Activating DC using current transformers

[Chris Gregory]

I can't remember when or where I saw it (seems to happen more and more) but I think it was either Popular Mechanics or FWW there was a design for using CT's to turn on your DC when you turned on a tool. I have been digging around and can't find it, does anybody out there have any ideas? It is a nice clean solution especially if you run your shop of a sub panel because it only requires two ct's for single phase and three for three phase, I can't remember if the original design had a delay timer for the shutdown but that would be a nice feature to have too.

Chris

[Jim Becker]

FW originally published an article on this topic and reprinted it in a compilation book called The Small Wood Shop in The Best of Fine Woodworking series. (1ISBN 1-56158-061-9) The latter may be out of print...my copy was in a bargan bin at the now-defunct Woodworker's Warehouse several years ago. The article in the book does not indicate what magazine issue it was originally published in. You might want to contact Taunton Press about that.

[Matt Meiser]

You might try your local library for the book Jim mentioned. Ours has a lot of books from that series.

[Frank Hagan]

I mentioned this in another thread, and I found the warning in the Oneida Gorilla DC I/O manual:




Single phase motors should only be started and stopped up to 4 - 10 times per hour. Starting single phase motors more frequently can cause heat build up and can cause the motor overload to trip or cause motor damage.


Have any of you found this to be true? I'm afraid that a voltage sensing switch would cycle the DC on and off too much.

I actually stop my table saw after every operation, which I have to do for safety reasons (can't really change the blade tilt or elevation with the blade spinning!) I'll leave the band saw running between cuts, but if I know I'm going to work on some other operation, off it goes.

[Edited to remove some weird formatting that happened when I did a cut and paste from the PDF file from Oneida]

[Matt Meiser]

Frank, I heard that starting and stopping was bad as well. I recently wired my blower to turn on when I open a blast gate. The side effect is that the system starts and stops more frequently, which is not good. I have a PLC that I use to control heating & cooling, and I'm thinking of using a timer in there to add a delay so that the blower keeps running for a minute or two after closing a gate, allowing me to move to another machine. Right now, I just open the second gate before closing the first. I also just let it run if I'm working at the same machine for a while, but still shut down the machine. Can't do that with the current transformers though.

[W Craig Wilson]

I've been using Penn State's Long Ranger (wired version) for about two years now. Works great, and is simple to implement. Blast gates cost a little more but not too bad.
By leaving the gate open, DC keeps running when tool is shut down - saves motor start cycles. I also have a 3 way blast gate manifold for my TS, Planer & Joiner - when I change tools, I open next gate before shutting last gate.

[Frank Hagan]

I'm thinking when I get my new DC, I'll just let it run while I'm working in the shop. I'll probably plan to try and group things like cutting, sanding and the like so they happen sequentially. I rarely get the opportunity to spend more than a couple of hours at a time anyway.

The Oneida I/O manual shows a Heavy Duty X-10 Appliance Module for their remote; I would think you could put one together with X-10 Modules from Radio Shack or from parts on-line.

[Rob Will]

I can't remember when or where I saw it (seems to happen more and more) but I think it was either Popular Mechanics or FWW there was a design for using CT's to turn on your DC when you turned on a tool. I have been digging around and can't find it, does anybody out there have any ideas? It is a nice clean solution especially if you run your shop of a sub panel because it only requires two ct's for single phase and three for three phase, I can't remember if the original design had a delay timer for the shutdown but that would be a nice feature to have too.

Chris

I have been thinking about the same thing. It seems to me that a time-delay relay would do the trick. If so equipped we would need a separate emergency shut off in case of some malfunction. i.e. your toupee gets sucked into the DC.
Somebody fill me in.......what sort of output can we expect from a CT and can it be used directly on a relay coil?

Rob

[Jamie Buxton]

It was FWW #143, pp 66-69.

The key element in the system is the current sensor. The author used one from SSAC (ssac.com). If you can't find the FWW article, you may find their web site informative.

[Jim Becker]

The Oneida I/O manual shows a Heavy Duty X-10 Appliance Module for their remote; I would think you could put one together with X-10 Modules from Radio Shack or from parts on-line.

I used to use the X-10 system with my Oneida setups...but kept burning out the HD switch modules...costly over time. Since then I "switched" to a contactor setup so that the 240v power cut-off for the motor is more appropriate for the application and control that via 120v. (I've posted about it here at SMC and so has Dennis Peacock who I patterned my system after) Although I used to have a switch at every major tool (multiple remotes) I now have a single switch centrally located in the shop and frankly, it's not a hassle. And I just let the system run while I move from machine to machine when that is practical and appropriate. That said, for a "weekend warrior", it's less likely that more frequent off/on cycles are going to be a problem, but the chance remains, nonetheless.

[Ken Garlock]

Chris, if you don't mind beating up on electrons, it is pretty easy to build your own current actuated switching system. All my stationary power tools are set up to activiate the cyclone dust collector when the tools is turned on.

