Purpose of coil of wire on motor?

I was wondering what the coil of the wire on the 393 motors are. It’s shielded by plastic, and its a metal rod with one coil of wire through it. It seems like it’s using induction, but why?

I thought I’d add a little to jpearman’s reference.

Like the link points out, the purpose is to filter out high frequencies.

The reason THAT is useful is the fact that the micro-controllers use PWM to regulate power.

That, then, brings the question, what is PWM?

Pulse Width Modulation is used because of the efficiency of switching power vs linear circuits that control power. A linear circuit would permit control of direct current at any particular voltage, but at the expense of considerable heat, inefficiency and component cost (inefficiency being an enemy of battery life). Switches, on the other hand, are quite efficient, but they are either on or off.

However, if the switch is fast, one can switch power on and off to approximate reduced output power. For example, if the circuit powers 7 volts, but over time a switch is cycled on and off (and repeats), such that the switch is on half the overall time and off the other half of the overall time, the output would be the equivalent of about 3.5 volts. If the switch is on 1/4th of the time, but off 3/4ths of the time, that would be about 1.75 volts. The problem is that the switch creates a bit of a “shock” as the power pulses on.

That’s where the filter comes in.

You’ve probably noticed that the motors “whine”, especially at low power. You are hearing the motor’s structure reacting to occasional power on cycles. The more power being applied, the more power on cycles are being sent to the motor. The motors is less likely to “whine” at full power than at low power. The “pitch” of the “whine” is related to the rate of the pulses of the PWM.

The “whine” is merely annoying, but the spikes of power can create heat in the coils of the motor. The filter basically causes what is a square wave (an “instant” power on") into a slant, or a ramp up of power in each power on cycle. This reduces the “whine” a bit, but it reduces the initial “shock” of the power on cycle, which in turn reduces wear on brushes (in DC motors with commutators), reduces heat in the coils and can slightly increase the efficiency of the system.

The amount of this effect varies with the value of the inductor (measured in “henrys” or “microhenrys”).

If the coil is removed you might not notice any change at first. If you observed carefully, you might measure a shorter battery life (it would drain faster), you might notice a difference in speed at each power setting (i.e. the motor might be SLIGHTLY faster at 50% with the coil than without), and the “starting power” - or that point where the motor first starts to move when climbing from 0 power toward full power - is lower.

This last point is about how slowly you can get a motor to operate. They don’t move below about 10% power (varies by motor). Without the coil, the point at which the motor first starts to move may lower (or rise) slightly.