The Speed-range ratio for Drives is 20:1, 100:1 or 1000:1. This is an indication of how slow the Drive can run the motor, before the motor starts to overheat.
Drive with a 1000:1 ratio is good, as compared to 20:1 or 100:1.
The Altivar 61 and Altivar 71 Range of Drives have the Speed-range ratio of 1000:1 in closed loop and 100:1 in open loop operation.
The speed in RPM (Revolutions per Minute) of a 4 pole, 50 Hz Motor will be 1500 RPM.
If the Drive Speed-range is 1000:1, Drive can run the motor as slow as 1.5 RPM (1500/1000 = 1.5) without overheating.
For 100:1 it is 15rpm and for 20:1 it is 75rpm.
The higher the Speed-range ratio, the slower the motor can be operated safely without overheating.