Counter-Rotating Fan Motor

[ilostmydrink]

Hi, all!

I have what would seem to be a relatively simple problem. What I'm having trouble with is justifying my current conclusion.

We have a Reliance motor (fan blower motor) with the following nameplate information:

• Horsepower: 150 HP
• RPM: 1800 RPM
• FLA: 171.27 Amps
• Hz: 60 Hz
• Voltage: 460 V

Without getting into the gritty details, I have been asked if starting the fan with the fan counter-rotating at 33-RPM will trip the protective relaying. The relaying assumes Locked Rotor current for 4 seconds while the relaying is set at 5 seconds. As the 33-RPM will not generate more than a negligible amount of voltage determining what additional time is required to start the fan is the answer to the question. Unfortunately, this is not the type of application that I can eyeball the curve and say it is good without sufficient justification (there's no way the 33 RPM will add an additional second of starting time).

My question to the community: what is a good resource for formulas related to this problem? I know converting horsepower to acceleration is inaccurate, at best. I'll be reaching out to Baldor as well.

Thanks for the help!

[buford]

Why not just stop the fan from counter rotating, either mechanically or can you not make the drive hold it?