re: The "lie" in motor HP *may* be the max HP you can get from the motor before stall (which which would be a lie), which you might very occasionally use. Wouldn't think that would make sense for a shop vac. "
The way I heard it years ago is that the max-claimed HP of a shop-vac, etc. was actually achievable. It was the based on the max amperage the unit would draw if you jammed a screwdriver in the impeller. Just before the motor fried, you will have achieved maximum HP.
AC compressors are rated in RLA Run Load Amps and LRA Locked Rotor Amps. Of course when you're putting the freon in a system, you can watch the current draw increase as the compressor does more work. My favorite question to use to tax the mind of a neophyte is "Will the blower draw more or less current if the inlet is blocked off?"
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I was at an industrial plant that had a large pump on the water well -
10 HP or larger. The motor was on the surface with a shaft down the well to a centrifugal pump at the bottom. The plant engineer (actually a mechanical engineer) said when they first started the pump the motor current was too high, so he closed the discharge valve a little until the current was at the rated motor current. I was not happy with the idea and measured the motor current while I closed the discharge valve on a 4" submersible pump. He was right. Mechanical HP is basically flow rate times head pressure at the pump. Closing the valve raised the head pressure but apparently lowered the flow rate faster.
This really becomes apparent if you watch the current on an air compressor as the tank charges. On a 150PSI shop compressor the peak current is around 90-100 PSI, then it drops.
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