electrical question

[Ukraine Train]

If I want to run two 6VDC motors from a 12VDC source, can I hook them up in series like this so each motor only gets 6V?


This is for a Power Wheels Jeep that I'm doing a school project on. The way it's wired from the factory is two 6V batteries in parallel in HI range and in LO range the batteries are in series which adds up to 12V but under load the voltage drops to only 4V so I don't think the motors can handle a real 12V.

[charby]

No

[Ukraine Train]

Any suggestions on stepping the voltage down? I imagine a really big resistor would work.
Link to a wiring diagram I drew up: http://i23.photobucket.com/albums/b384/UkraineTrain/random/PB152020.jpg
There is a HI/LO switch and FWD/REV switch.

[Headless Thompson Gunner]

Probably, assuming the two motors are identical and are loaded equally.

The way it's wired from the factory is two 6V batteries in parallel in HI range and in LO range the batteries are in series which adds up to 12V but under load the voltage drops to only 3V so I don't think the motors can handle a real 12V.

This doesn't make sense, though.  All bets are off until you understand what's going on here.

[Ukraine Train]

Doesn't make much sense to me either but I verified it with a volt meter and that's what I got.

[Headless Thompson Gunner]

Is there any way to identify the motor specs?  Or the battery specs?


A high internal resistance inside the batteries might account for that sort of voltage drop when under load.  But then you'd expect the same voltage drop under load in the other configuration.

Can your meter measure resistance?  Or high current?

[Ukraine Train]

I can get the specs from the batteries tomorrow but the motors did not have anything stamped on them. I was also just thinking that when you put the batteries in series they are not able to put out as much current as when they are in parallel so the voltage drops due to the inability to meet the load the motors require. But what do I know, I'm an ME lol.

[Headless Thompson Gunner]

That's more or less correct.  

Batteries have an internal resistance.  Think of a battery as a voltage source and a resistor in series.  When the battery is disconected, no current flows and no voltage is dropped accross the internal resistance.  When you connect the battery and current starts to flow, there will be a voltage drop over the this internal resistor.  Ohm's Law.  The voltage at the terminals will decrease by (I*R) volts from the disconnected voltage.  The higher the current, the higher  the (I*R) voltage drop will be.

But it's pretty "odd" for a battery to drop from 12V down to 3V.  The internal resistance of the battery would have to be 4 times greater than the resistance of the motor.  That's pretty unlikely.  Unless the batteries are faulty.

[charby]

I can get the specs from the batteries tomorrow but the motors did not have anything stamped on them. I was also just thinking that when you put the batteries in series they are not able to put out as much current as when they are in parallel so the voltage drops due to the inability to meet the load the motors require. But what do I know, I'm an ME lol.

That is true the voltage will drop across the series, Look at getting a 6 volt motorcycle battery and wiring in parallel.

You could get a variable resister and with a meter check were it puts out 6 volts and create a stop so you don't go over 6volts.

Charby

[Brad Johnson]

Something is wrong with the flux capacitor.

What is the intended outcome? Maybe we can help you come up with a more elegant solution.

Brad

[Ukraine Train]

The objective of the project is to convert energy from one form to another. What we are doing is making the Power Wheel hybrid electric. We are building a trailer that it will pull. The trailer will have a lawnmower engine on it spinning an alternator. The alternator will power the Jeep.

[cfabe]

The power wheels already converts energy from one form to another... From chemical potential energy to mechanical kinetic energy.

If you get the right alternator, you can probably fudge the output voltage by adding a high power resistor in the right place in the wiring, so it drops the stator excitation voltage.

[280plus]

Just out of curiosity, where are you placing your probes to measure this 3 or 4 Volts? Across the batteries or across the motors?

And IIRC Ohm's Law only works for resistance type loads not inductance type loads.

I have a feeling hooking the motors in series will create problems due to them each switching polarity as they run. there is a "gap" in the voltage or current flow at the point between each switching of polarity due to the dead spots in the commutators, I tend to think this will mess things up. But I'm not sure.

If I understand correctly you are trying to charge 2 - 6V batteries with a 12V alternator and then using the batteries to run 2 - 6V motors. Is this correct?

If so why not get a 6 volt alternator instead? An old VW one would be 6V as well as a motorcycle. (Do motorcycles have alternators?)

I have a feeling you are right about the motors only being able to handle 6V too.

I assume the factory configuration has the motors wired in parallel?

Acually, now that I'm thinking about it you might be able to drop the right amount of voltage by simply using a light bulb in series with the motors. You'd have to experiment with different wattages until you got it close. Just a guess though.

[Ukraine Train]

We are eliminating the two 6v batteries. We will still have a small 12V battery as it is necessary to excite the field in the alternator. We will also run power from the alternator to the battery and then from the battery to the motors, the battery needs to be in the circuit to smooth out the ripple from the alternator. We are limited to a $50 budget and got the alternator and just about everything else for free from the school so I don't think we'll be able to find a 6V alternator cheap enough. We still need to buy some wheels for the trailer and wiring/terminals. Yes, the factory configuration has the motors in parallel.

[280plus]

Ah I see you are here with us. I just edited my post to say you MIGHT be abble to drop the extra 6V by putting a light bulb in series with the parrallel motors.

One other thing. I thought alternators had diodes that converted the AC from the alternator to DC? (bridge rectifier)

[Ukraine Train]

Yeah, it's rectified but I guess it's still not a "smooth" output. I'm not really sure on the technical stuff, just going but what I've read from a few different places.

