Back EMF of a motor is typically due to the motor acting as a generator. I find that it is easier to understand when you consider the operation of a DC motor. Picture a DC motor with a large inertial mass loading it. Once it has reached operating rotational speed the mass will contain a large amount of rotational energy. If you open the drive supply, the motor will continue to spin until all of the energy contained within the various masses is dissipated.
You will find that the DC voltage appearing across the disconnected motor is a small amount less than the supply at the moment of disconnection. This reading is the back EMF of that motor. Now, if you connect a resistive load to the motor terminals it will conduct a current that is equal to the back EMF divided by the resistance. I am neglecting the small contributions due to the series resistance and inductance of the motor to make the example easier to understand. Remember that the load inertial mass has energy due to the motors rotation rate and most of that stored energy can be extracted into a resistive load connected across the motor terminals once the supply is removed. Another interesting observation is that the back EMF is zero at the moment that the supply is first connected to the DC motor terminals when it is not spinning. This allows the incoming current to be very large and is limited by the resistance of the starting network. As the rotational speed increases the back EMF due to generator behavior rises until it approaches the magnitude of the input supply voltage. The current drawn by the motor falls as the speed increases until it reaches a final value, at which time the input DC power matches the mechanical load power supplied by the motor and its internal losses. With a DC motor the rotation rate of the motor will adjust itself depending upon the amount of mechanical power extracted by the motor loading and losses. The back EMF and winding resistance are the main parameters that determines that rate and back EMFis proportional to the rotational speed of the DC motor. I hope this explanation is helpful although it may be too wordy. The answer to your second question is that a DC motor is also a DC generator and that back EMF is associated with the generator operation. The laws of physics working through the conservation of energy tie the mechanical work delivered by a motor with the input electrical work originating within the supply. If the back EMF were to go away for some strange reason then this link would be broken and the system would not function as a motor. AC motors and generators are much more complex to explain but the principles are similar. Dave -----Original Message----- From: mixent <[email protected]> To: vortex-l <[email protected]> Sent: Sun, Feb 7, 2016 4:04 pm Subject: Re: [Vo]:at your mercy: please explain what BackEMF/Lenz Law is - and what would happen if a motor/generator could be built that is not subject to them? In reply to esa ruoho's message of Sun, 7 Feb 2016 22:34:38 +0200: Hi esa, [snip] Dave is perhaps better positioned to comment, but for what it's worth, I think you would end up with an excellent electrical heater. ;) If you want an OU motor, then the back EMF has to *exceed* the forward EMF, not be equal to zero. Back EMF is your friend, not your enemy. It is what ensures that the majority of the energy fed to an electric motor ends up as mechanical energy, not as heat in the windings. >So uhh noone has anything to say about this? > >--- >| Esa Ruoho | +358403703659 | http://fi.linkedin.com/in/esaruoho ><http://fi.linkedin.com/in/esaruoho> | >| http://lackluster.bandcamp.com <http://lackluster.bandcamp.com/> | >http://lackluster.org <http://lackluster.org/> | http://esaruoho.tumblr.com ><http://esaruoho.tumblr.com/> | >| http://twitter.com/esaruoho <http://twitter.com/esaruoho> | >http://facebook.com/LacklusterOfficial ><http://facebook.com/LacklusterOfficial> | > >> On 04 Feb 2016, at 23:59, Esa Ruoho <[email protected]> wrote: >> >> Hi, I'm putting this out here, hoping for some sort of layman's explanation >> of these two things. >> >> 1- What is Back EMF / Lenz Law? >> 2- What would happen if a Motor-generator could be built that was immune to >> BackEMF/Lenz Law/ (sometimes referred to as Back Torque)? >> >> >> >> -- >> --- >> http://twitter.com/esaruoho <http://twitter.com/esaruoho> // >> http://lackluster.bandcamp.com <http://lackluster.bandcamp.com/> // >> +358403703659 // >> skype:esajuhaniruoho // http://esaruoho.tumblr.com/ >> <http://esaruoho.tumblr.com/> // iMessage: [email protected] >> <mailto:[email protected]> // Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/project.html
