Why do brushed DC motors only use commutators, and not slip rings?

[cairoliu]

Commutators always generate unwanted sparks and bad EM interference.

If slip ring can replace commutator in DC motor, then electric vehicle industry will love it more than multi-phase AC induction motor.

 

[Merlin3189]

I'd say the clue is in the name - it commutes the polarity of the supply.
People can avoid the commutator by electronically switching the current, then they can use slip rings. But I get the impression that the modern preference in brushless DC motors is to use permanent magnet armatures and control current to the static windings.

 

[artis]

Commutators always generate unwanted sparks and bad EM interference.

If slip ring can replace commutator in DC motor, then electric vehicle industry will love it more than multi-phase AC induction motor.

I'd say the clue is in the name - it commutes the polarity of the supply.
People can avoid the commutator by electronically switching the current, then they can use slip rings. But I get the impression that the modern preference in brushless DC motors is to use permanent magnet armatures and control current to the static windings.

Exactly, the commutator is simply an electromechanical "SWITCH" it constantly switches certain windings on the rotor ON/OFF in order to always have a rotor magnetic field that is oriented such that it attracts to stator field pulling the rotor with it.

You can introduce solid state switches like transistors but then you lose the necessity for a current transfer method using slipping bodies that wear out and cause friction and dust, because then you can simply switch the currents and reverse the fields in the stator while the rotor field can be made static and is then dragged along by the help of the stator.

The only real advantage of collector motors that use commutators (they can be both AC and DC) is that they don't require rare earth metals for magnets , they use common cheap non toxic and widely available metals like copper, steel etc and they have a very high starting torque.
This is the reason they were used and are still used for railway locomotives.

They are cheap, powerful and easy to make, the downside is the commutators take up space that can't be used for power production so decreases the power to size/weight ratio and the same commutator introduces wear and dust + electrical noise because as it moves past the stationary brushes there are arcs jumping due to induction and switching of contacts , those arcs cause electrical interference, they also decrease the life of the commutator as they wear off metal from the surface.

 

[cairoliu]

You can introduce solid state switches like transistors but then you lose the necessity for a current transfer method using slipping bodies...

Not really.
Nowadays some EV motors still use a pair of slip rings, though many power transistors are used to switch current direction, so as to obtain a complicated multiple phase AC power supply to stator, not rotor.

Of course, two cases are exceptional: one is permanent magnet motor, another let field coil fixed on stator.

 

[artis]

Not really.
Nowadays some EV motors still use a pair of slip rings, though many power transistors are used to switch current direction, so as to obtain a complicated multiple phase AC power supply to stator, not rotor.

Of course, two cases are exceptional: one is permanent magnet motor, another let field coil fixed on stator.

Motors that use a commutator don't use semiconductors, for exactly the reason that the commutator does the job of semiconductors!
Those that use semiconductors all have no coils on rotor, they are either asynchronous induction type motors or synchronous type, the last ones have permanent magnets within rotor.

One of the reasons why electric cars don't use slip ring or commuator style motors is because in order to avoid the need for a gearbox, the motor is attached either directly to each wheel or to the axle (rear or front) via a differential gear, this means that the motor has to cover the whole speed range of the car with it's RPM which can get quite high when your driving say at 150 MPH. In Tesla's IIRC the motor rotor was going above 10k RPM easily.
Having a commutator at such speeds would mean it would wear out fast.

Back when commutator motors were used on almost all rail locomotives and trains this wasn't a problem because those motors were so big (the size of a smaller fridge) that they had a huge torque therefore their gear ratio wasn't as high as in electric cars therefore they turned at much lower RPM and having a commutator wasn't that big of a deal.

Besides wearing out and graphite dust the only real thing that determines whether a commutator style motor can be used is the rotor RPM, because at low RPM the commutator copper slot surface speed is not that high and the wear is minimal, while going to higher speed it becomes a problem.
One other important aspect is that at higher surface speeds, the commutator can start to arc over, this happens when the copper slots are moving past the brushes so fast that the disengaged slots with their inductive "kickback" manage to draw an arc that goes all around the commutator surface (especially if the commutator is worn and dirty with graphite dust between the slots) eventually the brushes simply short circuit themselves and burn out the commutator..

Watch this video, it shows exactly what I talk about.

 

[sophiecentaur]

Motors that use a commutator don't use semiconductors, for exactly the reason that the commutator does the job of semiconductors!

That's what I'd call "the modern answer". (You are putting it the wrong way round for me) For decades, there were no semiconductors available to do the job of commutation. The only way was to have a rotating electromagnet with a switching arrangement provided by the rotation of the commutator (segments) itself. The stator field was provided by either a permanent magnet or an electromagnet.
Many modern motors use a rotating permanent magnet (no supply needed to the armature) and the commutation is done by semiconductors which are provided with a signal to tell where the armature is in its rotation. Old fashioned ('synchronous') clock motors used a permanent magnet in the armature and they would follow the 50 (/60) Hz mains frequency.
We could also discuss Induction Motors which don't use any electrical connection to the armature but induce a field in the armature which chases the field provided by the stator. These motors are pretty much the most powerful available - the only disadvantage is that speed control is difficult.