PDA

View Full Version : E-Stop switch?



torker
01-15-2007, 08:59 AM
Hey guys! I have a "twist- lock type E-stop switch that I was going to use before my VFD. There'll be 240V running into the switch. There is only one pole in this switch. I just looked in the catalog link that Willy provided and the e-stop switches they show all seem to have only one pole as well. So do you only disconnect the power to one hot wire for these in a 240 volt deal? I'm thinking no here...
Thanks!
Russ

Rusty Marlin
01-15-2007, 09:02 AM
The e-stop should be controlling a relay that cuts main power when the e-stop switch is activated (pushed in).

torker
01-15-2007, 09:25 AM
The e-stop should be controlling a relay that cuts main power when the e-stop switch is activated (pushed in).
Sheesh! Sounds like another box. Dang! I just made up a cool plate and mount for the VFD and E-stop boxes. Now I'll have to change some stuff...unless the relay will fit in the E-stop box as well.
Thanks!
I'll check with the local electrical supply rapists and see if they'll take my first born as a downpayment on a relay :D
Russ

Weston Bye
01-15-2007, 09:32 AM
What Rusty said. The relay, also called a motor starter, should interrupt the 1 or 3-phase power to the VFD. Ideally the circuit should include a start button wired in series with the e-stop then to the coil of the relay. An extra contact on the relay is wired in parallel with only the start button to hold or "seal" the circuit when the start button is released. Push the e-stop to open the circuit to the relay coil, allowing the relay to open the seal contact.

The twist-lock feature helps to prevent inadvertent starting. The start button should have a flush or guarded or recessed head so you can't bump it and start the machine.

About the relay:
The simplest circuit uses the line voltage in the control circuit, with the coil voltage being the same as that supplied to the VFD. Just connect one leg to the start switch and the other leg to one of the coil terminals, with the above described circuit in between.
However, this circuit has certain drawbacks if all the circuitry is not contained in one enclosure. If external wiring is not protected a cut wire may present you with the line voltage and a lethal shock. Also, this circuit may violate the electrical code in certain localities.

A better option would be to add a small transformer or DC power supply, powered by the line voltage to power the control circuit and select the motor starter relay coil voltage to suit.

This has the added safety advantage that if power fails while the machine is running, the start button has to be pushed to restart rather than starting in its own if you forget to turn it off.

Also, things like guard switches can be added in series with the e-stop for other safety measures.

Willy
01-15-2007, 09:36 AM
What Rusty Marlin said,most manual switches aren't designed to handle the current that you will be controlling.Should not be too expensive ,but like you say another PITA.

mbensema
01-15-2007, 09:36 AM
Torker, your VFD will have a low voltage input that can be used with the E-stop. Check the manual on which input it is and there should be a schematic on how to wire and program it. Usually the way it works is the VFD requires voltage into this input before it will start running and if it looses voltage when the switch is tripped, it will shut the drive down. It is very dangerous to use the e-stop on the leads to the motor, it is meant to be used only with control voltage and will most likely fail fairly quickly if wired inline.

Weston Bye
01-15-2007, 09:49 AM
Torker, your VFD will have a low voltage input that can be used with the E-stop. Check the manual on which input it is and there should be a schematic on how to wire and program it. Usually the way it works is the VFD requires voltage into this input before it will start running and if it looses voltage when the switch is tripped, it will shut the drive down. It is very dangerous to use the e-stop on the leads to the motor, it is meant to be used only with control voltage and will most likely fail fairly quickly if wired inline.

Be aware that in the (unlikely) event that the VFD has an electronic control malfunction the e-stop may not be effective.

thistle
01-15-2007, 10:16 AM
I dont think you want to have an emergency stop switch on the 220 single phase line feeding the VFD .