[280plus]

okie doke, I GTG, just realized I'm late. But the more I think about it a light bulb (regular household) might do the trick. You'll have to experiment with different wattages to get the right output. The bulb MAY NOT EVEN LIGHT UP! But it will definitely suck up some voltage.

[Ukraine Train]

Hmm that's a good idea with the bulb. I guess we'll try it and see if we let the smoke out.

[Azrael256]

The 3v reading off the battery makes perfect sense.  Remember, anything you do with electricity can be modeled with water or compressed air.  Imagine an air ratchet running on 12psi (a small one, no doubt!).  When you check the standing pressure on the line, it will be 12psi (the pressure difference between the air hose and the atmosphere), and thus the 12v from the battery (the differential, called potential, between the anode and cathode).  When you actually start turning the ratchet the line pressure will decrease because the pressure is being let out as it is doing work.  There is still a higher pressure in the air hose than there is in the outside atmosphere (or the whole system would shut down), but there is only so much pressure that is not being put to use at any given moment.  The same goes for the motor.  It is basically just electron pressure running down little copper hoses.

An alternator's output is both rectified and regulated.  The two are distinctly different.  A rectifier converts AC to DC, whereas a regulator maintains a constant voltage.  Alternators must be regulated because the voltage they produce is dependent on the rpm of the engine.  Obviously things would get real exciting if the voltage varied wildly as your speed changed.  What you need is a voltage regulator.  You'll also want a capacitor (probably a hefty one) between the output voltage and the ground to reduce the ripple from the alternator.

[280plus]

Hmm that's a good idea with the bulb. I guess we'll try it and see if we let the smoke out.

Yes, as we all know it NEEDS that magic smoke in there to work.

Azrael is absolutely right. Strangely enough electricity and water behave exactly the same. I guess air would also be the same but I'd never heard that before.

IIRC the higher the wattage the lower the resistance. I'm trying to recall the PIE charts but it's been a while since I did that stuff. Be aware though you cannot read the resistance of a light bulb cold and get the numbers to work. Resistance goes up as the element heats up so you can only calculate actual resistance with E/I = R

Good luck! Let us know how you make out.

[Guest]

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmlaw.html

http://www.physics.uoguelph.ca/tutorials/ohm/index.html

[280plus]

Wow, good links sm! Thanks! I remembered the charts" E= I x R and P = E x I

P stands for power and is expressed as watts. I = amperage

I just wanted to add 2 things here:

What is being used to prevent the alternator from overcharging the battery?

and...

Batteries can explode and scatter acid everywhere, especially when you're doing something that they are not necessarily designed to do. PLEASE make sure you have some substantial cover over the battery to contain it should it decide to let go.

I'm also thinkingyou have some pretty fat wire going to the motors. If so the bulb might not be able to handle the current. I'm interested in hearing what happens. If so you can always try placing several bulbs in parallel and then putting that circuit in series with the motors. Parallel circuits in series. Wish I was there to mess around with it with you  

[Guest]

I took a Electronics for PC class in an earlier semester.

Mal H and Art  are due Credit for those links posted. I just could not edit post earlier for some reason, glitch in the get-along perhaps.

My instructor hated the textbook, so we did handouts and took lecture notes. My lab partener was a Lady Doctor, we were the two old pharts in the class.  We, as suggested by instructor, and fueled by our own initiative looked up Info and various subject matters.  Art made a suggestion on Search Criteria, and IIRC sent me some links.

Mal H, is really really smart in this area. Great in Math, and in My Class we did a LOT of math as one might expect.

Mal H pointed me to some great resources, made some great points. He did NOT give me answers. He DID agree with me on learning how to do some math without a calc...math is not my strong point, and I never took Trig. Not a pre-req for the class, still we used it [sine-waves]. Mal H did look at some problems, and my answers. I just on my own looked up the math and physics sources. I never took Physics either...I see now how "This Stuff" Relates to each other...
Even when I took Medical Classes, such as Anatomy & Physiology, Principles of Electricity, Physics, Chemistry, and other disciplines were part of the whole package.

I am not that smart - I see the original post and would have to find my notes to better recall and understand. I may need practice and experience on classes now behind me. Then again Art and Mal [ and others] keep telling me "Part of the College eal is the fact you learned to read a question, and learned to find the information".

I aced "Electricity for 4 year olds".  Cloth covered cords under lamp tables in the corner...Be Sure to grab the plug itself. If'n you grab the cloth insulation and pull exposing wires, and wittle fingers touch said wires...ZAP, Sparks, Knots on head, and lamps fall off tables...That was AC circuits. For DC circuits I touched tongue to 9V battery...
Carbon batteries burn skin, and Car batteries put holes in jeans...

Never said I was smart, still learning, always will...

I progressed quick from age 4.   Now I am older and having taking some classes...

"It is a wonder  little Boys actually make it to Adulthood" - pax

[Brad Johnson]

I think the 6v idea might be a simpler solution, and easier to implement. Use a 6v generator instead of an alternator and you won't have to have standing voltage to excite the field. Power it off a small engine. A weedeater or chainsaw engine might do for what little current it would take to run the powerwheel, and it would be smaller and lighter than a lawnmower engine. Heck, for that matter you could use an old outboard engine (which would also look really neat - you could run it off the prop shaft or something equally odd just for the hack of it). A generator would also allow you could also regulate current with RPM, possibly eliminating the need for other types of current controls to regulate speed.

Keep up the good work, and keep us posted.

Brad

[280plus]

Or get Art Eatman to build an engine for you.