Contactor Damage on VFD Input 8

[TeamIE]

Equipment Design:

Motor; VFD; Line Contactor; Start; Machine Stop; E-Stop; Safety Whip Stop. The machine start stop is part of the e-stop loop. The VFD's inhibit is hardwared to run at power up. Normal equipment operation causes the Line Contactor to apply power to the VFD, and it causes the Line Contactor to remove power from the VFD. The Line Contactor is controled by the E-stop loop. In other words, the Machine Stop, E-Stop, and the Safety Whip Stop are all wired in series.

Damage:

Several times the Line Contator's contacts have been fused together, causing the Line Contactor to remain engaged so that all Stop features do not work. at other times, the contacts are so pitted that no contact is made.

Questions:

1.) If your company designed this circuitry, what would you have to say about it?

2.) What information is available on-line that would comment on Contactor damage related to VFD start/stopping?

3.) Is there any US code that restricts this design for VFD starting or safe-stops?

Thanks much!

 

[jraef]

Do a keyword search of this site and you will find several well discussed threads on this very subject. The concensus seems to be that although there is no governing standard, there are "best pratices" depending on the specifics of the application.

As a generality however, there should not be a problem with opening an in-line contactor on the VFD as far as the contactor is concerned. I would hazzard a guess that someone skimped on the contact rating, perhapse falsely assuming that since the VFD was soft starting the motor at 100% FLA, they didn't need to use a contactor rated for Across-the-Line (DOL) starting. That would be wrong. Since the potential exists for that contactor to interrupt the motor at FLA, the contactor must be rated just the same as if it were starting ATL.

"Venditori de oleum-vipera non vigere excordis populi"

 

[sreid]

The opens and shorts in the power contactor are caused by the inrush current to the VFD supply capacitor. Most small VFD's use a NTC power thermistor to limit the inrush current. Larger VFD's usually have a current limiting resistor that is shorted by a relay contact shortly after power is applied. These current limiting functions may have stopped working in your VFD.

Zero voltage crossing solid state relays can reduce the inrush current (they can't turn on at peak line voltage). Some argue that this is still not safe (semiconductors can fail shorted)but I think it is better than welded relay contacts. There are specific safety relays which are designed to guarantee contact opening when de-energised (but they are pricey). Their contacts can still be fried open circuit.

 

[jraef]

Thats a good point sreid. Hadn't thought of a component failure in the VFD. I voted you a star.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Marke]

Hello TeamIE

Frequent switching of a contactor on the input of a drive can cuase stress on the drive and premature failure of the input rectifier and soft charge resistor. You should ensure that the contactor operation is within the Drive manufactures recommendations.

There is a high charging current into the drive when you first close the contactor. This will cause heating of the contacts. When the drive is operating at full load, the shape factor of the waveform is such that there is more heating per amp than is usual for a sinusoidal current.
Both these factors result in the contacts running much hotter than would be the case with a resistive load. If the contactor is selected for the input current of the drive with a resistive rating (AC1), then you will have problems. Select the contactor for an AC1 rating at least 30% higher than the maximum input current to the drive and the problems will generally go away unless there is another problem.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[TeamIE]

"Do a keyword search of this site and you will find several well discussed threads on this very subject." - Yeah, I did this and found exactly what you said, there is not consistant information on this.

"These current limiting functions may have stopped working in your VFD." - No problems have surfaced yet on the VFD's, although this is a concern. So far the damage is only on the contactor. The real concern has to do with the fact that this is the safe-stop loop, and has actually failed so that the equipment has not shut off. (Obvious hazards)

Great feedback from everyone though. Many comments harmonize with our own. Thanks!

Any further comments on the design of having this contactor energize/deenergize everytime the equipment is started and stopped. This seems like a lazy, quick-and-dirty, least amount of work and wiring method of accomplishing the start/stop operation. (Inhibit command on VFD hardwired on.)

 

[raisinbran]

TeamIE,
I agree that the contactor may be undersized, but this type of problem may also be caused by contactor "chatter" which, in turn may be caused by low voltage, intermittent connections, etc.
Regardless of the cause, if there is a specific safety concern, you may want these series "stop contacts" to also de-energize the drive control in addition to de-energizing the contactor.
This should improve the reliability of the stop circuit and may help to minimize the occurance of contactor tip welding.
Good Luck.

 

[panelman]

I'd do the normal start/stop function via the drive controls and keep the contactor for E stop function only

 

[ScottyUK]

I'd second Panelman's suggestion, as I've previously commented in most of the threads on this topic in the past.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[busbar]

 
NEMA standard ICS2-2000 parts 1 and 2 might shed light on contactor ratings and “durability.” Industrial Control and Systems: Controllers, Contactors…

http://www.nema.org/index_nema.cfm/1427/C10ECC19-3209-4937-B078EFB592D00F7A

[Electronic version currently available at zero cost.]

 

[DickDV]

The best advice in this string is from panelman and ScottyUK. It is always bad practice to do routine start/stop functions by powering up and down the drive input. Not only will you have contactor problems but you will experience shortened VFD life as well.

Just as they have said, put routine (frequent) start/stops in the low voltage drive control circuit and e stops in an input contactor control loop. And, please, for the sake of safety, get rid of that run-on-power-up setting in the drive. Unless this is a remote pumping station or a roof top air handler, it results in serious hazard and is never a good idea around production machinery.

 

[mc5w]

You need to open up your VFD and see if the contactor around the precharge resistor is NOT fused shut. Also check to see if it is not closing too soon. You also might consider installing a 3 phase line reactance between the drive and the contactor to help control inrush current and possible repetitive surges during normal operation. When the input bridge rectifier communtates from one diode to the next there is a momentary short circuit and the line reatance will keep that at a resonable value.

Similar Problem:

Your inrush current may be more than you think. 277 volt electronic ballasts have nasty inrush problems. Inrush current for one line of ballasts from General Electric has 600% to 3,600% depending on the number of lamps and the ballast factor. I determined this by taking one apart (Used about 5 gallons of diesel fuel to get rid of tar) and determined that this 2 lamp ballast used power supply capacitors rated for 4 lamp operation and there were 2 blank space for lamp inverters. After calculating inrush current I figured that no more thean 16 of these ballasts can be connected to a 20 amp light switch regardless of the number of lamps per ballast and the ballast factor.

I informed my customer as to how they can control this problem including substituting 32 amp inductive rated contactors and 50 amp solid state relays. Since most electronic ballasts are made by about 4 Chinese factories most other brands probably have the same crummy engineering.

This is to say that you need to measure or calculate the inrush current of your drive to determine if you need an oversize contactor.

Mike Cole, mc5w@earthlink.net