A disconnect to isolate the VFD yes,asuming it is used when the vfd is not running the motor ie motor stopped , vfd on stand by

an emergency stop should be wired in to the vfds aux strip

sch
01-15-2007, 10:58 AM
The Estop on my VFD ramps down the VFD output at the same rate as simply shutting off the VFD. It is not instantaneous, nor can it be without hazard to the VFD. The VFD power supply must be drained down before the VFD shuts off power to the motor. The VFD must deal both with its own power supply and the power dumped into it by the motor on shut down. Doing this requires time, usually at least 0.5sec to several seconds depending on how the VFD is programmed, the dump resistor(s) if any extrinsic to the VFD and the load on the VFD. Think of it like an Estop on a table saw.

Evan
01-15-2007, 11:03 AM
Legally an e-stop must interrupt the main power input.

DR
01-15-2007, 11:10 AM
Legally an e-stop must interrupt the main power input.


On CNC machines the e-stop will usually only power down the axis drives.

Weston Bye
01-15-2007, 11:19 AM
Legally an e-stop must interrupt the main power input.

Correct, in principle. However, I have seen a trend in automated equipment toward interrupting only the motion producing circuits. This from both US and Canadian machine tool builders. The various limit switches, photo sensors pushbuttons, etc. all retain control power while in the event of an e-stop, the motors, valves, heaters or anything else that could cause motion or injury are powered down. This is different from what I learned years ago in designing and building automation, and makes it doubly important to shut down and lockout the main disconnect or excersize extreme caution when doing electrical troubleshooting.

gzig5
01-15-2007, 12:22 PM
The contacts in an e-stop push button are not designed to handle the mains power to a VFD. It should control a properly rated contactor supplying the drive.
As Wes said, common practice is to remove the mains supply for the DC bus or other motion producing power sources such as pnuematics, while leaving the control power on the controller and sensors. As an additional level of safety, new servos are incorporating reduntant internal safety contacts that disconnect the IGBT gate inputs. This allows higher SIL level compliance that is being pushed by various governmental agencies.
Greg

lazlo
01-15-2007, 12:49 PM
I dont think you want to have an emergency stop switch on the 220 single phase line feeding the VFD.

an emergency stop should be wired in to the vfds aux strip

VFD's have a specific low-voltage E-Stop input. Read the Fine Manual -- they show how to wire the E-Stop. ;)

Also, make sure you use shielded signal wire, with the shield grounded to the VFD's ground.

Evan
01-15-2007, 02:38 PM
It's ok to leave low voltage logic powered up. Low voltage is anything below 40 to 50 volts depending on local codes. All of the machines I have worked on, and they are numerous, have a main power relay that disconnects all input power to the drives. Solid state relays are not acceptable, they must be mechanical contactors. For smaller single phase equipment it is acceptable for the interlock or e-stop to switch the main power. For anything over 12.5 amps a relay must be used.

The e-stop or interlock must be fast acting, without time delays.

The electrical codes are extremely fussy about this sort of thing. It doesn't make a real difference in a home shop (other than your own safety) but Russ has at least one employee and should be adhering to the code.

nheng
01-15-2007, 03:16 PM
From John Stevenson's side of the pond, a pretty good safety / stop paper:

http://www.robin-carver.com/rjcarver/pages/resources/emergencystop.pdf

It goes into several categories of e-stop and the characteristics of each ... covers some of the things mentioned here like free running stop vs. braking, etc.

nheng
01-15-2007, 03:29 PM
Regarding e-stops and brakes, this UK document for a horizontal boring machine says:

"The emergency stop should actuate the brake (if fitted) before cutting off the power supply."

http://www.hse.gov.uk/PUBNS/eis28.pdf

Mike Burdick
01-15-2007, 03:30 PM
Legally an e-stop must interrupt the main power input.


...The e-stop or interlock must be fast acting, without time delays.

The electrical codes are extremely fussy about this sort of thing. It doesn't make a real difference in a home shop (other than your own safety) but Russ has at least one employee and should be adhering to the code.
Hmmm... that's different!

As far as I know the Electrical Codes ONLY require that a disconnect be placed in site and within 50 feet of a machine or be capable of being locked in the off position. A cord and plug-in will meet this requirement.

