Snubber design and safety - Page 1
Forum section: electronics / projects

author: iXod
author_rank: Frequent Contributor
timestamp: April 07, 2018, 02:39:50 am
content: I'm building a motor test station for a hydraulic repair shop. The user will hook up AC induction motors (120, 240 single and 3 phase) to test hydraulic pumps. The tester will utilize magnetic contactors to energize the motor connection leads with alligator clips and insulating boots.I am considering using snubbers across each pair of contacts in the contactors but I'm a little put off because of a safety concern.These RC snubbers (I haven't decided whether to buy or build) will pass some current to the output when the contactor is de-energized. Normally in an enclosed, no-human interaction design I would probably go ahead and install these without a second thought. But with the likelihood that these motor connection leads and alligator clips will be handled while making connections for the test procedure I'm obviously concerned about any current through the operator to ground.Are there alternative means to suppress contact arcing that don't involve shunt snubbers that might conduct current when the contacts are open?Suggestions welcome.Thanks.

author: jbb
author_rank: Super Contributor
timestamp: April 07, 2018, 04:11:25 am
content: Quote from: iXod on April 07, 2018, 02:39:50 amOK, there may be some safety issues to do with unprotected connectors and whatever they clip on to. Careful design of the complete testing station will be required to manage that (hint: a safety door with interlock switch would be nice).Contractors are good for this sort of test because they open a nice physical gap. But an important part of such a design is to consider the impact of component failures, e.g. sometimes a contractor will fail to open (e.g. welded contacts). If at all possible, the test should be designed such that no one will die even if one component fails in the worst possible way.I suggest the following:Snubbers may not be required if using a heavy duty contractor rated for motor starting.If snubbers are necessary, don't put them across the switch contacts. If a snubber fails short circuit, it could apply mains to the output. Consider MOVs or similar to shut currents to neutral or earth (which should be safe to touch).Use an extra contactor to short the three output leads to protective earth when the switch is in the 'safe' position. Connect your big green 'safe' light to the auxiliary terminals of this contractor so it only lights up once the output leads are shorted to ground.Disclaimer: these are just my thoughts. The actual implementation will be up to you as you are working on the complete system. Depending on your location there could be all sorts of special requirements (e.g. inspection, certification) or scary liabilities.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 07, 2018, 03:27:38 pm
content: How about a MOV instead?Same rating as you'd use across the line.Obviously, for the same reason, these will pass mains transients. Should be uncommon. In any case, treat the output aslive, unless the equipment is fully de-energized.Tim

author: ConKbot
author_rank: Super Contributor
timestamp: April 07, 2018, 11:26:05 pm
content: Few options.1)Use an oversized contactor, meant for a motor larger than the panel will be capable of running and just let it take the hit.2) use 2 contactors in series, one functions as a motor starter, and you can add snubbers of choice to it, and a second contactor to provide physical isolation that the snubbers bypass on the first. It won't typically break current, so the coil should burn out before the contacts fail.Remember that safety contactors exist, all of the contacts are mechanically linked, and if one set of contacts weld somehow, the auxiliary contacts won't change positions. Useful for a safety interlock or indicator.Also be sure to check into your requirements for physical isolation distance. Some contactors may meet the requirements, some may not. Plus you'll have to consider failure modes for the contactor and what is controlling the contactors. A fused or non-fused disconnect (I.e. the wall type disconnect switch) on the output of the machine could serve as disconnect also. However that would count on the operator to have to flip the switch before reaching for the leads.

author: floobydust
author_rank: Super Contributor
timestamp: April 08, 2018, 12:08:20 am
content: AC motor contactors are designed for arcing, don't put a snubber across contacts unless you have a safe work procedure to switch off the disconnect to deal with the leakage currents.Because human hands are on the motor's wiring and possibly output shaft, you will have to have a lockout for safety, and E-stop if phase-sequence (rotation) is wrong.I use VFD's to test hydraulic pumps, with an extra (safety) output contactor.Less drama if there are problems with the pump/hoses etc. and you can make a pump curve too.

author: iXod
author_rank: Frequent Contributor
timestamp: April 08, 2018, 12:39:18 am
content: Quote from: floobydust on April 08, 2018, 12:08:20 amI like this idea. Can you expand a little?Do you set output to 100 percent (50 or 60 hz)? Then close the safety contactor? Will VFDs take instant load like that?Or is your operation different?

author: floobydust
author_rank: Super Contributor
timestamp: April 08, 2018, 06:35:56 pm
content: In a manufacturing environment, I did hydraulic and water pump test bench designs. Most 10-100HP 3-phase.A VFD allows variable speed and you can set a torque limit. Software profiled a pump in real-time with speed, flow, pressure.You can go above synchronous in speed, limited to constant power. 0-60Hz usually.Because operators use (hands) a snipe and pipe wrench to attach the pumps to the test bench electric motor, I added to the VFD a safety output-contactor, which would ensure the motor was disconnected. I added a light tower to show operators when it was safe to attach/detach pumps.You can hit the STOP button on a VFD but it's firmware is not safety-critical and people could get injured or killed if the VFD did anything.I programmed the VFD to wait 1 second after START was pressed, to give the contactor time to engage first.Pressing STOP, the VFD stops quickly and the contactor takes longer to disengage. So no contact wear really.Your approach to use a contactor is all or none, and usually someone forgets to tighten a hose or screws, so that would be scary.

author: CopperCone
author_rank: Super Contributor
timestamp: April 09, 2018, 04:22:25 pm
content: Honestly based on your procedure that the technicians use snipes to connect the pumps, you should have a seperate workbech where a cord is fixed to the pump, then your hysraulic test bench where the pump is connected to the hydraulics and thrn plugged in.By making sure you fix the cord to the pump, i assume by screw terminals, you basically remove any reason for the technician to handle and sort of bare conductor that can for whatever reason be enrgized. At this point even if your control system completely fucks up, your operator isonly handling a large plug.Depending on your plug choice it can be very safe.Then all your saftey systems become secondary in nature rather then critical. If you want th bafoon proof it dont let any screw drivers or other electrical tools anywhere near the hydraulic work bench to prevent momentary lapse of jidgement accidents and clear legal protection for yourself.I mean, we all probobly renove the battery from a sawzall before changing the blade right?A recommendation i have for design of a workbench is to put high density polyethylene rails down on the bench surfaces and the transport cart to minimuze how difficult it is to relocate heavy things. Also will prevent something in sitting in a pool of water due to a slight leak. This way you can maximize sliding. If you get the right height on your transport cart this whole relocation procedure will become easy.Having gotten soaked around high voltages i recommend that every measure should be taken even if it slows things xown a bitShould not be touching high energy mechanical or electrical elements when just a switch is controlling it. Even in electrical work your supposed to use a lockout tag when doing mains stuff even after the circuit breaker is disabled.It seems ok to connect hydraulics at the work bench since it seems unavoidable towards how the test is conducted and theoretically even with a leak the entire device should be grounded. Might want to leave specialized tools for hooking up the hydraulic process water though, to discourage using something like a adjustable wrench to adjust some kinsa shaft or whatever, only leave the sizes used for the connectors on the hoses.Should be designed so that if your shoot high pressure water into the motor there is no operator hazard other then slipping. Leak detector could not hurt either on the table top. Should be a dry area.