3 phase motor monitoring

[xJAyx]

Can a single phase current monitor be used to determine if a 3 phase motor is running? I don't need an analog indication, only running or not running. Under normal conditions, I am confident that the single phase current switch will be fine, but my concern comes if I blow a fuse on one phase. It's an old panel so the motor is fused individually on each leg.
What will happen if I blow one phase? Will the other phases blow from the load change?

 

[dpc]

Not necessarily. Sometimes the motor will continue to run in single-phase mode until it overheats and fails. Sometimes the overload relay will trip the motor.

Normally, motor running is determined by the status of the motor starter contactor. Auxiliary contacts from the contactor indicate if the starter is on or off.

The current monitor would provide a backup to this. If the fuse blows on the phase being monitored and you get indication that motor is off, what is this information used for?

 

[xJAyx]

This will be used for confirmation that a fan is running in a purging application, due to the age of the panel a contactor feedback is not reliable enough because of a lack of auxiliary indication from the fuses.

 

[dbaird]

Check out this website:

http://www.loadcontrols.com/products/products.html

They have some relative inexpensive power sensing products for motors.

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.

 

[dpc]

I'd probably think about an air flow switch for verification of a purge blower or fan being operational. Sometimes the motor runs but the fan doesn't.

But if the panel is so unreliable, maybe efforts should be focused there.

 

[itsmoked]

I'm with dpc. If this is a fan check for safety purging you need to monitor the actual parameter of interest, the air flow. Fan blades can be tossed, belts broken, dampers closed, errant plastic bags, etc.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

A differential pressure switch would typically be used for your application to insure that the enclosure has positive pressure.

 

[xJAyx]

Thanks everyone so far. Using an instrument in the process isn't an option here. Normally we use an encoder type sensor, but for this application we can't install it. The blade is connected directly to the motor shaft and it's not set up as a blower, it's a heat and process gas circulation fan. It's not a safety rated check (otherwise I wouldn't even bother trying to monitor only one phase) and the panel reliability isn't an issue, it's a system UL rating thing.

 

[xJAyx]

I should rephrase the reliability point, the system is perfectly reliable, but feedback from the contactor would not provide a positive indication that the fan is running because the fuses may be blown, but the contactor would still be pulled in.

 

[dpc]

Normally the control power for the contactor is taken from the incoming power, so if the fuses blow, the contactor is going to drop out.

 

[ScottyUK]

Unless it's the third phase which blows. In my part of the world 'stuff happens' most days.

Have you considered a fuse monitor?

http://library.abb.com/global/scot/...1dd65257115003df8e1/$File/1SCC314001C0201.pdf

Sorry about the mess the link is in - anyone know how to avoid this problem?




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If we learn from our mistakes I'm getting a great education!

 

[fangas]

Scotty,

Try

http://WWW.tinyurl.com

ED

 

[ScottyUK]

Thanks - I'll give that a try.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

The three phase monitors used by the refrigeration industry are about the same price as single phase monitors. For a modest price you can get over current protection, voltage unbalance protection, short cycle protection, (if you need it) power contact failure protection and a log of the last 15 or 25 trips.
Or, use a sail switch as dpc or itsmoked suggested. You can fabricate one from a few square inches of thin metal of a type that will survive your environment and a limit switch. Cheap rugged and dependable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

If your system runs in a stable way, meaning not much normal load variance, you can use an over-under current monitor on one leg and tell if you lost a phase on any of the 3. Let's say you put the sensor on phase A. If you lose Phase A, you get no ( or extremely low) current on that phase. If you lose Phases B or C, you will get higher current on Phase A.

More accurate than that is to use a Power Factor monitor on one phase in the same way. Current fluctuates with voltage, power factor does not.

 

[khatri]

I don't know specifications / ratings of your motor and fuses installed and if your power system is neutral grounded. IEC 60269 specifies guidelines for fuse applications.

If you install properly selected motor rated dual rating fuses, these will protect your machine on single phasing by blowing one of the remaining two fuses, since the current in remaining two phases rises to almost twice.

Govind
Electrical Engineer
FFC, Pakistan

 

[jraef]

Hmmm... It might happen, but in my opinion that is not even remotely reliable as a method of detecting a single phasing condition for a 3 phase motor. It is entirely dependent upon the motor loading at the time the single phasing condition occurs. In addition, in many countries fuse selection is governed by installation codes which may or may not allow selection in a range that would detect a single phase condition in time to avoid motor damage. I would not recommend that as a viable option.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

BTW Welcome to Eng-Tips khatri.

Thank you for participating.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LaffeyDD724]

xJAyx what is the motor voltage?
What is the supply voltage?
Are they the same?
Are you trying to determine if the fan motor is running or are you trying to determine if it is single phasing or both?
Tom

None of us is as smart as all of us.

 

[LaffeyDD724]

Check out this URL and see if these devices will help solve your phase loss concerns. (Single phasing) They appear to be very economical.

http://www.squared.com/us/products/...88A85256CB60056D979/$file/phaselossrelays.htm

Tom

None of us is as smart as all of us.