battery resistor

[Mdkanz]

I have a motor that needs a sub-c battery and i was wondering if a resistor would make it be able to work with 2/3A cells. right now it can only run with sub-c's because it even makes them hot so it drains them really fast.

also if a resistor would work how many ohms would it need to be.

 

[marcbarker]

it can only run with sub-c's because it even makes them hot so it drains them really fast.

Why does it get hot and drain really fast?

Also, is the motor stalled? Does the motor spin too fast?

 

[crutschow]

It's the battery voltage, not its size that determines the motor current. Thus you can use as large a battery as you want, as long as it has the same voltage.

If you want the battery to last longer you need a larger battery, not a smaller one.

 

[Mdkanz]

It gets hot because it is such a powerful motor.

 

[marcbarker]

Need a bit more information, otherwise there's the risk of attracting the wrong kind of authorative answer.

Why does is this motor considered gettnig "too hot"? My vacuum cleaner motor is very powerful and it doesn't get too hot

Is there anything else you can tell us? (i.e. is the motor stalling?, or does it spin too fast?) I ask, because the supply voltage may be too high.

What is precisely the concern over the heat?

Can the motor itself tolerate the heat without being excessively stressed?

 

[Mdkanz]

the motor is for an rc car and every body i know that has one knows that the battery and motor get hot. i tried the motor with a AA cell pack and it started sizzling because the voltage draw was so high. so i want a motor that runs on a sub-c to work with a 2/3AA without exploding.

 

[Chippie]

the motor is for an rc car and every body i know that has one knows that the battery and motor get hot. i tried the motor with a AA cell pack and it started sizzling because the voltage draw was so high. so i want a motor that runs on a sub-c to work with a 2/3AA without exploding.

So far we have determined that you are running a motor on a battery pack of unknown voltage but with a number of cells of differing capacity...

We need to know the motor voltage and the number of cells in the battery packs you are using...

I'm guessing the motor voltage is too low for the battery packs you are using...

Provide the anwers to the questions asked and perhaps a solution to your problem may be forthcoming.

 

[marcbarker]

Reading between the lines of what's written by OP, I can take a guess at what's going on now.

In standard form, the RC model works OK, but the motor and battery are getting "hot". Some heat pickup is inevitable in an electric powered RC, but I think a motor reaching, say 100 degC is normal, the RCMr's here can confirm this or correct me. The battery pack is going to get warm too. If I remember rightly, there are 'high temperature' battery packs and motors specially developed for competition RC. I'll take a good guess the OP's RC has been buillt down to a price, and is in need of improvement.

Battery pack power tools get "hot" too, but they are only used intermittently. I suspect in the RC, the motor and battery don't get much cooling. I'll guess the motor is inside a plastic housing. If it was a thermal path (metal) between the motor frame and ambient it to cool it, the motor would lose most of the heat pick up. Again, the OP knows more about the motor cooling (if any).

There's no mention by OP that the RC's motor "spins too fast" (with wheels off ground). I take that as meaning the battery cells are not too many for the motor. I'll guess it's two-cells. Is that right?

I can see why smaller batteries are tried in a bid to reduce the heat build up. I'll guess that at first when it was tried, it "solved the problem" really well, that is until the AA cells melted their way out of the battery holder!

I see the logic of trying that, and why the OP's next step was to reinstate the original battery and add a ballast resistor instead. Instead of regulating the current using the AA cell's internal resistance that is. Rheostat regulation has been tradtionally used for slot-car racing, the hand controllers are air cooled and the resistance wire can sometimes glow red hot inside.

So, my 0.02c is, the quickest easiest solution is to put the RC back to standard configuration, but add a "ballast resistor". The easiest way of doing this is do it the same way that automobile air blower fan speed control is done, a small coil of resistance wire in series with the motor, and with the coil cooled by the air flow. For resistance wire, you can ask a welder to give you a few feet of MIG welding wire, it's 0.6 mm diameter and needs crimp termination. I guess you'll need a couple of a few inches in a tight coil.

