Undervoltage Protection

http://wiki.answers.com/Q/What_is_the_function_of_an_under_voltage_relay

This question has been asked before but it's easier to say it again than find the other thread, when the voltage falls so low that it is not possible to induce the rotor of an induction motor to rotate, current will still flow through the windings. With no back-EMF the only limiting factor will be the resistance of the stator windings which will overheat and burn out.

Why don't you take a course in electronics????? PLease!!!!!!!!!!!!

I concur, given coolyaar's numerous previous questions and the fact he appears to actually WORK in the power industry.

Scary. Are we keeping someone unqualified for a position employed in the power industry? Don't we get part of his paycheck or something? On second thought I want no part of that (possible) deception.

This statement, "however undervoltage has no harm to any equipment" would tend to reinforce your misgivings about this "member".

Undervoltage relay had saved the live to many motors through the world and myself from the responsibility point of view, please check technical documents about it and you will change your mind.

bye

why? because replacing burned out components is costly and unnecessary if you have a properly designed and constructed circuit. was this asked on April 1st? are you joking?

The OP's assertion that undervoltage does no harm is not correct.

A wee tale to illustrate :

A couple of years back, I worked as technical support engineer for a distributor of electrical switchgear. In this role, I got to sort out all manner of electrical problems over the phone, often for electricians who were stumped with a problem on site.

One morning I took a call from an electrician who had installed a 4KW irrigation pump which was started DOL using a 7.5KW starter. The contactor inside the DOL starter had lasted less than a week before its contacts were reduced to a barely-recognisable melted mess.

After discussing the installation with him for a bit, the penny dropped - the pump was installed down the back of the farm, some 1500m from the distribution point, and the voltage dip on startup was enough to make the contactor chatter instead of closing cleanly.

I said to the electrician that I thought that he had a voltage sag on startup, and asked him if he would consider using an undervoltage relay. His reply? "No, we already tried that but couldn't get it to hold in"!!!

After some further discussion, he agreed to fit a soft-starter and this seemed to fix the problem.