Inverters can be adapted from stall right up to CHP ranges, only design limits are feedback device and proper cooling aspects to the machine load profile. Some applications require extended low speed performance. Normally the variable frequency drive (VFD) supplier indicates speed range capability based on type of VFD and feedback device required.
There may be some confusion between frequency and motor speed.
Open loop, typical speed regulation is plus or minus 0.5%, which means about 9RPM over the range of an 1800rpm motor, meaning that a frequency output of, say 6hz, could give an RPM output of between 171 and 189 rpm.
Closed loop, that error shrinks to, typically, 0.01% of full range, or about 0.18 rpm.
VFD frequency range is 0Hz to 400Hz, you can set whatever you want. The use of a closed loop system with a typical VFD nowadays should let you attain 100% torque or more at zero speed.
But Whenever you run a motor at low speed the problem that will occur is that the motor will overheat, but there is a solution for that if you have a motor where there is a separate fan to cool the motor and the fan is not connected to the shaft of the motor. Therefore you can lower the speed to 10 Hz and even less. So go on and find your application frequency / speed limits.
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