Measuring Lead Length

We are going to have to change out some very old drives we have and the integrator needs to know the lead lenth from the drive to the motor to decide what stuff we need on the output side.

They say the lead length makes a difference in using sine filters, load reactors, Chokes, DV/DT filters, Reflected Voltage Traps,

I have no idea which of these methods are best. Is therea tool we can buy to measure the lead length as there are lots of them and parts are in conduit and some in cable tray and difficult to get to to measure in any manual way.

Is there a way to measure them with a meter when we have a shutdown day and find out the length? This is armored tray cable going to the motors now but it will be replaced with VFD cable.

Any good way I can take these measurements?

What type of drives are you talking about, ac or dc. We use ac drives and run simply from the drive out to the motor. Motors are 460 volts. We would need to know more about your existing application to offer more advice. Our cables are cut to length as needed.

Wire length is important in determining whether a reactor is needed between the drive output and the motor. You may estimate the wire length by following the routing of the wire. If your wire is too long, then you'll damage your motor windings if you don't install a reactor.

if its to long to measure manually get a load reactor!

Do a google search for "Cable length meter." Connect a lead to each end and viola, it tells you how long the cable is.

Fluke makes them, so do several dozen other manufacturers.

Check Mitchell instruments.

Amazon has got one listed too.

Those look like specialized Ohm meters to me.

I'm guessing the cable is to long to connect to both ends. You could tie two of the three leads together at one end, measure the resistance, and from a chart of resistance vs. wire size calculate the length and divide by two.

If there is a lot of EMI/RFI on the wires it will affect your reading.

On the other hand, the length doesn't have to be dead nuts. You can probably get close enough by pacing off the length along the floor underneath the cable trays etc. and add the vertical rise.

as the wires are also copper same as the windings in the motor the vfd will have to know how much copper is outside motor, has to do with filtering, and control of speed.
so the size and length is important within 10%.

TConnolly said:

Do a google search for "Cable length meter." Connect a lead to each end and viola, it tells you how long the cable is.

Fluke makes them, so do several dozen other manufacturers.

Check Mitchell instruments.

Amazon has got one listed too.

Click to expand...

We have one of these for measing the lengh of cable left on a roll and such but it won't work here as the 2 cable ends have to be close enough to get both leads on.

Would this type of meter work? http://www.tequipment.net/Megger655510F.html

It finds the length to a fault but I wonder if I could use it to just measure. I am really suprised someone does not make something like I need.

Be tricky!

Tim Ganz said:

We have one of these for measing the lengh of cable left on a roll and such but it won't work here as the 2 cable ends have to be close enough to get both leads on.

Click to expand...

It will work, but first you have to disconnect two of the motor power leads at the motor end and connect them to each other. Then you can use the meter, or a plain old multi-meter and a resistance chart, with the meter leads connected to the two motor leads at the VFD end. The result will be the length of cable out and back, which is twice the length of the cable between the motor and the VFD.

Tom, that is a workable method. It seems to depend on the resistance per foot of wire. That reminds me of another way that might work, since this is a VFD. If the VFD is running, then it probably has internal voltage and current measurements. If so, and they are accessible, then you could use my Voltage Drop spreadsheet to find the wire length. It is a guess-and-test method where you adjust the wire length input that you want to find, and check to see if it produces the desired output voltage.

With the VFD running at 100% speed, simply fill in the raceway type, wire type and size, and VFD supply voltage (actual measured VFD input voltage) on the spreadsheet. Then read the VFD output current and enter it in the spreadsheet in the box for "Load Current". Now you can make a guess for the "Length of Circuit" and enter that (100', 150', 200'?). Then read the spreadsheet "Volts at the Load" and compare to the output voltage shown by the VFD. Adjust the spreadsheet "Length of Circuit" until the spreadsheet calculated voltage closely matches the VFD measured voltage. That should find the wire length within +/- 10%. If the VFD has an internal mesaurement of Power Factor for the motor, then setting that factor in the spreadsheet wouuld make the Length determination more accurate.

With a little work, I could modify the spreadsheet to take the voltage In and voltage Out and calculate the Wire Length directly, but it would not be worth the effort for one case.