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I'm new to electricity.

Textbooks on brushed DC motor talk about inductive kickback when we DISCONNECT the power supply from the motor (and the stuff to protect against it e.g. by adding a diode/snubber). While when a motor is operating, only back emf caused by the rotation matters.

But from what I observe, isn't the commutator of a brushed DC Motor already functions as an on/off switch that keeps turning on/off the current from the winding? Shouldn't there be some inductive kickback due to sudden removal of current from the winding when commutator switches current direction? Instead of when turning off the power switch, does the inductive kickback issue matter when a dc motor is connected to a power supply and rotating? If yes, what kind of protection is required in the power supply circuit?

Textbooks always model a dc motor's equivalent circuit as 3 electrical components: inductor + resistor + back emf source. Why there is nothing related to inductive kickback caused by the commutator?

JavaMan
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  • Flyback diode and/or snubber. – winny May 08 '17 at 11:36
  • Because the kick back is generated due to inductivity and not from the commutator. – Marko Buršič May 08 '17 at 13:04
  • @Marko Buršič an inductor connected "statically" to a circuit is not the same as an inductor keep automatically switched on and off by the circuit itself. The former one contribute inductance, the latter one contribute voltage spike that depends on how the auto-switch operates – JavaMan May 08 '17 at 18:32

2 Answers2

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Yes, a mechanical commutator causes inductive kickback when switching off the current in a winding.

However, this kickback isn't in a place where it can hurt your circuit. Think about it. If the kickback results in high enough voltage to arc across the contacts (which almost certainly happens when the contacts initially separate), then from the circuit's point of view the contacts are still closed, maybe just with some extra resistance in series.

The problem with inductive kickback due to mechanical commutation isn't damage to your circuit, but lots of nasty voltage spikes that radiate interference. This is a nasty problem because it's hard to add snubbers in the right place. These would have to be across the rotating coils. That is done sometimes, but is not a easy mechanical problem due to the potentially high forces from the rotation.

Olin Lathrop
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  • Um... Didn't you just contradict yourself? The arcing injects rather large voltage spikes back into the brushes which in turn will be seen by the driver. All mute mind you since you should have kickback diodes anyway. – Trevor_G May 08 '17 at 17:07
  • @Trevor: Having kickback diodes to protect your circuit is a good idea when driving anything that can be inductive. They prevent the circuit from getting damaged when *you shut off the inductor*. When the device itself shuts off the inductor, then the resulting voltage spikes aren't necessarily able to get back to your circuit. Such is the case with a mechanically commutated motor. – Olin Lathrop May 08 '17 at 17:10
  • I agree with you, but "aren't necessarily able" has killed a lot of circuitry over the years :) – Trevor_G May 08 '17 at 17:12
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The commutator doesn't switch on/off, rather it commutates the windings. enter image description here

The transient phenomena can be observed right at the commutator, which short circuits a segment before the current is reversed at respective segment. Therefore all the kickback voltage is shorted and the current is recirculated trough the commutator. The upper and lower path, as depicted remains at constant current, so no further transient can be observed there.

We can freely use a circuit with inductor, resistor and voltage source as equivalent circuit. The kickback phenomena would be observed when the motor is switched on/off by an external switch, only.

Marko Buršič
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