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Choice of VSD and motor.

Guest Anonymous

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At home I have a metal turning lathe driven by a 550w output, 50hz, 4 pole (1400rpm) single phase capacitor start induction motor.


Instead of changing the belts to change the lathe speed I want to install an electronic speed control system.


I gather from reading the posts that a 3 phase motor controlled from a single phase in three phase out VSD is the way to go.


I need to keep torque at the very low speed settings (i.e 40rpm).


Do you think I should I buy a 550w 3 phase motor set up (exact equivalent) or should I go for a more powerful set up?


Also I get a lot of vibration from the cheapie motor. Will a 3 phase motor be smoother do you know?



Hope you can help.


Thanks for the forum.




Steven Vine

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Hello Steven


Yes, the most popular option is to replace the single phase motor with a three phase motor and a single phase in, three phase out inverter.

The potential difficulty that you may have here, is that below rated motor speed, the motor wil only produce rated torque. i.e. the effective power rating of the motor drops with the speed. If you use a mechanical speed changer, the shaft torque increases by the speed changer ratio. i.e. with a 10 : 1 gear box, you can get ten times the rated motor torque output.

One solution is to oversize the three phase motor. Additionally, if you have some mechanical speed reduction and over speed the motor to get you normal output speed, you will get a higher output torque. i.e. use a 3 : 2 reduction and then operate the motor a 50% overspeed for standard output speed, you will get a 50% increase in torque at low speeds.


Best regards,

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