How DC Motor works?

Let us discuss about DC motors which are the most integral part of any moving machine starting from toys to robotic models. This topic may extend to more than one post as there are different types of motors which we will like to understand as they have different usage. 

It’s has been observed that electric current and magnetic field interact with each other and this dependency gives us an opportunity use them in electric motors and electric generators. As we saw in basic science chapter, that a current carrying conductor when comes under magnetic field, it experiences a force on itself till there is electric current in the wire. The force vanishes if the follow of electric current is stopped in wire. This phenomenon can be rightly denoted by Fleming's Left hand Rule. To understand the rule, stretch your left hand thumb, index finger and middle finger in such a way that all three are perpendicular to each other. If the index finger indicates the direction of magnetic field (from North to South pole) and the middle finger indicates the direction of current in the wire, then the direction of your thumb indicates the direction of force exerted on the wire. This is has been shown in picture here.

It has been observed that the magnitude of force is maximum if the direction of flow of current is perpendicular to the direction of magnetic field. Using this theory, let us see how a current carrying coil kept inside a magnetic field feels force and then rotate the shaft of motor. Refer the diagram where we have a magnetic field present due to a permanent magnet with North pole at left and South pole at right. So the direction of magnetic lines of forces is from left to right. We have placed a rectangular coil of insulated copper wire in middle attached to shaft of motor which blue in color. We have supplied current in the coil with the help of a cell and the direction of current is shown with red arrows. Now stretch your left hand fingers as suggested in Fleming's Left Hand rule above. Align your index finger from your left to right and middle finger towards yourself. In this condition your thumb will automatically point upwards. So this position aligns with the left side of the rectangular coil, hence the force direction is shown upwards in the picture.

Now for the right side of the coil, align your left hand fingers so that your index finger still point from your left to right. Middle finger now points away from you as the direction of current in right side of coil is away from you. Now observe that your thumb indicates downwards, hence the force on right hand side of coil has been shown in downward direction.

So these two forces, upward on left side and downward on right side of coil forces the coil to rotate in clockwise direction. After half rotation is completed, again the coils reach on same position and similar action happens, this keeps the coil to rotate till the current is supplied from outside. And since the coil is attached with motor shaft, the shaft also rotates along with coil. 

You must be wondering that if the coil is rotating, the wire through which we are supplying current will get twisted. Your worry is correct; to avoid this, actual construction of motor uses cylindrical contacts and carbon or metal brushes to supply current. The cylindrical contacts are connected to coil and attached to motor shaft to rotate with it and the brushes are stationary to supply current from outside as shown here. The picture shows here the construction of a motor from inside. Permanent magnets in pink and blue color are fixed. The shaft (in blue) contains a soft iron core (in green) on which we have coil of insulated copper wire in Orange. There is a small cylindrical insulating material attached to shaft shown in pink color contains two semicircular metallic contacts shown on yellow color. These semicircular contacts do not touch each other and are connected to the both ends of coil. Two fixed carbon or metal brushes are in contact with these semicircular contacts and the external supply is given through these fixed brushes. Now the semicircular contacts, circular insulator and soft iron core with coil rotates and keep getting current through brushes which are just touching the semicircular contacts. This is how a DC motor is constructed.

Using this Fleming's Left Hand rule, we have observed one point that if direction of current is changed, the direction of force on conductor also changes. This property helps us to change the direction of rotation of motor by changing the direction of supply to a DC motor. So if we reverse the DC supply i.e. if we interchange the positive and negative terminal of cell, motor shaft will rotate in opposite direction. But for changing direction of AC motor we have different method, not as simple as this.

DC Motor Symbol

There is one more property which effect the magnitude of force applied on coil of motor or current carrying conductor in magnetic field. Force on wire is directly proportional to strength of magnetic field and strength of current flowing through wire. Since we use permanent magnet in DC motor, hence we don’t have option to increase or decrease the magnetic field strength. So, we can change the supply current to change the magnitude of force on current carrying wire of coil of motor. The more the force applied on coil, the more is the speed at which the shaft rotates and vice-e-verse. So, by controlling current through motor coil we can control its speed of rotation.

The speed of motor is measured in RPM (revolution per minute). We have few basic specifications of a motor i.e. operating speed i.e. RPM, voltage at which it should be operated. Maximum current that it will draw when operated at rated voltage. Last one is amount of torque it creates when operated at rated voltage and current. Torque is a twisting force created by any rotating object or applied on any object externally. We will discuss more in next topic where we will learn about geared motors.

That’s all about basic of DC motor. Since this topic has already become big, we will learn about some types of motors in next topic, which we use in electronic appliances or projects. Keep watching and share your feedback. You may also subscribe through email so that you get email alert when a new article is posted.

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