Are you using the inverter in vector or v/f mode (see section 5.1.2 of manual). I think it needs to be in v/f mode (n002 = 0, which is the default), however because this motor is not designed to run below 1500rpm some of the other defaults are inappropriate. You'll need to set values n011=205, n012=173, n014=53 , n016=53, n017= 127