Need help with wiring motors in series and in parallel

[matt621]

Hello,

I'm working on my kids ride on cars. He has two of them.

On the first it was 6V and came with one motor on one wheel. I updated it to 2 motors, one on each wheel and 12Volts. I put in a DPDT switch so he could select high and low. On high the motors are run in parallel and on low they are in series. Works great. No issues.

The other car came as 12v with 2 motors one on each wheel. It has a "turbo boost" button that switches between series and parallel like I did on his other car but it's a momentary switch can came from the factory that way. Works fine with the factory motors.

But boys being boys and dads being dads... we want to go faster (and the factory gearbox died so I had to replace that anyway) so...

I bought two new "super fantastic, double whammy, going to last forever" motors along with new gear boxes. When I installed the new motors... on low, only one motor runs. The other motor runs for a second in one direction (I think it's reverse of the other wheel) then changes direction and then stops completely, all at the same time, the 2nd motor runs in forward all the time fine. As soon as we hit the turbo boost, both motors run fine in forward.

The problem is, in non-boost (series) it's a total one wheel wonder. The factory motors did not do this. Both applied power equally to both wheels. In this first car that I modified to have 2 motors and switch from series to parallel, it works fine, both motors drive equally regardless of how it's wired.

The company I bought the "super fantastic, double whammy, going to list forever" motors says this is normal. My basic understanding of motors (and experience with both cars) is this that this is not normal. Can someone explain to me what is happening and maybe how to fix it?

As a one wheel wonder it's useless. It just sits and spins the one wheel and does not go anyplace.

Thank you!

 

[Tom.G]

They may be Induction motors with electronics in them. When wired in series, the switching of the electronics in one of them interferes with the other one. A copy of the data sheet would help us sort things out, or a link to the make and model number. So far there is not much to go on besides speculation.

 

[matt621]

I think they are these with a different label on them:
https://traxxas.com/products/parts/motors/titan775
I know they are called 775 motors.

 

[Tom.G]

The Forum on the Traxxis site (post #21 at: https://traxxas.com/forums/showthread.php?443546-Titan-775-specs) suggest the motor is made by Mabuchi.
http://www.mabuchi-motor.co.jp/cgi-bin/catalog/e_catalog.cgi?CAT_ID=rs_775vcwc

If that is indeed the motor, I too don't see how/why your situation is happening. The motor is described as a plain DC motor with brushes and only a slightly advanced (3 degree) timing advance.

Hopefully a DC motor expert on this site can jump in and explain. Anyone?

Meanwhile, could you post a photo of the actual motor, or at least the make, model, size and any other markings on the label?

 

[CWatters]

This is what I think is happening..

In series the voltage across the motors adds up to the supply voltage. So the voltage across one affects the other. The motor voltage also depends on the motors back emf/rpm. So if motor L starts spinning faster than motor R the voltage across motor L will be higher than motor R. The current through both motors is the same so motor L has more power than motor R and spins faster and faster while motor R spins slower and slower. The speed of each motor diverges from the other. To confirm try measuring the voltage across each motor while it's in this funny state.

I think the problem should resolve itself if you can load up the spinning tyre by getting it better traction. Slowing one motor should speed up the other (I think).

 

[jim hardy]

I think the problem should resolve itself if you can load up the spinning tyre by getting it better traction. Slowing one motor should speed up the other (I think).

that sounds right to me.

motors in series must have same current hence make same torque
if you grab the spinning wheel does the other one start and run?

The other motor runs for a second in one direction(I think it's reverse of the other wheel)

that suggests that in series connection one is wired backwards? Double check your switch wiring.

 

[sophiecentaur]

The series / parallel / single trick works well with resistive loads - such as with heaters (Hi Med Low) but you can't even guess what will happen with motors (or even with unequal light bulbs)

 

[matt621]

What is especially weird is the one motor will start to run for a few seconds and then reverse direction for a second and then stop completely. I have tried to load them with the motors installed in the car and my son riding it, but it just spins the one wheel like heck and the other does nothing. Only hitting the switch to run them in parallel solves the problem. However his other car does not exhibit this problem and the factory motors do not exhibit this problem.

I'm thinking about just running them both in parallel all the time and putting in a scooter controller. But it irks me that I can't understand what the problem is. I will test the voltage as indicated and report back.

Thanks!

