VFD Location

[joan271273]

Is there any difference on locating VFD's close to their loads or can they be located +100' from their loads. I had read at one time that there was an issue when working with VFD'and the cable lenght,etc.
Is this correct or is this no longer applicable with new generation VFD's

 

[NCTHAI]

It is recommended to put the VFD near to driven equipments. Check with OEM for maximum distance that they can be kept. There will be harmonics generated, you need to consider power cable capacity and derate them as required.

NC

 

[jraef]

It's a complex issue that needs more explanation that I am willing to give here (again) but in general, long distances between the motor and drive are bad. The way VFDs work creates what are called "standing waves" in the cables and the longer they are, the higher the voltage pulses can be when they reach the motor. the pulses then can be high enough to damage the motor insulation. There are lots of ways to mitigate it if you have no other choice, but the simplest thing to do is to keep the distance as short as possible.

Search this site for lots and lots of discussion of this topic.

 

[controlsdude]

Locally
If vfd mounted locally, noise problems are a lot less. Usually reactors are not needed due to the noise levels are low due to the short vfd power runs to the motor. Making changes to VFDs that are not centrally located can be a pain.
Expense in making an enclosure for each individual VFD instead of a central panel.
On the flip side you have to bring your control wiring out to the vfd from your main plc panel.

Centrally located
Have to look hard at cable runs to the motors, line reactors needed, special conduit runs or shielded cable needed. Grounding is critical
VFD programming is easier due to location in one panel

If you have your drives on Devicenet, I thought this was nice but the added expense of the interface card adds costs.

 

[TestBeforeTouch]

If you have a high starting torque load, voltage drop associated with a long cable run can also be a problem on a VFD. As an example, at a starting frequency of 4 hz a VFD may only provide ~30 volts but the motor may draw up to 150% of the motors full load amps. So in order to keep voltage drop to 10% (81% torque) you can only lose 3 volts. This problem can be overcome by using voltage boost, changing the V/Hz ratio, using bigger wire or by putting the VFD near the motor.

 

[ozmosis]

As close as possible to the motor is generally better for technical reasons and cost (if you are using screened cable). However, a point not mentioned is the environment where the load is. If, for example, the motor is outside on a rooftop or a cruasher motor in a quarry etc, then it might be more practical and cost effective to mount the VFD in a cleaner or safer environment away from the motor. Restrictions on motor cable length are both technical (as mentioned) but also, depending on your country location, limited to compliance to EMC restrictions. Countries in Europe for instance and Australia where compliance with EMC laws is mandatory require the manufacturer to provide limits on motor cable length to ensure compliance with the specific radiated and conducted levels of EMC categories.
If you are in the US then you are probably ok, nobody seems to care about EMC and nobody seems to fit filters..another long discussion.

 

[jraef]

If you are in the US then you are probably ok, nobody seems to care about EMC and nobody seems to fit filters..another long discussion.

Sure we do. It's just that without a directive saying it MUST be done, the bean counters don't want to pay extra for it. That wouldn't be a problem if it were not for the fat that VFDs have become such a "me too" product that every ignorant belt salesman will give them a quote. When someone points out that the better proposal includes filtering and the el-cheapo one doesn't, the bean counter says "Is there some law that says we have to have it?"

Frustrating...

 

[electricuwe]

If it is a small drive it may trip on overcurrent with a long cable and without filtering.

 

[joan271273]

Thanks for all replies. My drives are to handle fan type loads between 10-40hp.
I am reviewing manufacurer drives O&M books to find distance data,but nothing yet.
I google the site but found limited data, still searching.

 

[ozmosis]

Jeff
I should have added your point that there are a few diligent folk in the US who are fully aware of the issues of EMI and that until it becomes law, then it will go by the wayside.
It has frustrated me in the past dealing with the US, China and other countries not bound by the law that imply that as there is no law then there is no problem...

joan271273: let the forum know your drive type and you could get an answer straight away. There are people here who work (or have worked) for the majority of VFD companies.

 

[joan271273]

Thanks.

I am dealing with ABB drives

 

[SteveWag]

I run into this quite often when supplying power to well pumps (some are almost 1000 below ground level). I use dv/dt style load reactors and set the drive for the lowest switching frequency. The only problem was with a Cutler-Hammer unit that required two reactors in series (30 HP, 480 volt, line length of 2500 feet). Franklin Electric has a white paper on this subject.
Steve

 

[ozmosis]

check out this link on the ABB library for drive manuals:

http://www.abb.com/product/ap/seitp322/f559f9c6126b7eb2c125734e001e9145.aspx