# Can anyone explain this?



## Voltswagen (Nov 13, 2008)

My 11 yr. old son got an Electric Motor Kit from Edmund Scientific for Christmas.
You build it yourself and it runs on a standard D Cell battery. It really took me back to my childhood days building Crystal radios, motors and the like.
Anyway, I was teaching him how to use a Multi Meter as part of the learning experience. We set the meter for 20v DC and read the battery which was
about 1.55 volts. Then we read the voltage at the brushes which was 1.50 volts. Then he started the motor and while it was running he read the voltage again at the brushes. The reading was bouncing around 2.1 - 2.8 volts. Why would it be more?
Could anyone explain this?
Thanks - Roy


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## Georgia Tech (Dec 5, 2008)

Voltswagen said:


> My 11 yr. old son got an Electric Motor Kit from Edmund Scientific for Christmas.
> You build it yourself and it runs on a standard D Cell battery. It really took me back to my childhood days building Crystal radios, motors and the like.
> Anyway, I was teaching him how to use a Multi Meter as part of the learning experience. We set the meter for 20v DC and read the battery which was
> about 1.55 volts. Then we read the voltage at the brushes which was 1.50 volts. Then he started the motor and while it was running he read the voltage again at the brushes. The reading was bouncing around 2.1 - 2.8 volts. Why would it be more?
> ...


Well this as you well know deifes the laws of Physics....It seems that the needle must have some sort of mechanical momentum and over swing on the comutator noise of some sort......Need a scope to see what going on....but you do know that commutator will make noise spikes higher than the power supply...


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## TheSGC (Nov 15, 2007)

Georgia Tech said:


> Well this as you well know deifes the laws of Physics....It seems that the needle must have some sort of mechanical momentum and over swing on the comutator noise of some sort......Need a scope to see what going on....but you do know that commutator will make noise spikes higher than the power supply...


Yep, it would definitely have to be ringing. 2x the voltage is not that bad actually. I have estimated that a normal EV motor would have ringing in excess of 8 times the normal voltage.


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## Voltswagen (Nov 13, 2008)

So if I have this right.......the "ringing" causes a false voltage reading on the meter and an ocilliscope would filter out the ringing and provide a correct voltage reading which would be battery voltage or less.


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## Georgia Tech (Dec 5, 2008)

Voltswagen said:


> So if I have this right.......the "ringing" causes a false voltage reading on the meter and an ocilliscope would filter out the ringing and provide a correct voltage reading which would be battery voltage or less.


No the scope would give you a good look at the commutator noise...You will see this noise frequency increase with RPM increasing......

if anything the analoge meter will in some way filter the noise....


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## Qmavam (Aug 17, 2008)

Voltswagen said:


> My 11 yr. old son got an Electric Motor Kit from Edmund Scientific for Christmas.
> You build it yourself and it runs on a standard D Cell battery. It really took me back to my childhood days building Crystal radios, motors and the like.
> Anyway, I was teaching him how to use a Multi Meter as part of the learning experience. We set the meter for 20v DC and read the battery which was
> about 1.55 volts. Then we read the voltage at the brushes which was 1.50 volts. Then he started the motor and while it was running he read the voltage again at the brushes. The reading was bouncing around 2.1 - 2.8 volts. Why would it be more?
> ...


 The motor consists of several coils of wire, when you pass current through them it builds a magnetic field around them. When you remove the current the magnetic field collapses and induces a current flow that causes
a voltage spike. Cars engines used this method (coil and points) to create a 30,000 volt spike from 12 volts, to fire the spark plugs.
This url has a demo. 
http://www.wisc-online.com/objects/index_tj.asp?objID=ACE5803
Mike


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## flybynight (Jan 2, 2009)

8X nominal voltage is quite common, really, and the largest reason most controllers fail over time. They can't handle the extra coming back from the motor. That's why a properly sized Brake Resistor in most drive systems only has a duty cycle of about 40%. That's also why you cannot efficiently use all of the regen power you generate. Just a thought......


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## COS (Dec 23, 2008)

It's called BEMF (Back Electromotive Force). What you're seeing is the collapsing field of force from the coils. Remember ELI the ICE man theory. In this case it's ELI where Voltage leads the current in the inductors/coils. There is a Quadratic equation used to calculate this from real to imaginary or polar to rectangular but I don't remember it offhand.
________
Weedmaps


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