Induction motors are designed to provide enough cooling air to keep the motor at a safe temperature in a range of ambient temperatures. If you run them on a VFD at low speeds, depending on the motor and the load, you can get to the point where the motor no longer has the required cooling air flow. (There is more documentation on larger motors, 25 hp and up than there is on small motors.) The motor also has less available torque at low speeds, is more inefficient further increasing its temperature. If you monitor the temperature of the motor, you should be safe. You should be able to place your hand lightly on the motor for 5 to 10 seconds. If you can't, it is getting too hot. Either speed it up for more cooling, or shut down until it has cooled.