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Author Topic: Speed control for a DC micro table saw?  (Read 1420 times)
N2NJO
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Posts: 141




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« on: May 25, 2012, 12:20:37 PM »

I have a Proxxon micro table saw that uses a 100 watt DC motor. There is a full wave bridge after the power switch. The motor runs at 7000rpm which is too fast for cutting plastics. Their older version did have the speed control with settings from 1 to 6. According to Proxxon, for that saw and the optional blades, a #3 setting is recommended.
I talked with tier support number in the US (the saw was designed in Germany and manufactured in Twain), they couldn't tell me anything more than what I already wrote above.

My question is, I assume the older design had the speed control after the inverter using DC, not before on the AC side. What type of control would work here that would be internal as opposed to a typical external AC router speed control?
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KD0REQ
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Posts: 971




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« Reply #1 on: May 25, 2012, 12:36:20 PM »

can't see why a standard Triac type speed control wouldn't work.

a Variac certainly would.

barring that, search for transistor model train speed control on Da Web, and make sure all your parts are uprated for voltage, and there is nothing with voltage on it that is finger-handy.  you might have to fiddle some bias resistor values if you start losing parts.  I'd test it out with a 150-watt light bulb first.

there is going to be a certain speed below which the motor will overheat.  one way to find it quick is to note what it takes to stall the motor as you start winding it down.  put a red line there on the control's range, and stay above that.
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VK2TIL
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Posts: 331




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« Reply #2 on: May 25, 2012, 02:31:47 PM »

There are lots of simple pulse-width modulator controller kits on the 'net and they are not expensive; Google "DC motor speed controller".

This was #1 hit; http://www.electronickits.com/kit/complete/motor/ck1400.htm

It has a very good description in the pdf notes.

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K5LXP
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« Reply #3 on: May 25, 2012, 03:17:22 PM »

If it's really a DC motor, it shouldn't need rectifiers (brushed motors run on AC or DC).  That would imply there's some kind of (DC) control circuitry.

Brushed motors don't lend themselves well to speed reduction and keeping useful torque.  PWM would be a good way to go, and I've had luck in some applications using a stepdown transformer.  You could try using an AC variable fan speed controller too.


Mark K5LXP
Albuquerque, NM
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KB1GMX
Member

Posts: 780




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« Reply #4 on: May 25, 2012, 03:56:18 PM »

Several errors noted.

It is a permanent magnet DC motor and will not run on AC (though it is brushed).
the so called universal motors run on AC or DC but have field windings that are powered.

There are AC in to DC out motor controls available check Granger or McMaster.

I use a 90V permanent magnet DC motor on my cheap drill press to replace the
induction motor and a motor control that uses AC in and DC out. Names like
Dayton and Leeson come to mind among others.  These are basically simple PWM
(using AC line freq) with voltage and current feedback controllers.  By using feedback
they tend to maintain torque under load.

There are a class of DC motors (brushless) that use multiphase variable frequency
AC from controllers.  This is not from what I know one of those.


Allison
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N2NJO
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Posts: 141




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« Reply #5 on: May 25, 2012, 09:01:47 PM »

KD0REQ; It's a 120vdc motor. That circuit board is only a full wave bridge with a couple of caps and a filter.

VK2TIL; That is only good up to 100 volts.

KB1GMX; I asked if it is a universal motor, but they didn't know. All what was on the motor itself states 100 watts.  I also asked if it is the same motor as the older version, but that wasn't known either. The parts list for the older model shows the speed control board, but has no part number to it.
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KB3HG
Member

Posts: 404




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« Reply #6 on: May 25, 2012, 09:03:00 PM »

KISS Keeping it Simple method High speed motor and plastic.

100 Watt DC Motor
Bridge rectifier
AC Feed
120VAC feed

Inferred:
120HZ
160 V DC peaks (theoretical)
approx 1 amp peak pulses close enough i=e/r
approximate motor impedance 160 ohms  z=e/i

option 1 a 600 w light dimmer or variable fan speed controls (Google wood stove fan controls)

option 2 a Variable power resistor, real old books showed putting a light socket in series and screw in different wattage bulbs.
          each lamp having a different voltage drop. back to Ohms law double the resistance half the current ect

Just a Thought,

Tom Kb3hg
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N2NJO
Member

Posts: 141




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« Reply #7 on: May 25, 2012, 09:07:42 PM »

I would go for something internal and a little most high tech than that.  Wink
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VK2TIL
Member

Posts: 331




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« Reply #8 on: May 25, 2012, 11:45:49 PM »

Voltage was not specified in the initial post.

The IRF530 is the component that sets the voltage rating of that randomly-chosen circuit; there are plenty of high-voltage MosFETs that could be used in its place.
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KB3HG
Member

Posts: 404




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« Reply #9 on: May 26, 2012, 12:39:23 AM »

No, Voltage was not specified. Since he called it a table saw, I have not seen to many of them that was not 120VAC at less than 1/4 hp let alone 100 watts.  120 VAC is the common voltage in the states.

The fan controllers are semiconductor based 

http://www.ebay.com/itm/Plug-In-Fan-Speed-Controller-Fireplace-Blower-Motor-Power-Control-Dial-A-Temp-/280740479279?pt=LH_DefaultDomain_0&hash=item415d6fc   

another :
http://www.ebay.com/itm/Fireplace-Stove-Blower-Fan-Variable-Rheostat-Speed-Ctrl-/130666141621?pt=US_Replacement_Parts&hash=item1e6c4f17b5#ht_1026wt_952


Many levels of technology are available to your question. Simple usually precludes cost, reliability and ease of use.
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