DC MOTOR

www.physicsmodels.in The D.C. Motor as a machine was a greatly useful invention, and it took in a Direct Current (DC) and churned out a rotating motion. Like a fan. Think of it as an ‘input-output machine’. DC means Direct Current, from a battery producing a constant voltage (which won’t be a sinusoidal wave like Alternating current). This Direct Current is the input into a conductor. A conductor can be any metallic piece. In a Motor, the metal is shapedlike a rectangle, running around a kind of boundary, and having a gap in the centre. What’s important is that the DC we feed in as input, should enter from one sideof the conductor , travel all around the boundary, and come right back to the other side. The pieces of metal conductor are called segments.To complete any current flow , we need a circuit without any open ends, so we have to connect the end of the conductor segments to another metallic part called slip-rings (we’ll come back to this later), and then connect using wires , then the wires should be connected to Battery terminals. Now you have a current flow running all the way round and back to where it started. The (DC) current flow will always start from the positive terminal and come back to the negative terminal of the battery. This also means that the way you choose to connect the battery to the wires, slip-rings and conductor will make the Motor turn either clock-wise or anti-clockwise. Why? That’s because the rotation direction will be as perFleming’s Left Hand Rule.

Now, the conductor segments should rotate in the space between two magnets where the North Pole and South Pole face each other. Every bar magnet will have a North Pole and a South Pole. So, if we choose two magnets and put them the right way facing each other, you get a North Pole facing a South Pole. This causes a magnetic field, and the magnetic field lines simply start off from the North Pole and enter into the South Pole of the facing magnet. The conductor when it rotates, should cut across the magnetic lines, like a car cutting across lanes in dense traffic. The more it cuts, the better for the DC Motor.

One may have a question - What if you did not have these two poles facing each other? For any bar magnet, the magnetic field lines will start from North Pole and curve back into its own South Pole. We don’t want that in a DC Motor, we want to create straight looking magnetic field lines , kind of concentrated, passing through our metallic conductor, therefore we need to put a North and South pole facing each other. In fact, to make the magnetic field stronger, people put in a soft iron core between the poles, this concentrates the magnetic field.

This also means that the way you choose the placement of North and South Pole will make the Motor turn clock-wise or anti-clockwise. Why? That’s because the rotation direction will be as per Fleming’s Left Hand Rule.

That brings us back to Fleming, who explained the Left Hand Rule rule specific to Motors. See separate video to understand this. It’s all about directions. The three important directions are explained by this rule, to get very clear on which way the Motor should rotate.

a) Take the LEFT HAND, place it on a conductor or any part of a conductor. Then, point a forefinger towards the direction of Magnetic Field (North Pole -to- South Pole), and point the middle finger towards the direction of DC current flow on a conductor segment, then the output , the motion of the conductor segment , will have a direction shown by where the thumb is pointing. All three fingers are at mutually 90 degrees.

b) Now, in the animation presented, try it out yourself. I have already shown Left hands in the animation. You may have to twist your left hand a bit, to place it correctly on the conductor segment. The magnetic field direction is always constant from North to South Pole, but the current flow direction is opposite for the two conductor segments.

c) A question: why does’nt the far end of the conductor segment, the L-shaped part, also move up or down? That’s because, for this small portion, although the conductor is cutting the magnetic lines of force, when we try to apply Fleming’s Left Hand rule, the direction of current is not perpendicular to the direction of magnetic Field. So, this portion does’nt see a Force perpendicular to axis of rotation, hence does’nt add to torque of the machine.

d) The Left Hand Rule , even if we forget to remember, can be got from the Right Hand Thumb Rule. Not to get confused here, the Right Hand Thumb Rule is shown ny the rings around each portion of the conductor segments. Place your Right Hand on the conductor, and the curling fingers will show the direction of magnetic field, caused by the current flow. The current flow also causes a magnetic field. This is distinct and separate from the magnetic field created by the North and South Poles magnets. Now, superimpose both magnetic fields in terms of directions. Take the LEFT side conductor segment in the animation. For this specific arrangement of current flow, The superimposition of magnetic fields adds up on top of the conductor, it means they are stronger on top of the conductor. The superimposition seems to be opposing below the conductor, the net magnetic field effect is small. That will imply the conductor will be pushed from strong to weak side, i.e. it will move downwards. Which matches with the direction shown by Fleming’s Left Hand Rule.

e) Hence, in this physicsmodel animation, the way magnets and battery terminals are arranged, the left part of the conductor will always move downwards. The right part of the conductor will always move upward. That’s why we see an anti-clockwise rotation. There is no confusion about the motion.

f) If for a trial, you want to inter-change the battery by reversing terminal positions, then negative terminal comes on left, and positive terminal comes on the right. The current flow direction will always happen from positive to negative terminal, that won’t change.So, simply put your Left Hand correctly on each conductor segment. You will get the right direction of rotation, and it should be clockwise.

g) If for a trial,you want to inter-change the positions of North and South Pole magnets. Well, the magnetic field direction will always be from North to South, so here it will be from right to left. Again, use Fleming’s left Hand Rule.

h) Practice a number of times so that whatever the position of battery and Magnetic field, you are confidently able to get the correct direction of rotations.

Why does the conductor keep rotating round and round, and not stop ?

This is achieved by slip-rings. There are two slip-rings, LEFT half and RIGHT half. You have to see the 3D Animation to understand this. In the given sketch, when the conductor segment rotates anti-clockwise and crosses the bottom portion, it stops contacting the left half slip-ring, and then starts to contact the right half slip-ring. Immediately, this conductor stops getting current from positive terminal , and gets connected to the slip-ring which is connected to negative terminal of battery. The conductor has a BRUSH (like a toothbrush) which helps it to move against the slip-rings. The slip-rings don’t rotate, they are stationary. Now, our conductor that moved to the other side, gets a new direction of current flow. Putting Fleming’s Left Hand Rule , we see that it must move upward. The same segment of conductor which was earlier moving downward now starts moving upward. A similar thing is happening to the other conductor segment too. That’s why, the segments keep rotating about a central axis. Every motor has a shaft, which rotates about this central axis of rotation with an angular velocity, or rotations per minute (rpm).

You have understood the importance of slip-rings. In terms of power and torque for this DC Motor’s rotation , it all comes down to how much Force is acting on the edge of each conductor segment to push it around the central axis. This Force ‘F’ is proportional to what we do. Example- how strong a magnet you put in there, how many magnetic poles, how strong a current you passed through the conductor, how long a conductor is placed between the magnets. Nothing comes for free. The more you put, as an ‘input’, the more you will get as an ‘output’ [ power and torque] The torque is an equation: T=k {∅I} . In simple words,

k is a constant for a given machine, depending on no. of poles, length of conductors, etc.

∅ = Magnetic flux . (between North and South Poles)

I = Current flowing.

To understand the working of slip-rings, observe the video at various positions of conductor

Trust this was useful to you.