# Back EMF



## madderscience (Jun 28, 2008)

I am no motor expert, but I know that field weakening can be used on shunt wound motors to increase maximum RPM at the same voltage. The drawback however is a loss of efficiency and/or power. You can't reduce back EMF without reducing the magnetic fields which create the mechanical power that the motor produces. 

I suppose you could modify a series motor to reduce the number of turns in its field, but there are already only a few big turns of wire there so probably not practical.

Basically it boils down to not being able to get something for nothing. To increase the range of an EV, you have to make it more efficient or make it able to carry more energy. Even the "worst" EV conversion motors easily make 80% peak efficiency and many meet or top 90% so there is not a whole lot you can do there to get more range as far as the motor is concerned. Its all in getting a better battery, or a sleeker, lighter chassis.


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## GerhardRP (Nov 17, 2009)

deanbo said:


> If it was possible to reduce back EMF (is this possible?) would it make a great deal of difference to the range of an EV?


Just a reminder that back EMF is not a loss mechanism. The power delivered by a motor is equal to the armature current times the back EMF.
Gerhard


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## major (Apr 4, 2008)

deanbo said:


> If it was possible to reduce back EMF (is this possible?) would it make a great deal of difference to the range of an EV?


Hi deanbo,

Answer to your question is no.

Sure you can reduce back EMF. If you're working with an existing armature, either reduce the flux or reduce the RPM. If you can alter the armature, reduce the number of conductors.



GerhardRP said:


> Just a reminder that back EMF is not a loss mechanism. The power delivered by a motor is equal to the armature current times the back EMF.


Gerhard is correct.

major


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## aeroscott (Jan 5, 2008)

higher voltage will overcome limiting amps of back emf , giving higher torque (amps) at higher rpms .


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## HighTech (Nov 12, 2008)

deanbo said:


> If it was possible to reduce back EMF (is this possible?) would it make a great deal of difference to the range of an EV?


No, because you will still lose a certain amount of electricity either way, whether you insulated the wiring or not, you will still lose electricity because it is going through a line of friction and its going to create heat. The motor itself you have to keep open too so it can cool. So its not like a hot water line in which you can try to keep the heat captured to save energy, electricity creates heat and on the motor it has to be cooled or the heat will damage the motor over time. Heat on Electronics or motors is not a good thing, thus you want to keep the motor cool, but if you are worried about personal safety from back EMF, then putting a good rubber base on the firewall and insulating the wiring more will help. Still some will get through, but it will be much less that can harm you over time. However, we get that from our ICE alternators as well and no harm has become of us, so its just FYI and it makes no difference. Just more current means more back EMF.


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## rillip3 (Jun 19, 2009)

HighTech said:


> ... if you are worried about personal safety from back EMF, then putting a good rubber base on the firewall and insulating the wiring more will help. Still some will get through, but it will be much less that can harm you over time. However, we get that from our ICE alternators as well and no harm has become of us, so its just FYI and it makes no difference. Just more current means more back EMF.


That doesn't jibe with my understanding of electricity and magnetism. While rubber is an electric insulator, the way you typically harden something from magnetism is with a conductor (often lead). I seem to recall that any metal will block EMF under a certain frequency in the ultraviolet spectrum. I don't see how rubber would interact with the magnetic field.


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## HighTech (Nov 12, 2008)

rillip3 said:


> That doesn't jibe with my understanding of electricity and magnetism. While rubber is an electric insulator, the way you typically harden something from magnetism is with a conductor (often lead). I seem to recall that any metal will block EMF under a certain frequency in the ultraviolet spectrum. I don't see how rubber would interact with the magnetic field.


Well it is because EMF is not like a microwave where certain metals will block microwaves, but because of the different wave signals from EMF it can be insulated with certain products such as rubber to absorb the frequency, but yes there are certain metals that can redirect the frequency as well, but won't absorb. The rubber being an insulator will help absorb some of the EMF frequency, but not all, thus limiting the frequency to a lessor of a danger, if that is the case where a person is sitting above the motor in a mid motor vehicle or where they may be putting large sums of current into a motor and wants that extra precaution.


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## Duncan (Dec 8, 2008)

Hi Guys
There seems to some confusion here,

Back EMF is the voltage caused by the motor spinning - it is basically a DC effect and stays inside the motor circuit


Electro-Magnetic-Fields
Is a broader thing and is basically a radiated effect related to light and radio waves

totally harmless until it gets to the level that it is causing significant heating inside your tissues - like in a microwave


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## umurali2000 (May 3, 2010)

Back EMF depends on the magnetic field ( flux density in airgap ), number of turns. Hence by varying the above two parameters Back EMF will be vary.


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