Inductive and resistive loads...

[st07]

Hey,

I was wondering what the exact difference is between an inductive and a resistive load. They reason I ask is because on the back of my energy meter, thee are ratings for the two different loads. The maximum rating for resistive is 13A, and inductive is 3A. I understand that the inrush current for an inductive load can be many time higher than its running load, but why would the current rating be so much less than resistive?

Many thanks

 

[333rocky333]

Afaik , a lot of stuff for inductive loads, especially switches are derated.
When switching inductive loads, they often spark more and damage the contacts etc.

http://www.markallen.com/teaching/ucsd/147a/lectures/lecture3/3.php

http://en.wikipedia.org/wiki/Snubber

 

[st07]

Afaik , a lot of stuff for inductive loads, especially switches are derated.

Thanks for the links. What do you mean by 'derated', though?

 

[ericmark]

Oh is it really that simple? I wish. What happened to imagery numbers and J with argand diagrams and all those calculations?
Power factor correction I am sure came out of Merlin’s magic and apparent power and real power were just figments of my imagination?
I though it was all to do with phase shift between voltage and current and how more current can flow without doing useful work which is why we have VA and Watts and VA x Power factor correction = Watts which is a Joule per second and a Joule is 10 to the 7 ergs and an erg is 1 gram lifted on 1 cm or something like that and being metric there are 10 of one unit in the next but 4.2 joules in calorie and 980 grams in a Newton never did work out metric I was taught poundals, pounds and slugs.
Think I need to go back to school?
Eric

 

[JACKC]

That has got to be 1 of the worst links posted on this site.

 

[plugwash]

It's not so much inrush current with inductive loads (a pure inductor would have less inrush than a resistive load) as the voltage spike when you turn them off. That spike is not good for switches etc.

 

[Lectrician]

Oh is it really that simple? I wish. What happened to imagery numbers and J with argand diagrams and all those calculations?
Power factor correction I am sure came out of Merlin’s magic and apparent power and real power were just figments of my imagination?
I though it was all to do with phase shift between voltage and current and how more current can flow without doing useful work which is why we have VA and Watts and VA x Power factor correction = Watts which is a Joule per second and a Joule is 10 to the 7 ergs and an erg is 1 gram lifted on 1 cm or something like that and being metric there are 10 of one unit in the next but 4.2 joules in calorie and 980 grams in a Newton never did work out metric I was taught poundals, pounds and slugs.
Think I need to go back to school?
Eric

Yea, but current is still current!

If the inductive load needs more current to produce the same power then it uses more current. Thats why they derate things to compensate.

 

[Spark123]

I think the main issue is as 333rocky333 said above - the arcing caused by the high voltage as a result of the magnetism collapsing in an inductor.

 

[IJWS15]

It is all down to the phase angle.

 

[OilLecky]

I agree with plugwash. An inductive load resists all changes in current flow ( it's magnetic field either collapsing or expanding acts to maintain the status quo) So on switch-on the build up of current is slow but the inductive load tries to maintain the current flow when you switch off, thus causing arcing that is of a longer duration than with a purely resistive load. (Not quite the same as the inrush of a motor. That is because initially the motor is stationary and the magnetic fields are not yet developed, not producing "back-emf" and thus the windings are virtually a short circuit. This also applies to dc motors.)

I guess that a meter would continue clocking up "power use" for some time after the equipment had been switched off which is OK once in a while but if it was every few minutes then the 'lecky bill would soon mount up!

 

[plugwash]

It is all down to the phase angle.

Not really, that may be the case for stuff that is rated primerally based on what happens in the steady state but with switches of far more concern is what happens immediately after switch on or switch off.

 

[holmslaw]

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