Automated Controls for a Variable-Speed Dust Control Setup

Here's an extended discussion of the instrumentation and controls needed to enable a dust collection system to adjust to changes in the shop machinery in use from moment to moment.June 15, 2014

Question (WOODWEB Member) :
We are looking at the Ecogate Greenbox system to help reduce our electricity bill a bit. We have 2x55w extraction motors, and 19 blast gates on various machines. We have had a quote and it seems very high priced - $26K. I'm looking for a breakdown of the costs Ė does anyone have any?

Ecogate Greenbox 12x2
19 Blastgates
Inverters x2

Forum Responses
(Dust Collection and Safety Equipment Forum)
From Contributor H:
I've used my own shop-built automatic system for our dust collector for years. It's simple to construct, reliable and very inexpensive. It can incorporate air cylinder activated gates (again shop built) or manually opened gates. I have both in various locations throughout the shop. All you need to build this is a relay that is sized to your blower motor, a 24v AC transformer (doorbell transformer) and some magnetic reed switches (burglar alarm switches). You power your blower motor through the high voltage side of the relay which has a 24v AC coil. Any 24V AC current applied to the coil will close the contacts to the blower motor.

You then mount one of the magnetic switches to your dust gate. You want to use a switch that will close the contacts when the gate is opened. This is best done by mounting the switch on the housing and the magnet on the sliding gate. When the gate slides into the open position the magnet comes close to the switch and closes the 24v AC circuit, thus activating the coil on the relay and starting the blower. You put these magnetic reed switches on each dust gate and wire them in parallel so any switch will complete the circuit and start the blower. Since this is a low voltage switch circuit you can run as low as 22 gauge bell wire. There are more details and intricacies I've learned over the years. I've replaced the relay with a magnetic starter and a safety cut off switch in a central shop location. I've also changed my reed switch circuit to 24v DC as it puts less electrical stress on the switches. I also now have the secondary blower on my shop built 3-bag collection unit tied to the system.

From the original questioner:

Well done for doing that yourself, sounds like quite the project. The main issue for us is not just the gates opening and closing, but the extractor fans running at full tilt all the time all day. I need a way to reduce the speed when gates are shut and speed up when gates are open, like a variable speed controller on the motors controlled by a PLC, and some volt sensors on each machine to trigger the plc and set the motors to either speed up or slow down.

From Contributor H:
Yes that makes sense. You have a larger and more complex system than mine. I know about motor current sensing switches (I have one in use) but applying varying sensed current draw to a variable speed motor control (probably a frequency drive) would require a good bit of research for me to figure out how to build. Probably could be done though. I have a good friend who is an industrial electrician and I'm sure he would know how to pull this off. If you can find someone like that to talk to locally you might find that they can build if for less than the cost of an Ecogate system. It's probably a bit of a trade-off though - less money for a design/build system and more for the prepackaged and ready to go Ecogate unit. The question of course is how much less for the design/build system.

From the original questioner:
Exactly! I think that buying all the components would come to about 15k, and thatís the autogates, sensors, plc and variable drives/controllers. I have looked at a few open source solutions for the plc, with multiple outputs /inputs and one for each gate/voltage sensor. These allow for a signal to be interpreted by the volt sensor to trigger the plc. The plc is programmed to allow set parameters to fire off (like port 1 = 15hz or some other unit of measure), and send this to the motor. You can buy the hardware pretty inexpensive, and the software is free.

From Contributor T:
I have used Ecogate and regretted it. You can build far better blast gates yourself. I thoroughly recommend variable speed drive for your 55kw motors using inverters (not necessarily Ecogate's VSD). Your shop will be so much quieter and you will save a fortune in electricity. It is true that if you half the speed of your fan, its electricity consumption will reduce to 12%. I have measured this myself. Shop around for your variable speed drive. Ecogate was going to charge me 15,000 euro for one inverter panel. I ended up buying a panel with two inverters in it for 4,500 sterling.

I still use the Greenbox GB12 unit and am gradually replacing the Ecogate blastgates. I installed slave relays in all my machines to trigger their blast gates. You can't get a more positive signal than doing it this way. I have been working on my extraction system on and off for about ten years now and it has been more than worth it. The extraction comes on automatically when a machine is activated and the fan spins at a speed accordingly. No hiking, no noise and no large electric bills.

From contributor GD:
"I know about motor current sensing switches (I have one in use) but applying varying sensed current draw to a variable speed motor control (probably a frequency drive) would require a good bit of research for me to figure out how to build. Probably could be done though."

