Safety circuit and inverter

I have the following problem.

When I open a door or push the emergency button a motor driven by a VFD needs to stop.

I am using a safety-relay but how do I stop the VFD? Do I cut the power to the VFD (this is what we're doing now) or do I put a contact from the safety-relay in series with the run-command of the VFD or...

I've even seen a drawing from Pilz where they cut of two contactors (done by the safety-relay)on the output of the VFD.

Anyone knows?

physics, physics

There is only so fast you can stop any motor, without
destroying it or whatever is attached to it.

Attaching an output contactor to the output of the
VFD will remove the power to the motor, but, you
really need to ask yourself if this will stop the
motor. If its a fan, and its well balanced, it might
run for a while while spinning down.

OTOH, the drive might be better suited to bring the
motor to a very fast controlled ramp down, if it
has sufficiently sized braking resistors and an
input for selectable deceleration.

Beware, some of the newer VFD's need to have a load
attached, and can actually misfunction if the motor
is disconnected, so you need to consider this as well.

You can also look at a clutch/brake combination for
the driven load, since this is what is likely to be
stopped, not the motor itself.

THIS thread talks about the same problem.

I have since done a little more research myself.

Danfoss and Allen Bradley state that an output contactor is acceptable, but it should be combined with an "inhibit" input to the VFD so that the VFD enters coast-to-stop when the contactor is opened.
Siemens doesnt have a clear stance, I spoke with a Siemens person who wouldnt recommend an output contactor.
Older VFDs certainly do not like an output contactor.

It is also possible to put the contactor on the input side of the VFD.
There are two problems with this:
1. The VFD can continue to power the motor until it is discharged.
2. Most manufacturers warn against cycling power too fast, Telemecanique state that there must be 5 minutes pause !

About using only an input to the VFD: Some persons argue that the VFD can bring the machine to stop faster than when you just open the contactor. But I believe that this is simply not allowed. No VFD is certified as a safety device in the same way as a safety relay is.

Thierry, the answer to your question isn't whether to use one or the other, it is to use both the run/inhibit circuit AND the contactor. If an immediate E-stop of a VFD-controlled motor is likely to bring about a condition that is *less* safe than a controlled stop, safety codes almost always allow you to bring it to a controlled stop before opening the safety contactor. Here's how to do it:

* Exchange your existing Pilz safety relay - which I assume has only instantaneous safety contacts on it - for a different model that has both instantaneous safety contacts and timed safety contacts.

* Wire the VFD's "enable" or "run" (if you are using 2-wire control) circuit through the instantaneous contacts on the Pilz

* Wire the contactor's coil circuit through the timed contacts on the Pilz and adjust the timer's preset to be slightly longer than the VFD's decel time.

This method will allow the VFD to bring the motor to a controlled stop, then open the safety contactor. It's a lot less stressful to your VFD, your contactor and your process.

Here is circuit diagram view for Contactors control. Contactors are before VFD's.

As Jesper told, Voltage must be off enough of time before re-powering. Safety relays will set by delayed output in PLC.

As Jeff told, Safety relay itself includes off-delayd contact who is in the view under this.

Thanks everyone, seems that it isn't that simple after all.

I try the "sure" thing and exchange the Pilz relay as suggested.

Thanks

On the smaller VFDs I use if you interrupt the power to the drive, as in the case of a contactor preceding the drive, you kill any control the drive wanted to have over the motor. Contactors should go on the motor side of the drive with an auxiliary contact to control the drive. AB even states this in the 160 and PowerFlex manuals. Be careful not to create more problems than you are using something besides these drives.

I think the Run/Inhibit line is the way to go.

If it were a critical safety issue, then I might even consider an external brake of some sort.

My guess is that possibly this is used in make up air or for exhausting in an area, as opposed to something that is really a safety issue.

If safety is an issue, then the inhibit line, and possibly an external brake is the way to go.

I have never been a big fan of opening the contactor from the drive, as it could do some wierd stuph internally if there are some componets that are already bad.

Anyway, sounds like you have a handle on it.

regards.....casey

Casey, it is for a case erector.
When the operator opens the door(s) I wannna make sure there is no way the motor can run. There is an external brake on the motor. For now we kill the power to the inverter and to the brake. Just to be on the safe side (better safe than sorry) and because I am rebuiling the cabinet I'll think I'll use a safetyrelay with delayed outputs. Using the delayed outputs I'll shut off the output of the inverter. I've seen a similar application where they are using 2 contactors in serie to shut off the output. Probably overkill, but if I consider what a contactor costs I'm considering it...

emergency stop means stop the current flow to motor direct and the code says you will have to use a contactor just before the motor.
however when safety requires a brake you are allowed to use the converter as brake.
when releasing use a time delay to prevent the converter still charged when safety is ok .

shooter said:

however when safety requires a brake you are allowed to use the converter as brake.

Click to expand...

hmm, can you say where you have read that ?

I am possitive that for Cat 3 mechanical devices are mandatory in order to achieve safe standstill (EN 954-1). Electronic devices (and that means VFDs too) are not allowed.
If it is OK for lower categories I do not know.

There is a change in the regulations on the way.
It will allow electronic devices to bring a machine to a standstill, but only if the "device" is safety approved (like a safety relay).
This is allowed today in Germany, and is expected to be harmonized with other nations in 2005.

Jesper,

Example: When we have Lift, it could be better use VFD declaration and DC-dynamic braking before we close electro-mechanic brake in the motor. (We need dynamic braking that electro-mechanical brake have time to closing. In other case lift drops down les or more.)
After 'close barke' command, (a few hundred ms) we can switch off dynamic DC-brake. And after that, we switch off whole VFD from the power supply net.

Dynamic DC-braking is property of the VFD.

seppo,
what you describe is how the lift should stop during a normal stop.
Then it is certainly OK to use the VFD to achieve a precise stopping position.

In case of an E-stop you are not allowed to rely on the VFD.
Its possible to get better controled stop with a VFD, but it is a question of what the law says.

I understand the point of law. But sometimes is better break the law if we get better safety. If we read the law as wrote, maybe we 'kill' someone ?

Emergency Stop

Fact:
Each country has a different LAW on E-stops.
I actually disagree on many of the laws because they are not " as safe as they could be"

The current EU law (correct me if I am wrong), is that when an E stop is pressed, all the power to the controlling panel AND all other visible panels are simultaneously removed.. thus this renders any VFD powerless and all it's controls. The reasoning being (in part) is that any person being electrocuted from a remote source may be saved. Such items having inertia (controlled by a VFD or NOT should have an electrical/mechanical break) which can or cannot be controlled by the VFD. The ultimate provision is that on removal of power that the item of equipment stops immediately. On items of high inertia where it is impractical to have this provision, they must be inaccessible by fixed guards that are only removable by a time delay release switch such as are used on high inertia fans, rotating drums etc.

Does this help??

regards
Richard Christian