There are resources for the windings design. It might be cheaper and easier just to do a boost converter and since I already had this pulled up, I thought I would post it before I closed the tab.
This is how to design a variable boost converter:
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~Wishing You a Happy New Years Sean~
I can’t belive I forgot to say it until now!
Your posts in this thread have been so helpull in the past, I would like to offer your some dried fruit that I picked over the summer and dehydrated. Would you accept dried cherries by post as a token of my appriciaton?
With regard to slvaej4.pdf, I don’t understand.
Why did you post this PDF?
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Mainly For completeness of the discussion. It is potentially cheaper way to accomplish what you are trying to do. I believe it is always good to understand competing technologies even if you don’t choose to use them.
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As far as potentially cheaper ways: Here’s what I’ve got as a temporary solution to drive my universal motor appliances plugged into a 1500W car-battery inverter.
That’s an SL32 AC Current Inrush Limiter rated for 15amp and 10ohm (AKA NTC Thermistor) on the hot wire from the inverter before it goes into my full-bridge reftifier. My reftifier has a heat sink, but I notice the SL32 gets much hotter than the rectifier.
During testing I was unable to trip the circut no matter how hard I pushed the chainsaw, and used 500W running under no load. I say, this Current-Inrush-Limiter crimped the power flow to save battery without reducing the saws effectiveness. And, I usually need a 2500W inverter to run this saw!
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I have a new scheme to Limit Inrush Current for Universal (brushed) Motors operating on AC grid power. But, I’m not sure about the math.
My table saw has been tripping the 30A breaker and there is a Universal Motor powering it. I’d like to rectify power to the device to save energy, and adjust voltage to control speed.
The 30A, 120v table saw requires 3600W AC. Power is supplied in 60Hz.
Rectifying to DC cuts consumption 3600W AC / 2 = 1800W DC .
1800W / 60hz = 30 Joule / Hz
It means that every Hert (1/60 of a seoncd) , 30 Joule of energy are required to make 1800J in a second => 1800W.
But, on mains, I don’t need to supply power for the full second because some % of that time the mains AC current will be flowing in the right direction. I think 50% of the time? Therefor I need to store ~15J / Hz in a capacitor.
Farad = Joule / Volt^2
15J/120^2V = .001 Farad => 1,000 μF
If I could limit the power per Hz to 30J with a single 120V, 1,000uF capacitor across a full bridge rectifier then I would not trip the 30A mains circut breaker. However, power flow will not be limited during a portion of the AC cycle. Therefore, It will still be possible to trip the breaker with this circut.
Can you guys think of a way to Limit the power flow per Hz to the charge held in a single capacitor?
To switch power such that mains is not directly connected to load? Or is this impossible?
Unfortunatly, the table saw motor burst into flames after tripping the breaker for the last time. I blew the fire out with a leaf blower. Once the engine was cool, the flames stopped. This happened because the table saw fence was not aligned and caused the wood to rub against the blade more than cut. The friction lead to head buildup and eventually flames.
I don’t want that to happen again so I’ve built a new WCB (Wooden Circut Board) this snowy morning to Limit the current.
This one has a 1 Ohm 30A rated NTC Thermistor (AKA Inrush Current Limiter) as opposed to the 10 Ohm, 15A I use for my inverter.
I also include a 20uF capacitor across the bridge rectifier to smooth DC current
Tested with my bandsaw
With PWM motor speed control. Not active in the first pic.
The Osciliscope is showing 120Hz and the 20uF capacitor I added to smooth the DC barely affects it.
In this Pic I turned the PWM motor speed controller on. You can see it interupts power to the drvice ~250 times a second.
The motor did slow down a lot with the PWM speed control and the saw still cut at that slow speed, but I’m actually interested in doing the opposite: speeding the saw up because as you seen this type of saw is designed as a portable metal-saw and I’m using it on a stand to cut wood.
My nexy try will be with the 1000uF capacitor to reach higher speeds by reducing the time power transmisttion is interupted.