You can do it that way, however the result that you are trying to achieve, variable power to the dust collector based on need (to save money), is far better done from the other end. Simply install a vacuum (flow rate) sensor in the main duct a short ways before the DC. Wire it to your VFD. You will be measuring air flow rate (vacuum) and can set the VFD to maintain any desired flow rate without a complicated computation of the various machine motor loads.

From the original questioner:
I had also thought of this vac sensor idea, but having never actually seen one in the flesh. I was worried that the dust and shavings would foul the sensor and damage it. What kind of frequency drive would you use for this?

From Contributor T:
I've not tried the vac sensor on the main duct because some of my machines require a higher vacuum than others so I would have to create a high vacuum in the main duct when most of the machines wouldn't need it, e.g., I have a sheet saw that needs a high vacuum to try and reduce the amount of dust it throws out. Also, if a machine is a long way from the DC in comparison to other machines, there would need to be a high vacuum in the main duct to counteract the resistance from the long ducting for just that one machine. I can see that if the ducting were perfectly designed and each machine required the same vacuum, a vac sensor on the main duct would work but how many workshops are like this? I have chopped and changed our ducting over the years and it is far from perfect but I imagine this is the same for a lot of shops.

It's not that mathematically complicated to work out what speed a fan would need to run at for each machine or each combination of machines. After around half rated fan speed, the volume of air extracted is roughly proportional its speed. All I've done is effectively assigned a speed of fan to each machine but offset the half rated speed because below this speed I have assumed there is virtually no vacuum. I started small in a similar way but I ended up doing it all anyway.

From the original questioner:
So I have a pretty clear image in my mind what I want to do.

Can anyone suggest any of the following items and costs:

1. Variable Frequency Drive hardware x2.
2. Voltage Sensors x10.
3. PLC hardware and software to run.
4. Automatic blast gates.

From Contributor H:
As to dust gates, what are you using now? It may be easy and less expensive to convert them over to automatic with an air cylinder system.

From the original questioner:
We just have manual steel gates at the moment and to be honest theyíre not that great. I think what Iím going to do is small scale it all first, and get it all working correctly. Sometimes that helps the head understand it.

From Contributor H:
I have a few steel gates in the shop. They don't have the air cylinders on them but they could. They do have the low voltage reed switches to automatically turn on the dust collector when the gate is manually opened.

From contributor M:
I am not up to the intricacies of tackling the automatic gate system. All I need is to control a 10 and 9" gates that are high up and difficult to reach. I am thinking an air cylinder with a manual valve to operate each gate. Not an elegant solution but something I can handle. Do you have information you can share, such as what type of air cylinders and gates to use and perhaps where to find them and some pictures of how you have yours set up?

I am in the initial/planning stages of installing an Aget cyclone, #70SN70-D2-SP (15HP) with a baghouse after filter #FT40-D1-SP. It is possible that since I am only hooking up a beam saw (10" port), a ptp (8") and an edgebander (4 or 6", don't remember), the gates won't be necessary, but I would hate to have to come back to retrofit it. We are using a 7.5 HP Dustek now but when the 10" duct is open the other machines get little suction. Any feedback on this would also be appreciated. It is amazing the information shared here. I had never heard about the use of an inverter in a DC application. I will have to keep an eye in the cost of running this (for me) beast. Going from 7.5 to 15 HP would not necessarily double the electric consumption, would it?

From Contributor H:
If you really want to do this at a low price and easy then consider using a bungee cord to pull your existing gate closed. You can pull it open with a string/cable with an S-hook on the end to hold it open. If the gates are high up you'll just need a pivot point to hook a screw eye or pulley for turning the corner to come down to the floor. Then when you release the S-hook the bungee cord will pull the gate closed. This seems really low tech and perhaps even a little amateurish. However we use bungee cords on two of our gates to pull them closed. One is a gate that only has the cable and bungee and the other is a gate that has a one way cylinder that won't push the gate closed. Talk about mixing low tech with high tech. That gate is on our CNC and an output code from the controller activates the air cylinder for opening. Itís sort of a silly solution but very reliable.

From contributor M:
It actually sounds pretty good, especially for a short term solution (well, you know how that goes, once it works, it stays in place forever). I have a 10" gate fabricated and installed by a local sheet metal shop when the system was installed. To say it is reluctant to close would be kind. I had been thinking about springs of some sort but the bungee cord idea seems very practical.