Safety Circuits and Contact Ratings

[DDgitfiddle]

Gents,

I'll apologize in advance for the long-winded question. I am in the design phase for a DIY control box for a 3-axis CNC router, and I am struggling a little with the safety system. I want this the be safe, and I am at a stage where I can change my plan without any cost impact. I understand that I need to shut down any motors that are moving when I hit an e-stop. I also understand that the NFPA code would have me cut the power to the motors.

I am planning to use a monitored safety relay. I don't trust the "safety" circuits that are part of the BOB or motion controllers because they are all software controlled. I'm planning to use UCCNC, a UC300ETH, and an MB2 BOB. This setup has an issue with enabling outputs briefly on startup. The safety relay will solve this absolutely by forcing a hardware reset button push at startup. Here is an example of one relay I am considering:

PLC Hardware - Allen Bradley 440R-C23139 Series B, New Surplus Open

I am planning to use Teknic ClearPath SDSK motors. The ClearPath motors have an "enable" pin, but when I contact Teknic, they indicated that for safety I need to drop the motor power supply, not just the enable. That makes sense, also. I am planning to use the Teknic Amazon IPC-5 Power supply (Rated for 220VAC, 500W output, 900W output peak, and a less than 65A in-rush current (!!!!!)). Teknic indicates that you should not switch the DC side of the power supply because the contacts would be damaged, and that the AC side should be switched.

I am also planning to use a Hitachi WJ200-022SF VFD for the spindle. This VFD has a Gate Suppress function that seems to be designed to stop the motor according to code (per EN60204-1). I am planning to use this function because it appears to be the intention, and the contactor needed for the VFD would be pretty big.

So here are the questions I am running into:

1) What size relay contacts do I need to specify for the Teknic IPC-5 power supply? If I go by rated power, (500W), I would guess that I need about 2.5 - 3A at the contacts (accounting for efficiency in the power supply). If I look at the max power (900W), I would need about 4 - 5A. If I look at the max in-rush current, I will need 65A. That seems ridiculous. The safety relay has contacts rated at 6A/250V.

2) Assuming that I need to add a relay to switch the motor power supply, is this a good application for a solid state relay?

3) Am I over-thinking this? Should I just switch the enable signal on the ClearPath motors?

Thanks in advance for the help!

-Robert

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[Jim Dawson]

The E-Stop circuit should cut power to all of the output devices.

For inexpensive high power relays, I like these or similar. Google ''PRD relay'' https://www.automationdirect.com/adc...AD-PR40-2C-24D

Never use a solid state relay in a safety circuit, many times their failure mode is a dead short, this has the effect of it being turned on and you can't shut it off. I do like to use solid state relays with zero crossing turn on in series with an electromechanical relay to mitigate some of the inrush. These turn on when the sine wave crosses 0 volts and brings the power up to max over about 8 ms, gives the caps a chance to charge more slowly thus reducing the average current on the feed.

The enable signal should also be cut on an E-stop. And should not power back up until you press the Enable button after the E-stop condition is cleared.

[DDgitfiddle]

Thanks for the reply, Jim. I'll check out the relays you suggested. Would it make sense to use the safety relay to control a SSR? I suppose that the fail-short failure mode would essentially cause the same problem.

For the enable signals, my plan was to have a dedicated distribution block of 24VDC that would then distribute to each of the enable signals and return through the breakout board. I would then run a single line of 24VDC through the safety relay so I don't need as many contacts.

What contact rating do I really need for cutting out the power supply? Do I really need to aim for 65A/220V?

Thanks,
Robert

[Jim Dawson]

The 40 Amp PRD relays will more than switch your power supplys. The contact rating is for continuous duty, they will make or break much more current.

The only place I use SSRs is after an electromechanical relay or power switch. They have a very very low failure rate, but I have seen them fail. I would not rely on them for an E-stop circuit, besides I like to hear that satisfying ''clunk'' when an electromechanical relay pulls in, that way I know it did something.

The only problem with using the PRD relays for control power is that you have to add another small relay to switch the coil, because they don't have a third set of contacts for the seal in. The additional relay is to provide a ''seal in'' for the PRD coil, this is a standard two button control (start/stop) circuit.

[CitizenOfDreams]

A big contactor is nothing to be afraid of. In SCM Author machines, for instance, the safety relay is driving a contactor that energizes the 11kW spindle VFD.

[Al_The_Man]

If you use a 3 phase contactor ( I generally go for Telemecanique) you can control the VFD input with 2 contacts and could use the third for other 1ph motor supplies. As you do not switch the neutral.
There are several versions/ratings of Telemecanique contactors on Ebay.
If your Safety relay has other output contacts you could use one of these to advise your controller, Mach etc, that a E-stop has taken place, Otherwise an aux contact on the Contactor could be used.

Al.

[DDgitfiddle]

Thanks everyone for the replies. The safety relay I am looking at has 3 NO contacts and 2 NC contacts, and will switch 4A at 220VAC. It has the reset circuit built into it, so I don't think I need the "seal in" relay that Jim mentioned (if I am understanding correctly). I will be able to use one of the NC contacts to signal the controller about the e-stop. One NO contact will drive the contactor for the power supply. Al's thought to use a 3-phase contactor would simplify things a little and leave a contact on the safety relay for switching the enable signal to the motors, and another one for the VFD enable (from the BOB) if needed.

So in theory, I think this covers everything. When the system powers up, the safety relay will be deactivated until the reset button is pushed. This gives time for the computer, controller, BOB and power supplies to come up without turning on things that shouldn't turn on. I'm sure there are other ways to make this happen, maybe for a little less money, but this will work.

Thanks for the help!
-Robert

[Al_The_Man]

The Telemecanique line also have add on aux contacts that clip on the front of the contactor.
And other featured add-on's.
Al.

[ger21]

One thing that you might want to consider.
I'm running the reset button on my safety relay through a small relay, activated by the charge pump on the MB2. This prevents you from powering the system until UCCNC is up and running.

[DDgitfiddle]

Thanks for the idea, Gerry. I do want to do something along those lines. It will probably help to keep me from bringing up the system in the wrong order.
Thanks,
Robert

[CitizenOfDreams]

By the way, contactors sometimes DO get stuck in closed position. I just witnessed one myself. A tank of molten glue overheated from 280F to 460F was not a pretty picture.

[DDgitfiddle]

One thing that you might want to consider.
I'm running the reset button on my safety relay through a small relay, activated by the charge pump on the MB2. This prevents you from powering the system until UCCNC is up and running.

Gerry,
I finally have some time to put together a schematic before I buy any more components. Is it feasible to map one of the relays on the MB2 to the Charge Pump signal from UCCNC running a UC300ETH-5LPT? I'm just trying to use the available resources on the board if they make sense.

Thanks,
Robert

[ger21]

I don't think you can do that. But I'm far from an electronics expert. I guess you can assign the charge pump pin to the relay and see if it will activate it? It might work.

[DDgitfiddle]

That's what I was thinking. It doesn't really save much effort or money, but its worth a try. I don't have the boards in house yet, but I'll give it a try when I do. That will be a few weeks. Thanks!

-Robert

[ger21]

I know that when the charge pump is active, the MB2 activates the charge pump output. But I don't know if the charge pump signal itself can activate any other outputs. I may be able to check this tomorrow, by simple changing the port and pin numbers on my test rig.