Inductive Heating ????

[Dennis Alwon]

I received a pm from Gary123. Here is his question

Hey Dennis,
This is my first time posting here, my names Gary and I an working in a small NH paper mill and we has recently been changing over to alum conduit.

Two weeks ago i completed a 60' run of 2" with 6 1/0 conductors feeding a 200HP 460V 230FLA motor. The problem Im having is the conduit, conductors and soft start are heating up to between 140 and 165 degrees.


The conduit in connected at both ends with non-metallic flex and that where i think the problem lies. . . .The conduit is not bonded at either end so I am suspecting "inductive heating"

I welcome your thoughts and opinions,

Gary

 

[charlie b]

Unless there is an unsuspected wiring error somewhere along the line, it likely won't be inductive heating. The magnetic field that would exist outside a set of three phase conductors, even at so short a distance from the conductors as the conduit that surrounds them, would be very small. The field created by any one of the three phase conductors would be cancelled out by the fields created by the other two. If, for example, someone had made the wiring error of using two parallel 2" conduits, and placed the A and B phase conductors in one, and placed the C phase and ground conductors in the other, that could cause the symptom you described.

On the other hand, if there is something wrong with the motor itself, such that it is drawing a significantly different current in each of the three phases, then such an imbalanced set of currents could create a net magnetic field that could result in circulating currents flowing in the conduit. That could explain your symptom. So I would suggest measuring each of the three phase currents, and see if that tells you anything.

 

[bob]

The conduit in connected at both ends with non-metallic flex and that where i think the problem lies. . . .The conduit is not bonded at either end so I am suspecting "inductive heating"

The post states that there a 2 1/0 conductors per phase. If so where is the required EGC.
The 90C rating of 1/0 Cu(assumed) is 170 amps x 2 = 340 amps. Reduction for contineous loads is 340 x .8 =272 amps. Reduction for 6 conductors = 272 amps x .8 = 217 amps. Is the fact that this installation is not code compliant make a difference? AL conduit is not subject to inductive heating. The fact that all three phases are in the conduit would prevent this anyway.
The problem is the load is heating up the conductors and the conduit. Installer may need some competent assistance. Check the information below.

http://enews.penton.com/enews/electricalzone/codewatch/2007_08_27_august_27_2007_codewatch/view.html

 

[texie]

Unless there is an unsuspected wiring error somewhere along the line, it likely won't be inductive heating. The magnetic field that would exist outside a set of three phase conductors, even at so short a distance from the conductors as the conduit that surrounds them, would be very small. The field created by any one of the three phase conductors would be cancelled out by the fields created by the other two. If, for example, someone had made the wiring error of using two parallel 2" conduits, and placed the A and B phase conductors in one, and placed the C phase and ground conductors in the other, that could cause the symptom you described.

On the other hand, if there is something wrong with the motor itself, such that it is drawing a significantly different current in each of the three phases, then such an imbalanced set of currents could create a net magnetic field that could result in circulating currents flowing in the conduit. That could explain your symptom. So I would suggest measuring each of the three phase currents, and see if that tells you anything.

While I agree with your comments, wouldn't also it be the case that the conduit here is nonferrous and should not cause inductive heating in any event? I'm also wondering how the OP bonded the conduit given the description of the install.

 

[Dennis Alwon]

It does not sound like the conduit is bonded at all.

 

[don_resqcapt19]

The fact that the conduit is not bonded should not be the cause of the heating. The code required ampacity of the motor circuit conductors is 300 amps. The installed conductors have an amapcity of 272 amps. They will be running hot. In fact they will be running very close to their 75? ampacity and very close to 75?C or 167?F....

 

[gar]

120620-0639 EDT

Some guesses.

Power dissipation in the wires is about 10 W per foot length. Calculated from I²R.

I can not find a quick reference on the convection thermal transfer from a pipe in still air, no fan. There is also radiant transfer of heat energy from the surface of the pipe. One foot of 2 inch diameter is about 75 sq-in of surface area. This is a substantial area. For a wild guess assume 1 deg C/W/100 sq-in. That would mean about a 24 deg F rise over ambient.

My guess is that there is inductive heating and probably means substantial magnetic field unbalance in some form.

Relative to a previous post it should be noted that for inductive heating to occur it is only necessary that you have a closed loop conductor with resistance. Thus, a chunk of copper can be inductively heated.

.

 

[Besoeker]

120620-0639 EDT

Some guesses.

Power dissipation in the wires is about 10 W per foot length. Calculated from I²R.

I can not find a quick reference on the convection thermal transfer from a pipe in still air, no fan. There is also radiant transfer of heat energy from the surface of the pipe. One foot of 2 inch diameter is about 75 sq-in of surface area. This is a substantial area. For a wild guess assume 1 deg C/W/100 sq-in. That would mean about a 24 deg F rise over ambient.

gar

The value I use for calculating surface dissipation from painted steel enclosures is 5.5 W/m²/degC.
At that, the 10W per foot that gives a rise of about 70F over ambient.

 

[gar]

120620-1117 EDT

Besoeker:

Thanks. That is much better than my wild guess.

Using your value the measurements of the original question may indicate there is not much inductive heating.

.

 

[bob]

120620-0639 EDT

Some guesses.

Power dissipation in the wires is about 10 W per foot length. Calculated from I²R.
One foot of 2 inch diameter is about 75 sq-in of surface area. This is a substantial area. For a wild guess assume 1 deg C/W/100 sq-in. That would mean about a 24 deg F rise over ambient.

My guess is that there is inductive heating and probably means substantial magnetic field unbalance in some form.

