I am curious as to the difference in current for a resisitive load(heater) at 220V 60Hz versus 50Hz. We are having problems with a product at a site in South Africa that operates on 220V 50Hz where the resistive heaters are burning out(opening) after a few weeks or months. We designed and tested the product on 220V 60Hz. The lower frequency will result in a higher RMS current but I am not sure how to calculate it?
50 Hz Versus 60 Hz Current Difference
- Thread starter Tony Batt
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gar
Senior Member
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- Ann Arbor, Michigan
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- EE
080627-1403 EST
Tony Batt:
If the RMS current is of a constant value, then DC or AC of any frequency makes no difference in the average heating of a resistor of a constant value.
There is a region of very low frequency, below that of the thermal time constant of the resistor, where the temperature will vary with the AC waveform and in this case there will be more temperature rise on the peaks of the AC waveform than from DC of the same RMS value. If I had an AC source of 1 cycle per minute and an ordinary electric heater I would see substantial temperature ripple.
I doubt that your thermal time constant is anywhere close to 20 MS (50 HZ) and actually you would think in terms of 1/2 cycles or 10 MS for the heating ripple.
.
Tony Batt:
If the RMS current is of a constant value, then DC or AC of any frequency makes no difference in the average heating of a resistor of a constant value.
There is a region of very low frequency, below that of the thermal time constant of the resistor, where the temperature will vary with the AC waveform and in this case there will be more temperature rise on the peaks of the AC waveform than from DC of the same RMS value. If I had an AC source of 1 cycle per minute and an ordinary electric heater I would see substantial temperature ripple.
I doubt that your thermal time constant is anywhere close to 20 MS (50 HZ) and actually you would think in terms of 1/2 cycles or 10 MS for the heating ripple.
.
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
I agree, resistive elements don't know for frequency (unless extreme as in the example given).
But it may be that you have a temperature control device that is not functioning properly. How are the heaters controlled? If it is a contactor, is the contactor stuck closed because of a bad coil and not disconnecting when it should? if it is a proportional zero-cross SCR controller, it could be way off in it's timing and over-driving your heater, making it run too hot.
But it may be that you have a temperature control device that is not functioning properly. How are the heaters controlled? If it is a contactor, is the contactor stuck closed because of a bad coil and not disconnecting when it should? if it is a proportional zero-cross SCR controller, it could be way off in it's timing and over-driving your heater, making it run too hot.
OK, thanks for helping me remember AC 101.
The heater circuit is a PID controlled loop with a thermocouple and triac output. The temperature is controlling properly within a few degrees. We have control over output% to the heater, however the thing we are struggling with is the reason why this particular product in South Africa has a failure of 4-5 heaters in a year as compared to no failures for all other instruments, mostly of which are located in the US. We believe it to be a site specific issue( thus my original inquiry about 50Hz). The power grid in South Africa is known for having issues and we are considering having a power line monitor to check the quality of the power over time for issues.
The heater circuit is a PID controlled loop with a thermocouple and triac output. The temperature is controlling properly within a few degrees. We have control over output% to the heater, however the thing we are struggling with is the reason why this particular product in South Africa has a failure of 4-5 heaters in a year as compared to no failures for all other instruments, mostly of which are located in the US. We believe it to be a site specific issue( thus my original inquiry about 50Hz). The power grid in South Africa is known for having issues and we are considering having a power line monitor to check the quality of the power over time for issues.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
I, too, agree. Is it possible the controller is frequency dependent or sensitive?gar said:
The heaters are designed for 220VAC. We are told the voltage at the site is 220V but are waiting for confirmation on this and are opting to install a power line monitor on the AC lineto check the quality of the incoming power. Power in S. Africa is apparently hurrendous with brown outs and rolling power outages.
gar
Senior Member
- Location
- Ann Arbor, Michigan
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- EE
080627-1859 EST
If you have a DC switching supply I would look at failures in this and/or the sensor system as the likely cause.
.
If you have a DC switching supply I would look at failures in this and/or the sensor system as the likely cause.
.
gar
Senior Member
- Location
- Ann Arbor, Michigan
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- EE
080627-1903 EST
The switching supply is what will be concerned with the AC input voltage and frequency. The output voltage and current to the heaters will be controlled by the switching supply and the sensor control to it. In all probability the input AC is rectified, filtered to moderately ripple free DC, and then converted to moderately high frequency AC.
