Measuring motor current? 4

[nmark]

If I have a motor 150' away from the contactor controlling it and I measure current at the peckerhead and then take a second measurement at the contactor assuming no change in load and regardless of wiring methods should the measurements be identical?
Is it relevant where you measure current in an AC circuit(for example at the breaker versus at the load)?

Thanks

 

[itsmoked]

Theorectically they will be the same.

That said they can be different.

Your clamp-on measures the magnetic flux that passes thru its aperture. Unlike a shunt or direct connect meters that measure the actual current thru the shunt a clamp-on measures "what it finds in its aperture". This means its readings can vary by the angle the wire runs thru it. The location the wire runs thru the clamp-on, and whether you have high magnetic fields in the same area, or lots of iron nearby.

Generally the readings would not vary enough for anyone to actually care. The motor loading can shift slightly and it would cause a much larger shift than perhaps any of the aforemented details.

So measure where you will..

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

In general it shouldn't make any difference. If you find a big difference in current at each end of cable, you really need to ask yourself where that current is going.

You might Google Kirchhoff's Current Law....

 

[nmark]

What effect could having multiple loads in the same raceway have on the measured current?
Could I expect the measured current to be lower if wiring to loads was physically seperated?
I have a panel with 13 motor starters in it motor nameplate
FLA-24A but most motors are drawing between 26-27 amps the wiring for the 13 motors is split between 2 raceways for 50' + 100 feet in a wireway with all 13 motor leads.

Thanks

 

[davidbeach]

Where do you think the current would be going? Charging current isn't an issue at this voltage. Current in equals current out or there is a fault. Other circuits don't matter.

 

[nmark]

Why does the NEC require derating current carrying conductors installed in the same raceway?

 

[davidbeach]

Conductor ampacity is a thermal rating. Additional conductors in the raceway contribute additional heat meaning that none can create as much heat as they could if there were only one circuit in the conduit without the temperature in the conduit becoming too high. All of this to keep the final temperature from exceeding the thermal rating of the insulation (the conductor itself is good for much higher temperatures).

The derating has nothing to do with any violations of Kirchhoff's Current Law.

 

[nmark]

Thanks davidbeach that answers my questions and clears some of the fog in my head.

 

[jraef]

Did you measure the voltage at either end to see if there is a drop? If there is a drop at the motor, the conductors may be undersized and you could be losing current in heating of the cables.

 

[Skogsgurra]

Too late at night, Jeff? Do you really lose current that way? I know that you know you don't.

Gunnar Englund

http://www.gke.org

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

 

[itsmoked]

Jeffs just trying to trick us.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[davidbeach]

Jeff,

No, I don't think so. Kirchhoff wouldn't like that and you wouldn't to make Kirchhoff mad now would you?

 

[halgross]

The current measurements will be the same regardless of where in the AC circuit it is measured. A clamp-on meter can yield different measurements simple by how centered the cable is in the opening and the angle at which the meter is held. Shunts aren't usually used in AC circuits. AC currents higher than can be measured with a series connected meter are measured with a current transformer and secondary meter. The current transformer is connected to a ammeter usually with a 1 or 5 amp movement. The motor cable either passes thru the current transformer (donut type) or is connected in series with a bar type current transformer. The current transformer can be mounted near the contactor or in the motor connector box. Current will be the same either way.

 

[peebee]

A water pipe is a pretty good analogy. The water flow at this end of the pipe should be the same as the flow at the other end of the pipe.

If you get different flow measurements, that either means that you have a tee fitting in there somewhere that you forgot about, or you have a leak.

Electrically, if you get different current readings, either you have another load tapped into the middle of your feeder run, or you have some leakage current (small leak), or you have a short circuit (big leak).

"Big leak" short circuits are obviously a problem.

"Small leaks" might include insulation breakdown, which can quickly turn into big leaks.... or they could also be capacitive coupling, which is generally not a problem.

 

[jraef]

Derr derr derr... yes, it was late at night. Gotta quit doing that...

 

[electricpete]

I think jeff was right the first time. Kirchoffs current law does not consider the inverse reactive current associated with the modial interaction of magneto-reluctance and capacitive directance. It makes a difference whether the motor is fed from a retro encabulator.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[Marke]

If you have static power factor correction connected to the motor circuit, you will get a different current if you make your measurements either side of the capacitor connection.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

Time for PLS:es

Jeff gets one for interesting new theory.
Pete gets one for another interesting theory - and very well formulated.
Mark gets one for being on the mark.

Gunnar Englund

http://www.gke.org

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

 

[jraef]

I like the fact that I got a PLS for a blurry-eyed 1/2 brained response that flew in the face of established laws of physics. Life is good...