Altivar "SCF"

[ptonsparky]

This can best be described as a failure waiting to happen.
Your are lucky the drive is oversized so it is capable of having a load slammed on to it.

And I told them, you aren't supposed to do that. It won't last.

 

[Jraef]

The 'run' command is there continually. It's jumpered. There is no command from the starter.

When powered up the display goes from 0 to 60 in the 3 second ramp time.

OK, then what they are possibly doing is that the drives have a feature turned on INTENDED to prevent stalling, wherein it responds to a rapid rate of change in load by automatically overriding and decreasing the speed reference command until the drive can handle it, then ramping it back up as soon as it can. Different mfrs call it different things, I usually call it "Stall Prevention". That's not how it is SUPPOSED to work and you are dancing on the edge of disaster, but I can see it working under the right circumstances.

An analogy would be that you have your car engine running at max RPM with the clutch in and you slowly let the clutch out. In the analogy of course you have a clutch that can physically catch on fire, but electronically, it probably will not, so long as the instantaneous change in load is not to the point of the Short Circuit protection feature of the VFD (which is usually hard coded in the firmware and non-adjustable). So in this case with the smaller motors, the instant jump in current is within the VFD's capabilities and the stall prevention works, but with the larger motor, you exceed the trip threshold of the SC protection in the drive to avoid damaging the transistors, so it shuts itself off.

 

[ptonsparky]

So the installer, 14-15 years ago totally lucked out?

 

[jim dungar]

So the installer, 14-15 years ago totally lucked out?

Yes.

 

[Jraef]

So the installer, 14-15 years ago totally lucked out?

They may have done it on purpose knowing that it would work on the smaller motor, but in all likelihood yes, they probably lucked out.