Motor Contactors - Line side or load side of VSD? 3

[adam1234]

Hi Everyone,

I'm a sales guy who has worked in the automation arena for quite a few years now, particularly machine safety. I am not an Engineer (even though my business card says I am), but I hope you can accommodate me for a short while and help me out with a question I have.

I recently had a discussion with a client about motor contactors. I have always thought it was best practice to have them switching the line side of the VSD. The client sees no problem having them on the load side.

This question sparked a fair bit of debate when I took it back to the office about new vs old technology drives, the effects on IGBTs, and so on.

I'd like to hear your thoughts on "best practice" in this area if you could spare a few minutes...

Regards,

Adam.

 

[Skogsgurra]

Line side has always been best practice. Not only best practice, but also prescribed by almost all manufacturers of VFDs. According to some manufacturers, a contactor between VFD and motor is always a no no.

The safety switch needed in many installation is usually placed bewteen VFD and motor and, in such cases, has an early opening switch that shall switch off the VFD before the main contacts open.

There are many reasons for this. One is that we want things to happen in an orderly way. Another is that opening a circuit with full load causes voltage transients that are supposed to be bad for the drive's semiconductors. Closing a contactor on an active VFD is also supposed to be a problem because contact bounce could generate similar transients.

A scalar drive will generally not have a problem with internal control if run without load, it just outputs whatever frequency and voltage that has been set. Also when no motor is connected.

A vector drive will probably be very confused when it doesn't find the load it has been told shall be there. Most drives have algorithms that handle this situation in som predetermined way. The most common (I think) is to report "control error" or any similar failure.

So, it depends a lot. If there are several motors that either shall run or stop and all have the same frequency, then one VFD and one contactor (and thermal protection) for each individual motor may be used. But I would put heavy snubbers or MOVs on the VFD in such a case.

There are applications where this has been done. A bakery with light conveyors in several sections is one succesful example that I have seen. But I would say that the main rule is: "No contactor between drive and motor".

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

Using a contactor on the load side but only switching it when the VFD is off is not a problem at all. There are however at least two main dangers of putting a contactor on the load side of a VFD involving switching under loaded conditions.

1. Opening the contactor under load can cause damaging voltage spikes on the transistors. As contacts open, the air gap acts like a capacitor and the voltage across it rises rapidly, creating what is called a high dv/dt (delta voltage over delta time; rapid rate of rise). That is why Skogsgura mentions having good spike protection if you plan on doing it. The damage it incurs is incremental, so although it appears you may be "getting away with it" for a while, you are causing stress to the silicon in the transistor layers, leading to eventual premature failure. There is no way to quantify that so that you can predict it or say "it's OK for X number of operations", but it is widely accepted to be a bad idea.

2. Closing when the VFD output is already on. If you are running a VFD and it is already at speed, closing a contactor into a motor is going to put that motor into an Across-the-Line (DOL) start condition. So the current is going to surge and that can severely damage the transistors as well (dI/dt r delta current (I) over delta time). At low speeds the surge current will be lower, but still may be higher than what the transistors can safely deliver.

So it depends on what your purpose is for having a contactor on the load side. If it is for safety purposes, you can easily design your circuit such that under normal circumstances, you ALWAYS turn off the transistors (Base Block or Coast-to-Stop) before releasing the contactor coil. That only takes a fraction of a second. Then in a dire emergency you don't worry about that and just open it, i.e. with a Safety Relay, but the risk of transistor damage is kept to a minimum.

If however you are planning on opening and closing a load contactor (or contactors) on a regular basis while the drive is running, expect to replace that drive often. Better to use the drive to turn the motor on and off, then isolate the VFD if need be with a line contactor.

 

[edison123]

Gunnar & Jeff - LPS posts.

 

[adam1234]

Thanks Gunnar & Jeff for the reply.
Much appreciated.

Regards,

Adam.

 

[edison123]

BTW, that was my new sig line I was trying. They are not reflective of your posts

 

[jraef]

Whew! Had me worried there.

 

[LionelHutz]

To expand on the other answers a little further.

There should be an input on the VFD that will immediately shut-off the output. It is likely called enable but it could be something else. You want to use an aux. contact with your switch or contactor to open the enable before the main contacts open. That way, the drive will be off when the contacts open and it can't be started again while open.

 

[burnt2x]

Having a contactor at the load side (after the VFD) was a solution done long time ago in a plant where I used to work with to prevent inadvertent meggering of the VFD output side by sleepy electricians(hence damaging the VFD's). We all knew the danger of a contactor dropping out with VFD up and running so we made sure we interlocked the contactor to only open when the VFD is "OFF" and VFD interlocked not to be operable if the same load side contactor is not "ON".

 

[samanele]

I have seen VFD installed with both line side & load side contactors. On the input side of Line side contactor even a SDF is provided.
Dont we need similar switch (If safety for VFD damage by motor meggering is must)at the output side considering the output contactor contact welding?
Also where there is practice to provide bypass contactor (where VFD failure is not tolerable for longer time), then I think input & output contactor both shall be there. It will provide isolation (faster)from faulty VFD to run the motor in bypass mode in DOL.
Can we replace line side contactor by switch (SDF) in VFD without bypass?
Regards
Sam

 

[jraef]

samanele,
All of the circumstances you describe are related to switching when the VFD is not running. So no problems there.


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