Invertor Current Monitoring Relay

What sort of device is the motor driving? What features of the device and what it is connected to might be easier to use to infer low inverter current, irrespective of frequency? What are the characteristics of the device that the motor is driving; for example, if it operates at <5% top speed, will the low current at that speed require a trip and how does it compare to a low current at 100% speed?

Motor is direct drive to a fan. Once commissioned fan will run at fixed speed which is normally in the50 to 80% speed range. When we use a DOL fan we can determine when a fan blade or part of a fan blade has gone AWOL using an undercurrent relay. The fan rotation is also monitored by a rotation relay and a DP switch is fitted across the fan. We have had problems with using a DP switch due to the gases being moved contaminating the seals.

Fan power, and therefore current, varies with something like the cube of the tip speed. So a current trip set-up for 50% speed might not operate at 80% speed for the same AWOL fault.

What is the nature of the contaminant gases? What size is the duct each side of the fan?

I agree with PWSlack, a current monitor on the output of a VFD is not going to mean much, in all likelihood you will have a lot of nuisance tripping. What level would you set it at and how would that relate to current at any given speed? Let's say you set it at 30% curreent. An exterior relay would have no way of knowing if 30% current on a fan was at full speed or half speed, yet at half speed, 30% of full speed current may actually mean an overload! (At 50% speed, current would be the cube of the speed reduction, to .5 x .5 x .5 = 12.5% current)

The VFD itself is going to be much better at detecting at reacting to a loss of load (assuming that is what you need to know) because it can be programed to detect a RELATIVE loss of load, since it knows what speed it is driving the motor at. If you want the current monitor because of the possibility to run the motor DOL if the VFD is off-line, then just locate it in the isolated portion of the DOL circuit, so it will not be exposed to the PWM output of the VFD.

But if you insist on pursuing the folly of putting a current monitor on the output of a VFD, there are some that are rated for that. Veris Industries makes one called a "Hawkeye" current monitor that has a PWM capable CT.

http://www.veris.com/Category/Current-spcMonitoring/Variable-spcFrequency-spcSensor.aspx