Distance between drive &motor

[ohmhead]

Well question ? Vfd distance from drive to motor cable length ?

What is the rule of thumb is there one ?

Different lengths for different manufactures no one is the same ?

We know its better to be near a motor and most say use a filter or line conditioner to help with harmonics and PWM voltage spikes but give me your input to the most effective ways of installing a vfd at a distance from motor i like to hear what you think ?

I read three grounds in a conduit wrapped in a spiral twisting method of a three phase motor circuit hooked to a VFD improve long length harmonics effects & voltage peaks as voltage can get near twice the normal level of input voltage due to impedance of the wire to the motor due to distance between drive and motor .

What your thoughts?

 

[masterinbama]

Cerro makes a shielded cable especially designed for VFD's I recommend it on any run over 25'. It's similar to this.

http://www.sabcable.com/keyword-items.php?kwID=83

 

[__dan]

Line reactor

Line reactor

The vfd does not like the added capacitance of a long run of wire. The manufacturer will usually recommend a line reactor close, next to the vfd output, then you should be OK with a long load side run.

I would ask the manufacturer's rep. about the application and put a line reactor in, load side of the vfd, if that's what they recommend.

 

[Besoeker]

Well question ? Vfd distance from drive to motor cable length ?

What is the rule of thumb is there one ?

Different lengths for different manufactures no one is the same ?

We know its better to be near a motor and most say use a filter or line conditioner to help with harmonics and PWM voltage spikes but give me your input to the most effective ways of installing a vfd at a distance from motor i like to hear what you think ?

I read three grounds in a conduit wrapped in a spiral twisting method of a three phase motor circuit hooked to a VFD improve long length harmonics effects & voltage peaks as voltage can get near twice the normal level of input voltage due to impedance of the wire to the motor due to distance between drive and motor .

What your thoughts?

Gambica and REMA (Rotating Electrical Machines Association) both well-respected sources in the UK, have produced a publication covering this.
In summary, their recommendation is symmetrical shielded multicore cable.
To achieve this either requires the cable to contain only the three phases with the PE (ground) run separately or the cable may contain three PE conductors in a symmetrical arrangement with the three phase conductors.

What is frowned upon (deprecated is the word they use) is individual conductors in a steel conduit. They don't mention PVC conduit but I think they'd like that even less.

On cable length, the suggest contacting the manufacturer for anything above 50m (metres, not miles) or about 160 ft.

 

[ohmhead]

Well i know from your posts you are a good source of information on drives we have a job starting up were they didnt engineer!!!

They Did not plan on the VFD locations near the motors and may need a drive 100 feet to 200 feet away these would be 480 volts 3 phase from 15 hp to 200 hp to AHU units or MAU units & S R chiller pumps thur out the building .


Ive never seen them more than 25 feet and mostly less so the question came up from this issue .


I hear long length makes insulation crack and brittle also motor insulation due to standing waves on the line in question whats the worst that can happen and what suggestions ?

Thanks for the help

 

[Besoeker]

They Did not plan on the VFD locations near the motors and may need a drive 100 feet to 200 feet away these would be 480 volts 3 phase from 15 hp to 200 hp to AHU units or MAU units & S R chiller pumps thur out the building .


Ive never seen them more than 25 feet and mostly less so the question came up from this issue .


I hear long length makes insulation crack and brittle also motor insulation due to standing waves on the line in question whats the worst that can happen and what suggestions ?

Thanks for the help

There are probably three common problems associated with variable frequency inverters and cage motors.

1. Bearing currents.
2. Overvoltage transients.
3. Excessive rate of change of voltage (dv/dt).

I haven't had any experience of bearing problems but I have on a number of occasions with the other two.

The typical inverter puts out a series of pulses with a very fast rise time.
Here is voltage rise time measured at the output terminals of an inverter.

When this is sent down a cable it usually produces a ringing effect like this:

The first is very fast and what the motor would see if it was on a very short cable run. The second was, from memory about a 30m (~100 ft) cable run from drive to motor. It has significant overshoot. Neither the fast rise time nor the overshoot is particularly good for the motor insulation. The effects produce degradation of the motor insulation and failure after a time. How long that time is depends on a number of factors. I was asked to look at one where the customer was experiencing a failure rate of about one motor a month. There were four motors on the site and within the first six months of operation all had failed. The problem was resolved by fitting drive output reactors which is something we do as a matter of course.

FWIW, the voltage waveforms shown are real, not theoretical. They are what I actually measured on installed equipment.
The resolution on the first isn't so good. The Voltage axis is 0-1000V. The time axis is 0-20us.

