This has probably been asked before, I just can't find it.
For the sake of simplicity, I'm assuming the armature has the coils and the stator is just a permanent magnet.

1: A motor is turned on, current energizes the armature's coil.
2: The armature rotates thus losing contact with the electrical source.
3: The armature's coil is re-energized by the electrical source in the reverse polarity as before.
4: Repeat 2 and 3.

During step 2, the armature's coil becomes open circuit. Because the armature contains an inductor, it follows that as it's magnetic field breaks down, a (theoretically) infinite voltage develops across the circuit, thus causing a spark across the contacts.
Alternatively, if the time between the disconnection and reconnection of the armature is too little for a spark to form, as soon as the armature reconnects to the electrical supply, there will be a short between the electrical power of the armature's collapsing magnetic field and the electrical source; which is now attempting to energize the armature's coil in the reverse polarity.
Now I know many children's toys have a small capacitor on the back of the motor (220nf ceramic through-hole, is the favorite.) This may be used to help handle this switching effect.

So, is my understanding of what's going on here correct, or am I missing something?
Do DC motors typically short or spark during normal operation? If so, how does this effect the lifespan of the motor?

Thanks!

Addendum: I did not know at the time of writing this question the term "Back-EMF" so I called it "shorting". I have since been straightened out on this point.

 

There is no short circuit but there will be sparking between the brushes and the commutator as with any switch, but especially with an inductive load due to the back emf as you described.

 

Concerning lifetime…

The ordinary commutated AC or DC motor uses consumable brushes and requires maintenance that includes the occasional replacement of those brushes and periodic cleaning of the commutator.

In practice, it is often if not almost always the case that a small AC motorized appliance (e.g. power drill) comes to the end of its perceived useful life with few if any brush changes and probably no disassembly for maintenance.

Smaller low voltage DC motors don’t cause enough sparking to require the consumable brushes and so use bifurcated wipers and generally don’t have a way to open them to perform maintenance. Larger DC motors are generally found in industrial setting and so tend to get regular maintenance.

 

What happens as a commutator rotates is that the connection shifts to the next segment which powers the next winding. The sole exception is the experimental motors that only have one rotor winding and only two commutator segments, but those are not real-world useful motors.

 

Do DC motors typically short or spark during normal operation? If so, how does this effect the lifespan of the motor?

Thanks!

What you may be confused with is that any typical DC motor presents a very high load at switch on, due to the fact the current is only limited by the very low resistance of the armature winding.
As the armature RPM increases, BEMF is generated which opposes the applied voltage.
All the commutator does is switch in the next winding in order to keep the applied field at the optimum point in relation to the magnetic stator pole.

 

What happens as a commutator rotates is that the connection shifts to the next segment which powers the next winding. The sole exception is the experimental motors that only have one rotor winding and only two commutator segments, but those are not real-world useful motors.

Please give an example of one of those "experimental motors". A link to one will suffice.

 

Please give an example of one of those "experimental motors". A link to one will suffice.

I could be wrong but I assumed he meant the sort of motor used in classroom demonstrations, like this one.

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What happens as a commutator rotates is that the connection shifts to the next segment which powers the next winding. The sole exception is the experimental motors that only have one rotor winding and only two commutator segments, but those are not real-world useful motors.

All commercially available motors have three or more coils meaning there are three contacts minimum on the commutator. When power is switching from one rotating coil to the next you see a spark because, as others have already stated, there's back electro motive forces (BEMF). In other words, if you take a single coil and energize it, then disconnect the power you'll get a spark. I used to work for an industrial lighting service that had a bunch of heavy as hell transformers. Taking a 12 volt wet cell car type battery and connecting it to the secondary on the transformer there would be a small spark. When pulling the wire apart there would be a HUGE spark and pop. Close to the pop of a firecracker, probably about 75% as loud.

As the motor rotates, each time one of those coils is de-energized you're seeing that spark. And since the number of coils, three in this case, there are never any short circuits going on.

 

I could be wrong but I assumed he meant the sort of motor used in classroom demonstrations, like this one.

View attachment 260979​

That is the only type I've ever seen. I still have one I built when I was a boy. But those aren't what I would ever consider "experimental". They are illustrative or teaching type of motor, so a child can learn about magnetism and motors.

 

That is the only type I've ever seen. I still have one I built when I was a boy. But those aren't what I would ever consider "experimental". They are illustrative or teaching type of motor, so a child can learn about magnetism and motors.

RIght, it's a demonstrator but it's not much of a stretch to call it "experimental" if you can't recall the proper name. It's very informal, but people do talk about kids doing "experiments" in school when they wire a battery to a motor and it spins.

 

RIght, it's a demonstrator but it's not much of a stretch to call it "experimental" if you can't recall the proper name. It's very informal, but people do talk about kids doing "experiments" in school when they wire a battery to a motor and it spins.

When we built one in our sixth grade science class it was an experiment because we, the students, did not know how it would work out. We used big nails and thin wire and it certainly was an experiment that demonstrated both electricity and magnetism.
The one Yaakov shows is a much better looking version, but it uses exactly the same scheme

 

Theone Yaakov shows is a much better looking version,

Jeez ! I though @Yaakov had changed his first name for the second !

 

Jeez ! I though @Yaakov had changed his first name for the second !

I missed it. But my imagination is running wild right now.

 

It was simply an error of not hitting the space-bar between words. Mostly I catch the fat-finger goofs and the run together words but whan I am tired typos can slip past. PLUS, on this HP keyboard on this portable computer there is no space between keys. And I DO have big fingers.