easier Motor Replacement

Motor replacement is simplified as the contactor is close by.

Cons



Distributed Control

Control components are spread out, making troubleshooting and maintenance more difficult.

Environmental Considerations

Motor locations can be harsh (dust, moisture, temperature extremes). Requires enclosures rated for the environment.

Higher Initial Cost

Requires more enclosures to house the contactors/relays near the motors.

Potentially More Complex Wiring

Requires running control signal wires (24V) to each motor location.

Power Distribution

Need to distribute 208V to each motor location.

Recommendation

For a 100ft distance, the best choice depends on the specific environment and the number of motors.

Few Motors (1-2)

Mounting near the PLC might be more economical, especially if the environment is clean. Ensure proper shielding and wire gauge for the control signals to minimize noise and voltage drop.

Multiple Motors (3+)

Mounting near the motors is likely the better choice. The savings in power cable costs and reduced voltage drop will often outweigh the cost of the additional enclosures. Use high-quality enclosures rated for the environment. Run dedicated home runs for each motor's power circuit back to the main panel.

Important Considerations



Voltage Drop Calculations

Always perform voltage drop calculations for both the power and control circuits to ensure proper operation.

Code Compliance

Follow all applicable electrical codes and standards (NEC, etc.).

Grounding

Proper grounding is essential for safety and noise reduction.

Shielding

Use shielded cable for control signals, especially if running alongside power cables.

Fusing

Properly fuse both the control and power circuits.

Bonus Question: Multi-Motor 3-Phase SSRs> "Does anyone make an "off the shelf" 3 phase solid state relay capable of controlling multiple motors from single device? (Boss says "If there ain't, there oughta be!").... I'm not convinced of the need for such other than saying it can be done. I would suspect it would be a fairly expensive device and only useful in limited situations."Answer

While you might be able to find a custom-built SSR that handles multiple small motors simultaneously,

no, there is no common, readily available "off-the-shelf" product that does this.Why it's not a good idea, and why it's rare



Inrush Current Summation

The primary reason is the motor inrush current. If you start multiple motors simultaneously, the SSR would have to handle the sum of the inrush currents of all the motors. This is a huge burden on the SSR and would require massive oversizing, making it very expensive and impractical. Even staggering the motor starts by milliseconds would likely not be enough to mitigate the issue, because inrush current is generally milliseconds-long.

Fault Isolation

If one motor fails (short circuit), it would likely take out all the motors controlled by the same SSR. Individual fusing of the motors would help somewhat, but the SSR would still be stressed.

Complexity and Cost

Designing and manufacturing such a device would be very complex and expensive. The market for it would be extremely small, as most applications are better served by individual contactors or SSRs for each motor.

Heat Dissipation

Managing the heat generated by a single SSR handling the load of multiple motors would be a significant challenge.

Lack of Standardization

There is no standardization for this type of device, making it difficult to find and support.

Why your boss is wrong (with all due respect)

Your boss's sentiment of "If there ain't, there oughta be!" is well-intentioned, but in this case, it's not practical due to the inherent technical challenges and lack of market demand. It's a classic case of trying to solve a problem that doesn't exist in a cost-effective manner.

Better Alternatives



Individual Contactors/SSRs

The standard and recommended approach is to use separate contactors (or SSRs, if justified) for each motor. This provides better fault isolation, easier troubleshooting, and more reliable operation.

Motor Starter Panel

For multiple motors, a pre-built motor starter panel is often the best solution. These panels include all the necessary components (contactors, overload protection, fuses, etc.) and are designed to meet all applicable codes and standards.*

Variable Frequency Drives (VFDs): If speed control is required, VFDs are a much better solution than trying to switch multiple motors on and off with a single SSR.In conclusion, I would strongly advise against pursuing the multi-motor SSR idea. It is not a practical or cost-effective solution. Stick with the standard industry practice of using individual contactors or SSRs for each motor, or consider a motor starter panel for multiple motors. Flag for review