e-stop, safety relay for VFD motor

[liew_sc]

any advise on circuit for e-stop safety relay and
frequency inverter controlled 400 kg flipping
machine.

direct cutting of power to the motor cause heavy
mechanical stress.

is it still consider safe if we place a off
delay timer to delay cutting off power to the
motor (letting the inverter to decelerate till stop)

what is you normal configuration ?

 

[PLucas]

The emergency stops on our equipment inputs into the drives to put them into rapid braking, but the emergency stop also slams the mechanical brakes onto whatever motor(s) is (are) running at the time.

An emergency stop is what it says it is AN EMERGENCY STOP!!!! All movements should stop the instant it is pressed.

A time delay on this circuit is definately not the way to go, a little stress on the machines when someones life is at stake is a small price to pay.

If your company are considering allowing a time delay to be placed into the estop circuit, I thank God that I don't work there!

Paul

 

[Steve Etter]

By my way of thinking (and I believe OSHA's, too), Emergency Stop is purely a safety device and it means just that: Stop for an Emergency! Do It Now! It should ONLY be used for emergency purposes.

It is debatable as to whether things like regenerative braking should or should not be used or whether nip rolls and the like should be powered open or not, but that is another question all together.

So anyway, when an E-stop is used I don't care too much about mechanical stress, unless of course this in itself creates a new safety hazard. Such issues absolutely must be taken into account during the machine design so that and E-stop can be safely provided. If they are not, then it is a poor design and one that should not be accepted. Period. No ifs, ands, or buts.

For the remainder of my speech, I use large web-handling machinery as my basis of thinking:

Because not all rapid machine stops have to be E-stops, a Rapid Stop can also be incorporated. This type of stop should be labeled just that way and should then incorporate the deceleration you describe. This is usually intended to bring the machine to an orderly halt as rapidly as allowable to maintain as much machine and product integrity as possible. This can be used any time an operator deems it necessary.

Lastly, there is a Normal Stop. This is a stop that allows everything to ramp down together and is the type of stop the Machine operator should normally use. If done well, this type of stop will usually allow the machine to be restarted with minimal setup or operator intervention.

That's my position anyway.

Steve

 

[Jim Dungar]

NFPA 79 is the "Electrical Standard for Industrial Machinery" put out by the same group that does the National Electrical Code in the US (http://catalog.nfpa.org).

They define three categories of stops (not exact wording):
Category 0: stopping by immediate removal of power (i.e. uncontrolled stop)
Category 1: a controlled stop where power is left to the machine actuators until stop is achieved then power is removed electromechanically
Category 2: a controlled stop where power is maintained after stopping (i.e. a normal stop)

NFPA 79 allows Emergency Stopping to be either category 0 or 1 based on the needs of the application.

So unless an agency (like OSHA) prohibits it, controlled stopping before removal of power is/should be acceptable for emergency stops.

 

[DickDV]

E Stop

At least in the upper Midwest where I work, OHSA rarely defines an E-Stop as a stop so immediate as to cause mechanical damage. They generally define stop in terms of a maximum amount of travel after engaging E-Stop. On rolls, for example, it could be 1/4 turn or some other value.

It seems that each application has a different target to meet and, as a drive application specialist, I regard it the customer's responsibility to know exactly what target they are required to meet and to inform me accordingly.

From this information a properly designed system can be selected. In some cases, the braking hp is higher than the motoring hp. The system design naturally has to reflect this.

 

[Billando]

I recently worked in a plant that used E-stops for all stoppages.This was creating havoc with V.F.Ds. Dropping the power off them and then trying to restart them before they were ready to go again. The drives would go to fault.There were a couple of problems to overcome. But the main fix was to put machine stops (take away the enable signal on the drive but not power them down)that were in the right positions so that the operators would use them. The E-Stops then were only used as they should be in emergencies. All of a sudden no more problem.

 

[DickDV]

E-Stops on VFD's

Using E-stops to cut power to the VFD is generally bad practice for the following reasons:

First, as you mention, an immediate restart will often result in a fault or even scrambled drive brains because the initialization period hasn't been completed.

Second, nothing stresses a drive power circuit more that power-up. Needlessly powering down for routine or even E=stops will shorten drive life.

Third, and maybe most importantly, killing power on the drive will often leave the motor to coast to a stop. This can be dangerously long. The drive electronics can often bring the machine to a safe stop FASTER if it remains powered up and in control of the motor. This is particularly true if snubber braking equipment is part of the system. Other drives use DC injection to make the motor act as a brake which requires input power to activate. Other drives use a clever scheme called flux braking to saturate the motor with braking energy but this also requires input power.

In my view, it is far better to leave the drive in control, invoke an E-stop input on the drive and allow the drive to manage electronic braking or mechanical braking via its output relays, whichever your system has.

There is always some concern for safety under power failure conditions but, normally, a machine will coast to stop when power fails. The chance of a safety mandated E-stop occurring at the same moment as a power failure is usually very small (cranes and hoists are a major exception). If that is a concern then a power release-spring set safety brake needs to be installed on the machine with its controls interlocked with the VFD to avoid the possibility of the drive trying to run while the brake is set.

 

[gbradley]

Bad Practice?

First, as you mention, an immediate restart will often result in a fault or even scrambled drive brains because the initialization period hasn't been completed.

Excuse my ignorance, but don't most VFD's Have Huge Capacitors. The Woods drives that we use, sense a loss of power, and always come back nicely after a loss of power.
We've had dozens of presses using them for years, and every single E-Stop cuts power to the VFD.
Of course We don't power down the VFD on routine stops, but All of our E-Stops Power down the VFD.
Leaving Power to a VFD Drive after hitting an E-Stop just seems spooky to me.

 

[Alan Case]

As far as I know most OH&S standards DO NOT allow an E stop function to be controlled by a semi conductor device ie VFD (safety PLCs excepted). You must use a positively driven contactor, and the Estop is for that reason, an emergency ie as your co-worker disappears into the machine, not to shut the machine down for a smoke break. Regards Alan

 

[DickDV]

E stop

Drives do have large capacitors on their DC bus but that doesn't really solve any problems. Once the DC bus voltage drops to the equivalent of rated input AC power less 10%, the processor is switched off.

Under power-up, once the DC bus recharges to the above minimum voltage, the processor on most drives does an initialization run and also a diagnostic routine. If a run signal comes in during this preliminary process, usually it will be ignored. However, I have seen on many different brands of drives an occasional misqueue when a run is received before the processor is done with preliminaries and it goes off into the ozone somewhere or worse. That is one of the important purposes of drive READY relays. It won't pick up until the prelim's are done so you can use it to block early run signals.

I don't have direct experience with TB WOODS drives but, I'll bet they ignor early run signals too.

 

[Dave Williams]

Alan is correct in his statement regarding the use of solid state devices for e-stops, it must mechanically break the circuit. My experience has been that most OEM's, at least the ones we deal with, interupt the enable circuit, but leave the drive powered.