Controlling speed of motor using VFD

Question

Motor is 0.75Kw, 3Ph, 400V and the rated RPM is 1380.

How much speed I can reduce using the VFD

Can I operate at 300rpm while it is designed for 1380rpm with worm gear assebly.

Answer 1

For 1380 RPM rated speed at 50 Hz, the motor must have a synchronous speed of 1500 RPM. Synchronous speed = 120 x f / poles. The number of poles is an even number, usually 2, 4 or 6 for a small motor. The possible values of synchronous RPM would be 3000, 1500 and 1000. Rated speed is synchronous speed minus slip. Since slip is usually 1.5 to 4 percent of synchronous speed, the usual speed would be about 1440 to 1480 RPM. Your motor is close to that at 1380 RPM, but 120 RPM slip makes it an eight percent slip motor. That would mean that it is a high slip motor rather than a standard motor. High slip motors are usually selected for loads that have a very high starting torque. That may be of some concern when using a VFD. You should certainly select a sensorless vector VFD rather than a V/Hz control VFD. It should be a VFD designed constant torque operation rather than for fan and pump duty. It might be a good idea to select a VFD rated for a 1 kW motor.

For 300 RPM the operating frequency would be between 50 X 300/1500 = 10 Hz and 50 X (300 + 120)/1500 = 14 Hz. That should be no problem for any VFD. Motor cooling at 22% of rated speed may be of some concern. Since it is a high-slip motor, it may have a larger than normal housing an enhanced self-cooling because of the increased slip losses. That might be enough to allow it to operate at the low speed without overheating. If the low-speed operation is not continuous, that would also help.

For VFD operation with an unusually high starting torque, it is generally better to select a standard motor of the next larger size. Using a high slip motor does not provide higher starting torque when using a VFD. Another approach is to use a 1000 RPM motor and operating it between 15 Hz and 75 Hz.

Answer 2

Yes you can. The output from the VFD is input frequency independant and can vary between 0 rpm ( external cooling required) and some 10 to 20% over the design speed of the motor. Modern VFD's allow you to make a fully automated setup. However if the requirement become very specific manual setup might be needed.

Answer 3

It is not reccomended to spin the motor bellow 50% rated speed, because it will overheat. You can mount an additional blower fan to continuosly run the motor at low speed.
The VFD with V/f control won't work bellow 15Hz (50hz nominal), you may need vector contol VFD to run down to 5Hz - but this is also some comercial data, it is reccomended that you don't go lower thazn 15Hz in either way.