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[Anonymous]

I hated to post something so technical because it tends to confuse people. You cannot solve for the current draw in a motor unless you know the back emf. Back emf is related to the armature angular velocity. This is not a strictly resistive circuit where r = v*i. I was trying to simplify the results so we didn't get into this type of discussion. If you were confused with that paper - don't feel bad. There are lots of electrical engineers who do not fully understand dc motors.

You say p=i*r. That is well and good for resistors. Capacitors, inductors and non-linear circuits have different governing equations. For example in an incuctor the equation v = L*didt or the voltage = the inductance * the time derivative of the current.

Let's just say that having a low battery voltage will cause the starter motor to draw more current which is bad.

I don't think I will ever post anything technical about electronics here again as it just causes more problems than it solves. I did this once before and had many people who didn't know didly criticize my post. Back emf is critical to understanding the current voltage relationship in a dc motor.

[Anonymous]

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Hap and Swanny,

I thank you both for your explanations Frankly I wouldn't have posed the question if I hadn't wanted a more thorough and technical explanation. An answer that is too simple may limit understanding. An answer that is too technical may bore the uninterested. An answer that has both will probably suit most.

Your explanations have made things a lot clearer. Thanks again.

[Hap Call]

I hated to post something so technical because it tends to confuse people. You cannot solve for the current draw in a motor unless you know the back emf. Back emf is related to the armature angular velocity. This is not a strictly resistive circuit where r = v*i. I was trying to simplify the results so we didn't get into this type of discussion. If you were confused with that paper - don't feel bad. There are lots of electrical engineers who do not fully understand dc motors.

You say p=i*r. That is well and good for resistors. Capacitors, inductors and non-linear circuits have different governing equations. For example in an incuctor the equation v = L*didt or the voltage = the inductance * the time derivative of the current.

Let's just say that having a low battery voltage will cause the starter motor to draw more current which is bad.

I don't think I will ever post anything technical about electronics here again as it just causes more problems than it solves. I did this once before and had many people who didn't know didly criticize my post. Back emf is critical to understanding the current voltage relationship in a dc motor.

Swanny, I do not want you to stop posting on technical issues...you raise many valid points. Back EMF is very critical in motor design and application and I really do understand it. I actually understand the paper you gave the link to BUT...when a paper opens with a cross product of two functions I believe it becomes too in-depth for most folks to understand. Most people have no idea that small omega (the little w looking symbol in the paper) is angular velocity normally measured in radians! I was trying to keep it simple enough for the layman to understand and by using straightforward Ohm's law and the power equation makes it a little more palatable.

Honestly, it is cool to hear about back EMF in an application other than those dealing with variable speed drive control and field loss functions. I hope you continue to post your explanations! I apologize if I slighted you in any way.

Oh yeah, one other thing… your explanation on back EMF was one of the best I have ever seen.

Junkman, sorry about the confusion. Swanny’s explanation is the most technically correct between his and mine. I was probably trying to keep it too simple.

Hap

[earlfor]

Well Swanny, I hope you'll reconsider. I found your answer very informative and interesting. It sure would become boring around here if everyone only posted answers that were common knowledge. There are many in the forums that work in specialized fields and could make posts such as yours. That is a GOOD thing.
Keep it up. :-)

Earl


[quote="Swanny"]
I don't think I will ever post anything technical about electronics here again as it just causes more problems than it solves.

[Anonymous]

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The back emf factor is an interesting one. I am not an engineer but, I recall such a term from physics 101 many years ago. Sounds like in this application it would be emf induced by the coil passing through the field of the permanent magnet. I didn't realize it would play such a role in stabalizing current. Is the back emf in essence what keeps the armature from continuing to accelerate to infinity? If i*i*r=(heat generated from resistance of the wire), then any increase in current will cause the heat to increase by a factor of i raised to the power of 2 (an exponential increase, wow!!). I would think that TIME also plays a big role in how much heat is generated in the armature coil. When the armature slows down, the brushes have a longer contact time, thus allowing more heat build up in the coil.

Thanks for the discussion.