You may be applying AC or DC to the heaters and it does not make much difference but DC will require ouput diodes that waste power.
Look closely at your circuit and the control algorithm. What is the maximum RMS current the switching supply can apply to the heaters. In what physical environment are the heaters located?
.
The switching supply is what will be concerned with the AC input voltage and frequency. The output voltage and current to the heaters will be controlled by the switching supply and the sensor control to it. In all probability the input AC is rectified, filtered to moderately ripple free DC, and then converted to moderately high frequency AC.
You may be applying AC or DC to the heaters and it does not make much difference but DC will require ouput diodes that waste power.
Look closely at your circuit and the control algorithm. What is the maximum RMS current the switching supply can apply to the heaters. In what physical environment are the heaters located?
.
brian john
Senior Member
- Location
- Kilmarnock, Va
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- Retired after 52 years in the trade.
The real difference is about 10 HZ.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Thank God you're here. :roll:brian john said:
grin
AC line voltage is applied to the heaters. The DC switching power supply supplies only the control circuitry to the SBC(single board computer) PC104 stack which is operating fine. The equipment that the heaters are located in are in a temperature controlled laboratory envioronment.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080630-1128 EST
Tony:
Can you provide a better discription of the power part of the circuit? Why was the switching supply mentioned if it does not relate to the power to the resistive load?
Reading back thru your posts I find that you may be saying that there is a 220 V 50 Hz AC supply, a Triac, and a resistive heater load that I now assume are in a series loop with no other components in that loop.
Most heating elements will have a thermal time constant longer than 1/60 or 1/50 second and therefore should not care whether it is 60 or 50 Hz.
Suppose the Triac remained on continuously, then would the heater fail? If not then you have an environmental problem if the voltage does not go above whatever maximum voltage the heater can continuously tolerate.
Next assume the design was not that conservative. Is this a bang-bang, or a phase modulation servo system? May not be important but would be useful to know. In either case if the controller failed for a short time and kept the Triac on, then the heater could burnout.
Assuming ambient temperature is reasonably constant and you do not change the content of whtever is inside of this heater where your temperature sensor is, then there should be no great changes in heater current averaged over a moderately short time period. Any sort of current monitor, average or RMS reading, with a time constant of 100 to 500 MS (a standard analog meter movement or typical digital meter) measuring the heater current should allow you to see if abnormal current changes exist. Obviously it should be a recording meter. Probably a computer with appropriate harware and software.
A bang-bang (on-off) controller might have a longer time constant than the above (100 to 500 MS), but recording the measurements would still allow observation of abnormal conditions.
Consider this example. A system with only temperature monitor and no current monitor, a consrvative design for 220 V such that full on of the heater would not cause burnout, and an assumption input line voltage was moderately constant at 220 V, and a long time constant from heater to temperature sensor such that this time constant is long relative to a burnout time if overvoltage is applied. Now fail the one assumption about line voltage and have a voltage that rises to 1.4 times normal, then power input is doubled and the heater fails.
If the controller loop included current or voltage measurement, then line voltage would have no effect unless it went high enough to fail the Triac.
.
Tony:
Can you provide a better discription of the power part of the circuit? Why was the switching supply mentioned if it does not relate to the power to the resistive load?
Reading back thru your posts I find that you may be saying that there is a 220 V 50 Hz AC supply, a Triac, and a resistive heater load that I now assume are in a series loop with no other components in that loop.
Most heating elements will have a thermal time constant longer than 1/60 or 1/50 second and therefore should not care whether it is 60 or 50 Hz.
Suppose the Triac remained on continuously, then would the heater fail? If not then you have an environmental problem if the voltage does not go above whatever maximum voltage the heater can continuously tolerate.
Next assume the design was not that conservative. Is this a bang-bang, or a phase modulation servo system? May not be important but would be useful to know. In either case if the controller failed for a short time and kept the Triac on, then the heater could burnout.
