SERVO (brand) 350 VDC variable reluctance motor wiring questions …

[Mike Braddock]

I am preparing to retrofit a 1995 SERVO brand CNC knee mill because one of the amplifier/motion control boards is damaged. Like many similar machines, SERVO no longer supports the controller and parts are not readily available. General info about the machine can be found here and info specific to the motors/controller can be found here.

My question concerns the suitability of the existing servomotors for this project. They are 350VDC 6A variable reluctance motors. The DC bus in the amplifier cabinet is 300 VDC. The armature has 3 coils, and each coil has a pair of wires that return to the amplifier (which is shot). I found enough info on VR-style motors to know they are (1) not very common any more and (2) fairly low performance. Their performance should be suitable for this retrofit, as the machine has acme leadscrews instead of modern ballscrews, so the peak velocities and accelerations will be intentionally low. I am just trying to figure out what type of amplifiers I should be looking for, or whether I need to back up and toss these motors in the dumpster too . Since most DC amplifiers have outputs for 3 motor phases, can I just tie one leg of each coil to the ground bus? I wouldn’t think this would work, but I’m not sure. Thanks for any input you can provide!

Mike

[Al_The_Man]

Originally Posted by Mike Braddock I am just trying to figure out what type of amplifiers I should be looking for, or whether I need to back up and toss these motors in the dumpster too . Since most DC amplifiers have outputs for 3 motor phases, can I just tie one leg of each coil to the ground bus? I wouldn’t think this would work, but I’m not sure. Thanks for any input you can provide!

Personally I would look for some 250-300v BLDC motors in order to use the existing supply, AMC make drives in excess of this voltage.
I would not risk a good drive by connecting the motor to the ground bus.
Just by connecting them together provides a star point. But then you will need commutation to turn them into BLDC.
Al.

[Mike Braddock]

Al,

Thanks for the quick reply! Do you know how the commutation is usually handled on variable reluctance type motor? There are two cables coming off the motor. One has the 6 conductors I was speaking of in the previous post (3 pairs of coil wires). The other has 10 conductors. I was assuming 8 of these would be for a differential encoder with an index pulse and two would be a tach output. Unfortunately, the manufacturer didn't include any real wiring schematics with the machine, so I was planning to use an o-scope to try to decipher the wiring. Now I know my original assumptions were wrong (3 motor wires for the 3 phases, 3 for commutation hall effects), so I'm wondering if 3 of the wires in the 10 conductor cable might be hall effects for commutation, or even if a variable reluctance motor uses halls for commutation? If it does, I guess that would imply the encoders are single ended as well.

I need to do more research and figure out how commutation is handles on VR-type motors. I completely agree that if there are no encoders for standard BLDC type commutation, it's not worth adding and I should look for other motors.

I really appreciate you help Al!

Mike

[Al_The_Man]

I am not all that familiar with Variable Reluctance motors, it has always been my understanding that they had more similarities to steppers, but lost attraction in favour of modern stepper technology. I believe, like steppers, they do not use any commutation feedback.
I would tend to use something a bit more predictable.
Al.

[Mike Braddock]

I haven't bought any amplifiers yet, so do you have any suggestions on a few good servo motors? I'm planning to either control it with a Delta Tau PMAC/UMAC I already have off another project or perhaps trying out the EMC2. Would you go AC or DC brushless? I haven't noticed any cogging at low speeds, so I think either type of motor should be fine. I do have some SIMIPOWER Powerblock amps that are rated for 200/240V but I think they would be overkill for this project. Thanks in advance for any input!

Mike