Relative to a previous post it should be noted that for inductive heating to occur it is only necessary that you have a closed loop conductor with resistance. Thus, a chunk of copper can be inductively heated.

.

The resistance for 1/0 copper is given in table 8 as 0.100 per 1000 ft @ 77F and 0.122 @ 167F. For 60 ft the resistance is 0.006. Watts = I²R = 230²x 0.006 = 317 w/phase.
So for the 3 phase conductors the total is 3 x 317 = 951 watts. Using the 167 degree resistance the watts = 1162 watts. Closer to 20 watts per ft.

*** So what is being inductively heated. Certainly not the conduit.
*** What chunk are you referring too?

 

[Dennis Alwon]

Well I don't know what happened to Gary. I sent him a link but I was hoping he would fill in some details. Oh well.

 

[hillbilly1]

The original poster didn't say, but if steel mineralac straps were used to support the conduit, could it possibly act as a choke if they are screwed to a non-metallic structure?

 

[gar]

120620-1223 EDT

Bob:

Put a conductive material in an oscillating magnetic field and currents are induced in the material. Those currents in combination with the resistance of the material cause heating. Eddy current losses. These are not just limited to magnetic materials.
http://en.wikipedia.org/wiki/Eddy_current

Why not the conduit if it is a conductive material?

I believe there are 6 conductors, not 3. Thus, your calculation approximates mine when you double the area.

.

 

[Besoeker]

The resistance for 1/0 copper is given in table 8 as 0.100 per 1000 ft @ 77F and 0.122 @ 167F. For 60 ft the resistance is 0.006. Watts = I²R = 230²x 0.006 = 317 w/phase.
So for the 3 phase conductors the total is 3 x 317 = 951 watts. Using the 167 degree resistance the watts = 1162 watts. Closer to 20 watts per ft.

Per the OP, there are six conductors. One might reasonably assume that these are arranged as two conductors per phase.

Edit:
Gar posted while I was typing, answering phone calls, checking this evening's TV schedule, having a beer, shooting the breeze with the missus.........
Excuses, excuses, excuses........I know....

 

[bob]

120620-1223 EDT

I believe there are 6 conductors, not 3. Thus, your calculation approximates mine when you double the area.

.

You are correct. Forgot about the parallel conductors. This article indicates that AL is not affected.
http://enews.penton.com/enews/elect...27_august_27_2007_codewatch/view.html#a070820

 

[jumper]

Gar posted while I was typing, answering phone calls, checking this evening's TV schedule, having a beer, shooting the breeze with the missus.........

So what's on the telly tonight?

Regards,
Del Boy.

PS. My neighbor, a Scot also, says he loves that show.

 

[Besoeker]

So what's on the telly tonight?

Regards,
Del Boy.

PS. My neighbor, a Scot also, says he loves that show.

Tonight there's a soap that the missus watches. Eastenders. London's east end that is. I call it Beastenders.
The schedule has been disrupted by Euro 2012. Bladdy European football. Soccer to you. I can't stand it. Soccer in any way shape or form. It seems to engender the mob mentality in the fans and, particularly for England fans, drunkenness and violence.
For sport, I watch F1 motor racing. The drivers need to have incredible levels of fitness, strength, stamina and split second reflexes. And it's very technical. What more could one want for an engineer who is also a petrol head?!

There is now an electrical element to it. KERS. Kinetic energy recovery system. Some of the energy that would otherwise be dissipated thermally in brakes is used to recharge the system electrically and available for use at other times to give a 60kW (80HP boost). Good for overtaking.

And Del Boy sells dodgy electrical goods as well as other things.

 

[gar]

120620-1303 EDT

Bob:

From your referenced article:

Aluminum conduit, locknuts, and enclosures carry eddy currents, but because aluminum is nonferrous, it doesn't heat up.

That is wrong. It probably should have been worded differently. If there is current with resistance, then there is heat. Maybe not as much heat, but there are heat losses. I do not know the relative difference in inductive heating between an aluminum and steel conduit of comparable cross-section. Apparently it is quite different based on the quoted comment. In a relative sense the statement is probably OK.

The reason Beldfoil shielded wire is made with an aluminum shield anodized on one side is to provide a total wrapped cable, but without a shorted turn surrounding the wires. This is to reduce losses. Aluminum oxide is an insulator (anodizing).

In transformers with ferromagnetic core materials there are two types of losses. Hysteresis and Eddy current. Hysteresis losses are determined by the material and the amount of the material. The Eddy current losses are reduced by increasing the resistance of the material. This is done in several ways --- choice of material, making parts of the core smaller (thinning laminations and powdered iron in a binder). Orientation of material thickness relative to the magnetic field is a big factor in Eddy current loss.

In a transformer made with two different core materials, both of 29 gauge --- USS Transformer 72, and Hipernik --- the Eddy current losses are the same. But the hysteresis losses are quite different. USS has about 5 times the hysteresis loss as Hipernik. At 60 Hz the dominate loss type shifts from one material to the other.

.

 

[jumper]

There is now an electrical element to it. KERS. Kinetic energy recovery system. Some of the energy that would otherwise be dissipated thermally in brakes is used to recharge the system electrically and available for use at other times to give a 60kW (80HP boost). Good for overtaking.

I have to look that up, sounds interesting.

And Del Boy sells dodgy electrical goods as well as other things.

I swear, I did not print those UL labels-I got no idea where they came from.

 

[Besoeker]

I have to look that up, sounds interesting.

A link:
http://www.formula1.com/inside_f1/understanding_the_sport/8763.html