Assuming ambient temperature is reasonably constant and you do not change the content of whtever is inside of this heater where your temperature sensor is, then there should be no great changes in heater current averaged over a moderately short time period. Any sort of current monitor, average or RMS reading, with a time constant of 100 to 500 MS (a standard analog meter movement or typical digital meter) measuring the heater current should allow you to see if abnormal current changes exist. Obviously it should be a recording meter. Probably a computer with appropriate harware and software.
A bang-bang (on-off) controller might have a longer time constant than the above (100 to 500 MS), but recording the measurements would still allow observation of abnormal conditions.
Consider this example. A system with only temperature monitor and no current monitor, a consrvative design for 220 V such that full on of the heater would not cause burnout, and an assumption input line voltage was moderately constant at 220 V, and a long time constant from heater to temperature sensor such that this time constant is long relative to a burnout time if overvoltage is applied. Now fail the one assumption about line voltage and have a voltage that rises to 1.4 times normal, then power input is doubled and the heater fails.
If the controller loop included current or voltage measurement, then line voltage would have no effect unless it went high enough to fail the Triac.
.
keepitsimplestupid
Member
The final control element matters. If Zero crossing switched, you should not have problems, if ON/OFF your going to put stress on the heater, if phase angle fired, frequency will matter.
If you really want extremely good heater lifetime, replace the final control element with a DC switching power supply with voltage control input.
You can convert 0-20mA to a voltage with a resistor. The output of the controller generally has to be isolated to work. www.xantrex.com makes suitable supplies.
Eurotherm, BTW makes excellent controllers, both PID and power. Some incorporate current limiting, which helps against failures.
But, Lifetime of the heaters will be infinately better with DC applied from experience.
If you really want extremely good heater lifetime, replace the final control element with a DC switching power supply with voltage control input.
You can convert 0-20mA to a voltage with a resistor. The output of the controller generally has to be isolated to work. www.xantrex.com makes suitable supplies.
Eurotherm, BTW makes excellent controllers, both PID and power. Some incorporate current limiting, which helps against failures.
But, Lifetime of the heaters will be infinately better with DC applied from experience.
weressl
Esteemed Member
brian john said:
Yeah, but I think the problem in this case is that the current churns in a different direction below the equator....
iaov
Senior Member
- Location
- Rhinelander WI
The electrons spin couter clockwise there, don't they?:grin:weressl said:Yeah, but I think the problem in this case is that the current churns in a different direction below the equator....
B
bthielen
Guest
Another consideration.
What about the shutdown sequence of the heaters? If the heat generated by the heating elements can't be dissipated efficiently, the heaters will not last long. For example, if you apply power to your electric water heater without first filling it with water, the heaters will open rather quickly because the heat won't be handled efficiently. If your heating system is subjected to frequent shutdowns without following proper sequence, the heaters could be overheating due to residual buildup.
Some of our equipment incorporates heat tunnels where we use hot air to heatshrink plastic film on the product. If these tunnels are shut down via an emergency stop the heaters and blowers are stopped instantly and heat is trapped around the heating elements causing premature breakdown. We design our systems with a normal shutdown sequence but our customers don't always follow our recommendations.
Another problem can be frequent startups from cold. We have found that customers who run 24-7 achieve longer life out of their heaters vs those that run one or two shifts per day. The thermal expansion caused by cooling to ambient and then reheating to operating temperature on a daily basis is harder on the heaters than to maintain a more constant temperature.
Bob
What about the shutdown sequence of the heaters? If the heat generated by the heating elements can't be dissipated efficiently, the heaters will not last long. For example, if you apply power to your electric water heater without first filling it with water, the heaters will open rather quickly because the heat won't be handled efficiently. If your heating system is subjected to frequent shutdowns without following proper sequence, the heaters could be overheating due to residual buildup.
Some of our equipment incorporates heat tunnels where we use hot air to heatshrink plastic film on the product. If these tunnels are shut down via an emergency stop the heaters and blowers are stopped instantly and heat is trapped around the heating elements causing premature breakdown. We design our systems with a normal shutdown sequence but our customers don't always follow our recommendations.
Another problem can be frequent startups from cold. We have found that customers who run 24-7 achieve longer life out of their heaters vs those that run one or two shifts per day. The thermal expansion caused by cooling to ambient and then reheating to operating temperature on a daily basis is harder on the heaters than to maintain a more constant temperature.
Bob
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