Subject: Re: DC motor voltage

From: [email-masked](Dale A. Keltie )
Date: 29 Jul 1996 03:01:09 GMT

In <[email-masked]> [email-masked] (Kurt Rieder)
writes: 
>
>I have noticed that some tools which are powered by motors with 
>continuously variable RPM also specify either 110 or 220 volts.
>An example are the Sherline lathe and mill where it is stated
>that the dual voltage is automatically compensated for by the
>control system... no changes required.
>
>Can anyone describe how this wide range of voltage is accommodated ?
>Surely there is not a step down transformer included ?  Is there 
>some way to do this with the usual rectifier diodes, diac and SCR
>speed control system for a DC motor without burning it out at double
>the rated voltage ?  My reason for asking this is that I have a
>couple of very nice 48V DC (PM) motors that I would very much like 
>to run on 110 volts with such a variable speed system.
> 

Kurt -

Controllers that automatically compensate for a wide range of input
voltage utilize armature voltage feedback to (1) sense the speed of the
motor and (2) limit average armature voltage to a value safe for the
motor.  Most controllers I've seen of this type are designed
specifically for the motor with which they are used (and that motor is
generally rated for 90 V.D.C.), and so would be unsuitable for your
application unless modified to limit the average armature voltage to 48
V.D.C.  I don't believe it would be a particularly complicated
modification however if that helps.

Regards,
Dale

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From: [email-masked] (Paul Amaranth)
Date: 29 Jul 1996 15:22:10 GMT

In article <[email-masked]> PPIERCE <[email-masked]> writes:
[snip]
>I'd really like a softer start up of my gear head lathe.  I grew up fooling
>(and I do mean fooling) with a 10" Logan flat belt drive.  That sudden startup 
>of the gearrhead and the noise of gears still gets to me after a year.
>
>I'm just finishing up a table feed (PIM) with a windshield wiper motor that looks 
>(to me) to be promising.
>
>Paul in AJ AZ
>
>

The latest Projects in Metal has an article about adapting an automobile
air conditioner clutch to a small lathe to serve as both a clutch and a
spindle brake.  I thought it was a pretty clever adaptation.

I'll be changing addresses soon and probably will lose posting ability
for a while.  You can get to me at [email-masked]
--
Paul Amaranth  Manager User Services - office: (810) 370 4541 (also voicemail)
               (e-mail)     [email-masked]         
               URL http://www.acs.oakland.edu/~amaranth

---

From: Glen Harris <[email-masked]>
Date: Wed, 31 Jul 1996 13:06:01 +1000

> to run on 110 volts with such a variable speed system.

  The method varies, but a simple system is to use a variac, a cheap
one is a light dimmer, to drop the voltage down, then rectify the AC
through a bridge rectifier.  This is four diodes arranged so that
positive on either AC input goes to one output and negative to the
other.  Use a 5 or 10 Amp bridge and a 10,000 uF smoothing capacitor.

  You now have whatever voltage you need, from 0 to the RMS peak
of the AC voltage thanks to the capacitor.  eg: 110*sqrt(2)=155V
I use transformers to drop the voltage, but the dimmer works in
a lot of cases.

  Now, we have to make a Pulse Width Modulated (PWM) signal.  Get a 
555 timer chip and set it up to give a fixed frequency, variable
duty cycle ouput.  This is an extremely common chip and the circuit
for this can be found in many data books and electronics texts.
The duty cycle can be varied from 1% to 99% on-to-off ratio for each
pulse, using a variable resistor.  All of this circuit runs on a
lower voltage, say 5 to 35 volts, and only a few milliamps, so use
low power, cheap components from Dick Smith, Rat Shack, Tandy, etc.
The frequency should be more than 1000Hz, preferably 10 to 100
thousand for smooth operation of the motor.  20,000 is a common
frequency used in high tech radio controlled car speed controllers.

  You still have to switch the high voltage/current to the motor, so
connect the output of the 555 to one or more Field Effect Transistors
(Fets), which can switch large currents quite easily.  You might have
to put a small drive transistor between the 555 and the Fets.

  Connect one lead of the motor to the positive supply coming from
the rectifier, the other to the input of the Fets, and the output
of the Fets to the negative of the rectifier.

  You now have a variable voltage, variable speed, PWM controlled
motor.  There are more complex designes, but this is one that can
be made by a beginner, unlike the Sherline one.

  Have fun, and don't kill anyone but yourself.

(This information is provided off the top of my head, and I make no
guarantees, or take any reponsibitily for damage or injury related
to the use or misuse of this information)

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From: [email-masked] (Kurt Rieder)
Date: 28 Jul 1996 06:09:58 GMT

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From: [email-masked] (CBear81438)
Date: 28 Jul 1996 12:56:45 -0400

In article <[email-masked]>, [email-masked] (Kurt
Rieder) writes:

>  Is there 

Yes there is, you can use a variac, or a controller.   You caould use a
variac then run it through a bridge rectifier to supply your motors.   For
the best power. you need a DC motor controller which gives the motor a DC
pulse and the speed is worked out by the length of the pulse and the
frequency.

I just finished installing a 1.25 Hp PM motor on a 15 in Mill Drill, it
works great.   It replaced a 2 Hp unit with no problem.  It will drill a
1.25 in hole in 3/8 steel at 20 RPM or mill Al at 6000 RPM with only one
belt change.

Cecil in OK

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From: PPIERCE <[email-masked]>
Date: Sun, 28 Jul 1996 14:13:41 -0700

CBear81438 wrote:
> ........

Tell us more..........

Paul in AJ AZ

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From: [email-masked] (WRitt)
Date: 29 Jul 1996 03:28:24 -0400

>1.25 in hole in 3/8 steel at 20 RPM or mill Al at 6000 RPM with only one

I would like to hear more  about  this instalation. Where do you find PM
DC motors in the  1+ HP range at surplus?

Bill Rittner