Heater Element Impedance

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philly

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Will a heater element that is mostly resitive contain an inductance which contributes to the overall impedance. The reason I ask, is becasue if I measure the impedance of a heater element with my meter the impedance I get does not match up with the current that I am measuring. Measured current is less than theoretical calculated current.

Seeing this leads me to believe that there is an inductance in the elements that is contributing to a greater impedance that I can measure with my meter. Does a mostly resistive element such as a heater have much more reactive impedance?
 
It is certainly possible, and it depends mostly on the way the element is constructed. Everything metallic has inductance. Your wrist watch and my wedding ring each has inductance, and the two have mutual capacitance. But is it enough to make a measurable difference in current? That's hard to say. It may be nothing more than a matter of accurancy in your measuring instruments.
 
charlie b said:
It is certainly possible, and it depends mostly on the way the element is constructed. Everything metallic has inductance. Your wrist watch and my wedding ring each has inductance, and the two have mutual capacitance. But is it enough to make a measurable difference in current? That's hard to say. It may be nothing more than a matter of accurancy in your measuring instruments.
Click to expand...

Could a measured 260ohm vs a theoretical 468ohm be a matter of meter accuracy?
 
philly said:
Will a heater element that is mostly resitive contain an inductance which contributes to the overall impedance. The reason I ask, is becasue if I measure the impedance of a heater element with my meter the impedance I get does not match up with the current that I am measuring. Measured current is less than theoretical calculated current.

Seeing this leads me to believe that there is an inductance in the elements that is contributing to a greater impedance that I can measure with my meter. Does a mostly resistive element such as a heater have much more reactive impedance?
Click to expand...

It will have some. A wire loop has some inductance.
But I would expect that the resistance for heater elements would be far greater than the reactance.
What you measure with most standard meters is resistance rather than impedance.
Bear in mind that most conductive metals have a positive temperature coefficient. Resistance increases with temperature. Your measured cold resistance may well be lower than when the heater is at operating temperature.
That could explain the apparent disparity between your measured resistance and calculated current.
 
philly said:
Could a measured 260ohm vs a theoretical 468ohm be a matter of meter accuracy?
Click to expand...
Doubtful. But you didn't give us any numbers the first time around. You also haven't described your metering process, nor said whether the heater is single phase or three phase. The number I get by dividing 468 by 260 is suspiciously close to the square root of three, so I would certainly need more information, before I could give any explanations.
 
philly said:
Could a measured 260ohm vs a theoretical 468ohm be a matter of meter accuracy?
Click to expand...
not real likely.

however, it might be possible that at the operating temperature the resistance is that much higher than at room temperature. especially if it is an air cooled element.
 
Besoeker said:
Bear in mind that most conductive metals have a positive temperature coefficient. Resistance increases with temperature. Your measured cold resistance may well be lower than when the heater is at operating temperature.
That could explain the apparent disparity between your measured resistance and calculated current.
Click to expand...

Good point! This could be the case. The heaters I am referring to are 3-phase heaters rated for 1460W. I belive the three heater elements are wired in delta. Each heater element would be rated for 486.6W. The line currents we are reading for each heater are 1.7A which makes sense for the overall 1460W heater rating I would then expect the current across each heater element to be 1.7A / 1.73 = 1.0A.

With all that said taking the 480V L-L across each coil and dividing it by the measured impedance of 260ohm gives a current across the element of 1.8A which is higher than the line current. I also tried to derive the load current across element using power and still do not come up with the measured 260ohm. Maybe it is the temperature coef.

On another note if these elements are being controlled by a starter with overloads what should the overload setting be. What does the NEC say for heaters? Should 125% be added to full current?
 
And that brings me back to my question about your measurement process. How are you taking your readings? Specifically, if you are measuring resistance of a heater that is wired as a three phase delta, are you disconnecting all the leads, and measuring each element separately? If instead you are measuring phase to phase with the heater fully assembled, then you are measuring the resistance of one element as it sits in parallel with the other two that are, in turn, in series with each other. That would throw off the readings, and might account for the anamoly you are describing.

Also, are you taking current readings on each phase by using a clamp on ammeter internal to the heater, or are you reading each phase of the branch circuit that feeds the heater?
 
philly said:
Will a heater element that is mostly resitive contain an inductance which contributes to the overall impedance. The reason I ask, is becasue if I measure the impedance of a heater element with my meter the impedance I get does not match up with the current that I am measuring. Measured current is less than theoretical calculated current.

Seeing this leads me to believe that there is an inductance in the elements that is contributing to a greater impedance that I can measure with my meter. Does a mostly resistive element such as a heater have much more reactive impedance?
Click to expand...

Impedance value would be negligible.