 

[Chippie]

In standard form, the RC model works OK, but the motor and battery are getting "hot". Some heat pickup is inevitable in an electric powered RC, but I think a motor reaching, say 100 degC is normal, the RCMr's here can confirm this or correct me.

Running a motor at those sort of temperatures isnt good for the magnets.
Continued running will lead to demagnetisation...

It is well known with brushless motors that heating the motor beyond 80 ish degrees will permanently damage the motor....

 

[smanches]

If this is a typical Losi/Tamyia/Etc RC vehicle, then the motor and battery packs will get hot. They are designed for a 4 minute duration, after which both the motor and the battery need to cool down. It is never recommended to run two batteries in a row, otherwise you might overheat the motor.

The motors will get too hot to touch, and although it's not the greatest for the motor, this is competition. Worth the price to most.

They are most like a 6 cell sub-C pack (7.2v) with associated motor. Unless it's the newer style brushless motors.

 

[marcbarker]

I would of thought normal motor magnets' Curie Point would be too low for competition RC motors, don't they use high temperature magnets?

i.e. this high **broken link removed**

 

[Mdkanz]

They are most like a 6 cell sub-C pack (7.2v) with associated motor. Unless it's the newer style brushless motors.

yes this is correct. i put a motor from a 1/10 scale car in a 1/16 scale car and the 1/16 scale only fits 6 cell 2/3A pack. so the 2/3A pack does not have a big enough drain rate to use the motor, that is why i thought a resistor on the battery might make it work.

 

[Chippie]

Ahhh....the mists are clearing...

The cell count is the same, its the capacity that's different hence the heating effect on the pack...due to the current drain...

 

[marcbarker]

cell count is the same, its the capacity that's different hence the heating effect on the pack...due to the current drain...

Is this right? When smaller cellpack was tried, the model still needs the same amps the bigger cellpack was giving before to give the same performance. Smaller cellpack has more internal R. For the same amps, more R = more I2R heat. If this extra R is in the cellpack, that's where the extra heat appears.

If more R was added (away from motor and battery), it would reduce the overall motor current, lessen the motor power and the heat in the battery, yes. But would doing that be a waste of power that could be going to the wheels instead? Is there any motor amps controller?

 

[Mdkanz]

Is there any motor amps controller?

No, The only way i could do that is not pushing full throttle.

The cell count is the same, its the capacity that's different hence the heating effect on the pack...due to the current drain...

No, the capacity is the same it is the type of cells that is the problem.

 

[marcbarker]

No, The only way i could do that is not pushing full throttle.

Add a servo-controlled rheostat? My Tamiya RC I had 30 years ago had a ceramic one, I guess it's a PWM nowadays.

 

[smanches]

It sounds like the AA cells are not NiCads? Maybe NiMh? They aren't going to be able to supply the current the motor needs. You could get away with NiCad AA batteries, but the run time is going to be SHORT.

Or get a different motor that is designed to run on AA NiMh cells.

 

[marcbarker]

Or get a different motor that is designed to run on AA NiMh cells.

This is interesting, can you tell us more?

 

[smanches]

This is interesting, can you tell us more?

Get a motor that does not draw as much current. More windings will do this, however you'll lose torque in the process. The motor that he's trying to use is for very high torque, but that takes a lot of current that the batteries cannot supply.

 

[mneary]

Let me summarize my understanding:

i put a motor from a 1/10 scale car in a 1/16 scale car and the 1/16 scale only fits 6 cell 2/3A pack. so the 2/3A pack does not have a big enough drain rate to use the motor

and the 2/3A pack is getting too hot. Solutions already proposed aren't acceptable because

No, The only way i could do that is not pushing full throttle.

You've given no thought as to why the 1/16 motor was supplied with the 1/16 car (2/3-A pack) and the 1/10 motor came with the 1/10 car (sub-C pack)?

Yes the 1/10 motor fits in the 1/16 car, but it blows up the tiny batteries. Running at less than full throttle is not an option. Now, what was the question?