Subject: Re: DC motor voltage

From: [email-masked] (Richard Griffin)
Date: 29 Jul 1996 12:24:48 +0100

CBear81438([email-masked]) replies to Kurt Rieder:

>>  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.
>> 

>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.

I offer some backbround information:

Autoranging power supplies (the switched-mode sort that are used in most
electronic apparatus) cope by using an input topology that requires no 
transformers or adjustment for mains voltage. They simply rectify whatever
mains voltage is present then apply it to an oscillator (operating at around
50 to 75 KHz) that creates high frequency pulses to a transformer. 
The output of the transformer is rectified to produce DC and a feedback system 
controls the oscillator to provide voltage stabilisation.

Since you require pulsed DC for the motor at around 100Hz I would guess that a
controller could be built that rectified whatever mains was available, used it
to drive an oscillator that pulsed energy into a reservoir capacitor until it
had enough energy in it for one pulse to the motor. This is then released and 
the reservoir recharged. As long as energy can be put into the reservoir faster
than it is taken out all is well. The hard bit is designing the oscillator 
since it usually involves inductive circuits with finite turn-off times and
huge back-emfs that cause overdissipation in the driver transistors. 

Mucking around with home-built switched mode supplies is usually to be 
discouraged - 350V DC rectified straight from the main bites!

Richard Griffin.

---

From: PPIERCE <[email-masked]>
Date: Wed, 31 Jul 1996 20:53:40 -0700

Henry Kroker wrote:
> 
> Richard Griffin wrote:..........

> I agree. Switched power is tricky. You can however run off the line using
> an scr bridge. I service a model with brake accel torque, and aramture
> current control. It's an amasingly simple circuit. Completly analog,
> no transformers, and no hard to get parts.
> 
> It will run up to 2HP 120-240 and you set min and max outputs.
> 
> henry

Come on Henry...  Give us a commercial.  I sure won't flame you.
Probably lots of others would welcome your advice.
Tell us the name of the unit so we can look into it.

Thanks..

Paul in AJ AZ

---

From: [email-masked] (darylb)
Date: 1 Aug 1996 21:23:06 GMT

In article <[email-masked]>, [email-masked] (Kurt
Rieder) wrote:
> Hi... I'm back again... after starting this thread, I'm amazed at all
> the ground that has been covered... but still not clear on what would 
> be a simple solution to my problem.  Allow me to restate the question.
> 
> I want to run a 48 VDC PM motors from the house 110 VAC power.  I know
> that I could use a variac or step down transformer, but either would
> be bulkier, heavier, costlier than the motor.  I do know how to build 
> a conventional SCR speed control for a DC motor.  This would give a
> 120 Hz pulse rate, and delay of SCR turn-on would get rid of the lower
> voltage part of the half wave and keep the high voltage peaks (after 
> the SCR turns on) which would be close to 3X the motors voltage rating.
> 
> I see that Sherline uses a 90 VDC PM motor on their tools and that they 
> can run the same tool on 110 OR 220 VAC by merely changing the plug that
> goes into the wall socket for their export model.  What they do, seems to
> be the answer to my problem.  How do they get rid of the extra voltage ?
> I don't think we are talking about a switch mode power supply here.  But
> if someone could look inside a Sherline box... it would be very helpfull.
> Seems like if they can run a 90V motor on 220V, I should be able to run
> a 48V motor on 110V ?  What do I need ?  Or can I just allow the SCR control
> to limit motor current to a safe value and forget about voltage (keeping my
> fingers crossed and hope that the armatur winding don't arc over). ???
> I surely don't want to build a switch mode power supply.

If I were doing this on the cheap I would use a dimmer to control the wall
AC. That would be fed into a bridge followed by an inductor (say 1mH) and
cap (say 6800uF).  Just be sure all parts including the dimmer can handle
the current. Given the lack of transformer you might want a dimmer that
starts from zero rather than pop on/off. This will (if started slowly)
limit the surge current into your filter cap. The extra 20V shouldn't be
deadly for a 90V motor and the savings of no transformer is large. 
I did this for a 24VDC motor (I had to use a transformer) where I needed
variable speed and smooth operation. It worked fine. Switched mode
supplies (currently designing my first) are for high efficiency and
compact size (switching power at 128,000Hz say, means much smaller
transformers - smaller power/cycle) which you likely don't need.

-- 
Clouds are high flying fogs.

---

From: [email-masked] (Ray Spinhirne)
Date: 2 Aug 1996 19:04:10 GMT

darylb ([email-masked]) wrote:
: In article <[email-masked]>, [email-masked] (Kurt
: Rieder) wrote:
: > Hi... I'm back again... after starting this thread, I'm amazed at all
: > the ground that has been covered... but still not clear on what would 
: > be a simple solution to my problem.  Allow me to restate the question.
: > 
: > I want to run a 48 VDC PM motors from the house 110 VAC power.  I know
: > that I could use a variac or step down transformer, but either would
: > be bulkier, heavier, costlier than the motor.  I do know how to build 
: > a conventional SCR speed control for a DC motor.  This would give a
: > 120 Hz pulse rate, and delay of SCR turn-on would get rid of the lower
: > voltage part of the half wave and keep the high voltage peaks (after 
: > the SCR turns on) which would be close to 3X the motors voltage rating.
: > 
: > I see that Sherline uses a 90 VDC PM motor on their tools and that they 
: > can run the same tool on 110 OR 220 VAC by merely changing the plug that
: > goes into the wall socket for their export model.  What they do, seems to
: > be the answer to my problem.  How do they get rid of the extra voltage ?
: > I don't think we are talking about a switch mode power supply here.  But
: > if someone could look inside a Sherline box... it would be very helpfull.
: > Seems like if they can run a 90V motor on 220V, I should be able to run
: > a 48V motor on 110V ?  What do I need ?  Or can I just allow the SCR control
: > to limit motor current to a safe value and forget about voltage (keeping my
: > fingers crossed and hope that the armatur winding don't arc over). ???
: > I surely don't want to build a switch mode power supply.

: If I were doing this on the cheap I would use a dimmer to control the wall
: AC. That would be fed into a bridge followed by an inductor (say 1mH) and
: cap (say 6800uF).  Just be sure all parts including the dimmer can handle
: the current. Given the lack of transformer you might want a dimmer that
: starts from zero rather than pop on/off. This will (if started slowly)
: limit the surge current into your filter cap. The extra 20V shouldn't be
: deadly for a 90V motor and the savings of no transformer is large. 
: I did this for a 24VDC motor (I had to use a transformer) where I needed
: variable speed and smooth operation. It worked fine. Switched mode
: supplies (currently designing my first) are for high efficiency and
: compact size (switching power at 128,000Hz say, means much smaller
: transformers - smaller power/cycle) which you likely don't need.

: -- 
: Clouds are high flying fogs.

I would make sure to use the configuration abouve (without transforver)
only an a circuit protected by a GFI breaker or outlet.