Electric DC motor, commutator, carbon brushes, and magnets

In summary, a commutator is a common part of most DC motors, it reverses the current flow in the coil to produce a rotating force, and it is important to ensure that the current direction remains the same during rotation.
  • #1
alexandria
169
2

Homework Statement


upload_2016-4-14_1-16-34.png


Homework Equations


no equations required

The Attempt at a Solution


so here are my answers, i just want to know if they are correct.
also, I am really confused about the function of the commutator in the DC motor, the answer i wrote for part b) was based on research off random websites that didnt really make sense, i don't actually understand how a commutator functions and its purpose, can someone please explain in simple terms.
thanks for the help!

78.

a) the answer is Clockwise,
i used the right-hand rule for the motor principle to figure out the direction of the magnetic force acting on the conductor. As shown below:

upload_2016-4-14_1-14-47.png


b)

A commutator is a common part of most direct current-rotating machines. The commutator functions by reversing the direction of the current in the coil, in order to produce a steady rotating force. Together with the carbon brushes, the commutator ensures that the current flowing through the coil is in the same direction as the magnetic field, so the motor will rotate continuously.

c) The Carbon brushes that are pressed against each side of the commutator are there to conduct the current throughout the motor. The main function of carbon brushes is to deliver or collect current from the rotating part of the electric motor. These brushes are pressed against the commutator, but not firmly attached to it, so when the commutator and coil begin to rotate, the brushes remain stationary, while still conducting electricity throughout the circuit.

c) The whole concept of the DC motor is based on the idea that whenever a current carrying conductor (such as a coil) is placed within an external magnetic field, it will experience a mechanical force. How is this ‘external’ magnetic field established? This is where permanent magnets come into play. The permanent magnets simply create an external magnetic field that the coil’s magnetic field can interact with, resulting in a force that ultimately causes the conductor (in this case, the coil) to rotate.
 
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  • #2
alexandria said:
i used the right-hand rule for the motor principle
It's actually the left-hand rule. Right hand rule is for the generator principle.
alexandria said:
A commutator is a common part of most direct current-rotating machines. The commutator functions by reversing the direction of the current in the coil, in order to produce a steady rotating force. Together with the carbon brushes, the commutator ensures that the current flowing through the coil is in the same direction as the magnetic field, so the motor will rotate continuously.
Just after the coil rotates through 90°, the commutator segments flip their polarity and current direction in the coil-sides is reversed. This ensures same current direction under each pole, which produces a undirectional torque and the motor keeps running. If there were no commutators, the motor would stop after 90° rotation. You can see it using the left hand rule.
There are animations available on youtube which will help you visualize the motor action.
 
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  • #3
the lesson I am doing called it the right hand rule??
upload_2016-4-14_8-58-2.png

thanks for the explanation, but i have one question, why would the motor stop after a 90 degree rotation (assuming the commutator isn't there). isn't there still a force acting on the coil that would force it to move in some way, since it is placed withing an external magnetic field?
 
  • #4
alexandria said:
the lesson I am doing called it the right hand rule??
I don't know where they got it from:wink:. But it's definitely the left hand rule. Right hand rule is for generator action. It follows from Lenz's law.
alexandria said:
why would the motor stop after a 90 degree rotation (assuming the commutator isn't there). isn't there still a force acting on the coil that would force it to move in some way,
The force on the coil reverses its direction just after crossing 90° because current direction remains the same but the coil is now under opposite pole.This pushes the coil back and the motor stops. The sole purpose of motor effect is to bring the coil in such a position that there will be zero torque and it will reach equilibrium. This position is reached after 90° rotation(no net torque since the forces are vertical) . It is like a magnetic dipole placed in external magnetic field, which tries to attain equilibrium. To keep the motor running, direction of current in the coil-sides is reversed by the commutators at the instant of change of pole. Commutators are also called "mechanical rectifiers".
I believe an animation is worth a thousand words:smile:.
 
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  • #5
thanks so much, the animation really helped! :smile:
so once the motor spins to 90 degrees, the brushes will lose contact with the commutator, and the current will stop flowing through the motor. So what happens is that the coil spins until the left hand commutator makes contact with the right-hand carbon brush, and the right hand commutator makes contact with the left-hand carbon brush. This means the current is switched, allowing a continuous force to rotate the motor.
this is my understanding so far, i also used this other helpful video, for anyone having trouble with commutators.
 
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  • #6
alexandria said:
so once the motor spins to 90 degrees, the brushes will lose contact with the commutator, and the current will stop flowing through the motor.
Practically, the commutators don't lose contact with the brushes, instead, they momentarily "short out" the brushes at the 90 degree position, so no current flows through the coil. But it keeps moving due to its inertia and the commutators move ahead. Once the coil-sides get under the poles, motor action takes place again and the motor keeps on running.
 
  • #7
great explanation thanks :)
 
  • #8
alexandria said:
great explanation thanks :)
Glad it helped!:smile:
 

1. What is an electric DC motor?

An electric DC motor is a device that converts electrical energy into mechanical energy. It works by using the principle of electromagnetism, where the interaction between a magnetic field and an electric current produces rotational motion.

2. What is a commutator in an electric DC motor?

A commutator is a mechanical switch that is used to reverse the direction of the electric current in an electric DC motor. It consists of a series of copper segments that are connected to the armature windings of the motor, allowing for the continuous rotation of the motor in one direction.

3. Why are carbon brushes used in electric DC motors?

Carbon brushes are used in electric DC motors to provide a constant contact between the commutator and the armature windings. They are made of a conductive material, such as carbon, that can withstand the high speeds and friction of the rotating commutator, ensuring efficient transfer of electrical energy.

4. How do magnets play a role in electric DC motors?

Magnets are an essential component of an electric DC motor. They are placed on the stator, which is the stationary part of the motor, and create a magnetic field. This field interacts with the electric current in the armature windings, causing the motor to rotate.

5. What are the benefits of using an electric DC motor?

Electric DC motors have several benefits, including high efficiency, low maintenance, and precise control of speed and torque. They are also compact, lightweight, and can operate at various speeds, making them suitable for a wide range of applications, from small household appliances to industrial machinery.

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