IMO VXSM75-1 Instruction Manual page 76

(2) The motor rotates but the speed does not change.
The change in the rotation speed of the motor is
also small in the following cases.
— "F01 "Frequency command 1" and C30
"Frequency command 2" are set at "3" and a
signal is input from both of control terminals
12 and C1, and there is no change in the
sum of them.
— The load is excessively large and the torque
limit and current limit functions are activated.
Troubleshooting 7-6