What is this component for?

[DamoRC]

Can someone help me understand what the function of the coil is on the input leads to this small DC motor. The coil is formed from magnet wire wrapped around plastic. There is no conductor running through the plastic therefore all the current to the motor needs to run through this coil. My only thought is that it smooths the PWM signals from the speed controller but I don't understand how. Thanks in advance.

(note the coil is cut in this pic as I was confirming if there was a conductor running through the plastic).

**broken link removed**

 

[chrisjpitt]

It is an inductor and is used to block high frequency (pulse) noise generated by the motor (mainly from the brush/commutator sparks) travelling along the connecting wires and being radiated as EMI (which may cause interference to the RC receiver or nearby radios/TVs). The small chip capacitors mounted on the motor PCB do a similar job of reducing the motor noise by shorting them to the can (each wire has its own capacitor to can, plus one across the wires).

Although the coils are there to reduce interference, they do smooth the PWM signal very slightly (round off the sharp edges), but they (and the capacitors) would have to be much bigger to do this properly.

 

[duffy]

I see it's broken. If you want to fix it, unwind a turn, scrape the enamel off, and solder it back on. The tiny loss in inductance won't affect things.

 

[DamoRC]

Got it. Thank you.

 

[Hero999]

It's not wound around plastic, it's ferrite.

 

[PhillDubya]

they do smooth the PWM signal very slightly (round off the sharp edges)

How does it do that? The edges of pulse widths are actually sinusoidal correct? So I am assuming it cuts off the high frequency spikes of the harmonics themselves?

 

[Hero999]

That's not true.

The inductor simply blocks the spikes created by the commutator.

 

[chrisjpitt]

That's not true.

The inductor simply blocks the spikes created by the commutator.

It is true that the capacitor/inductor filter is there to block the spikes from the commutator passing in to the motor wiring and not to smooth the PWM drive - but it is still a low pass (L-C) filter and as such will affect, however slightly, the square wave PWM signal passing to the motor.

 

[Hero999]

It won't affect it that much, what's the value of the inductor and the PWM frequency?

If the inductor is 1µH and the PWM is 20kHz, even at the 8th harmonic (160kHz) the impedance is only 1R which is just 2R for the two inductors in series.