Old & New impedance
- Thread starter eestudent
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- Massachusetts
Well the impedance of an incandescent lamp changes greatly at the end of it life.
Mayimbe
Senior Member
- Location
- Horsham, UK
I believe they dont remain the same. Material degradation matter.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
The electrical impedance of things such as motors and transformers will remain approximately constant over the life of the equipment, though if you look closely enough you are certain to see changes.
In terms of the design of the electrical installation (circuit size, short circuit current, switch ratings, etc.) these changes in impedance can be ignored.
Examining these subtle changes in impedance can have significant value for preventative maintenance.
A concrete example would be the change in contact resistance of a terminal caused by repeated expansion and contraction of the various parts, this expansion and contraction caused by changing temperature as load is applied and removed. This very small contact resistance makes up only a small portion of the entire circuit impedance, and is ignored from a design point of view. But if you look at the voltage drop across such a contact when under load, you can detect and correct problems before they become serious.
A similar example would be the insulation resistance in a motor, which goes down over time. Since the insulation resistance is much greater than the line-line impedance of the motor, changes in the insulation resistance are negligible in terms of the current flowing through the motor in use. But insulation resistance measurements are a key maintenance tool for large motors.
-Jon
In terms of the design of the electrical installation (circuit size, short circuit current, switch ratings, etc.) these changes in impedance can be ignored.
Examining these subtle changes in impedance can have significant value for preventative maintenance.
A concrete example would be the change in contact resistance of a terminal caused by repeated expansion and contraction of the various parts, this expansion and contraction caused by changing temperature as load is applied and removed. This very small contact resistance makes up only a small portion of the entire circuit impedance, and is ignored from a design point of view. But if you look at the voltage drop across such a contact when under load, you can detect and correct problems before they become serious.
A similar example would be the insulation resistance in a motor, which goes down over time. Since the insulation resistance is much greater than the line-line impedance of the motor, changes in the insulation resistance are negligible in terms of the current flowing through the motor in use. But insulation resistance measurements are a key maintenance tool for large motors.
-Jon
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Virtually all impedances change over time; in fact it is one preventative maintenance method to periodically check them to detect impending failures on many classes of equipment, including motors.
K8MHZ
Senior Member
- Occupation
- Electrician
The impedance of a motor will change as bearings fail, also. Anything that causes excess slip will decrease the impedance of a motor.
The impedance of contacts will increase over time due to corrosion, pitting and contact area mismatch.
The impedance of electric heating devices will change over time as oxidation takes it's toll.
Actually, if a device was designed that would never ever change impedance it would be easy to make it last forever.
One example is some old porcelain switches I have that are about 100 years old. They were taken out of service due to a remodel. These switches worked perfect for something like 96 years, and still work. They weigh a pound or so and the contacts are huge and the gear inside is not only very strong but made of all non-corrosive metal or porcelain. I would guess that the impedance these switches have now is the same as the day they were installed. That is the exception to the rule.
The impedance of contacts will increase over time due to corrosion, pitting and contact area mismatch.
The impedance of electric heating devices will change over time as oxidation takes it's toll.
Actually, if a device was designed that would never ever change impedance it would be easy to make it last forever.
One example is some old porcelain switches I have that are about 100 years old. They were taken out of service due to a remodel. These switches worked perfect for something like 96 years, and still work. They weigh a pound or so and the contacts are huge and the gear inside is not only very strong but made of all non-corrosive metal or porcelain. I would guess that the impedance these switches have now is the same as the day they were installed. That is the exception to the rule.
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
FWIW, batteries change impedance (a lot, sometimes) over time. On some medical cells, we'd measure it over the expected life of the cell, and it was a fairly accrurate predictor of end-of-life, esp compared to voltage under load. In this case, z (@about 200Hz?) would start to rise way before e would start to drop.
K8MHZ
Senior Member
- Occupation
- Electrician
That's interesting.
I knew of the existence of internal resistance in a battery, but internal impedance is new to me. Since I use many batteries I would like to know more about this. Sometimes I get pulls from various sources and this seems like something I need to bone up on.
Can you PM me with more details as to not hijack this thread?
Thanks!
cadpoint
Senior Member
- Location
- Durham, NC
Why not write a Saturady/ Sunday thread & broaden the scope of this subject, which could include life cycles and the math!
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