 

[ohmhead]

Well Besoeker thanks thats interesting i think we are going to suggest if they go with this plan some filtering next question ive read a little on 18 pulse drives and also the zigzag transformer to delta wye filtering systems which is the best in your opinion?

no.1 zigzag transformer ?

no.2 line conditioners ?


no.3 18 pulse vfd ?

 

[dbuckley]

Worth reminding folks: many VFD applications are controlled by a low voltage or data circuits and that these can be arbirarily long wihout causing issues. So put the VFD by the motor and run long data cables as necessary.

 

[Besoeker]

Well Besoeker thanks thats interesting i think we are going to suggest if they go with this plan some filtering next question ive read a little on 18 pulse drives and also the zigzag transformer to delta wye filtering systems which is the best in your opinion?

no.1 zigzag transformer ?

no.2 line conditioners ?


no.3 18 pulse vfd ?

These all deal with the input to the drive, not the output from it, so have little bearing on your original question about cable length.

That said, drive input line current has a significant harmonic content and this may need to be given consideration but it is a different issue.
As usual, there is no "one size fits all" solution. It needs some calculations on the magnitude of the harmonics and how they would affect the supply voltage to decide if it could be a problem and, if it is, how best to solve it.

 

[ohmhead]

These all deal with the input to the drive, not the output from it, so have little bearing on your original question about cable length.

That said, drive input line current has a significant harmonic content and this may need to be given consideration but it is a different issue.
As usual, there is no "one size fits all" solution. It needs some calculations on the magnitude of the harmonics and how they would affect the supply voltage to decide if it could be a problem and, if it is, how best to solve it.

Well i guess will get with Johnson Controls they supply the vfds seems like the vfd manufacture can do the calculations for us .

The project has no room for a number of vfd s meaning no electrical clearance in these spaces so its a tuff job in this area .

So as the electrical contractor we dont supply anything but iam very interested in what we have to install for the mechanical contractor and what it involves in the process .

So we need a engineer to do the math on length of cables to motor ?

Basically how or what will he need from us besides the length ?

 

[Besoeker]

Worth reminding folks: many VFD applications are controlled by a low voltage or data circuits and that these can be arbirarily long wihout causing issues. So put the VFD by the motor and run long data cables as necessary.

It's a fair point. But sometimes not practical or accepted practice here.
Most the VSD systems we have produced form part of a motor control centre suite of panels and located in a drive room or switch room.
There are often practical reasons for this approach. The motor may be located in an environment that is unsuitable for the drive. Paper mills, cement works, and petrochem plants are examples.

There are sometimes exceptions though, and one comes to mind.
I and our workshop foreman were on our way to modify and re-test a drive.
We got about half way and I got a call from the customer.
"We have to call the job off."
"OK, but why?"
"It's raining."
Well, it was - quite heavily.
It took me a moment or two reflect on that and consider why it was relevant.
Then the penny dropped.
The drive was outside, open to the elements, but not manufactured for that environment.
I'm an old fellow and, after 40 some years in the game, you don't get too many surprises. But that was the first, and only time, I had a job rained off.

 

[BAHTAH]

Well question ? Vfd distance from drive to motor cable length ?

What is the rule of thumb is there one ?

Different lengths for different manufactures no one is the same ?


What your thoughts?

I have been involved with many well installations where the distance to the pump (depth of well) was several hundred feet. Add that to the distance from the well head to the drive location and you can see that most applications had considerable lengths. We used Danfoss Drives exclusively because they are good up to 1000ft without modification. You may want to check them out and talk with their engineers. They have a great warranty program and we never had any problems. I think that ABB Drives also have a standard cable distance that is possibly equal. I liked many of the standard features of the Danfoss Drives that are specific to wells.

 

[TxEngr]

Since the distances are already engineered, there are a few things you can still do, most of which have already been discussed:

1. Purchase cable designed specifically for VFD applications, e.g. three symmetrical conductors with three symmetrically arranged grounds in a continuous armor. This helps.
2. Install a reactor on the load side to help with the standing wave issue.
3. Purchase inverter duty rated motors. I'm assuming you're working with 480V so the insulation system on the motor should withstand a minumim of 1600V and preferably 1800 or 2000V.
3. If you still need help, Allen Bradley makes a motor termination kit that you install at the motor to protect it from the standing waves. Not cheap but a lot cheaper than replacing motors every month. Here's a link to some info from A-B that includes info on the motor terminator http://literature.rockwellautomation.com/idc/groups/literature/documents/br/1204-br000_-en-p.pdf
The other problem you can run into (and I have) is bearing damage due to circulating shaft currents. This can be helped by using insulated bearings on the motor (one end only) and insulated couplings to the equipment. You can also get a grounding brush mounted to the motor that rides on the shaft to eliminate the shaft currents.

Good luck with your application.