Confirmed Shut Down Signal - Motor Starter

I would measure the motion of whatever the motor moves. The motor's back EMF can give false indications that the contactor is still providing power.

I agree with you. KISS is a perfect mantra to follow in engineering. However, without knowing why any complication is desired, one should not dismiss exploring how to implement the complication. This should be part of an engineer's "due diligence."

Agree, but it depends on the situation. If the location is a small factory, putting sensors for every possibility can be expensive in addition to being complex.

If the place were a naval warship, for example, with ample budget, putting many sensors aids in fast, practically instant, identification of problems during critical periods.

I know these are extremes, but I'm just giving examples. (",)

regards,

Vulcan

Use a zero speed switch attached to the motor shaft. This method is used in mission critical applications where the motor status must be known. This is in addition to other interlock contacts on the motor starter and computer. There are also motor-amp relays to detect when the motor is running.

Take your pick.

Link to Zero Speed Switches:

https://en.wikipedia.org/wiki/Zero_speed_switch

Current relays sense motor current.

They are relatively cheap. Here is a link:

https://aax-us-iad.amazon.com/x/c/RP4NCY6ARhiKHghU6I9vMfsAAAGX00DbIAEAAAH0AQBvbm9fdHhuX2JpZDMgICBvbm9fdHhuX2ltcDIgICBLlCI5/clv1_CEuOPUxokZA2iHrVCqEB8in3bipefoEpl40V2y6qxdsp8X8TzRyqaPw18nFItlO9gqEp25IbkHjNfU0FDGcAQUc7FSOWPOXcMkYATW9NTFn7TJY5okfq8YkGXCQqXOW6XJfef1DnuNIJHEkq3Unra0sKKlFODMtmGRY-HqMnUhKFyLqDhUk3Q0G-Kp7vPuogBnoOdtqahJ1BBWh2KcNaYfAXkcOsktIiVBKEZpAkjwMt7q9FwOmESBUKnUiigUVjPGfAC8yX5UA_jkcRbvRyNDOlc1SrkTbAINay3LDpTZggYGK6x_-Bxha2RBL7HaCgzvZB4u9xV8zKzKHAB6flWYBr2M4vcWMQvbQVh815CphB8mZHVW6DwyPsyq46JkObEhZsgFvToGUXvw_JVZCnWlZ-ebPbtvdAeMIqIFVbnwt8B71cgj1qMWtpvpZiUG8nY4SyYPWk5sClcDYcSGzOvIYtSnTSvx-EYzl3epIjppmNPXAGnqim7CzLM3dxAoiqyu5koFxLCqoWjB-lmFPA1SCAUilOaF1fMWvaWDjCYpjXbHy4J3V28CQ-GWr4SSOzS6chmsdfaxfIsoBIrONWPQz6Nh1ozhd6yboxr9eXhkQR7NO9yvc1UUX66A6HVfWXWDsXatkYLDxxo8waCZl1ghok0oWxYhxDBUUfta4s6r8wekgEMyj5szUy_gqFY9IF1D0xzQll_Ohh1LMwLHyuNRIk-Z3MNKDwx6HjAQgYXgDKoS38fvNukBhdssQ7F8W_25w8RiLZmBTszxn5Lg__PHE8hE_wWDdjn8piTdwSeXjL9nZxZs0q7Rhg9Ijlc3iDduS9eVJY8VvUuc5TX9IQp6yunhJFidSsKO2Nl8u-qFKqwDj97m4s2gGVhFi9hRuir6FhrfUapHjlKZs4Yvo5IuH49fSF0Q4bKT0_PArZtBHlig5EFkJ7aA5ufyH_uDujlmbuWYOetYDstLbF7ijrZNl8N_2IYICS1z_LenBCaftvezMldBdVdIjM8CTrbCkOQ18ZGVGcY84autcjj-2oxQolhgZkSvSXvVp7yO1nqSofD0_7XN-naSmhNt0jKdweN3_mZqqLHTocCvjhG5SFlv7hXWXUwoAwCNIGHQ5uOfKAEnGvQudjqxxIC6Cp5KCglZ9fYtHSoT4dAeZQ5HBBXWoIVdDYQUDQDoAOtcrCk6ZFhyZruFWEiJ0KowZEVuciEGW0IQGv9kiraD4shxiCOcuz8NtHprRT4zWBybZ1F0HNC3AeUWin00GUpzsDDI7RUZk3utqkTkDQZ4t1dJQBg2brsvAf5qOHGVSCuyefC81gRJSZls4/http://www.amazon.com/dp/B07P87TG8F/ref=syn_sd_onsite_desktop_0?ie=UTF8&psc=1&pf_rd_p=4500617b-0cc7-45a9-8bde-f455bfff54d3&pf_rd_r=CNYFW00JAEYTEAVKCB46&pd_rd_wg=xL4ZW&pd_rd_w=Fpr63&pd_rd_r=16f29ba8-a485-4d21-9580-7b4886680d93&aref=AaYOs1k8V3

I just found out about 5 years ago that there is a term for this when i was integrating europeon equipment with Siemens control to the existing process equipment with Allen-Bradley control.

