You've hit upon a good assessment of the problem and the limitations of each approach. You're right, simply monitoring power from the disconnect only tells you if power exists to the motor, not that the motor is actually running. And you're also right about the limitations of a standard CT.Here's a breakdown of possible solutions, considering cost, complexity, and reliability, along with my recommended approach

1. Current Transformer (CT) with Signal Conditioner (Recommended)
How it works

You're on the right track with a CT. However, you need a

signal conditioner to take the low AC voltage output of the CT (mV range) and convert it into a standard industrial signal like 4-20mA or 0-10V DC. This signal then feeds into a trip amplifier or PLC input that activates your alarm relay. Pros



Reliable indication of motor operation

Directly measures current draw. If the motor stalls, the current will either drop (underload) or increase (overload, eventually).

Relatively easy to install

CT clamps around a motor lead, wiring to the conditioner, wiring from the conditioner to your alarm.

Adjustable sensitivity

Signal conditioners often have adjustable trip points, allowing you to fine-tune the alarm threshold. You can set it to alarm if the current drops below a certain level (indicating underload, belt failure, etc.) or rises above a certain level (overload, impending failure).

Cons



Cost

CTs are relatively inexpensive, but the signal conditioner adds cost.

Complexity

More components than a simple voltage relay.

Why it's recommended

This provides the most reliable and informative indication of motor operation. The adjustable sensitivity allows for predictive maintenance by detecting changes in motor load over time.

Detailed steps for the CT with Signal Conditioner Approach


Select a CT

Choose a CT with an appropriate current range for the motor you're monitoring. You'll need to know the motor's full-load amps (FLA) rating. The CT's primary current rating should be higher than the FLA. A common secondary rating is 5A. Ensure the CT is sized appropriately for the wire you'll be clamping it around.2.

Select a Signal Conditioner

Choose a signal conditioner that accepts the CT's secondary output (e.g., 0-5A AC) and converts it to a suitable output (e.g., 4-20mA DC, 0-10V DC). Look for one with adjustable trip points and isolation. Isolation is critical for safety and preventing ground loops.3.

Wiring

Clamp the CT around one of the motor leads.

Important Safety Note

Never open-circuit the secondary of a CT while the primary is energized! This can create dangerously high voltages. Connect the CT secondary wires to the input of the signal conditioner. Observe polarity (if applicable). Connect the output of the signal conditioner to a trip amplifier or PLC input. Configure the trip amplifier or PLC input to activate your alarm relay when the signal falls outside the acceptable range.4.

Calibration

Use an amp meter to measure the current draw of the fan while it is running under normal conditions and use this measurement to set the current range of your signal conditioner so you can set the appropriate trip points for the signal conditioner.

2. Vibration Sensor



How it Works

Attaches to the fan housing or motor casing and detects vibration. When the motor stops or the fan becomes unbalanced, the vibration level changes, triggering an alarm.

Pros

Directly measures motor/fan activity. Can detect issues like bearing failure or impeller imbalance before the motor completely fails.

Cons



Susceptible to external vibrations

Need to filter out background noise.

Corrosive environment

Ensure the sensor is rated for the caustic area.

Installation

May require careful placement for accurate readings.

Cost

More expensive than a CT and signal conditioner.

3. Airflow Sensor



How it works

Measures the airflow in the exhaust duct. If the fan stops, airflow drops, triggering an alarm.

Pros

Directly measures the result of the fan's operation: airflow.

Cons



Caustic Environment

You'll need a very durable and corrosion-resistant sensor.

Installation

Can be complex to install in the ductwork.

Cost

Often more expensive than other solutions.

False Alarms

Duct blockages, damper failures, etc., can trigger false alarms.

4. Voltage Relay (Less Recommended, but Simpler) How it works

Uses a voltage relay wired to the motor starter coil. If the starter coil is energized (meaning the motor should be running), the relay is energized, and the alarm is suppressed. If the starter coil loses power (due to a trip, overload, or failure), the relay de-energizes, triggering the alarm.

Pros



Simple and inexpensive

Easy to install and maintain.

Cons



Limited information

Only tells you if the starter is trying to start the motor. It doesn't tell you if the motor is actually turning, if the belts are broken, or if the fan blades are seized. It only detects a complete loss of power to the starter coil. Doesn't detect motor underload or overload conditions.

Relies on starter operation

If the starter fails in a way that still provides power to the motor, but the motor isn't running correctly, this won't trigger an alarm.

5. Motor Current Signature Analysis (Advanced - for Predictive Maintenance) How it works

Uses sophisticated equipment to analyze the current waveform of the motor. Changes in the waveform can indicate bearing problems, broken rotor bars, winding faults, and other issues before they cause a complete failure.

Pros



Predictive Maintenance

Detects problems early, allowing for planned repairs.

Comprehensive motor health assessment. Cons



High Cost

Requires specialized equipment and trained personnel.

Periodic measurements

Not a continuous monitoring solution unless you invest in very expensive, permanently installed equipment.

Important Considerations



Hazardous Location Rating

Since this is a caustic environment, ensure that all devices (CT, signal conditioner, vibration sensor, relays, enclosures, etc.) are properly rated for the hazardous location (if applicable). This is absolutely critical for safety.

Power Supply

Ensure you have a reliable and clean power supply for your alarm system. A UPS (Uninterruptible Power Supply) is recommended to keep the alarm system running during power outages.

Audible and Visual Alarms

Use both audible and visual alarms for maximum effectiveness.

Testing

Thoroughly test the entire alarm system after installation and periodically thereafter to ensure it is functioning correctly.

In Summary

For the best balance of reliability, cost, and information, I strongly recommend the

Current Transformer (CT) with Signal Conditioner approach. It provides a robust and adjustable method for detecting motor problems and can be easily integrated into an existing alarm system. If you have a higher budget and want to proactively detect issues before* they lead to a complete failure, consider a vibration sensor. Always prioritize safety by using properly rated equipment for the caustic environment. Flag for review