"E-Stop"

On a small 2HP belt sander powered off of a VFD I wanted to put an across-the-line "E-stop" on the incoming 240Vac (1P) and was hoping to not have to add a control transformer and contactor; In other words just wire in a two pole switch.

I'm not seeing any contact blocks for 30mm switches rated for more than 6A. I was thinking more in the lines of 20A / 250 Vac

Am I dreaming?

The drive itself most likely has accommodations for a E-stop, and one that has the ability to brake quickly, versus killing the input power, which makes very little sense...IMHO.

The title of this thread may possible get it closed as it doesn't clearly explain what you're asking.

Stuart

If you want to directly switch the power as an E-stop, a better option could be a magnetic paddle switch.

These have the advantage of requiring a manual reset after a power failure so your machine can't unexpectedly restart if power returns or you plug it back in - probably not an issue with a VFD system though.

No reason you can't use an E-stop with a 230V contactor if you want to take that route.

Cyclotronguy said:

On a small 2HP belt sander powered off of a VFD I wanted to put an across-the-line "E-stop" on the incoming 240Vac (1P) and was hoping to not have to add a control transformer and contactor; In other words just wire in a two pole switch.

I'm not seeing any contact blocks for 30mm switches rated for more than 6A. I was thinking more in the lines of 20A / 250 Vac

Am I dreaming?

Click to expand...

What Stuart said. Wrong place. GOOD place for a "disconnect". I have both, right at the operator position.

Otherwise, you want the "brains" of a VFD (or DC Drive) to remain powered so they can do their programmed best at rapidly braking to a stop. Cannot do ANYTHING predictable if no power to their "smarts".

Belt sander of what size and configuration? Clothing could get caught? Probably not. So rapid braking maybe not a factor at all?

Now, think "lathe" instead of "belt sander" for a moment.

If/as/when/where you NEED that assured rapid stop, there should be a spring-actuated mechanical brake held-off by power. Most, not all, that would be at the motor.

THEN .. if the VFD (or DC drive) were to go nuts - say off a cut or broken control wire - and send RPM of a heavy or imbalanced workpiece in the direction of low-Earth orbit?

Slap THAT sort of E-stop, ALL power is cut, and the brake saves the day.

That's where you came in this door. Cutting ALL power.

But should you also have a brake? Pass. I know nada about your situation.

Your shop. Your needs assessment. Your risk assessment. Your budget as well. Your plan to address them.

The technology is the easy part.

So.... what kind of an "E-stop" is wanted? The "real safety nazi" type, or just a button to stop the thing?

There are requirements for a real safety E-stop, that may not be satisfied by a command to the VFD. A stop that breaks power like the "STO" switches (Safe torque Off)do, where the gate drive power is cut, might satisfy some levels.

Someone else can correct me, but IIRC the "full monte" E-stop has to cut power to the machine.

thermite said:

Otherwise, you want the "brains" of a VFD (or DC Drive) to remain powered so they can do their programmed best at rapidly braking to a stop. Cannot do ANYTHING predictable if no power to their "smarts".

Click to expand...

I disagree. The only time a malfunctioning electronic device behaves predictably is when it has no power. Coasting to a stop will end a lot of problems, and just having another switch that does the same thing as the stop button, which is the first thing an operator would push, would be useless. Also, I doubt a VFD running a belt sander even has much braking capacity to begin with.

I do agree that magnetically released brakes are the real answer, with all power cut.

Use a regular mechanical motor starter with a big red paddle. Rated to plenty of current and you can disconnect it easily. It's not a mushroom shaped button but it really doesn't need to be.

Strostkovy said:

..doubt a VFD running a belt sander even has much braking capacity to begin with.

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Probably ZERO. But "control Voltage" grade E-Stops are as little as five bucks, need only be in series with "ENABLE" .. "RUN" ...or "something like that".. depending on VFD.

Line power switching is going to be more expensive.

I do agree that magnetically released brakes are the real answer, with all power cut.

Click to expand...

"Depends".. Some machine-tools that would just make it harder for a person caught-up or already injured to get "untrapped" and off to medical care.

Use a regular mechanical motor starter with a big red paddle. Rated to plenty of current and you can disconnect it easily. It's not a mushroom shaped button but it really doesn't need to be.