That term is called ’Potential Free Signal’.

Thanks to AI, here is an explaination.

A "potential-free" or "dry" contact refers to a relay contact or switch that is electrically isolated from the device's power source. It doesn't have a voltage potential across it until an external circuit is connected. Essentially, it's a switch that can be used to control another circuit without being directly connected to the power supply of the device it's part of. Here's a more detailed explanation:

• Isolation: Potential-free contacts are electrically isolated from the device's power source. This means the contacts themselves don't carry any voltage from the device's internal circuitry.
• External Power: To use a potential-free contact, you need to provide your own external power source to the circuit it controls. The contact acts as a switch to turn that external power on or off.
• Applications: They are used in various applications, including:
• Interlocks: Ensuring a sequence of operations by preventing one part of a system from starting before another is ready.
• Status Indication: Providing feedback on the state of a device to a control system (e.g., a PLC).
• Safety Circuits: In safety systems, potential-free contacts can be used to ensure that a machine is in a safe state before it can be operated.
• Hazardous Environments: Where sparks could ignite a flammable atmosphere, potential-free contacts are used to prevent such ignition.
• Control Systems: They are frequently used in PLCs (Programmable Logic Controllers) and other control systems to switch external loads.
• Examples: Relays, switches, and sensor contacts can all be designed as potential-free.

This sounds like a it’ll be solved within the ladder logic.

see what the state is, and check the instiments down stream such as to flow rate, pressure, etc…

Yes, that's what we did. We never did have issues with stopping, but we did have them for starting.

Our motors are plug-in and, one time, the operator complained that he'd already started the pump and the display showed the motor was running, but there was no flow. The technician's Mark I Eyeball found that the motor had been unplugged. The operator was given a warning for marking the motor as checked in the startup checklist, but we were also requested to put a provision for sensing if the motor was plugged in. It involved replacing the socket with another socket with a built-in limit switch. I was close to retirement by then so I don't know if that was implemented.

You could mount a volt/ammeter on the motor line in...

If it's too far away to see, you could add a little wifi camera...

A confirmed shutdown signal from a DOL (Direct-On-Line) motor starter, especially at low voltage, is typically achieved using an auxiliary contact on the contactor. This contact, wired in series with the stop button and potentially a safety relay, provides feedback to the control system (like a PLC) confirming the motor has indeed stopped when the stop signal is initiated. Here's a breakdown:

1. 1. Contactor Auxiliary Contact: DOL starters use a contactor to switch the motor on and off. The contactor has auxiliary contacts, which are normally open (NO) or normally closed (NC) contacts that mirror the state of the main contacts.
2. 2. Stop Circuit: When the stop button is pressed, it breaks the circuit, de-energizing the contactor coil and opening the main power contacts to the motor.
3. 3. Confirmed Shutdown: The auxiliary contact (typically NC) is wired in series with the stop button and potentially a safety relay within the control circuit. When the contactor opens (motor stops), this auxiliary contact also opens, breaking the circuit and signaling to the PLC (or other control system) that the motor has stopped.
4. 4. Safety Relay: In more robust systems, especially those with safety requirements, a safety relay is used. The safety relay monitors the status of the auxiliary contact and other safety devices (like emergency stop buttons). It ensures a safe shutdown condition before allowing the motor to be restarted.
5. 5. Low Voltage Control: Low voltage (e.g., 24V) control circuits are often used for safety and to reduce the size and complexity of control wiring. The main power circuit for the motor is separate from the low voltage control circuit.
6. 6. PLC Integration: The feedback from the auxiliary contact (and safety relay if present) is fed into the PLC. This allows the PLC to monitor the motor's status and respond accordingly (e.g., display an "off" status, prevent other operations until the motor is confirmed stopped).

In essence, the auxiliary contact provides a reliable way to confirm the physical opening of the contactor, which is essential for ensuring a safe and controlled shutdown sequence.

I would use a set/reset flip flop in the PLC where the shutdown signal sets an indicator/alarm and is reset when the contactor open contact is made after a time delay if wished.

Mission critical situations, such as the Space Shuttle has multiple back ups on every system. It is expensive, but Nearly foolproof. Nothing is perfect, no matter how simple or complex. At some point you must accept that Murphy is ever vigilant and things will happen that no one could anticipate.

If the mission is super-critical, and there will be no means to repair it in case of failure and money is not a concern, you may get almost perfect results. The example of near perfection is the Voyager1 and Voyager2.They have far exceeded their mission objectives and are still going after all these years.

They are approaching the boundary of our sun's influence (the Heliopause)and are headed into interstellar space. Maybe someday an intelligent species may find one of them and wonder.

We may even develop a technology fast enough to intercept and retrieve them and still, we will wonder because the ancient technology was lost to history, a victim of the 6th great Earth extinction.

(Nothing is foolproof, because the fools are so ingenious.)

An ammeter could do that.

• Is the contactor de-energised?
• Is the current drawn zero?

If both statements are logically true, then by extension the motor isn't running.