# Principle of DC Motor Simulation

#### QUIZ

Q1) The conventional theory for current flow is being used to determine the direction of magnetic lines of force. Technician A says that the left-hand rule should be used. Technician B says that the right-hand rule should be used. Which technician is correct?

A. Technician A only
B. Technician B only
C. Both Technicians A and B
D. Neither Technician A nor B

Q2) A current in a magnetic field experiences a force. Which of the following statements is NOT true?

A. The force is at right angles to both the wire and the magnetic field.
B. The greatest force is experienced when the wire is at right angles to the magnetic field.
C. If the wire is parallel to the magnetic field, then no force will be experienced.
D. The Right Hand Thumb Rule can be used to predict which way the force will act.

Fleming's Left Hand Rule is used to predict the direction of a force, a field, or a current

#### Principle of DC Motor Simulation

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#### The Motor Principle

What would life be like without the electric motor? Many devices depend on the electric motor: computers, fans, elevators, car windows, and amusement park rides, to name a few. How did the electric motor come to be?

Moving conductors with Electricity
Oersted’s discovery inspired much interest in electricity and magnetism among other scientists. When English physicist Michael Faraday saw that an electric current in a wire caused a compass needle to move, he was curious to see if the reverse would be true.
Could a magnetic field cause a current-carrying conductor to move? Not only did he succeed in showing that it could, but he was able to make the electrical conductor rotate. In 1821, Faraday supported a bar magnet in a pool of mercury, which is a good conductor of electricity. He then suspended a copper wire alongside the bar magnet, allowing the copper to make contact with the liquid mercury. The wire and the liquid mercury were connected to a power source to complete the circuit. When the circuit was connected, the wire rotated around the magnet. This was the first electric motor (Figure 1).

The copper wire in Faraday’s motor design moved because the magnetic field in the copper wire interacted with the magnetic field of the permanent bar magnet. Let us examine the interaction between the two fields. In Figure 2(a) there are two separate magnetic fields. One magnetic field is from a current-carrying conductor with the conventional current directed into the page. The other magnetic field is from the external magnets. Where the two interacting magnetic field lines are pointed in the same direction there is a repulsion force. Where the two interacting field lines are pointed in opposite directions there is an attraction force. The final result is that the conductor is forced downward, as shown in Figure 2(b).

Figure 2 (a) The magnetic field lines around a current-carrying conductor and two permanent magnets (b) The magnetic fields interact to force the conductor in a downward direction.

The movement of a current-carrying conductor in an external magnetic field is described by the motor principle. The motor principle states that a current-carrying conductor that cuts across external magnetic field lines experiences a force perpendicular to both the magnetic field and the direction of the electric current. The magnitude of this force depends on the magnitude of both the external field and the current, and the angle between the conductor and the magnetic field it cuts across.

Right-Hand rule for the Motor Principle
A third right-hand rule can be used as a tool to determine the direction of force acting on a current-carrying conductor. This time your hand is held flat with your thumb at a right angle to your fingers. The right-hand rule for the motor principle states that if the fingers of your open right hand point in the direction of the external magnetic field and your thumb points in the direction of the conventional current, then your palm faces in the direction of the force on the conductor (Figure 4).

Figure 4 The right-hand rule for the motor principle

The Analog Meter
One of the first practical uses of the motor principle was the development of meters for measuring electrical quantities. The motor principle was used to develop the galvanometer—a sensitive meter for measuring current. The first meters were analog.

Figure 5 (a) An analog meter and (b) a cross-section of an analog meter

Analog means that the reading is shown using a moving needle or pointer on a scale; there is no digital display. In the analog meter shown in Figure 5(a), you can see the looped conductor where the current enters. Note that the current is directed to the positive terminal, through the loop and then out of the negative terminal. The needle is perpendicular to the coil and fixed to it. The needle and the coil are free to rotate on an axle. The spring provides just the right amount of tension and does not let the needle continue forward. The scale is there to provide a spot to take readings from. Looking at the cross-sectional view in Figure 5(b), you can see the current directed into the page on the right side and out of the page on the left side. Using the right-hand rule for the motor principle, you can see that the loop is forced up on the left side and down on the right side. This causes the needle to rotate toward the right side of the scale. The main advantage of analog meters over digital meters is that it is much easier to see the rate at which changes in readings occur.