
VSD's and safety interlock circuits
#1
Posted 01 May 2002 - 04:31 AM
Should the contactors be on the input or the output?
#2
Posted 01 May 2002 - 08:37 PM
I would recommend a contactor on the input of the VSD rather than the output, although there is an argument that I can see for one on the output.
Putting a contactor on the output of the VSD will guarantee immediate removal of voltage from the motor which is a good thing, however the problems could be that some drives are easily damaged by switching on their output and that it is possible that the motor could be reconnected to the output of a drive that was operating above zero frequency and this could also damage the drive. (Effectively a DOL or Full Voltage Start on the output of a VSD)
Putting the contactor on the input of the drive would shut off all power to the motor, but there ould be a small delay due to the energy stored in the capacitors in the VSD power supply. This would be safe, would not damage the drive and would prevent DOL switching on the output of the VSD. The drive would always start from zero when the contactor was reclosed. You would probably have to reset an under voltage trip on the drive, but that could be automated if required.
My recommendation, contactor on the input.
Best regards,
Mark Empson | administrator
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#3
Posted 01 May 2002 - 11:41 PM
Thanks for the response.
There are other limitations of installing a contactor on the input side, and that is possible damage to the DC bus capacitor pre-charge circuit. The pre-charge circuit will be designed for a certain amount of operations per hour.
I have had suggested that using a contactor/contactors on the output with a late make/early break auxillary contact, wired back to the drive is acceptable.
I wanted to see if anyone else has had any ideas...
#4
Posted 01 May 2002 - 11:49 PM

Mark Empson | administrator
Skype Contact = markempson | phone +64 274 363 067
LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters
#5
Posted 20 September 2002 - 06:34 AM
The early break contact must break around 500mS before the main poles to be effective. Typically available 10mS early break aux contact isolators are USELESS as the residual motor field will not have decayed, resulting in an inductive kick 10mS later when the 3 main poles open. Poof! goes the VSD output!
I am yet to identify a suitable isolator that could be recommended.
The only practical solution is a lockable isolator, the expectation being that the person with the key understands the need for care.
BigMax
#6
Posted 04 October 2002 - 03:55 AM
#7
Posted 10 October 2002 - 12:34 AM
Electrical isolation, on the other hand, is a different story...
BigMax;
Most modern drives are designed to be opened under load with a standard auxillary off an isolator. Take a look at the users manual.
#8
Posted 13 October 2002 - 10:32 PM
If "modern drives are designed to be opened under load ", why do you need an auxilliary contact?
Cheers!
BigMax
#9
Posted 13 October 2002 - 11:10 PM
The reason an auxillary is required is to prevent the drive from tripping (overcurrent) or going in to current limit. It's not required to protect the power switching device(s).
#10
Posted 13 October 2002 - 11:35 PM
Looks like we will have to agree to disagree based on our individual experiences:)
My point is that, in my experience, typical late break / early make aux contacts don't prevent VSD tripping OR potential damage. Around 500ms time delays are required, as yet no cigar!
Unfortunately, I have seen contradicting claims from VSD manufacturers regarding this issue, when both use the SAME brand of 'intelligent' (ie self protecting) IGBT modules....
Somebody has to be mistaken (misleading?).......;b;
Cheers!
BigMax
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