 

[matt621]

I will make a little video showing it in action, however I have cut and spliced the wires so many times I'm out of wire so I have to rewire it and I'm going to use banana plugs and posts so I can just plug and unplug as needed. But I have to order some so it'll take me a few days to get back here. Thanks!

 

[jim hardy]

Can you draw a primitive schematic with Paint and post it with upload button ?
Or mark up a photo ?

What is especially weird is the one motor will start to run for a few seconds and then reverse direction for a second and then stop completely.

That does not sound like a description of how two motors wired in series would behave.
Sure sounds like there's a computer in there trying to think for you, meddling busybodies that they are.

Since you're rewiring anyway
while they're apart
wire them in series with some lampcord
and connect them across a battery , or better a battery charger that'll only make around six or ten amps

if they do any better, that's a clue.

Trailer light connectors might be a handy connector for you. A few bucks at Walmart automotive/RV department.

old jim

 

[Tom.G]

Hmmm... I wonder if one or both of the motors have an internal short to ground, i.e. the motor frame.

 

[matt621]

The seller says to "break them in" by running free wheel for 15 minutes. Which we did. With the car off the ground, and low power to the motors (motors in series) they both start to spin in the same direction, then w/in a few seconds the one will slow down to stop, and reverse for just a 1/4 to 1/2 revolution and then not move again. Meanwhile the other one is spinning just fine. If I grab the one spinning the other one (which had stopped) will take off and spin fine. However it's very easy to hear a difference. The first one that spins spins freely and w/o any effort. As soon as I grab it and transfer power to the other wheel (the one with the problem) it spins, yes, but you can hear the strain. It's like the gearbox is full of slug. (It's not, I pulled it apart and it seems fine inside. There is some drag, but it's normal for plastic gears in a plastic housing to have some drag, the geometry is "pretty close" and I even hesitate calling them "gear boxes." :-) )

If I remove the motors from the gear boxes, they both run fine. So there is some drag on that one motor that seems to be causing this.

Yes, Tom, initially that is what I was thinking, however if I remove the motors from the gearboxes, they seem to both run fine. It's only under drag/load that they do this.

 

[jim hardy]

reverse for just a 1/4 to 1/2 revolution and then not move again.

mechanical strain someplace unwinding as torque disappears ? I'll defer to genuine ME types .

As an old electrical guy .....
CWatters in #5 described how DC motors in series behave
it's from the same old two equations that describe basic DC motor speed and torque
Volts = K X Flux_{from field} X RPM
Torque = 7.04 X same K X Flux_{from field} X Armature Current
Flux is set by the permanent magnet field so is same in both motors
so in series where current is the same to both, the one spinning fastest hogs the voltage , reduces the current which reduces torque, and that makes the one with more drag to slow to a stop.

Is there a gearbox doctor in the house ?

old jim

 

[matt621]

Yes, I got a msg from the seller. They believe one of the gears is binding on the axle. I will pull the "gearbox" apart and check to see that the gear is spinning freely. I just hope it didn't gall the shaft from our test runs. Thanks.

(I put "gearbox" in quotes because I can't with a straight face call something with a plastic housing, nylon gears and no bearings a gearbox. ;-) )

Seriously, with such a drag, do you think we've damaged the motor(s)? The compartment was very, very hot. It was so hot I could feel the heat thru the plastic to the outside of the vehicle. When I pulled the seat to get to the battery you could feel the heat rushing out.

 

[jim hardy]

Seriously, with such a drag, do you think we've damaged the motor(s)?

sniff test.- do they small of burnt varnish ? Smoke residue around vents ?
The one that stalled was unable to move cooling air through itself, probably it'll be the worse of the two

if they still run smooth they're probably okay

 

[matt621]

No bad smell. I'm going to put a fan in there to exhaust the hot air anyway. also see if I canfind a heat sink to fit them. they are popular with the RC folks so it should be possible.
thanks

 

[Tom.G]

Try swapping the motors, that is which motor is on which gearbox. That should help track down the problem (unless there are two problems).

 

[matt621]

Yes, the the gearbox. The mount for the motor/pinion is not in the correct location. The pinion to first gear is binding, has zero backlash. It was so bad it wore down the center hub of 2nd gear (the pinion was pushing on first gear so hard it transferred the force to 2nd gear. First gear has a ball bearing, 2nd gear has only the gear with a hole in it and some grease. So now 2nd gear rattles it's so loose. But the main issue was the binding of the pinion to the first gear. Mounting hole is off a few hundreds.

Thanks for all the help. :-)