E-Stop Circuit

I suggest you start here:

www.osha.gov/Publications/osha3170.pdf

Good reference! But to answer his question, he really needs to know/tell more.

In general, the E-Stop must be arranged so that the machine does not start again as soon as the E-Stop is removed. There are several ways to do this but it may be undesirable to disconnect the power every time the E-Stop is used. There are better ways to disable the output in many systems for Safety Circuits. It depends on the drive that runs the motor. A good system will require a start button with a warning to take place before the motor can run again after an E-Stop.

Depending on what, where, and how, for instance, some people use an E-Stop as if it was a regular stop button. That can cause significant wear and tear if it is a power assisted E-Stop, as described. Disconnecting the power on each of these events may destroy a contactor in a matter of months. Been there, done that.

Part of the answer depends on how much other shielding is around moving parts. Frequently people take a short cut and use E-Stop in place of a Lock-Out-Tag-Out (LOTO) which is not appropriate.

The VFD is probably programmed to slowly ramp down and stop rather than perform an emergency hard stop because of the dynamic load on whatever the motor is driving?

I would hate to see or be near that motor if it did perform a hard stop!

There are so many reasons why yet we are just guessing here in the forum.

1. Check the machinery design specs?
2. Ask an engineer why?
3. Ask your maintenance personnel?

Good luck

NFPA 79 Electrical Standard for Industrial Machinery provides extensive coverage of this topic

In the US OSHA 1910 rules. As with any standard, first you must determine jurisdiction and scope. Read the scope and make sure your situation fits before trying to digest the details. You should also search for applicable ANSI standards relating to machine safety in your situation. Although these may be voluntary, you would be well advised to conform to them.

There may also be more stringent local regulations and rules.

Given that this machine has been in operation for 20 years, unless there is a compelling reason such as a failure of the existing E-stop controls or the machine is undergoing extensive modification, I would not consider any changes to the existing controls. Any modification should require a complete engineering analysis of the machine dynamics and failure modes.

Or an insurance audit indicating an upgrade/change.

Hi,

Not sure how this is covered within US standards, in IEC/ISO and AS there are two E-Stop categories. Stop category 0 is to remove all power and stop category 1 is to control the drive to a motion stopped condition. Even on some applications with smaller drives we will look at using a timed safety relay that removes the control to a VFD, or triggers a 'fast stop' condition to stop motion in a controlled method, then a set amount of time the VFD power is removed.

I did have a larger application where a chipper would coast for 30 mins if it was turned off, which is not ideal. We changed it to ramp down quickly, based on the DC bus regulation, and when we detected zero speed using feedback from the drive and safety control units, the interlocked access gate would allow entry and drop out the contactors.

It would be hard to say what you need to do from where we sit, but I would suggest this should be reviewed, and looked if it complies with current standards. It might be okay or it might scare you, but at least you will know and if needed plan for an upgrade.

Cheers,

Trevor.

The E stop de-energizes the main contactor after a delay as you said in the beginning of your post in case the drive has a "stuck on" failure. In the end of your post you contradict yourself saying it will coast to a stop, as said in the beginning it does not de-energize till the motor ramps to a E-stop which has a faster deceleration time setting.

Where did this idea of the contactor required come from? Perhaps from the NFPA requirement of total power removal, but that is not the emergency stop they are referring to, but the main disconnecting means. Or, perhaps from industrial safety departments?

We have built our control panels with them both ways for our customers, dependent on their specifications, and how willing I was to argue against removing power to the VFD, because many VFD manuals tell you never to suddenly remove the power from a VFD, especially from the output side of the VFD. I have usually won that argument with company safety departments, as the damage caused can be very real.

However, I have had company safety departments tell me I absolutely had to have a contactor controlled by the E-Stop in the circuit, and I have built them this way.

Note that Eaton is the VFD exception, and removes the arguing, as they build their combination units (disconnect/circuit breaker/e-stop circuit included) with a contactor on the output to open the power to the motor in an emergency stop. Not a good practice for most applications.

However the unit you have was built, continue to use it that way, especially since it appears to be the safest way to shut down your motor. Please be sure you don't use the E-Stop as a normal stopping device or a lock out device.

ABB VFD DC drive is a contradiction of terms. Is it a DC drive or an AC VFD?

That aside, the stopping modes are always a point of discussion. On many machines we work with the customer thinks of an "E-Stop" as stopping the machine as fast as possible - usually due to product jams or product web breakage. That is not an E-Stop for safety of personnel. Then there is a normal stop where the machine ramps up and down slowly so the process can maintain tension on the web in a controlled fashion. The next stop mode is a power off coast stop, which is used under the rare condition that there is a fire in a motor, electrical control has been lost, etc.

On some machines such as a rubber calender, there is often a specification that a stop mode be added that if a safety bar is tripped the machine stops within x seconds or inches so that a persons arm will not be pulled through the rolls. That mode usually also requires fail safe mechanical brakes.

We try to get the customer to accept "Fast Stop", "Normal Stop", "Power OFF Stop".

None of these stop modes are suitable for lock out to do maintenance on the machine. That requires actually opening circuit breakers and applying locks and safety blocks and pins per a written procedure, and the lock out is witnessed and signed off. Only after applying your personal lock to the lockout board are you allowed to enter the machine.

Your local authorities for workers safety, insurance requirements, etc, will all have their say. Quite probably any changes or review will require a sign-off from a professional engineer.

I would not recommend putting a contactor on an E-Stop circuit for the following reasons:

1) On the input side to the VFD, the initial current surge can be substantially larger than the rated motor current (assuming that the capacitors for the DC power supply are completely discharged). This will eat up the contacts much faster than you might expect. Eventually, the vaporised contact metal can result in a short circuit between the power lines resulting in large voltage spikes into the VFD.

2) On the output side, if the contacts open up under power an inductive spike from the motor can burn out the output transistors.

This is similar to the first reason but the difference is simply that the motor is the inductor you don't want to interrupt because V=L di/dt. On the input side, the power transformer that feeds the VFD is the inductor that can spike if you develop a fire inside the contactor. Once the motor has stopped, the output side can be open circuited without a problem. If the machine has a lot of inertia, that motor will be acting as a generator while trying to stop it. Braking resistors can help but fast deceleration can cause inertia to be applied to the frame or lift it off the floor. Not good when that happens!

1) Most modern VFD's have pre-charge circuits that significantly reduces the initial inrush current, often to a fraction of the actual running current.

VFD manufacturers usually don't recommend switching the input AC line as the normal start/stop mode. But all drives must be able to withstand the removal of AC power from the line side, as it is only a matter of time before that will happen.

Sorry about the (VFD) statement in the original post my brain was not engaged at that time. We had a corporate guy that is from Canada that visited and he was here for a purpose. To find a way to perform LOTO quickly, at the moment we loose 20 mins of production time every production change and in an average month this amounts to 48 hours total. He is proposing ac contactors on the input side of the dc drives on the production line, this with instrumentation to verification zero energy with one lock out station that performs the task of opening all contactor with verification. This means of LOTO is for production personnel only. This is a form of LOTO is accepted in Europe, I have not found this in the USA.

Going back to the original machine issue. We have a 2400vac fused disconnect with a vacuum contactor that feeds the transformer this drops the voltage to 500vac to feed the DC drive for this application. There is not a contactor for the 500vac nor is there a contactor on the drive armature output to the motor. The motor is a 600hp motor