Author Topic: Toroidal inductors in R/C Car starter box motor circuits  (Read 1338 times)

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Offline SuperchargedSoup

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Toroidal inductors in R/C Car starter box motor circuits
« on: July 11, 2016, 09:17:58 am »
I'm trying to understand the reasoning behind some companies choosing to implement toroidal inductors in the circuit for an RC Car starter box motor circuit (http://www.redrc.net/wp-content/uploads/2013/08/TLRStarterBox.jpg is a photo of a starter box, and http://puu.sh/pXISB/c07a00d65e.jpg is a photo of the underside of the same starter box with the inductors indicated by the red square). To give some context, the way these work is the RC car is placed on top of the box, the alignment pegs ensure the wheel aligns with the flywheel of the engine, the car is pushed down (connecting the terminals of a plunger switch, closing the circuit), starting the electric motor, which turns the rubber wheel, which when in contact with the engine's flywheel, turns the engine over and starts it.

I have one starter box where the circuit consists of a power source (4s LiPo) going through a rocker switch, a plunger switch, to the motor. No inductors are present in the circuit of this starter box. I haven't had any issues with this box.

I also have a starter box exactly the same as those pictured above, where inductors are used at each terminal of the motor (as pictured above, second image). Between the inductor and the motor terminals there is solid 12 AWG copper, which recently broke at the motor terminal, presumably due to a high current draw that heated the solder joint enough to melt it.

I understand the theory behind using an inductor to smooth the power input, but why introduce such a point of failure in an application where high current draws are likely to occur? Especially when my starter box sans inductors is 12 months older than the one with, and still running strong.

Would it be a bad idea to remove the inductors from this circuit?
 

Offline danadak

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Re: Toroidal inductors in R/C Car starter box motor circuits
« Reply #1 on: July 11, 2016, 12:12:28 pm »
Given its a torroidal one would venture a guess they are for maintain
current, torque, during starting. They also do have the effect of suppressing
noise.

You could always do a quick experiment and place a jumper across them and
see what effect that has on starting torque and behavior.


Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 

Online Kilrah

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Re: Toroidal inductors in R/C Car starter box motor circuits
« Reply #2 on: July 11, 2016, 12:46:12 pm »
If I had to hazard a guess I'd say a combination of reducing motor switching noise, avoiding excessive current rush when the circuit is closed and some kind of protection for the motor if it was stalled.
 

Offline johansen

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Re: Toroidal inductors in R/C Car starter box motor circuits
« Reply #3 on: July 11, 2016, 03:46:18 pm »
Given its a torroidal one would venture a guess they are for maintain
current, torque, during starting. They also do have the effect of suppressing
noise.

an inductor in series with a dc motor would have the opposite effect. it would add some trivial amount of an ac impedance which means it would take more time for the current to rise, when the motor's rpm dropped under increasing load.

but it would reduce motor brush noise, and it would be very difficult to measure its effect on the ouput of the motor, aside from perhaps, 1mOhm additional resistance.
« Last Edit: July 11, 2016, 03:48:28 pm by johansen »
 

Offline SuperchargedSoup

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Re: Toroidal inductors in R/C Car starter box motor circuits
« Reply #4 on: July 12, 2016, 07:10:21 am »
Given its a torroidal one would venture a guess they are for maintain
current, torque, during starting. They also do have the effect of suppressing
noise.

You could always do a quick experiment and place a jumper across them and
see what effect that has on starting torque and behavior.


Regards, Dana.

Dana, where exactly would I place the jumper lead? Before or after the inductors?


On a side note, I would say that these motors are powerful enough that they very rarely torque stall. Generally what happens if the engine's flywheel won't turn over is that the rubber wheel will slip. When pushing the car down on the box (and thus the wheel) we don't exert enough force to create enough friction for the motor to stall if the engine locks up.

One other thing to consider is we often install a 30A-40A STSP relay to avoid too much current passing through the small, fragile switches.

With these things in mind, would it be unwise to remove the inductors? Especially considering 80% of the starter boxes manufactured by other companies do not employ the use of inductors in any shape or form.
 


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