Wiring parallel/series question....

[wolfen]

I have a LRP V7.1 ESC which has no motor limit. Can I wire it Parallel with 2X green machine2 motors. I have it in series right now but I want some more speed. Thanks:)

[ViperGTSR1736]

you should post this in the electric forum. You will get better responses, and make it more clear what your question is. Just a suggestion!;)

[PCC]

Wired in parallel, the current draw is equivalent to half of the turns of a single motor. So, if you have two 12T motors, the ESC 'sees' a 6T motor. The question is, what is the true turn limit of your ESC? They publish that there's no limit but trying to run, say, two 6T motors in parallel will really test that out.

Wiring in series two 6T motors will be equivalent to a 12T.

[wolfen]

Im not for sure how many turns the green machine2 motor has but I dont think they are very low.

[ProjectTwin]

A post I made on another board regarding series and parallel wiring: (italics are not mine...Just quoted from another post)

Quote:
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Series or parallel wiring?

Parallel wiring provides full power to both motors at all times. Series wiring halves the amount of power you feed each motor which can double runtime but reduces power and speed by 50%. Series wiring also has a nasty side effect of allowing one motor to easily stall and simply pass all the current to the motor with the least resistance.



There is a bogus rumor floating around the internet that can trap inexperienced builders. The overall gist of it is that you can wire in series and get twice the runtime and still maintain all the power you had. This is totally false information and wishful thinking. No motor can develop full power by supplying it half the energy...think about it. It can't happen.

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I think there's something wrong with this...


Quote:
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Parallel wiring provides full power to both motors at all times.
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Power (P) equals I x E (current x voltage). A parallel circuit is identified by having more than one path for current flow. In a parallel circuit power is not constant, voltage is constant and the current across each branch in the circuit (across each motor in our case) will depend on the resistance of the resistor (motors for us) in that branch. If resistance goes up in one branch, you will have less current flow through that branch, and more current flow through the other branch. So power is dependent on the resistance in each branch.


Quote:
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Series wiring halves the amount of power you feed each motor, which can double runtime but reduces power and speed by 50%. Series wiring also has a nasty side effect of allowing one motor to easily stall and simply pass all the current to the motor with the least resistance.
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In a series circuit, current is constant and voltage is additive. If you are passing 30 Amps through a series circuit, that 30 Amps will be measured anywhere in the circuit. Voltage, however, is a different story. If you are sending 7.2 volts into the circuit, (as in our case), you won't measure 7.2 volts anywhere in the circuit unless you measure the voltage drop across both motors. Each motor will drop a given amount of voltage, depending on the load it's placed under, A.K.A. resistance. If one of the motors is dropping 5V, that leaves 2.2 volts to be supplied to the other motor, but both motors are seeing 30 Amps of current.

Freaky, eh?



Parallel Facts:
Voltage is constant
More than one path for current flow
Current is additive: the sum of the current through each branch will equal It, or total current
Current through each branch depends on resistance of that branch (current and resistance are inversely proportional...resistance goes up, current goes down)
In a parallel circuit, your equivalent resistance is always less than the resistance of the lowest value resistor in the circuit.

What's this mean for us? Both motors that are installed in our trucks will see 7.2V no matter what load is placed on them, but the current through each motor (branch of the parallel circuit) will depend on the load that motor is under. The more load the motor is under, the higher the resistance in that branch, and the higher the resistance, the lower the current through that branch will be. When one branch of the circuit has an increase in resistance, current will look for another path of travel. Current takes the path of least resistance.

