I have a Trane downdraft furnace in my barn. It gets its electric power from my 16KW Generac generator. Both are supplied natural gas direct from a gas well. Both were purchased used from an industrial products reseller, and installed by a qualified installer.

When I first connected my furnace and attempted to run it, I got mixed results. Igniter functioned perfectly, pilot lit after about a minute, gas valve opened, burners ignited with perfect flame, and the fan started about 3 minutes later(starting from ice cold). Unfortunately after about 20 seconds, the motor stopped for about 5 seconds then went into a very regular on/off cycle. The fan runs for about 2 seconds, not enough to achieve full speed, then quits for about 3 or 4 seconds. Meanwhile the burner remains lit, finally shutting off from what I presume is the high temp limit.

I reconnected motor direct to generator output, with exactly same results, going into an on/off cycle after about 20 seconds running. Thus the problem is the motor, not something else in the furnace. Do I need to replace this motor?

Before the motor, I would replace the capacitor(s).