View Full Version : Servo motor drive
Inspired by Macona's excellent guide to adding a servo motor to his Monarch, at http://www.practicalmachinist.com/vb/showthread.php?t=154967 I have an idea to do the same to a CNC lathe (Denford Easiturn) I have acquired as a box of bits. Hence a few questions...
1. Say the lathe originally had a 1hp (0.75 kW) motor. Should I be looking for a servo with the same power output?
2. Some motors I see do not quote a power output as such, only a torque. How does one convert between the two?
3. Servo drives above 1hp seem to require three phase input. I have 230v single phase available (in UK). Can I connect the output of a single-to-three phase inverter to the input of the drive?
Thanks for any input.
02-01-2009, 04:56 AM
The drives I have used do not have any phase detection circuitry in them so they all have worked on single phase for me. You may have to go in and set the max torque lower to stop the drive from sensing a low voltage condition and faulting out.
Another option is getting some more capacitors and add them to the input buss to compensate. It might be a good idea to feed these caps with another bridge rectifier to take some of the load off the drives main rectifier since it will be seeing more current than was intended. Of course these are all extreme conditions. I set my drive to 70% max torque and have not had any issues.
As for sizing the motor its going to depend on what you can find. HP is a function of torque and velocity. So if you have a 3k rpm rated servo to replace a 1800 rpm motor you will need to belt it down to get the same torque as the old motor. But one thing different with servo motors vs an induction motor with a VFD is they can be ran way past their rated current for a short time. That time is only limited by cooling. And thats why dedicated spindle servo motors have built in fans with air channels through the laminations.
Most servo motors have a darned near flat torque curve from 0 to full rated output and drop past their rated output. Most newer 3k rpm rated mitsubishi motors will run to about 4.5krpm. Thats why many times servo motors are rated by torque n•m or kgf•cm, there is rated torque and max torque. On the 2kw motor I have the rated torque is 9.5Nm and max 28.5Nm at 2000 rpm. The same motor in a 1000 rpm version is 19.1 rated 57.3 max, and 3000 RPM 6.37 rated and 19.1 max. So you can see how the HP (Torque x Velocity) remains constant for the motor frame size, just the pole count differs in the motor.
Lazlo here on the board has done a Servo conversion to his Bridgeport. He also used mitsubishi motors.
I have a 3.5kw motor sitting on a shelf to put into my lathe. Its the same face as the old 2kw motor but it is actually shorter. I have not been able to find the dedicated mitsubishi servo drive intended to be used with the motor so I found an Aerotech servo drive that can be configured to drive most any motor. I just need to replace the mitsubishi encoder with an encoder with hall tracks to send commutation info back to the drive. The drive itself is a pretty "dumb" drive as it has only analog interfaces and does not have the positional feedback like the "smart" mitsubishi drives. The aerotech drive does have a tach or encoder input so you can run the drive in velocity mode instead of torque mode.