ASD with OL problems

I have a customer with a toshiba S7 ASD on a 3HP 3PH motor. They have 1PH input voltage available. They were getting OL everytime they tried to run. I found the gear box connected to motor was binding up. While monitoring the amps it read 150% with gear box attached. Once disconnected the motor ran fine. I was assured by Toshiba that derating of drive for 1PH input was not needed as motor was larger than needed for app. This customer wants the end result to be 2 to 4 RPM (at final device). I was concerned with the original gear box that was 50-1. This would of put the actual motor RPM (nameplate rpm = 1800) at 100 to 200 rpm. I was concerned that the motor would surely overheat at this low speed as no external cooling fan was added. Bottom line is they now have a 250-1 gear box which should put us at an acceptable 500-1000 RPM at the motor. My question is, based on the info provided, should I be concerned with anything else torque related? They said the motor stopped to quick and sheared the key at the shaft. My plan is just to increase decel time. Could I be missing something?

The only other thing to check is if the load is constant torque or variable torque. Withconstant torque loads most VFDs are one frame size larger because of the higher current required at low speeds. If the motor is sufficiently oversized, this won't be a problem.

Thanks Tom. I believe the drive is a constant torque drive but I will confirm.

With that much ratio in the gearbox, the motor will hardly see the load. Gearbox friction will dominate so variable torque or constant torque will hardly make any difference.

Since I gather that this application is installed and working, the only real measure is motor amps at running speed and maximum load. If the drive has got that covered, the rating of the drive doesn't really matter.

I don't mean to be trite with this but, if this job is running, it's rather easy to judge where you are at.

Incidently, I meant to also say that changing the gearbox ratio was exactly the right thing to do, given the circumstances. In fact, I would have spec'ed an even higher ratio, the object being to have the motor between 60 and 80 Hz at highest machine speed.

But, at the price of gearboxes, you may very well have saved some money on the gearbox and still got the motor into an acceptable range.

Good Job!