A heater element heats up....duh....:D

Conductors will have different resistance at different treperatures. Based on the ampere rating you can calculate the heater resistance. If you know what temperature the heater will stabilize after startup then you should be able to calculate the resistance at ambient.
 
Philly -
charley and Besoeker have discussed temperature coeficients and measure methods - Let's look at those a little closer.

If you are measuring the heater resistance line to line (delta connected elements) and are getting 260 ohms, that makes each element about 390 ohms
260 = 390 (in parallel with) 2 x 390

I'll use nichrome for an example, it's a pretty common heater element. Nichrome has a positive temp coeficient (alpha) of .0004, units are 1/K (K = degrees Kelvin - yeah, I knew you knew that:)) If the heater temperature change was say 500C (900F) that would give you a resistance change of 20% (alpha X delta T = .2). So the hot resistance of each element is about 468 ohms. (I did a little back calculating to get the numbers to work out:wink:)

Somebody want to check my math - I didn't spend much time at it

As other have mentioned, check how you are measuring your initial resistance. Get back to us, I'm curious as to how it works out.

cf
 
090203-1307 EST

philly:

An old professor of mine would tell you to do some research and figure this out yourself.

First, you need to clearly define your circuit. After three posts I believe you have a a 480 delta system. What was the measured line-to-line voltage when you made the current measurement?

Exactly where did you measure the current? And was it 1.70 or 1.79?

How did you measure the resistance? And what did you measure? By that I mean the circuit?

What does the equation 1/X = (1/Y) + (1/2Y) mean to you?

I have a small single phase electric room heater that I use as a test load. Its nominal rating is 120 V 1500 W. That calculates to a resistance of 9.6 ohms. My measured room temperature resistance is 8.6 ohms from a Fluke 27 meter using the resistance range. Today it reads 9.7 ohms. Note: this Fluke only resolves 0.1 ohms. Hot at 118 V input the measured resistance is 10.6 ohms from current and voltage. Almost certainly this is made from Nichrome wire and most likely your heater is as well. Both my heater and yours probably have the wires operating at the same temperature. Thus, I would guess your hot resistance would be about 9% higher than your room temperature measurement.

On my heater I measure an inductance of about 100 microhenrys. At 60 Hz this is about 0.038 ohms inductive reactance. Thus, negligible.

The problem you have is not related to inductance, but to understanding a resistive network. You need to think about this and figure it out on your own.

Go back to some of your school books on DC and AC circuits.

.
 
OK guys I was indeed measuring the resistance with the elements connected in Delta and therefore reading the parallel network of the three elements. I dont know why I didn't see this as being obvious in the first place.

I do understand that 1/x = 1/Y + 1/2Y represents the equation for the total combined impedance with "Y" representing the unknown element impedances of each element. Solving for Y we get a resistance of 390ohms for each element impedance.

I also now see how this positive temperature coeeficient can effect this resistance reading, and it makes sense that this value would increase from the value I measured at room temp.

When we are taking the current measurements we are taking them on the line of the delta connected heaters. So we are measuring 1.7A on the line and would therfore then expect to see 1.73 times less across each heater element which is aprox 1A. I am using this 1A in all of my calculations for each individual heater. All this information now seems to make sense.

What about as far as setting the overloads for heaters? We are using solid state overloads.
 
philly said:
What about as far as setting the overloads for heaters? We are using solid state overloads.
Click to expand...

I have never liked overloads for OCP on resistance heating elements. One of the the questions I would ask is this overload part of an listed combination motor starter with an instantaneous only CB? Cause if it is, I really don't like it - 240.9 (2005).

As for the code issue, it matters what the heater application is. Look at 422, section II for some ideas. Articles 424, 426, 427 are others. Likely your NEC limit is 15A - smallest standard rating. 240.6 (2005)

Once you are by the NEC issue, the next question is what are you trying to protect against. This is a design issue. Overloads won't save a heater if if goes bad. The 15A CB will protect the conductors. If you are using the overloads as supplementary OCP, I'd say at least 125% at the minimum.

Interesting subject - A 1500W, 3ph, 480V heater is an oddity - appears very specialized. Maybe there are design concerns with the equipment the heaters are in.

cf
 
weressl said:
Where does the NEC requires OL protection for heaters?
Click to expand...

The way this application was designed, a starter was used to control the heaters turning on and off. There are five of these heaters located on a particular skid. These heaters and turned on/off all at the same time via a starter which is controlled by temperature inputs to the controller from the skid.

This is a typical size 2 starter with an MCP breaker and solid state overloads. The question came up as to how to set the overloads. I figured just taking the combined sum off the heater loads and setting this as the overload setting.
 
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