One of my previous posts (http://www.sawmillcreek.org/showthread.php?t=17855) show some pictures of my setup. Since the posting, I have added a 11,000 Mfd capacitor across the relays to hold the cyclone on for about 15 second after the tool is turned off. The current switch I bought from CR magnetics (http://www.crmagnetics.com/newprod/ProductView.asp?ProdName=CR9321)in St. Louis, MO.

[Frank Hagan]

I used to use the X-10 system with my Oneida setups...but kept burning out the HD switch modules...costly over time. Since then I "switched" to a contactor setup so that the 240v power cut-off for the motor is more appropriate for the application and control that via 120v. (I've posted about it here at SMC and so has Dennis Peacock who I patterned my system after) Although I used to have a switch at every major tool (multiple remotes) I now have a single switch centrally located in the shop and frankly, it's not a hassle. And I just let the system run while I move from machine to machine when that is practical and appropriate. That said, for a "weekend warrior", it's less likely that more frequent off/on cycles are going to be a problem, but the chance remains, nonetheless.

That's what I'm thinking ... a manual switch in my small shop won't be an inconvenience, especially for the limited lengths of time I actually get to spend. Its very rare that I'm in the shop for more than 3 - 4 hours at a time, and usually its an hour or two. I can easily see switching on the DC as part of my set up process, and then shutting it down when I know I'm finished making dust.

[Rob Will]

Ken, what do you think about using a current sensing relay with a built-in on delay?
This could feed a conventional off delay relay and in-turn a contactor for the DC. (?) This particular one goes to 60 amps.
Rob

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The CR4395 series Current Sensing Relay provides an effective and highly stable method for monitoring electrical current. The current-carrying wire is routed through the opening extending from the top of the case. When current reaches the level set by the trip point adjustment, the relay trips and starts the adjustable timer. After the timer cycles the electromechanical relay is energized. A precision voltage reference circuit ensures a highly repeatable trip point. Design of the power-on delay circuitry allows the supply power to be repeatedly cycled on and off, without affecting the stability of the current sensing operation

[Rob Will]

I'm trying to insert a pic of a relay for you to comment on.
If I have a pic on my drive how to insert it here?

For now, here's a link to a product page.

http://www.crmagnetics.com/newprod/ProductView.asp?ProdName=CR4395

Rob

[Chris Gregory]

Thanks gents you have definitely given me some food for thought, I hadn't considered the stop sart effect on the DC motor so I am thinking about a programmable timing delay on the shutdown possibly increasing the delay to 15 or 20 minutes would solve that problem. A kill switch would also be a good idea in case of a toupe or such getting sucked into the DC :D. I don't expect to be able to pony up the cash for a Clear Vue until the end of the month so I do have time to play around. We use to use a 555 IC timer chip for some projects (seems like eons ago) but that was a different application which was basically an adjustable on off timer, what we now need to add is a secondary circuit that will keep the relay latched for a predetermined time once power has been disconnected.

[Ken Garlock]

Ken, what do you think about using a current sensing relay with a built-in on delay?
This could feed a conventional off delay relay and in-turn a contactor for the DC. (?) This particular one goes to 60 amps.
Rob

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The CR4395 series Current Sensing Relay provides an effective and highly stable method for monitoring electrical current. The current-carrying wire is routed through the opening extending from the top of the case. When current reaches the level set by the trip point adjustment, the relay trips and starts the adjustable timer. After the timer cycles the electromechanical relay is energized. A precision voltage reference circuit ensures a highly repeatable trip point. Design of the power-on delay circuitry allows the supply power to be repeatedly cycled on and off, without affecting the stability of the current sensing operation

Without studying the 4395, it sounds like it has possibilites:) The model I used cost under $20 each. It is a simple on/off operation, I would expect the timer function to add a notible amount to the cost. Maybe I am reading incorrectly, but I think the delay is on the wrong side of the operation. I want the delay in turning off the relay thus keeping the cyclone turned on after the tool is off, not on the start up.:) The delayed turn off addresses the times when you are making a series of quick cuts, like on a miter saw. It also allows the cyclone to "clear it's throat" after the dust source stops.

[Rob Will]

I agree, I thought the on-delay would be better for the motors because they would not start at the same instant (inrush current / voltage drop).

How can we accomplish this at the main panel? i.e. running dedicated machine tool circuits collectively through one of these puppies? That way only one or two relays would be required? The off-delay can be a separate relay or built into the DC's magnetic starter (?)

Rob

[Ken Garlock]

Rob, I see your thinking.:) In my case, I have two separate 30 amp circuits from the panel. Even if you have the DC and saw on the same circuit, there is a good chance that the 30 amp breaker can handle the start up surge. The next question is can the breaker handle the steady state load? Here again, most likely. The saw will only be drawing max current under max load, a condition that probably doesn't happen frequently. To be safe with no worry, I would go for a separate circuit for the DC only.

I have one 30 amp circuit down the middle of the shop that supplies my cabinet saw, jointer, and router. Someday it will also power a planer. I don't expect to use any two of these tools at the same time, so the 30 amp rating should be just fine. The bandsaw is on a 240V 20 amp circuit that has multiple outlets along the wall.

