How to tell if a Motor is really running?

Bullzi · Jun 8, 2018

Hi Everyone,
I have a Fan that recently did not start running when it was suppose to. The PLC is monitoring the Contactor AUX contact to get its run status. The AUX must have been pulled in but for some reason the fan motor did not run. If the AUX did not pull in the system would have shut down on alarm but that didn't happen. The system was running normally but the fan was not running. The main breaker to the fan motor is also feeds the CVT for the panel so I know that was on or the system would have not ran at all.

This is a situation where I need the fan to be running for the equipment to operate safely. So I am looking for ideas on how to know the motor is running other than using the AUX contact. I was thinking of using a current switch on the wires to the motor. Anyone have experience with those?

Any ideas you have would be very helpful.
Thanks!

dmargineau · Jun 8, 2018

Without any 'smart' devices involved, I'd address the intended purpose of using the motor.

In your particular case, a fan, I'd use a simple air flow switch (paddle style); this will also ensure feedback for a catastrophic fan prop or drive belts failure when the motor is still running, however without moving any air.

g.mccormick · Jun 8, 2018

Flow switch to prove flow and/or a current switch. I've used both, current switch is easier to field install especially if your controls are in the same panel as the motor contactor since you would not need to install any conduit. Best to get one that has adjustable trip point and adjust it so that if the fan belt breaks the current will drop low enough to turn off the switch even with the motor running. Obviously both the current and flow proof would be better than just one.

LoganB · Jun 8, 2018

dmargineau said:
Without any 'smart' devices involved, I'd address the intended purpose of using the motor.

In your particular case, a fan, I'd use a simple air flow switch (paddle style); this will also ensure feedback for a catastrophic fan prop failure when the motor is still running, however without moving any air.

Agreed. If the fan being in motion is an important part of the safety of the process, you'll want to measure fan motion, not any form of measuring the motor alone. Depending on how fast the fan moves, you could also use a photosensor or some other type of prox that could detect the motion of the fins themselves. But I think the air flow switch would be the best choice.

geniusintraining · Jun 8, 2018

I have used both paddle air flow switches located in the duct work and prox switches that were located on the drive shaft... below x rpm they would error out

Here is a option http://www.mullerbeltex.com/process-monitoring-systems/speed-monitoring/

Firejo · Jun 8, 2018

Monitor the load (current draw) on the motor and airflow. The airflow will provide the best info as to the fan actually running but the current draw is also good info to have to help predict failures and/or maintenance needs.

thingstodo · Jun 8, 2018

Cheapest and easiest would likely be an analog current measurement. You can do the wiring in the starter, and it should not be a big cable run to your PLC.

Motor amps should be around 1/3 full load amps for just turning without load. Depending on what current you draw with the fan load, if you drop down under 1/2 you can alarm that the fan is not actually turning.

The air switch flow would be best, but it is a bit more money and takes some wiring to bring the signal back.

I hate switches - they are difficult to calibrate and can fail off or fail on. 4 -20 mA signals are what I use when I have the choice.

If this is a safety application, it's not really much more money for the analog signals compared to hurting someone, or having your plant fill with smoke!

Bit_Bucket_07 · Jun 8, 2018

g.mccormick said:
Flow switch to prove flow and/or a current switch. I've used both, current switch is easier to field install especially if your controls are in the same panel as the motor contactor since you would not need to install any conduit. Best to get one that has adjustable trip point and adjust it so that if the fan belt breaks the current will drop low enough to turn off the switch even with the motor running. Obviously both the current and flow proof would be better than just one.

The only problem I see with inferring air flow due to normal current draw is that a mechanical issue (such as a failing bearing) could be restricting airflow without dropping the fan amps.

g.mccormick · Jun 8, 2018

Bit_Bucket_07 said:
The only problem I see with inferring air flow due to normal current draw is that a mechanical issue (such as a failing bearing) could be restricting airflow without dropping the fan amps.

That is true. Measuring the airflow (proof or actual flow measurement) is best.

geniusintraining · Jun 8, 2018

Bit_Bucket_07 said:
The only problem I see with inferring air flow due to normal current draw is that a mechanical issue (such as a failing bearing) could be restricting airflow without dropping the fan amps.

Thats why we used all three in one of our processes, it was critical to maintaining LEL due to the highly flammable fumes that we were trying to evacuate from the area, its a nightmare from the maintenance standpoint... and then you would have the off shift jumpering everything because they are to lazy to climb stairs and check it out

daba · Jun 8, 2018

LoganB said:
......But I think the air flow switch would be the best choice.

I agree, air flow is the prime object for measurement. The fan may be running, but some ducting blocked somewhere could mean it's not producing air flow.

spaderkung · Jun 8, 2018

Fans that are belt driven then use specialized fan power monitor.

keithkyll · Jun 8, 2018

If it's ducted, use a Dwyer Photohelic differential switch. Main application I'm familiar with monitors flow in a HEPA filter system. Lets us know when it's time to change the HEPA.
Use low setpoint to know fan is running, high setpoint to know when filter is clogged.

jraef · Jun 8, 2018

Change to using a solid state overload relay that can give you an actual motor current signal back to your PLC. So instead of just monitoring the six contact, you look for a minimum amount of current to flow. Base that minimum value on the fan current when sufficient air is flowing, which will mean any dampers etc are opennas well (dampers closed = no flow = lowest load on the motor). This tells you what you need to know.

damica1 · Jun 8, 2018

Current Sensor (4-20ma output) $25.00. This would even show you if belts broke, because the current will be a different value when things are working correct and not correct.

How to tell if a Motor is really running?

Bullzi

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