Three major problems exist with both conventional methods of
motor control and VFD control.

(1) When a motor is directly coupled and run at low speed it
becomes very inefficient. One could go with a high torque motor that is
significantly larger than the standard (including higher cost), and would have
the same results most of the time.

(2) Typical motors do not cool themselves well at extremely
low speed. The added heat buildup in the windings can cause premature motor
failure. Check with the motor manufacturer for more details.

(3) The other issue is load inertia, which is larger than
that of the motor. These effects increase greatly with a transmission system
that is sloppy. A speed reducer or gearbox should be employed to solve this
problem. It will match the inertial changes and increase the efficiency.