3-phase monitoring

I got an application with a 37kw drive that needs an extra layer of output protection. This 'smart' drive single phased and over time burned up $12k worth of hoss motor and caused a bunch of downtime (because the motor still ran but growled and nullified magnetic encoders we were baffled as to the cause). I'm looking for a simple inexpensive monitor to make sure it doesn't happen again. My first thoughts is 3 CTs running 4-20mA to a Micrologix 1100 analog card but thats a lot of firepower for something simple like this.

Anybody have monitoring set up in an alternative way?

I learned the hard way too.. I install these on all 3 phase equipment now
http://www.automationdirect.com/adc...l_Relays/Phase_Monitoring_Relays/PMRU-1C-480A

Do these phase sensing devices work reliably on the PWM output from a VFD or only on the line power?

@bernie
I've bad experience with normal motor protection relays (bimetal) between the VSD and the motor. They trip unexpectedly, probably because of heating generated by the overharmonic currents.
The VSDs i've used have had built-in protection of phase loss and overutilization. But it is wise to use motors with PTC/NTC/KTY to have warning or trip if the motor is to hot (covered fan inlet or low speed).

Kalle

bernie_carlton said:

Do these phase sensing devices work reliably on the PWM output from a VFD or only on the line power?

Click to expand...

Looks like the answer is NO. http://www.automationdirect.com/static/specs/prosensephasemonitor.pdf
But at <$70, I have bookmarked the item for my next project, it's a lot cheaper than all the methods I have used in the past.

We use Load Controls UPC power monitors to read the power from 3 phase motors run by VFDs. We get a 4-20mA signal proportional to motor power.

We haven't utilized them as overload protective devices, but they do offer that functionality.

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

Why not use a standalone overload relay? I have done this in the past with excellent results. Allen Bradley, Sprecher & Schuh, Siemens all make overloads of this type. They detect phase loss as well as having thermal overload function. Use the relay to turn off the drive when it gets out of limits.