Electric motor design question?

[bobokines]

Re: Electric motor design question?

Hi there boys. It’s not to do with leading or trailing edges, it’s all about armature reaction.

This is the twisting of the magnetic flux caused by the rotating armature.

The carbon brush does two things.

1) It supplies current to the armature. In an ideal world the brush should have as low resistance as possible so as to reduce losses in the brush (and therefore heating).

2) to provide commutation. Commutation is the shorting out of the back emf induced in the armature coils. :- When a coil moves inside a magnetic field a voltage is induced across the ends of the coil. If this is shorted out then a current will flow producing a spark and heat.

When the brush moves over the commutator it will short out two or more adjacent segments. If there is a voltage present between these segments, then current will flow and the brush will heat up and a spark will form. The secret to good (spark free) commutation, is not to have any induced voltage across those commutator segment being shorted out by the brush.

On non-reaction brush gear motors, the brush gear is normally half way between the field coils. At this position, the brush will short out the coils that are passing between the field coil poles. ie no voltage is being induced as there is no magnetic flux present. Unless the motor is overloaded, the brush will not spark and will last for a long time. Also the motor will rotate at the same speed in both directions (given the same supply voltage)

On reaction brush gear, the brushes are twisted so that the commutation happens nearer to the magnetic field in one direction and further away in the other direction. This will allow much higher speed and higher torque in one direction than the other.

Because (in a washing machine) the speed is controlled by a tacho feedback, the varying speed (relative to direction of rotation) is not a problem.

During high speed rotation, the magnetic flux (produced by the field coils) is twisted by the rotating armature. (This is called armature reaction) The designer of the motor will have twisted the brush gear just enough that the brush will short out the commutator segment at the right moment in time so as to reduce sparking.

Another interesting point is that the higher the resistance across the face of the brush the better the commutation. This is why most reaction brushes are laminated… low resistance along its length to reduce losses and high resistance across the face to improve commutation.

Hope that helps a little

Bob

[iadom]

Re: Electric motor design question?

The Oracle has spoken. 8)

Thanks Bob. :tup:

[Martin]

Re: Electric motor design question?

Thanks indeed Bob 😀

Next time I see a burned out CESET motor I’ll think of all that twisting magnetic flux and back emf that once coursed through its feeble alloy frame. 😯

[cockney steve]

Re: Electric motor design question?

MR. BOBOKINES, That, Sir , was a splendid and comprehensive dissertation,from which I learned an awful lot. I’ll have a lot more respect for the humble laminated carbon, in future 😳

Thank you 😀

[Author]

Posts