Electric motor failure detection

+1 for flow switches (sail switch, paddle switch etc.). ULTIMATELY, what you want to know is if the fan is blowing air. The motor could be running just fine, but the shaft or belt broke. Once you know there is no air flow, you alert maintenance to go find out why.

If you are wanting to do it with electrical sensing of the motor, spend a little extra to get something that monitor kW, not just current. Current fluctuates with voltage, kW is ONLY associated with the actual load on the motor. So a current sensor may be satisfied by the motor current of a blocked fan because it sees active and reactive current as the same, a kW sensor would see the low PF to know that the load drop off too low.

That's a good point on the kW sensing switch. I have one area that the kW switch would work much better. At the taste water plant, we tried all different types of switches, like sail switches and they would fail or false trip. Some of it was the air they're pulling wasn't really good (They say its fresh air ).

Most of our fans are unrestricted fresh air and we want to just use it as a "fan proof". If it trips, most of the time its loose or missing belt. Who needs preventative maintenance!

jraef said:

+1 for flow switches (sail switch, paddle switch etc.). ULTIMATELY, what you want to know is if the fan is blowing air. The motor could be running just fine, but the shaft or belt broke. Once you know there is no air flow, you alert maintenance to go find out why.

If you are wanting to do it with electrical sensing of the motor, spend a little extra to get something that monitor kW, not just current. Current fluctuates with voltage, kW is ONLY associated with the actual load on the motor. So a current sensor may be satisfied by the motor current of a blocked fan because it sees active and reactive current as the same, a kW sensor would see the low PF to know that the load drop off too low.

Click to expand...

Interesting input, Your idea is Good. In your experience, which vendor would you recommend for this application. How is this sensor attached/installed to electric fan motor since there is no previous provision from the fan's design

How fancy do you want to go..
For only around $400..complete communications and control...
http://www.littelfuse.com/products/protection-relays-and-controls/protection-relays/motor-and-pump-protection/mp8000.aspx

Wow Nice !!!!

Yes, I really like the looks of this unit. Modbus TCP and Bluetooth built in! Just walk up to device and monitor it from your smartphone.
I will be using 5 on my next job.

A few questions
What is the reason that you want monitor the motor ?
Is the fan attached directly to the motor shaft or is there a belt and pulley arrangement driving the fan.

If you need to know that the motor and fan have in fact stopped so that maintaince personal can enter and do work. If that the case then that requires a safety rated zero speed switch attached to the fan long with all the necessary safety interlocks.
If you just want to know that a fan is in fact running when the when it should, then a standard motor starter with overload relay will do that if they are sized correctly.
Setup the plc to monitor both contactor closing and the status of the overload relay.
This is the way I always wire them anyway so I know when there is a problem.
If the contactor is closed and the overload relay is not tripped, the motor / fan is running normally

In a 3 phase system if you lost a single phase to the motor the motor will not start and will pull high current and trip the overload. The new overload relays monitor for phase lose as part of their function as well so they will trip on a phase loss.
If the motor starter installed correctly the contactor coil supply is run through the NC contacts of the overload relay so if it tripped the contactor will drop out and the feedback to the plc will show it correctly as not running.
If the motor is grounded again the motor overload or the short circuit will trip. If it is absolutely necessary that you know that there is power to the motor / the short circuit protection is not tripped then you can get breakers with aux contacts to monitor their status or install relays on the output of the breaker or the output of the motor starter.
One thing if the fan is connected to the motor through a belt and pulley arrangement then you will need to monitor the fan its self, if the belt breaks the fan will not be running while the motor continues to run normally. You could monitor the motor current, but that would require you to know the current draw on each fan in normal conditions. And set it up to trip accordingly. But keep in mind the motor current may change if the fan intake or output changes the airflow ( motor load is a function of the flow) so you may find yourself having to constantly adjusting things to keep it working.
The only way you could have a motor failure that wouldn’t trip the overload would be if at least 2 of the motor leads opened at the same exact time.
I try to use the KISS method where possible (Keep It Simple Stupid) no point in making things more complicated then need.

If you monitor the motor current and/or power, you will know if there is no load from the fan...
However... I would also recommend an airflow switch.

Current doughnut transformer to measure amps. Auxiliary contact used for motor start check logic.