Does anyone know if a motor's torque is dramatically reduced if it's controlled by a VFD and is set to run at a very low speed?

Here's the situation: We have a couple of 15 hp roof top process related exhaust fan motors that are VFD controlled and set to operate at about 30% of full speed (18hz). When the outside temps recently dropped to -10 degrees here these motors wouldn't start. After we warmed the fan bearings with a torch they started fine. We increased the fan speeds to 50% and haven't had a problem since.