[ZeroBacklash]

I need to run the spindle at low speed around 5 to 10 rpm. So I was thinking for going in for a 3phase, 8 pole motor and then add a gearbox to further reduce the rpm.

I need at least a 2.2kW motor here. I need to drive this not as a spindle but as an axis because I need position control. The motor will be running continuosly at 5 to 10 rpm.

My questions are:
1. Is a AC motor + gearbox + VFD + encoder a good fit for this application. I mean what would be the min, max speeds I can safely run the motor without overheating issues.

2. What about dc or ac servo motor or even a bldc motor.Are these well suited for low rpms / high torque. Or do I even need a gearbox for these too.

Thanks in advance.

[ger21]

Get an AC servo with the appropriate power. No gearbox required.

https://www.automationdirect.com/adc...m_(Med_Inertia)

[ZeroBacklash]

ger21

Get an AC servo with the appropriate power. No gearbox required.

Good choice but pricey. (package coming to $2000)
How about DMM AC servos their drive + ac servo is around $600.

[Jim Dawson]

I need at least a 2.2kW motor here. I need to drive this not as a spindle but as an axis because I need position control. The motor will be running continuosly at 5 to 10 rpm.

You describe two different conditions here. A 3 phase motor / VFD combination would work fine for continuous operation, but for position control you would need a servo or stepper.

If you geared down any standard 3 phase motor with a 150:1 gearbox it would get you into the RPM range you need with enough torque to twist about anything into a pretzel. You say you need a 2.2KW motor, more importantly what torque do you need? A 2.2KW motor geared down 150:1 would output more torque than my Diesel truck engine is rated at.

My questions are:
1. Is a AC motor + gearbox + VFD + encoder a good fit for this application. I mean what would be the min, max speeds I can safely run the motor without overheating issues.

2. What about dc or ac servo motor or even a bldc motor.Are these well suited for low rpms / high torque. Or do I even need a gearbox for these too.

Thanks in advance.

Again if you need positioning then a servo would be best option. Most servos will output near 100% torque at 0 RPM with a pretty flat torque curve through the RPM range.

[ZeroBacklash]

Again if you need positioning then a servo would be best option. Most servos will output near 100% torque at 0 RPM with a pretty flat torque curve through the RPM range.

How about a Automation Direct VFD GS3 series with encoder connected to a 3phase motor. Will this give sufficient accuracy to do position control?
My torque requirements are minimum 100Nm.
Unfortunately DMM servo's are not meeting that They have max torque of around 12Nm.

[Jim Dawson]

How about a Automation Direct VFD GS3 series with encoder connected to a 3phase motor. Will this give sufficient accuracy to do position control?
My torque requirements are minimum 100Nm.
Unfortunately DMM servo's are not meeting that They have max torque of around 12Nm.

Unfortunately no, I have a GS3 and encoder on the 7KW spindle motor on my lathe. I thought I might be able to get it to position, but the GS3 is just too ''loose'' to make that work, I played with it for days trying to make it work. When I can find one at my price, I'm going to replace the spindle motor with a 10KW servo.

Since you only need 10 rpm, a 10:1 gearbox would give you 120Nm, to be really safe go with a 15:1 gearbox and you would have 180Nm available.

[ZeroBacklash]

Unfortunately no, I have a GS3 and encoder on the 7KW spindle motor on my lathe. I thought I might be able to get it to position, but the GS3 is just too ''loose'' to make that work, I played with it for days trying to make it work. When I can find one at my price, I'm going to replace the spindle motor with a 10KW servo.

Since you only need 10 rpm, a 10:1 gearbox would give you 120Nm, to be really safe go with a 15:1 gearbox and you would have 180Nm available.

Now thats a dampener. I mean regarding the VFD. I was planning to use an 8 pole 3ph motor (already low on rpm) with a VFD and encoder and with the GS3 drive I thought I would get sufficient accuracy to do position control.

Is threre no way to use a 3 ph AC motor with VFD and get sufficient accuracy for spindle operations with position control even with a high resolution encoder?. How much accuracy were you getting from your system.?

[Jim Dawson]

Now thats a dampener. I mean regarding the VFD. I was planning to use an 8 pole 3ph motor (already low on rpm) with a VFD and encoder and with the GS3 drive I thought I would get sufficient accuracy to do position control.

Is threre no way to use a 3 ph AC motor with VFD and get sufficient accuracy for spindle operations with position control even with a high resolution encoder?. How much accuracy were you getting from your system.?

I am not able to position at all. I am using a Galil motion controller so I have a lot of control over the way the system reacts. Thus far, no matter what settings I use in the drive or in the motion controller I can not control any positioning move. I can make it run at a constant speed or even programmatically vary the speed from about 5 to 5500 RPM and it's rock solid at any set speed. But any time I adjust any PID parameter above zero I lose control, I tried every trick I know and I've been doing this for about 25 years. The drive just does not react to speed change commands fast enough to act as a servo positioning system.

One thing I have thought about doing is adding a DMM servo to the spindle drive to use for low speed positioning for mill/turn functions and add a electro-mechanical system to clutch it in when needed, that is one way I've see it done. I have 1.8KW DMM servos on the axes, live tooling drive, and turret drive. Been very happy with them. But what I will probably do is just install a 10KW servo motor and be done with the problem.

In your case, a servo with low backlash gearbox would be the ideal setup. Low RPM and positioning resolution in fractions of an arc second.

[Jim Dawson]

The AC servo, while slightly more expensive, would give you more control options and perhaps greater accuracy. If I were building this machine and had the budget, I would go with the servo option. The most expensive part of the servo system would probably be the gearbox, required in either case, the servo and drive is relatively inexpensive. As I recall the 1.8 KW DMM servo and drive are in the US$600 range. The low backlash servo gearbox is in the US$1200 range from Automation Direct, there may be other options.