Here's a neat trick. Get your clod, put in a battery, and stand it on it's rear. Hold the front bumper and place a foot on each rear tire...give the truck throttle. You'll notice that the front wheels will rotate, but at a much slower rate than if the rear were not under load. What has happened here is that you have made resistance in the rear motor reach it's highest point by putting the rear axle under a severe load. Remember how we said that equivalent resistance in parallel is always less than the smallest resistor? When you hold the rear axle and prevent it from rotating, you've basically made the resistance at that motor reach infinity, or act as an open. Now the Total Resistance of the circuit is just your front motor (Now one path of current flow…a series circuit). Seems like the motor should rotate faster, right? Not so. With just the one motor in operation, our total resistance is now higher than the two motors wired in parallel. (Req of a parallel is always less than the value of the smallest resistor) What will frazzle your brain more is that if you measure the voltage across the terminals of each motor, you'll get the applied voltage, 7.2V, even though the two axles are doing two entirely different things.

So wiring your Clod in Parallel will make the ESC see less overall resistance, resulting in more current flow and higher speeds. Since the two motors are located on separate axles and placed under separate amounts of load, you will not have the same power output from each motor.

Why?

Because Power = Current x Voltage.
We have the same voltage across each motor, due to the motors being in parallel, but the resistance is different in each motor, due to the loads the motors see, therefore the current through each motor will be different. This will happen several times a second.

Series Facts:
Voltage is additive: The sum of the voltage drops in the circuit must equal the applied voltage.
Current is constant.
Resistance is additive

When you wire a Clod in series, you've increased the load that the ESC sees, which automatically decreases current flow. With two motors on seperate axles, your load (resistance) is going to change constantly. As the load on a motor increases, more voltage will be "dropped" across that motor. Knowing our laws of series circuits, that means that there is now less voltage being applied to the other motor in our circuit but they're both seeing the same amount of current. Weird, eh?

The big problem with wiring your motors in series on separate axles is that as the load increases on one axle, the voltage drop across the motor on that axle increases. You can get to a point that all of the voltage applied to the circuit is dropped across this one motor. This is known as an open. Many of you may know that an open in series will halt any current flow through the entire circuit.

So which is best for you? Series or Parallel?

If you’re going to do heavy-duty crawling, or just want higher speed from the same motors, wire them in parallel.

If you want a good, long, runtime, series is your best bet, but performance will suffer. The truck will be slower, and will not climb as well as the exact same vehicle wired in parallel.

Here (http://www.teamnovak.com/Download/acrobat/rooster_superr.pdf) is a link to the Novak Super Rooster ESC manual, which contains wiring diagrams showing motors wired in both series and parallel.

Jason

[wolfen]

Thanks for all ther info, hope you didnt get blisters on your fingers from typing all of that. Im going to try it in parallel and see how it goes, I think this ESC can take it. thanks again:)

[Grant Tokumi]

ProjectTwin,

I haven't had time to absorb ALL that info in your post. hahahaha, but for now, I think there is some misleading info in it. The one that stands out in my mind is the clod example. I promise I'll reread the rest of the post when I go home from work to try and understand your point better.


Seems like the motor should rotate faster, right? Not so. With just the one motor in operation, our total resistance is now higher than the two motors wired in parallel.


I think total motor equivalent resistance goes down in that scenario not up. You've stalled the rear motors, which makes the system send large amounts of current through it. It heats up. That is like shorting the system (very low resistance), not having an open system. And that system is sending so much current to the stalled motor, it leaves smaller amount of current available to power the front motor. The battery is the cause of the slowdown of the front motor. It can't supply enough current to handle both the stalled rear motor and a full throttle front motor. If you were to hook a second battery in parallel with the other battery, the front motor will spin faster in your scenario because the second batt can supply the current that starving front motor needs.

The example I like to think of is a house circuit. All outlets are hooked in parallel. Having any one outlet open or closed does not affect the current flow or voltage seen in the other appliances that are turned on.

[Thunder VP]

Wolfen, you will be more than safe using your Green machines in parallel. They are 27 turn 24 degree stockers. Make sure you have a forward and reverse version if you have a Clod. (reverse goes in the rear)

Project, you made a lot of good points and your formulas are correct, but Grant is right. As you load a motor the resisance goes down and increases the current draw. A stalled motor is almost a dead short not an open circuit. A battery pack can only supply so much current before it own internal resistance starts dropping its output voltage. Thus slowing the unloaded motor in parallel hookups. In a series hookup the voltage drop will be equal (1/2 of battery volts) until one motor is loaded more than the other. The least loaded motor will pick up power as the more loaded motor looses it.