Click to expand...

Same a big side-lever (fused or not) "disconnect".

Rotary disconnect might be harder to operate under dire circumstances, but I'm good wit' dat' on a 10EE where there are several safety "off ramps" already downstream of it.

Never personally gave much thought to E-Stop on a belt sander.

AM WELL aware they can sand "body parts" off fast enough yer bleedin' before yah even FEEL the abrasive at it's work!

I just don't see where an E-Stop could "intercept stoopid" fast enough to prevent bloodshed?

"The big red button" is of course for those cases where you, or whoever hears the noise and comes on over, can't reach the regular off button, or can't both keep worse things from happening and reach the button at the same time.

That's the reason for the knee operated stop buttons on table saws, also. Sometimes the stop button is close to the start button that, although it is supposed to be the first/only one hit if you just slap the control box, \ a person might hit it, and then a different finger hits the start. Not always a positive stop in an emergency.

The E-stop is supposed to shut the machine off, latch, and require an intentional restart that involves re-setting the E-stop button mechanically.

JST said:

So.... what kind of an "E-stop" is wanted? The "real safety nazi" type, or just a button to stop the thing?

There are requirements for a real safety E-stop, that may not be satisfied by a command to the VFD. A stop that breaks power like the "STO" switches (Safe torque Off)do, where the gate drive power is cut, might satisfy some levels.

Someone else can correct me, but IIRC the "full monte" E-stop has to cut power to the machine.

Click to expand...

That is true, cutting full power to the machine is required for an E-Stop however the logic side of things is allowed to stay powered up. On most industrial caliber controls, the PLC and logic is powered from in front of the panel disconnect. Power to the output cards is inhibited and is part of the E-Stop circuitry.

In the case of a VFD, removing power from the VFD on the line side does not instantaneously remove power from the driven motor as the energy stored in the capacitor bank must be dissipated before motion stops. This method is not considered to be truly E-Stop safe.

This is why the STO is on the output side of the VFD as it disconnects power when activated. Most jurisdictions accept the STO as part of the E-Stop. It is good design to have the logic side of the VFD powered separately from the line power as you want the logic controller to be fully booted up before allowing motor motion.

Th OP's application is a little different than most industrial applications in that it is only 2hp with likely an appropriately sized VFD so the energy storage of the capacitor bank will be somewhat limited.

In this case a dead man switch would be appropriate so hands being free would not be necessary for activation, just removing your foot. If the OP truly is after safety, using a VFD that implements a true STO would be best as you instantaneously de-energize the motor and you still have the logic controller powered up.

If you use a switch on the line side of the VFD, every time you disconnect the power, the VFD must go through its boot up routine which will get rather frustrating. In addition you will either still need to use a Run command signal to start the VFD or else configure the VFD to be in Run at startup which really circumvents the whole idea of an E-Stop.

The energy stored in the capacitor bank is generally fairly negligible in terms of rotation; I may have to do some testing. I've seen safety-certified machines with safety relays and dual contactors in series feeding VFDs.

Some VFDs have the option of supplying 24VDC to keep the drive electronics live when mains is not present.

One intriguing option is that cutting line power prevents the drive accelerating (as it can't draw power) but the DC bus capacitors can still supply the electronics for a while. Many drives have a 'power-loss ride through' mode where they brake the motor to recharge the DC bus, aiming to keep the electronics live during a short power cut like a transfer switch operating.

This could probably be (ab)used to make the drive brake to a stop ASAP, while absolutely preventing acceleration because there's no AC energy source.

Thanks guys: I was going for the easy answer on a VFD powered 2HP 2" x 72" belt machine that I had originally wired into a manual fused disconnect.

I just ordered up a new enclose, control transformer, etc and switches so we have a big RED E-stop other than the fwd-off-rev on the VFD panel.

Ziggy2 said:

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If you use a switch on the line side of the VFD, every time you disconnect the power, the VFD must go through its boot up routine which will get rather frustrating. In addition you will either still need to use a Run command signal to start the VFD or else configure the VFD to be in Run at startup which really circumvents the whole idea of an E-Stop.

Click to expand...

The E-Stop to a line side cutoff should be no hassle, as you would not use it unless you had no access to the standard stop button.

E-stop would be a separate system.