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Author Topic: What size motors?  (Read 15058 times)

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Offline simpson36

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Re: What size motors?
« Reply #20 on: April 20, 2009, 05:43:13 AM »
Bill;  Just FYI, I don't take myself all that seriously, nor do I get emotional over forum chatter. My 'opinions' are no better or worse than anyone elses, however facts are facts and I welcome any challenges in that regard.

I would like some clarification on one of your comments. You mentioned 'inrush of current' to a servo motor.

Sounds like you are implying that a servos is unpowered when not moving. If that is so, how does it have any holding power?

As to steam power, I think better to use a twelve year old kid on a hand crank.  . .  you have to feed them anyway, and it keeps them out of trouble . . . >:D

BClemens

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Re: What size motors?
« Reply #21 on: April 20, 2009, 06:25:48 AM »
Well, it takes time to build up a magnetic field, and more time in relation to the size of and wire size of the armature. DC Servo motors are basically two pole motors with a wound armature with brushes to provide the current path....one large winding contacted at each end with a carbon brush. Takes time to build that field compared to a motor with independent windings and multi pole - commonly three in a light duty stepper. The step magnetic field is built up much faster and the motor has reacted much faster - lighter windings and multi sources for multi magnetic fields.

The starting current or 'inrush current' is a factor with any electric motor - AC, DC, or universal. That current is usually a minimum of twice the running current. The larger the motor the more starting current required.

Now, as I had said earlier; while the DC servo motor is absorbing current into its one large winding to build a starting field, the stepper has done its job - one measured and very accurate step. Of course you would size a motor appropriately but size for size and load for load the time of reactance is very factually faster with a stepper motor. But to carry a load at speed - the servo motor is much more efficient.

Now if snapping endmills or rapid travel of a table is the prime interest, then a servo motor is the way to go. If machine work even on aluminum is the prime importance, then a stepper motor is very likely very adequate, sized for the job. Both types of motors have their place - it just depends on how much money and time one wishes to devote to either converting a machine or machining hardened 17-5 stainless steel. One is a given, the other is totally for practicality. To use one of your parallels: why use a high speed V-8 when a lugger four cylinder will do the job?

Serious? Hell yes I'm serious. Intelligence is wasted on a smart ass who demeans those around him. A true engineer is first taught how to be humble - otherwise his talents never enter a prototype shop where he must deal with those with different talents and sometimes a different outlook on life. I'll work with a good machinist with one leg and blind in one eye before I will sit down to converse with an engineer with his head up his ass.

Bill C.

Offline Hood

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Re: What size motors?
« Reply #22 on: April 20, 2009, 06:40:07 AM »
Bill,
 where do AC servos (or DC Brushless) fall in relation to what you said above? From what I can see their construction is similar to steppers in that the coils are in the motor housing and the armatures have the magnets.
 I have some 4 pole, some 6 pole and mostly  8 pole AC Servos, any info on the difference between them with regards machine tools?
Hood

BClemens

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Re: What size motors?
« Reply #23 on: April 20, 2009, 07:18:45 AM »
Hood,

AC servo motors are the most efficient type for slow running and holding power because of their multi poles. The current does not differ with running or stationary (holding) since the major factor in their speed is frequency. They have a quiescent current value that is a factor in sizing a power source but again their speed is a function of the frequency of the applied current and also the number of poles. Yes, they are functionally very similar to a stepper motor so their reactance is very much faster than a DC servo motor.

An AC servo motor is basically in a locked rotor state for holding. The current maintains that state and is the design difference between them and a 'regular' AC motor which will heat up from its own magnetic feedback. The current in DC servo in a holding state can drop to zero if no feedback motion is sensed so they are basically off until either a command to feed is given or a rotation to resist is sensed by the encoder. The most prevalent difference is with the driver circuit. A straight DC motor driver must be tuned to the motor and its function for either holding current and/or running current and the associated chopper circuits in the drive or they 'sing' or over-current. That's not a problem with a stepper or AC servo motor since they are specifically designed for such applications.

I finally realized that I was over adjusting these Gecko 320's in a attempt to tune them. An oscilloscope with the diagram in the paperwork for the drive did the trick. That's when I began to realize that a stepper motor would be very well suited to do this job - no hassle.

Bill C.

Offline Hood

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Re: What size motors?
« Reply #24 on: April 20, 2009, 07:25:02 AM »
Bill,
 excellent info, much appreciated, as they say, every day is a school day, shame I wasnt so interested in learning when I was there ;D

Hood

BClemens

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Re: What size motors?
« Reply #25 on: April 20, 2009, 07:31:49 AM »
 ;D

Offline simpson36

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Re: What size motors?
« Reply #26 on: April 20, 2009, 09:37:34 AM »
Bill, my sincere apologies if you felt demeaned in some way.

My comments were not directed at anyone in particular. I think some of the advice dispensed is not so much 'wrong' as it may be outdated, and out of context.

I don't think it is helpful to say steppers are 'just as good' as a blanket statement. Especially when it is obviously a newbee asking the question. Steppers are appropriate if used in such and such application, WHICH IS . . . ..  blah blah blah   BECAUASE . .  blah blah blah.  Mo' better answer.

There is a lot of conflcting information around and I know that as a newbee, I had trouble sifting thru it, primarily because AS a newbee, I had no tools to distiguish opinion from fact, or to dvine what information was missing from an answer.  I made some expensive mistakes following incomplete advice.  It cost me several hundreds of dollars to learn what an 'isolated signal' is, even though I had asked the right questions and purchased the exact components I was told to use. So, this being a forum, I share my 'experience' such as it is, like everyone else.

This is a technical forum. It's all about facts, not about personalities or one's "outlook on life".  :-*

BClemens

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Re: What size motors?
« Reply #27 on: April 21, 2009, 05:20:55 AM »
Trouble understanding what you read? Then blame the person attempting to help you. Expensive mistakes could be avoided if a basic understanding of basic electronics, basic mechanics, and a bit of humility are present. It's easy to see why you are in a predicament and can't seem to get out - you think everyone attempting to help is giving you wrong information and trying to steer you wrong. All that folks here will do is attempt to help you out with what they know to be true. The technical facts about a device are wasted on someone with little capacity to absorb it.

You ask vague questions and expect a purely technical , full of facts answer. What is a fact to you anyhow?

Bill C.

Hope to help....

Offline simpson36

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Re: What size motors?
« Reply #28 on: April 21, 2009, 06:20:40 AM »
Expensive mistakes could be avoided if a basic understanding of basic electronics, basic mechanics, and a bit of humility are present.

Bill, I don't know how I can be more humble than to admit that I do NOT have a basic understanding of electronics. I usually preceed any electronics question with exactly that statement. Sometimes you guys with all the valuable experience forget that and assume we newbees can fill in the blanks.

Speaking only for myself, I do a lot of homework and try to understand a topic at least in a rudimentary way before I ask questions, but I have no way to really fill in the blanks on an asnwer that I get on electronics issues, and somtimes that can cost us real money. I just wanted to point that out so that people (not just you personally) can keep that in mind.

i.e. it is better for an experienced person like yourself to over explain than to short sheet the answer assuming the questioner knows the rest.

I hope this clarifies my intent. You are free to disagree, but I really do not think I had anything bad to say about you as a person, so I dont understand your reaction here.

BClemens

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Re: What size motors?
« Reply #29 on: April 21, 2009, 08:01:16 AM »
Let's end this....I'm as much to blame....sorry for that!

I'm attempting to post a diagram for you....doesn't seem to work for some reason, but I've been able to in the recent past. I'll try again later.

Bill C.