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VSD's and safety interlock circuits

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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,

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Hi Mark,

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...

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Yes, thats true, if you are anticipating a lot of switching on the input, then this could be a problem. If you used an early break contact on the contactor to panic stop the drive first, then that should be OK. It depends on the timing and also how the drive responds to this sort of treatment. Some are more tolerant than others. Have you spoken to the manufacturers of the drive?


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  • 4 months later...

This thread has been inactive for a while now, but I can't resist the 'old chesnut' VSD motor isolator / aux contact debarcle:mad:


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.



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  • 2 weeks later...
I'd like to add a late thought to this thread. I've always held to the philosophy that in the case of an emergency, nothing can stop the motor quicker than the drive itself (most drives having an E-Stop input anyway). How the drive handles the emergency condition is usually a programmable function - ramp down at max or coast. This assumes, of course, that the drive is healthy - you would be wise to tie in a line or load contactor to the NO output of the drive status relay. If it's an emergency you are concerned with, **beep** the torpedoes and and break it on the load side (drive output). Nobody at the inquiry is going to sympathize with your concerns to coddle the thyristors when weighed against the possible risk to human life...
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If you are looking at Safeguarding of Machinery (in Australia standard AS 4024.1 - 1996), the drive is allowed to stop the driven load. There is no requirement to remove power from the motor to bring the machine to a stop - you can use Electrodynamic braking systems to stop the machine.

Electrical isolation, on the other hand, is a different story...



Most modern drives are designed to be opened under load with a standard auxillary off an isolator. Take a look at the users manual.

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Mr TheDog,


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;





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