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Author Topic: Noobish Question On Motors & Drivers  (Read 12163 times)

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Offline Hood

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Re: Noobish Question On Motors & Drivers
« Reply #20 on: April 29, 2008, 07:27:32 PM »
You may not get the speed you want with steppers, depends what you want I suppose :D
 I have steppers on my Bridgeport and they are great but on my Lathe I have AC Servos, mainly because its a big brute and steppers wouldnt be big enough but also because I want the speed, especially the acceleration you can get with servos. I have just been testing out a 5.5KW AC Servo that I am putting on the Mills spindle, had it going 4000RPM CW and put it 4000RPM CCW, had the acceleration set to 160,000RPM/Sec and it did the reversal in 0.05seconds with no positional error reported in the software :). The mill I am doing at the moment also has AC Servos on the axis and dont think I would go back to slow steppers again after seeing what servos can do. Saying that however steppers can do a great job if you are not in a hurry and as has been said if they are sized correctly you wont have problems.
Hood

Offline Sam

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Re: Noobish Question On Motors & Drivers
« Reply #21 on: April 29, 2008, 07:58:50 PM »
Quote
comfortably run a 1" (25.4 mm) 4 flute endmill through carbon steel at a reasonable clip.
Yeah, that would definitely go beyond my description of a hobby machine, if theres any depth to that cut. The price for a good capable spindle alone would cost more than my entire setup.  :) Lets see those models you speak of!  8)
"CONFIDENCE: it's the feeling you experience before you fully understand the situation."
Re: Noobish Question On Motors & Drivers
« Reply #22 on: April 29, 2008, 09:42:17 PM »
I would like to qualify that and say I WOULD LIKE TO BE ABLE :)  I don't expect I can get there, but it is my target, I will be more than happy to see the machine milling aluminum with a 1/2 inch single flute, but I want to maintain my target of the 1" in steel.  I have to set a high goal, and if I fall short it will still do everything I want it to.

I will get my models into a some decent eDrawings and post them this weekend.

Offline stirling

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Re: Noobish Question On Motors & Drivers
« Reply #23 on: April 30, 2008, 06:20:59 AM »
trekky - don't be too easilly seduced by size - big is generally better - but not allways. Nothing comes for nothing - everything's a compromise. Large stepper motors can introduce problems such as difficult tuning due to large detent torque. etc. etc.

Question: why do we need lots of power in our drive motors? - answer: to push the tool through the stock? - WRONG! - We need power in the spindle for that. A good spindle will go through stock like a knife through butter - a bad spindle will never go through it no matter how hard we push. We need torque in our drive motors to accelerate the mass of the gantry and we need power to maintain that torque accross our speed range.

But what speed? - well - the right speed for the job, our spindle and cutter (with maybe some to spare - for luck!). OK so you have a rig that will travel at a Km/sec - but your spindle and tool will only cut at 1mm/hour - that was money well spent then!!!!

That said, Hood is absolutely right - accel is also hugely important - you can never have too much, because what we'd actually like is instant speed, i.e. infinite accel. I'm no expert on servos as I've said - but I think I'm right in saying that at lower speeds, steppers will out accelerate servos by a gazillion miles (per sec per sec ;D) because they have more low speed torque. To get torque out of a servo - they have to be spinning fast. - so what happens at low speeds? - in order to work - you have to gear them down. That makes me think I'd want servos for either really high speed applications or really heavy rigs - course - that's just my opinion.

So..... read up - then do the maths - it may turn out that you'll have way more power than you need from 360 inoz Nema 23's. It may turn out you need 800 inoz Nema 34's OR it may turn out you need to buy Hoods servos  ;D

Tip for the day - learn about inductance and what it means to your motors - it's far more important than a seductive holding torque. Remember holding torque only tells you how good the motor is at standing still ;)

Offline Hood

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Re: Noobish Question On Motors & Drivers
« Reply #24 on: April 30, 2008, 07:53:32 AM »

That said, Hood is absolutely right - accel is also hugely important - you can never have too much, because what we'd actually like is instant speed, i.e. infinite accel. I'm no expert on servos as I've said - but I think I'm right in saying that at lower speeds, steppers will out accelerate servos by a gazillion miles (per sec per sec ;D) because they have more low speed torque. To get torque out of a servo - they have to be spinning fast. - so what happens at low speeds? - in order to work - you have to gear them down. That makes me think I'd want servos for either really high speed applications or really heavy rigs - course - that's just my opinion.

 I have to disagree with that I am afraid and to back up my case I have attached a screenshot of the torque curve of the AC Servo that I am putting on as my spindle (the other AC Servos I have on the axis are exactly the same with regards to their torque). As you will see the continuous rated torque is constant from zero RPM up to the rated speed(about 2/3 of the max speed) and even after that the drop off is nothing like a stepper.
 I dont know whether DC servos are the same but I think they are but wont say they are definitely. As I said I have steppers on my Bridgeport series 1 CNC and the acceleration is noticeable, on the Lathe you dont notice it at all, it seems instantaneous.
 I am not sure why people gear their servos, might be that they have to do that to get the torque they require as their motors are smaller than they actually need, my lathe has big motors and is geared 1:1 and the latest mill will also be that way.
Hood
« Last Edit: April 30, 2008, 07:55:20 AM by Hood »

Offline stirling

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Re: Noobish Question On Motors & Drivers
« Reply #25 on: April 30, 2008, 09:07:38 AM »
Hi Hood - well I did say "I'm no expert on servos as I've said - but I think I'm right in saying..." - that'll teach me to stick to what I know  ::) ;D

.....That said....  ;D ;D ;D

Here's why I said what I said....

http://www.allbusiness.com/sector-92-public-administration/justice-order/1177040-1.html

Offline Hood

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Re: Noobish Question On Motors & Drivers
« Reply #26 on: April 30, 2008, 10:01:06 AM »
Yes but from a quick squint at it they are not comparing like for like. I may be wrong as I am in the middle of working on a boats engine at the moment and dont have time to thoroughly read it. What I am seeing is they are comparing the wattage of these motors and not the torque. Also I dont know how they get the servo as having its peak torque at 3000RPM as I cant see any diagrams or charts, maybe they just arent loading because I use Opera. As can be seen by the above chart I posted my motor actually drops off slightly at higher revs and below is another to support my case, this is a motor I have on the lathes X Axis. Also I have attached a quote from the Rockwell Automation manual for the motor

 What I can say for sure is you need to size the motor to your application and no matter whether its a stepper or a servo if you undersize you will get bad results. If you get the size right you will have a good system, just with different characteristics. Oh and just to show I am not Anti Stepper I have just made a coil winding machine for a customer. Money was not a problem with this build in as far as hardware was concerned but there was no way I was going to spec servos for it. Plain and simple reason was they were not needed so would have been a waste of money, the steppers are more than fast enough and as has been said if sized properly then accuracy is not an issue either.
Hood

Offline stirling

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Re: Noobish Question On Motors & Drivers
« Reply #27 on: April 30, 2008, 11:30:03 AM »
Hey Hood - No argument from me - you the servo man  ;D - I was just letting you know what I'd read.

Cheers

Ian

Offline Hood

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Re: Noobish Question On Motors & Drivers
« Reply #28 on: May 02, 2008, 04:57:40 AM »
I am certainly not an expert on servo motors and I am just hoping the torque curves are correct. However I very much doubt that Rockwell would dare put out false representations of what their motors can do.

Hood