I have a blower motor for a furnace. It's a 3 speed, PSC, 1/4hp, 1050rpm, 115 volt generic HVAC motor. My problem is that even at the lowest speed, the blower is moving too much air.

Is there any way to slow the rpms of this motor down further below it's lowest speed setting?

You could change the blower (supply house will size one for you) or add a damper.

Could reduce speed via belt/pulley arrangement, or insert an Autotransformer in series with the Line Input of the PSC Motor - as this is the common method of speed reduction on the PSC fan Motors (Autotransformer wound into the Stator of the Motor).

Scott35