VFD and Light Curtains 1

[DAVEZIMM]

Is there an industry standard means of connecting a light curtain with a VFD. I often design fractional VFD systems that rotate tables for operators to manufacture parts. I usually wire the light curtain to a safety relay, then tie two positively guided contactors from that relay to drop out the three phase power to the motor. The problem is that frequent (5-10 times a min) tripping of the incoming power to the VFD causes long startup times and I imagine is hard on the VFDs themselves. I can't find a manufacturers recommendation to safely disconnect the power to the motor on the load side of the VFD, and most installation manuals say not to do this. Any suggestions would be appreciated.

 

[cbarn24050]

Hi, you need to fit external current limiting resistors in the vfd supply if you are going to keep interupting the supply to reduce the inrush current. Breaking the motor side can damage the vfd if especialy if the motor is running. Some kind of cluch/brake assembly in the drive train would be a better way.

 

[niallnz]

Davezimm

Are your operators using the light curtain as a stop/start control instead of as a safety device. If they are using it as a stop/start why not connect it to the stop/start circuit and have a secondary safety system like a dead man switch that takes out the power supply.

I think in practice all modern VFD's have a current limiting/soft charging system built in, but check the manufacturer's guide on the acceptable number of power ups in a given time period.

 

[jbartos]

Suggestion: Visit

http://www.ae.ucdavis.edu/projweb/c...e/$file/16483+Variable+Frequency+Drive+03.pdf

(by copying it into the browser)
for: abrupt operation of VFDs due to abrupt switching of VFD bypass contactors.
Operational manuals are supposed to describe abrupt operations of VFDs.

 

[DAVEZIMM]

Thanks everyone for the responses. To clarify the questions asked, the light curtain is a primary safety device with the protected area needing to be entered sometimes 5 times a minute. The clutch idea was helpful if I could get the machine builders to incorporate it into their design. I've since discussed the application and also found out that redundant safety rated switches can feed a safety relay to effectively "mute" the curtain in the position that the operator needs to enter and still meet our local code requirements. The problem with just limiting inrush and allowing the drive to cycle each time is the re-initiation time of the drive extends the production cycle time.

 

[jbartos]

Suggestion: Please, verify that the equipment is designed to start - stop five times per minute. This kind of service is sometimes called a heavy duty service.

 

[ozmosis]

Whilst it may not be in the same 'fractional power' range of VFD you are looking for, Siemens manufacture a range of drives called Masterdrive that has an option(K80) called "Safety off". This is a function that supports Safety Category 3 according to EN954-1 and means you can dispense with motor side contactors as a means of isolating the VFD to the motor still with power to the VFD. So there is no time related issue of powering up the VFD as it is still powered up. Built inside the VFD is a safety relay with positively driven contacts using the N.O. contact to interrupt the power supply to an optocoupler/fiber-optic cable (internal in the drive) and hence preventing the output stage of the drive(the transistors) building up a phase sequence. It halts the firing to the switching devices basically. The N.C contact (a checkback contact) is used to report the switching status of the safety relay to the external control unit. There's a lot more to it but it does work well. I've used it on applications in theatres where multiple banks of motors are being switched between one VFD to control the stage lifting or curtain lifts etc. In this way the VFD's were kept 'alive' and a series of output contactors were switched in and out to the required banks of motors and then the VFD ramps up to the req'd speed.
I'm not sure where in the world you are but if you contact Siemens A&D (or in the USA it's Siemens Energy & Automation)and ask for the MCPM department I'm sure somebody will provide details.

 

[Tumppi]

About EN954-1.

Control Techniques UNIDRIVE SP got this feature as standard.
(NO options needed).It is caled SECURE DISABLE-function and it meets the requirements of EN954-1 : category 1, 2, and 3 for machine safety.

 

[ScottyUK]

Sed2developer & tumppi,

Thankyou for that information. I now have a solution looking for a problem, and I know that problem will officially land on my desk in about two or three months.

Regards to you both.

 

[jbartos]

Suggestion: Visit

http://80.253.105.183/features/secure.shtm

http://www.imakenews.com/powermation/e_article000105246.cfm

http://www.controltechniques.com/products/sp_secure.htm

etc. for more info on secure disable unidrive

 

[jraef]

Just as an add-on the the info from Sed2developer & tumppi, many other VFD manufacturers have this feature, it is also referred to as a Base Block command. It works by instantaneously iterrupting the Base signals of the transistors. It is actually a faster shut down means than a contactor, hence the ability to use it in safety systems. Be careful though, it is not the same as having an "air gap" isolation / service disconnect of the load in that a shorted transistor is still dangerous. Typical useage is to have the Base Block function precede a contactor opening to provide maximum safety isolation if someone is going to service the drive.

Quando Omni Flunkus Moritati

 

[ozmosis]

jraef. Whilst a lot of drives on the market have a function to disable the pulses to the base driver circuit, there are not many with a Category 3 approved circuit to prevent unauthourised movement of the motor.. The Siemens and Control Techniques drives have integrated hardware that means it is classed as a safety circuit. Other drives on the market, as you indicate, use the OFF command only when interlocked with a mechanical contactor on the load side. So not many drives have this Cat 3 feature!. Having said this, even with this feature in the Siemens/CT drive, you should never be servicing the drive. You still have high voltages both on the incoming AC circuit and also the DC link voltage is still active. The drive is not 'safe' in that case and if ever you wish to work on the drive, you should isolate the AC incomer.

 

[QUIPPDUDE]

The concept of using an inherently safe input on a VFD and dispensing with an input or output contactor sounds great. The safety Standard the above mentioned drives comply with is EN954-1 category 3 required for EC Machinery.
As far as I know this is NOT a recognized standard in the U.S.A.
Does anyone know whether this approach to safety has been approved for use by the relevant organizations [OSHA, NFPA79 etc. etc]

 

[jbartos]

Comment. USA standards are selectively adding the IEC, EN, etc. contents or their intent as they find appropriate. Usually, the complete foreign standard adoption is not possible or practical.
However, the manufacturers, e.g. Carlo Gavazzi

http://gavazzi-automation.com/ac/pdf/CGSol03-ENG.pdf

are adopting foreign standards according to their needs (for "industrial automatic doors&quot.