[Rob Will]

Rob, I see your thinking.:) In my case, I have two separate 30 amp circuits from the panel. Even if you have the DC and saw on the same circuit, there is a good chance that the 30 amp breaker can handle the start up surge. The next question is can the breaker handle the steady state load? Here again, most likely. The saw will only be drawing max current under max load, a condition that probably doesn't happen frequently. To be safe with no worry, I would go for a separate circuit for the DC only.

I have one 30 amp circuit down the middle of the shop that supplies my cabinet saw, jointer, and router. Someday it will also power a planer. I don't expect to use any two of these tools at the same time, so the 30 amp rating should be just fine. The bandsaw is on a 240V 20 amp circuit that has multiple outlets along the wall.

Ken,
How many circiuts / how large of a wire can you get through the sensors that you have? (how big is the hole?)

I think you are right about the cost.....besides that, it would be simpler to have both on and off delay stacked right on the DC's magnetic starter.
I have some 3ph machines to hook up, so as Chris said by using a sub panel for the tools we can sense current in the feeder wires to that panel.

Rob

[nic obie]

Ken,

Thank you for the capacitor tip.

My DC is controlled by blast gates at each machine. 10 or 15 sec would be long enough to get to another machine and open it's blast gate before the DC turns off.

I use a 24v transformer to power the control circut that activates a 240v mag contactor. Would a 11,000 mfd capacitor still be the appropriate size to use?

Thanks again

nic

[Ken Garlock]

Rob, I think you are on track. I had easy access to my power lines at each tool, so I went with the senor near the tool. The idea of putting the senor in the subpanel sounds good to me, but, as aways, be careful. The hole for the wire to pass through is 9/32, a bit over 1/4". The entire senor is 1 1/16 high, 1 1/8 wide, and 7/8 deep. There are a pair of 8 inch pigtails coming out the housing.

Nic, I don't think you can use the capacitor trick in your application, because of the current draw by the 24V coil in the relay.

The theory behind using a capacitor is based upon a concept call time-constants. One time constant is defined, after all the math smoke clears, as the time in seconds it takes to discharge a capacitor 63.4%. The 2nd time constant is the time it takes to discharge the capacitor another 63.2% of the remaining charge on the capacitor. As you can see, if you keep taking 63.2% of the remaining, you can go on forever. In practice, 3 time constants gets you close enough for most applications, up around 98 or 99 percent discharge on the capacitor.

What does this mean to your project? First off one time constant in seconds is computed by multiplying the resistance in millions of ohms used to discharge the capacitor times the capacitance of the capacitor measured in micro farads. When I installed my capacitors, I first measured the current used by my two solid state relays. I was drawing 7 milliamp at 12 volts DC. (DC is a must for this application!!!!) Ohms law told me that the two relays together represented the equivalent of about 1716 ohms. To figure the time-constant I multiplied .001715 megohms by 11,000 micro-farads. I came up with 18.9 seconds for the value of one time-constant. Since I didn't have access to a graphing voltmeter and ammeter, I used the 11,000 Mfd capacitor a trial value. Viola, the dust collector stayed on for 14 seconds, that was close enough for me.:cool:

After that lengthy discussion, the heart of your problem is two items. First, you must have a DC voltage supplying your relay. Capacitors want to act like short circuits with AC and will not hold a charge, at least for our application. Second, you need to know the effective resistance of the coil of the relay, IE, you need to know how much current in amps the coil draws. You know the voltage, 24V; after measuring, you know the current draw. From ohms law, R= E/I, you get the coil resistance in ohms. Then you can play with various values of capacitance until you arrive at a time constant value you are happy with. Since the time constant is the product of ohms and farads, if either is too small, you have a situation that is not practical to implement. Once you get over 8000 or 10,000 MFD, you need to start putting the capacitors in parallel. Ken's instantanous rule of thumb is that if you start getting capacitor values above 25,000 MFD, or your resistance value gets under 1000 ohms, you need regroup, and try something else.:confused:

For all the EEs that want to critique this posting, unless there are glaring errors, forget it. :rolleyes: No, I didn't talk about multiple time constants playing into the discharge. No, I didn't address the holding voltage for the relay, about 3+ volts in my SSR case.

[Rob Will]

Ken,
CR Magnetics shows a larger version of the sensor that you have.

http://www.crmagnetics.com/9321.pdf

Using this larger version on the feeders to the sub panel...... what do we then need to power a conventional delay-off relay?

Rob

[Chris Gregory]

Thanks Ken, Rob also thanks for the link on the delay timer I am also looking at some relays that come with a simple dial timer that you preset for the delay for the relay to stay latched once power has been disconnected once I nail them down I'll pass that info on.

In my case the CT's at the subpanel are a no brainer because I will be building a portable supanel looking ahead to the possibilty of setting up shop on site (should I officially decide to get back into the business again that is the route I would prefer to take) and this would just make life easier all round, I would then run either Festool or Fein vacs on the house circuits for the smaller power tools.

[nic obie]

Thanks a bunch Ken, I thought the 24v ac control circut would mess things up. Probably couldn't use a bridge to dc either.

I know enough to know when to ask.
(should I change my sig to that?)