View Full Version : AC or DC Servos?
07-16-2010, 10:07 AM
AC or DC Servos?
I've an old Bridgeport series one cnc that needs redoing. I'm trying to develop a plan of attack which is difficult because to develop a plan one needs a decent knowledge level of the different options and I'm at the begining flat part of the curve knowing almost nothing :(
anyway, in reading posts here and vendors web sites it seems like there's a big price difference between DC and AC servo prices
Can you guys explain the advantages of one over the other; what performance advantages are there to AC to justify the higher price?
07-16-2010, 10:38 AM
What controller are you using?
Many of the popular step/dir type drives are still principally DC, AFAIK.
A couple of obvious advantages of AC is no brushes and smaller frame, due to the PM rotor and the windings on the frame, which can be cooled easier.
If using analogue ±10vdc control, there are many manuf such as AMC, Copley and AeroTech.
If mix and matching motors and drives, it is much easier to make a match with DC.
AC or even DCBL have some kind of commutation required which may be harder to find a compatible match between drive and motor.
DC can still offer good performance, brush wear should be checked every so often depending on usage, to prevent costly motor replacement.
One other thing to keep in mind if going with DC and ±10vdc control, DCBL drives can be used just as effectively with DC motors, this allows converting to DCBL motor if a DC motor fails down the road.
07-16-2010, 12:15 PM
What controller are you using?
thanks for the insight....i'm so much at the beginning of this journey i don't know what control options are available let along what to use....I've been trying to read up, was on Kelings site and was puzzled by the huge price differences - 799 for an AC servo but 1159 for a complete DC 3 axis kit for 1200 so thats where the Q came from.
At 1100 oz, is the enough for my bport? this kit seems reasonably priced:
Then there's trying to rationalize the difference between the centroid 11k kit and the Kelings kit. i know with the centroid one everything is done for you, but still with that big a difference i strongly suspect there a large performance difference that my newbiness isn't picking up on. Not that i want to spend 10k, but would like to know what i'm missing
I'd love to get some opinions and suggestions on what direction to go - where they best performance/cost balance is.....unlike my usual frugal shop approach I know prepared to spend some dollars.
Its a frustrating process. Now's when i need know it all to make decisions; its also when i know the least
07-16-2010, 01:43 PM
For a home shop, unless you are doing very high speed machining, The DC option should be fine.
Motors of the same torque, generally perform the same at average maching speeds.
Traditionally the B.P./XLO style knee mills used around 4.5 Nm SEM motors with 2:1 reduction.
These are fairly beefy motors.
So 1100oz-in (7.76Nm?) should be ample.
Another advantage now with Torque mode drives you do not require a tach on the motor as in original drives.
What are the motors like you have on now?
It is possible they and the power supply could be reusable?
07-16-2010, 02:48 PM
they're the original steppers.
The power supply might be reusable but the controller's dead right now.....first decision is do I try to get it working as is? Everything i read suggests this not a good idea, or at least not a great longer term solution, vs putting in a new controller. If its a new controller I probably want to go servos rather than steppers (is the current thinking)..... after that a i get bit lost, so far i've found Kelings at 1200, Ajax at 6000 and Centroid at 11 or 12k.....and there probably many others.
Be aware that the quoted torque on Keling motors is peak and constant is only 1.6Nm
07-17-2010, 12:51 PM
If I had my druthers I would go for brushless servos. You can get brushless that will work with Step/direction inputs, step/direstion or analog inputs.
I have worked with brushed and brushless and the brushless servos give a lot more performance out of a much smaller package.
Step and direction only drives - Mitsubishi and Omron both sell this type of drive. easy to setup and they are easy to get tuned for your application.
Yaskawa sells drives that can be setup either step/direction or analog.
I have worked with all three brands - I liked the Omron and Yaskawa better than the Mitsubishi - I thought they were better made.
Automation Direct also has a line of Brushless servos - have never used them just saw them in their catalog and on-line. A bit lower in price.
The real nice thing with these drives is that they can be tuned and tweeked from a computer as well as from the front panel. I have had one application where the drive wanted to oscillate - just had to toggle the auto-tune switch on for a couple minutes and then flick it off and the drive worked fine from then on. No oscilliscope required and real easy to reset to default values if you screw up.