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To update the thread with my result: using a 30A NTC Thermistor as an In-Rush Current Limiter to protect my 30A breaker from triping was an ill conceived idea. The breaker did not trip, but the router I was using burned up. The NTC In-Rush Current Limiter should to be sized for the appliance’s current draw.
I built a new rectifier with a 2200uf capacitor. It’s making 170VDC from 120VAC.
The bandsaw I use, pictured previously, is rated 10A (1200W). Running on this reftifier it draws 5.25 Amp and the voltage supplied by the rectifier drops to 150. 5.25 x 150 = 787W Rectified :
As shown, DC power is uninterupted with the 2200uF capacitor. I think it’s helps the saw run smoother by applying constant pressure to the blade. Next time, I will omit the NTC Thermistor when using grid power.
Snow is falling outside…
I’ve been running my power tools on uninterrupted DC power for a week, here’s what I learned:
- In many cases DC power will destroy the device’s switch. In every case where the switch burned, the motor continues to spin and caint be switched off.
- The bandsaw blade does not slip the wheel. Due to consistant tension there is no momentary loss of friction with the wheel.
When working with DC power, I vary the voltage from 80-240V.
- 100V - Optimize for longevity. decent speed, small spark from commutator/ brushes - widely used in Japan
- 120V - Kind of in a rush: sounding rough
- 170V - Longevity no consideration. Brushes emitting large colorful sparks
- 200V - Motor audibly stressed
- 240V - Dange
Let me show you how I fixed the switch and run the tool at 100V with another WCB:
This photo shows a method for transforming the voltage of an AC system down that uses a pair of capacitors known as capacitive voltage division. The two capacitors work in AC by collecting a charge on one capacitor. The charge is then mirrored to the other capacitor, but this capacitor has a larger capacitance. Therefore, every Hert the Voltage supplied to charge the first capacitor is then divided across the capacitance of second capacitor – lowering the output voltage. With the pictured set-up I swaped capacitor-combinations to tune the output to my load. Nota Bene: Capacitors are connected negative to negative in this method.
From before, we know ~1000uF every Hz is required to run a 10amp saw. Thus, I used a 2200uF and a 1000uF capacitor to 40%-divide the voltage output from 170V → 100V. Then power goes thru the rectifier and runs the motor a medium speed.
But what about the switch? Why do the switches burn out after just a moment of DC power?
I assume it’s amperage related and installed a 120V 40A solid-state relay to handle the load ▼
This photo shows an example of the following modifications and their estimated cost from the manufacture:
- Capacitive Voltage Adjustment ( Optional ) - 3,500 Yen
- Rectifier - 250 Rupee
- Smoothing-Capacitor for Uninterrupted DC ( Optional ) - 1,750 Yen
- Switch upgrade to solid-state relay ( Switch AC not DC ) 115 Renminbi
Estimated cost is less than fifty-bucks inclu. shipping, duties, etc.
As you can see, the wires are detachable via lever-nuts and the cord is clamped down with a grub screw that is turned on with an Allen-wrench head identical in shape to a pencil (1/4" hex key). If I decide to remove the saw later-on I can easily add a longer cable.
The 1000uF and greater capacitors have enough power to spot weld. I wired the circut carefully: capacitors are only charged when the tool is running.
Final point about my band-saw: This type of hand held band-saw originally used blade guides that bend the blade 90deg to cross the cutting edge and then bend 90deg back onto the opposite wheel. For stationary use, that is not a feature and I removed the original blade bearings. The wooden bearing blocks shown are from oil-soaked hickory. Since that upgrade I have not broken any blades.
When I started this thread I said that I would not consider three-phase motors due expense, but I feel like I have taken these Universal Motors nearly as far as they can go. Unless I can come up with some way to supply high-voltage but limit electric arcing across the brushes I will be looking at three-phase power for my next testa.
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DC has a higher resistance because the wire builds up a ‘skin’… Pulsed dc and ac, the split second the current is off, the resistance in the wire drops to zero again. Resistance creates heat so it melts. That is why switches and wire have both an ac and dc rating that are significantly different values.
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Are you familiar with arc welding? 
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