How to drive servo motor (newby Q)

[K Battenbough]

Hi Folks

Im looking at buying an ac servo motor to replace the noisey hydraulic varispeed system on my lathe and was wondering how would I drive it? All I need is start/stop foward and reverse, it is a 3 phase motor and I have 3 phase power in the workshop, do I simply wire it to a vfd? im not sure of the layout of things so could somebody describe the connection from wall socket to the motor?, Is it as simple as wall socket to vfd to motor? are the controls on the vfd? the motor in question looks like it has an encoder on it which im guessing would become redundant as my lathe is manual, im totally in the dark when it comes to things like this and would appriciate any help.

Thanks
KB

[Al_The_Man]

I would avoid attempting to drive an P.M. AC servo motor from a VFD, I tried it with less than stellar results, I have not heard of anyone yet doing it successfully.
I would put a three phase motor on the lathe, unless there is already one on there, and feed it with a VFD.
If you want to use an AC servo or BLDC, you will need a suitable drive that will work in the velocity or tach mode, and possess the right commutation feedback signals that you require with a AC servo.
Some drives will accept an encoder to be read as a tach.
Al.

[TOTALLYRC]

I am with Al on this one, in the fact that I would go with an VFD and a three phase motor. I would also recomend a sensorless vector VFD as they have better low speed torque than a Volts/hertz drive.

I am extremely happy with the hitachi sj200 on my mill and will be getting one for my new cnc conversion lathe as soon as I can find some cash floating around.

There are controls on the face of the vfd but setting up a remote control station is the better way to go, this way the VFD can be away from the chips and the controls can be located exactly where you want them to be.

On my current lathe I have a medium radioshack project box with a potentiometer, and several switches. The pot controls the speed. I have a pushbutton for stop and one for start. I have a toggle switch for fwd/rev, plus a toggle for run or jog which works with the start button. Jog speed is preset in the drive parameters on mine. There are several different ways of setting up the operator sation depending on how you want to do it and what the vfd has for options.

Since from your post the current motor is 3 phase and you have three phase power in the shop it is a simple as wall to vfd to motor.
The encoder may or may not need to be used as some vfd's will accept encoder input to help with the speed regualation
If you have more questions, feel free to ask.

Depending on the gearing of the spindle to the motor, you may not have enough torque at the lower speeds, which is why I suggest a sensorless vector drive. On the newer machines designed from the ground up to just have a vfd to control speed, they normally run more hp than you would expect so that there is enough low speed torque to do things like threading.

If the lathe has a back gear or multispeed gearing in conjunction with the varispeed, then you may be OK.
Mike