07-17-2010, 11:03 PM
I like brushless servos as well. Mitsubishis I have actually made a machine with. I have a couple Yaskawas as well but I have not done anything with them yet. Whats nice about the AC Servo packages is that a lot of them allow pulse input which will tie right into a parallel port. Mitsubishi drives that end with an -A or -AN and Yaskawa drives that have a P in the last part of the model have pulsing input options.
Omron drives are relabeled Yaskawas.
There are often sellers on ebay that sell packages of three drives, motors, and cables. 750watt is the biggest you could need on your machine. You could even use 400s. That still over 1/2hp continuous. And up to three times that for short periods.
But brushed drives work well too. I have gotten 400ipm rapids on my big mill no problem. I cant do that right now because of parallel port limitations.
Here is a comparison of different non-commercial servo drives:
07-18-2010, 10:12 AM
Thanks for all the input and help.
What happens if you go from say Mitsubishi or Yaskawa to what Macona calls non commerical like Gecko, other than price how would the differences show up?
07-18-2010, 10:23 AM
Gecko do not make AC drives AFAIK, only DC servo.
But they are probably cheaper than the higher end AC route.
As I already mentioned, DC also gives a bit more flexibility in motor choice for a given drive.
07-18-2010, 03:50 PM
Well, the gecko drive just sucks compared to the other DC servo drives out there. Tuning is a pain. You need a scope to tune them. Plus they do the dither approach to servo control where the motor bounces back and forth from encoder positions. This means the servos sit there humming when they are not being use. One other issue is they dont error out when they fault. They just loose position and you are stuck chasing down whats going on.
The other, newer DC drives are a whole lot better. Drives like the Dugong and Whale3 allow you to monitor and tune the drive while in motion with a PC.
The Granite drive will control brushless motors. And one of the drives will control DC motors as well.
What do you get by going with Mitsubishi or Yaskawa over these guys? You get a company that is not going anywhere for one thing. Any of these other companies could go belly-up at any moment. I had bought these boards called Pixies from one guy for my mill. They allow you to step/dir control analog servos. Work great but he quit making them. If anything happens to one of them I will be in trouble.
The commercial drives have full auto tune, variable gearing, and a whole slew of other features. Also you dont need external power supplies. Each drive has its own power supply. Just provide it with 120v or 240v (Depending on the drive) and go. Pretty much plug and play. The motors are also matched to the drives so these really is no configuration. Most of the newer motors use serial interface encoders which contain the motor info. Also with the serial encoders there is a battery backup that will monitor motor position even when the machine is off.
One thing about the Japanese drives though is they are configured for their motors only. They will not drive another companies motor. The American and European drives can be programmed in detail to run just about any motor. Drives like Allen Bradley, Aerotech, Pac Sci, Kollmorgen, etc.
But when you start mixing and matching drives you got to watch out. Drive voltages, position feedback. Does the motor have resolver feedback, DC Tach, encoder, abscoder, serial inc encoder, serial, abs encoder.
Thats why I recommend just getting a set of motors off ebay. It will save you so much time and headaches.
Here is a set of three motors, 750 watts each, a little overkill. Made by samsung but they might be made by Yaskawa.
07-18-2010, 11:16 PM
as Hood correctly points out, those Kelings are peak torque....once i compared apples to apples the price difference between DC and AC didn't disappear but sure shrunk
Macona that was a very helpful description, looks like it comes with breakout board and everything - if power supply is self contained basically all it needs is a computer? If I'm understanding it properly this looks like what Ajax sells for $5-6? great price, reading further i see the parts are used but tested, has anyone bought this particular package or from this vendor?
07-18-2010, 11:28 PM
I have not bought from them nor have I used samsung drives. It looks like they tear apart old semiconductor manufacturing equipment.
I would go through the manual for the drives and make double sure they accept pulse input control. Some drives will have indexing but no pulse or analog control, some will have a network bus system. It just varies depending on the application.
There is a good chunk of wiring to do still. These drives usually have separate inputs for the control circuitry and the bus power. You need an individual contactor for each drive as well as a circuit breaker for each drive. The manual should spec the breaker size. The contactor is controlled by the drive itself and opens the contactor in a fault condition.