[ProjectTwin]

ACK!


I saw it as acting as an open..with raises the Req of the parallel circuit, reducing current through the unloaded motor.

In parallel, when one component shorts, it kills the entire parallel circuit...all current is passed through the shorted branch.

Since it's in parallel, we can't measure the voltage, it will be constant. To measure current we'd have to open the circuit, which defeats our purpose.

So are we getting an open and Req is going up, or are we getting close to a short and just a smidgen of current is going to the unloaded motor?

Maybe I should stop typing at 1 or 2 am! :p

Jason

[Grant Tokumi]

Ok Jason, Took a while, but I read the whole thing. :) The only thing that seems incorrect (to me) is the stuff based on what I mentioned earlier. Stalling the motor decreases resistance and increases current.


So are we getting an open and Req is going up, or are we getting close to a short and just a smidgen of current is going to the unloaded motor?


I'm not so sure its "close to short", but yes relatively low resistance, which draws all the current and makes the motor hot. I've heard it called "stall current". Stall current is the worst case scenario and puts the most strain on the motors, esc, batts, wires, etc. It drains the battery the fastest. Severe overgearing is almost the same as stalling. The motor has a tough time turning. Everything gets hot. Lots of current.

And yeah, smaller amounts of current goes to the unloaded motor. If the battery could handle the stall current PLUS the current necessary to turn the unloaded motor, then you'd probably not see a decrease in the unloaded motor speed. But as ThunderVP mentions, the increased battery internal resistance caused by the high current draw is making the unloaded motor act the way it does.

I like that sort of writeups though. I can tell lots of thought went into it. Really makes me think too. Awesome. And I'm not saying I'm guarantee 100% corrent either. My thoughts are based on reasoning, observation, testing. Not directly from formal training or education. So I always have to question myself too to make sure I'm not jumping to wrong conclusions.

Its getting close to 1am, I better stop before I start saying wacko stuff. :)

[ProjectTwin]

We talked about this at work today.

Finally came to this conclusion:
The rear motor can't be a complete short, because if it were, there'd be no current making it's way to the front motor.

It can't be open, because if it were, it would not try to turn once you remove the load on the motor...


Odd.

Still interesting discussion. Good heads up, Grant!

Jason

[wolfen]

Thunder,
Thanks for the info on the motors, I couldnt find any info on the net, I have wired them in parallel and they seem to be working fine. Big diff. from them being in series.:)

[Thunder VP]

When a motor is stalled the resistance is equal to the resistance of the wire on two poles of the armature. (very low) As the motor turns through the magnets an opposing current is inducted back into the windings that increases the relative resistance of the motor. The faster it turns the higher the resistance. ie at low rpms (unloaded) a 1 volt increase will give a higher rpm increase than it will once the motor is running at a higher speed.
Crazy ehh?

[Grant Tokumi]

Thunder,
Interesting. One of the things that puzzled me for years is how a solid piece of wire (windings in the motor) can have varying resistance. I still don't completely understand, but your post helps me some. That word "induction" puts me back to the college days in electrical physics class. It was one of those words that freaked me out because I could not solidly grasp the concept. Probably cause I used that class to catch up on my sleep after staying up all night fixing and playing rc cars. hahaha. I swear, rc cars taught me more high education level concepts than the classes themselves. Too much formulas, not enough real life examples/analogies. I try to use rc car knowledge to bridge that gap between theory and reality. Tough thing to accomplish. After digesting your post for a couple years and stumbling across other induction related info, perhaps I might understand induction to my desired comfort level.

Thanks.
Your example with the 1 volt would help explain why electrics have such a strong punch off the line.