Now OSHA may require something different regarding the intent of the E-Switch; my guess would be that they are more concerned about shutting off the drives etc. for worker safety as stated by others. Most VFD's have a low voltage means to cut power to the drives and require a "physical reset" to start them again. Russ, check OSHA 's rules regarding an e-stop and you may find that you do not need to buy a contactor as your VFD has everything you need already.

JCHannum
01-15-2007, 03:46 PM
In most three phase installations, the E-stop button is merely a stop button that drops out the magnetic starter for a specific motor or set of motors. It does not interrupt the main power supply, nor is it required to. Most have a twist lock or other type of maintained contact that must be physically cleared before the machine can be restarted. There may be several in various locations through out the machine or line.

If the machine uses other power sources, such as pneumatics, the e-stop will usually dump that also, taking all pressure off the system.

E-stops are not to be confused with safety disconnects, interlocks and lockouts, which are a different animal that will protect against exposure to live circuits when an electrical panel is open, or physically disconnect the power to a specific motor or piece of equipment.

The use of e-stops with VFD's becomes tricky as the point at which the current is interrupted may cause damage to the controller, and manufacturer's recommendations should be followed.

Evan
01-15-2007, 04:21 PM
The electrical code that I am familiar with is the Canadian code which is also what Russ must comply with. US regs may vary and I know that in some cases the Canadian code is stricter.

The machines I worked on over the years included everything from dinky little office machines that you could pick up with one hand to machines that had full CNC control and weighed 3000 lbs with 3 phase 50 amp power.

In the large machines especially the main power off button meets the criteria for an e-stop, both in design, location, appearance and function. On these machines severe personal injury is possible if the machine is run with the panels open and a cheater in place. There have been accidents that resulted in arm amputation. Many of those machines also contained high voltage pulse discharge capacitor systems storing hundreds of thousands of watt seconds of power.

On all those machines the e-stop main power switch and the interlock switches are on the same circuit. It is a loop circuit that requires all contacts to be made in series for the main power relay to be actuated. An open in any part of the circuit prevents the main power relay from closing and opens it if closed.

Problems with suddenly removing power from a VFD are the VFD designers problem and if it is any good it must be designed to allow for that eventuality. Any integral e-stop function in the VFD must remove input power from the VFD via a mechanical relay to qualify as an e-stop otherwise it is merely a motor on/off switch, not an e-stop. Also, if it uses a mechanical relay that relay must also have secondary contacts that will serve to remove power from any other drives in the machine to qualify as an e-stop.

In most cases coasting is not allowed if significant coasting will take place. In that case some sort of braking is to be used. All of the machines I worked on would come to a complete halt in less than 0.2 second.

[added]

Most people aren't familiar with the large duplicating equipment Xerox makes. Here is a pic of one of the bigger machines similar to what I worked on.

http://vts.bc.ca/pics/docu.jpg

Willy
01-15-2007, 04:36 PM
I have an Allen Bradley vfd wired as Evan has suggested and when I hit the e-stop the low voltage fault code flashes,the machine stops instantly,it does not ramp down.It would be dangerous if the motor was to take several seconds to slow down.I have had this particular vfd wired this way for about three years with no ill effects.

DR
01-15-2007, 05:22 PM
Hmmm....just remembered something, not that it's especially pertinent to this discussion.

A good many CNC machine tools (even the ones from the biggies) are not UL approved. This topic came up when a large manufacturer was buying new machines and insisted they all have the UL sticker. I never heard the end of the story, last I heard they were having a hard time finding suitable machines with approval.

Evan
01-15-2007, 05:29 PM
It isn't that hard here to obtain CSA approval for a machine without it, if the machine is constructed properly. My wife imported an expensive special stud welder from Europe and it didn't have approval. She had to have it inspected and several minor modifications had to be made such as adding a star washer to the frame ground lug bolt and a vent hole screen put in. It is then signed off with a one off approval.