Subject: Motor speed control

From: [email-masked] (Michael Covington)
Date: 24 Mar 1994 20:39:27 GMT

Have there been any recent advances in speed control of AC induction
motors?  Like ICs that do the waveform synthesis for you, or something?

My application is a scroll saw with a 1750-rpm, 2-amp motor (120 VAC
60 Hz).  I'd like to be able to get down to half of that speed, or lower.

I've done speed changes of a few percent for telescope drives, but
I presume that if I took the frequency down to 50% of normal, I'd have
to reduce the voltage, too, to prevent saturation.  Right?

For that matter, how do the variable-speed and dual-speed
motors on such things as Delta and Sears scroll saws work?
They do not appear to be induction motors -- are they?  

-- 
< Michael A. Covington, Assc Rsch Scientist, Artificial Intelligence Programs >
< The University of Georgia, Athens, GA 30602-7415 USA    [email-masked] >
  <   Unless specifically indicated, I am not speaking for the University.  >
For information about any U.Ga. graduate program, email [email-masked].

---

From: [email-masked] (PW. Wheeler)
Date: Thu, 31 Mar 1994 13:26:51 GMT

Michael Covington ([email-masked]) wrote:
: Have there been any recent advances in speed control of AC induction
: motors?  Like ICs that do the waveform synthesis for you, or something?

: My application is a scroll saw with a 1750-rpm, 2-amp motor (120 VAC
: 60 Hz).  I'd like to be able to get down to half of that speed, or lower.

You can buy an inverter off the shelf which would do the job without any 
problem.  They can go down to speeds of a couple of Hz and up to 200Hz with
no problems, as well as having an array of protection including over current.

******************************************************************************
            Pat Wheeler                 Email [email-masked]
******************************************************************************
******************************************************************************
            I speak only for myself, and not even that sometimes.
******************************************************************************

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Apr  3 09:06:49 EDT 1994
Article: 4385 of rec.crafts.metalworking
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From: [email-masked] (Gary Paisley)
Newsgroups: rec.crafts.metalworking
Subject: Re: Looking for a crucible
Date: 31 Mar 1994 22:07:40 GMT
Organization: Schlumberger Laboratory for Computer Science
Lines: 8
Message-ID: <[email-masked]>
References: <rkh.764833658@teal>
NNTP-Posting-Host: exodus.austin.slcs.slb.com
In-reply-to: [email-masked]'s message of Mon, 28 Mar 1994 05:47:38 GMT

>Where would I buy a small crucible?  I need something big enough to melt
>aluminum cans in, for example (i.e. not "lab" size).  I have looked through 
>the Yellow pages for my area (Boulder/Denver Colorado) and I don't see 
>anything.

Well, Golden is just over the hill, try Coors Ceramics...

gary

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From: [email-masked] (Michael Covington)
Date: 1 Apr 1994 14:59:17 GMT

>no problems, as well as having an array of protection including over current.

Where?

Do they vary the voltage appropriately to keep the motor from saturating
at low frequencies?

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From: [email-masked] (JAMES COPP)
Date: 2 Apr 1994 03:59:04 GMT

Michael Covington ([email-masked]) wrote:
: In article <[email-masked]> [email-masked] (PW. Wheeler) writes:
: >Michael Covington ([email-masked]) wrote:
: >: Have there been any recent advances in speed control of AC induction
: >: motors?  Like ICs that do the waveform synthesis for you, or something?
: >: My application is a scroll saw with a 1750-rpm, 2-amp motor (120 VAC
: >: 60 Hz).  I'd like to be able to get down to half of that speed, or lower.
: >
: >You can buy an inverter off the shelf which would do the job without any 
: >problem.  They can go down to speeds of a couple of Hz and up to 200Hz with
: >no problems, as well as having an array of protection including over current.

: Where?

: Do they vary the voltage appropriately to keep the motor from saturating
: at low frequencies?

