What will be minumum safest speed of induction motor?
Is there any standard minimum speed details avaiable?

Hello rseee

The minimum safe speed is zero!!I gues that you are referring to slowing down a standard induction motor with a VFD.
The answer is not very definite as it depends on a number of factors.

As you slow the motor down, the cooling reduces dramatically. - The fan becomes much less effective a lower speeds.

If the motor is operating at full rating, it will need to run close to full speed to get sufficient cooling, however many loads reduce the torque requirement with the square of the speed, so part speed is not such an issue.
At any speed, there are two major losses in a motor, iron loss and copper loss. If the shaft load is reduced, the copper loss reduces with the square of the current reduction down to close to magnetising current.
The iron loss is independent of the load coupled to the motor.

If you operate a motor under open shaft conditions, it will still generate heat due to the iron loss plus the copper loss causd by the magnetizing current.
With small motors, the magnetising current is high and the iron loss is high also.
This can result in the motor overheating at low speed, even with no load on the motor shaft.

With small motors, I would recommend that you do not operate below 15Hz.

If you need to have a wide speed control range, you are better to select a slower motor so that it operates above line frequency at full machine speed, for example run the motor ovwer the range 20 - 80 Hx to get a 4 : 1 range rather than 15 - 60 Hz, or 12.5 - 50Hz.

If you do need to operate the motor at low speed for extended periods of time, you should apply auxiliary cooling from an external cooling fan.

Best regards,

Thanks for the nice reply

Is there any limitations in VFD parameters like minimum speed should not be set as zero?