Machine safety. 2

[Lawtons]

Variable frequency drive safety.


Q1 If I use a variable frequency drive as the main drive on a machine, do I need an isolation contactor between the drive and the motor?

Q2 Will opening the e-stop relay that turns off the drive logic have the same affect, as far as safety is concerned, as turning off the coil on a motor starter?

Q3 And if I do have to add a contactor, what do I do if I’m using the drives braking feature to stop the machine?



Q4 I’m starting to see E-stop safety modules used in new machines. What are they for, and who says I have to use them? What standards do I use for machine safety? I've been wiring machines for 20 years and I still can't get a straight answer.

 

[panelman]

1. No, and generally held to be a bad idea. If you need one for E-srot reasons, put it before the drive.
2. No, because software is involved and drives can fail on
3. Put the contactor upstream of the drive and only open it if an E-stop is opperated
4. For CE marking all machines must have risk assessment done, level of risk determins sort of safety system needed. Safety relays are for high risk and have duplication, positivly guided contacts, error checking and self diagnosis built in

hope that helps

 

[jraef]

Added to panelman's excellent response:
Re; Q2, you can also look for a feature on VFDs called a Base Block. Many VFDs use this as the equivalent of "opening the coil" since it is a hard-wired interruption of the transistor firing circuit, therby foresaking any "software glitches" in the VFD.

Quando Omni Flunkus Moritati

 

[Marke]

But you can still have bad leakage in one transistor that could be lethal if you got onto the motor terminals without the input side isolated!
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[panelman]

jraef...do i get a gold star?

re the base block idea, similar things are available for PLC software. Really it all depends on the risk assessment, some jobs just need a standard relay contact in the contactor circuit, others need e-stop contactors in series and powerfull safety relays

Marke....don't confuse isolation and E-stopping, they are not the same. Isolation makes it safe to work on, E Stop makes it stop.

 

[jbartos]

Suggestion to Lawtons (Industrial) May 8, 2003 marked ///\\Variable frequency drive safety.
Q1 If I use a variable frequency drive as the main drive on a machine, do I need an isolation contactor between the drive and the motor?
///It depends on the application. It is offered by some manufacturers. There some marginal safety in its application, namely, the contactor isolates the galvanic path between the VFD and Motor from whatever may come from VFD when an upstream safety switch or power is properly disconnected. The traditional safety motor disconnect near the motor is often preferred and included in electrical safety code. Code exceptions come next.\\\
Q2 Will opening the e-stop relay that turns off the drive logic have the same affect, as far as safety is concerned, as turning off the coil on a motor starter?
///The safety mandates to have the motor starter coil controlled by E-Stop. There are special rules pertaining to e-stops, their design, e.g. mushrooms, locations, etc. E.g. See NEMA 79.\\Q3 And if I do have to add a contactor, what do I do if I’m using the drives braking feature to stop the machine?
///It depends on the application. If the application requires it, then it should be upstream from VFD. It is assumed that the electrical safety takes the precedence to the braking feature.\\
Q4 I’m starting to see E-stop safety modules used in new machines. What are they for, and who says I have to use them? What standards do I use for machine safety? I've been wiring machines for 20 years and I still can't get a straight answer.
///The E-Stop emphasis is mandated by Safety Codes/Standards. These are enforced more and more. Many of the Safety rule advancements are coming from Europe and IEC, CE, EN. The purpose of those is that the operating personnel is safe in case that a human or hardware error happens.\\

 

[GusD]

Hi Lawtons

Q1=It is not required that you have an output contactor
between drive and motor.
We have some VFDs with isolation contactors and others without.We do it for safety reasons rather than electrical concerns.If your plant lock out system requires isolation between motor and drive,than you install one.

Q2=Your E-stop does not replace the isolation offered by the output contactor.It may be ok in some plants .

Q3-I beleive that if you have a dynamic breaking feature
you would have to time delay the opening of the OContactor.

Q4=The E-stops are their for the purpose of personnel safety.Their need is dependent on the type of machinery being used and the customer's requirements.
There are some safety standards regulated by Elect Codes
and others by OH&S regulations.

GusD

 

[Lawtons]

Perhaps I didn’t make my questions clear enough. Let’s take a printing press as an example. During the setup process of a printing press the operators are constantly putting their hands in the machine. Before doing so they must press an E-stop. As far as the maintenance of the motor, or the machine in general, lockouts are used.

 

[TheDOG]

As far as I can tell, there are two issues for emergency stopping:
1. Electrical isolation
2. Motion isolation

1. Electrical isolation can be achieved by an isolation contactor. An isolation contactor on the output side of the VSD is more effective than one on the input. The reason for this is that the DC bus capacitors keep the bus voltage "up" even when the supply is removed. This can (dependant on motor loading) maintain power to the motor for significant time.
2. In Australia (which is where I am) there is a standard called AS/NZS 4024.1 - Safeguarding of machinery. It states that "An emergency stop device when operated shall stop the machine as quickly as possible and apply a brake where provided." It also states the an emergency stop shall not be relied on as a means of isolation or immobilization. A VSD will be able to bring any motor motion to rest quicker than removing the supply. For this reason, an isolation contactor is not required for motion isolation.

Of course you will have to check your local codes.

 

[wired1]

Lawtons: Take a look at NFPA 79 - Electrical Standard for Industrial Machinery. The standard from Australia makes perfect sense to me. Years ago, I cut my teeth on trying to stop a servo axis that was moving a high inertia load at a pretty good clip. I started out by simply cutting the power to the drive on an Estop. When that happened, you better stand clear because there was going to be big crash. The solution was to let the drive stop the motor and then cut the power after a time delay (from a pneumatic timer). That was the safest.

 

[chartist]

Hello Lawtons,

Here in the UK, and the EU, we have the PUWER regulations.

Google these.

Of particular interest to you would be BSEN 60204 / IEC 60204 - Safety of Machinery - Electrical equipment of machines.

A manufacturer, Pilz, also produces a useful guide book.

Hope this points you in the right direction.

 

[jbartos]

Suggestion: Visit

http://www.simcotech.com/machinesafety.htm

for: Light curtains in addition to E-stops

http://www.sea.siemens.com/contrlbu/upld/files/p719.pdf

for: E-stop button shape

http://www.robotics-technology.com/safety/honeywell/

http://www.ab.com

etc. for more info

 

[GSI1]

If you want an E stop then the contactor must be between the drive and the motor. Drives have been know to fail ON. And with the new VFDs as lons as their is power on the DC buss the drive can run. This could lead to the motor running for some time after the AC line is disconnected.
If you want to be safe then you must disconnect the motor from the drive. Interlock the contactor to the drive so the drive can't start unless the contactor is closed.