: -- 
: < Michael A. Covington, Assc Rsch Scientist, Artificial Intelligence Programs >
: < The University of Georgia, Athens, GA 30602-7415 USA    [email-masked] >
:   <   Unless specifically indicated, I am not speaking for the University.  >
: For information about any U.Ga. graduate program, email [email-masked].

Check out your local Allen-Bradley dealer. Woods also offers small adjustable
speed AC drives. And yes these units maintain constant volts/Hz (ie constant
torque) over the operating range, or as near to it as possible.

Hope this helps,
Leigh Copp
[email-masked]

---

From: [email-masked] (Michael Covington)
Date: 2 Apr 1994 21:12:27 GMT

>: >: 60 Hz).  I'd like to be able to get down to half of that speed, or lower.
[Someone replied:]
>torque) over the operating range, or as near to it as possible.

How expensive are such things?  What I really want to know, of course, is
how they work, so as to build my own... man~ana...

---

From: [email-masked] (Raymond C. Minich grad)
Date: Thu, 7 Apr 1994 20:54:59 GMT

>how they work, so as to build my own... man~ana...

We just purchased a slat bed conveyor with a 2 HP Leeson AC motor (3-phase) 
that's speed controlled by a control unit made by AC-TECH.  They're using a 
uP to do all the waveform construction, but I've not yet determined what 
they are using as the final control element.  This unit is really fancy, it 
will even calculate how much energy (kw-hrs) it has drawn off the power line 
since it was last turned on.  This is a 3-phase 480 volt unit but they also 
have single phase 120v and 220v units available.

I too am interested in speed control for a woodworking device (my table saw) 
but for a different reason.  I bought this Sears 10" direct drive (yes -flex 
shaft from motor to blade) from a friend knowing it had a built-in flaw with 
designs on circumventing the problem.  The problem is that the flex shaft/
10" blade acts as a mass spring system, and when you turn on the 1-1/2 HP 
motor, the whole blade assembly jerks and chucks for about 1.5 seconds until 
the blade gets up to speed.  The starting torque on the flex shaft winds it 
up, the shaft jumps around in the conduit, and the flex shaft winds up and 
back until the blade gets to near operating speed, at which time the 
oscillations die out and the blade runs smooth.  It's really noticeable when 
you run a long extension cord to the motor, given the increased voltage drop 
of the extension cord, it takes 2-3 seconds for the blade to come up to 
speed, with the oscillations extending accordingly.  

The nice people at Sears say they have never heard of this problem.  I've 
checked the specs on, and the condition of the flex-shaft, and it's all 
within spec.

Now, if I could bring the motor up to speed with a variable frequency drive, 
that would fix the problem.  Anybody got any "hobbyist" type schematics 
lying around that they'd reccommend?

(P.S. - I tried using a 10-amp variac to start and run the motor, and the 
last thing I saw was smoke coming from the variac.  Test failed, haven't 
looked to see if the variac is dead or not.  Do not try this at home, it 
can be expensive.)

Any comments appreciated and thanks in advance.

Regards
RCM

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From: [email-masked] (Narla R Reddy)
Date: 8 Apr 1994 14:28:14 GMT

>you run a long extension cord to the motor, given the increased voltage drop 
                                                                 ^^^^^^^^^^^^
I am surprised to know you get voltage drop across an extension cord. Are you
drawing more than the rated current?

>Any comments appreciated and thanks in advance.
RAJA REDDY

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--

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From: [email-masked] (Bob Meyer)
Date: 8 Apr 1994 17:42:19 GMT

In article <[email-masked]>, [email-masked] (Narla R Reddy) writes:
|> In article <[email-masked]> [email-masked] (Raymond C. Minich     grad) writes:
|> >>How expensive are such things?  What I really want to know, of course, is
|> >>how they work, so as to build my own... man~ana...
|> >
Hi folks,
Does anybody have a schematic for a speed control for a universal motor
1.5 hp (router).  I know I can buy one for $30, but it seems like more fun
to make one, if not cheaper.
Thanks,
Bob Meyer