ELECTRIC MOTORS Lesson 11

The motor principle  Michael Faraday was the first person to create a device that used electromagnets with a permanent magnet to apply a directed force. This device was the first electric motor and it used the motor principle.

 Two magnetic fields interact with each other to produce a force. If the conductor that carries a current cuts through an existing magnetic field, it experiences a force perpendicular to both the conductor’s charge flow and the external magnetic field. The strength of this force depends on the strength of the external magnetic field and the current through the conductor.

 The direction of the force of the force can be predicted by using a left hand rule

Left Hand Rule #3 (LHR#3)  Open your left hand so that your fingers point in the direction of the magnetic field (from north to south). Rotate the middle finger so that it points in the direction of electron (-) current flow. The orientation of the thumb the direction of the force produced.

Electric Motors  The electric motor is a device that directs electric force full circle, without stopping part way. A DC current motor is safe, efficient, and cost effective. In a DC motor, the electromagnet changes its polarity once every rotation. This polarity change is made because the split ring communicator reverses the current flow in the armature.

How the armature works  Stage 1:  The current enters the black half-ring and leaves the gray half-ring, causing the split ring and the attached loop to rotate in a clockwise direction.

How the armature works  Stage 2:  The black and gray conducting halves of the ring lose contact with the black and blue electrodes. At this point, the insulating portion of each ring is in direct contact with the electrodes. No current is present in the attached loop. The loop’s momentum carries it forward until the black half-ring is now in contact with the negative electrode.

How the armature works  Stage 3:  With the black half-ring in contact with the negative electrode, the current in the attached loop is now reversed. Use the left hand rule to verify that this allows the rotation to continue in the original direction.

How the armature works  Stage 4:  The motion continues in the same direction. The rotational motion will be continuous with the split ring communicator attached. The rotating loop in the magnetic field may also be used as the basis for the galvanometer.

The Simple Induction AC Motor  There are several types of AC motors that exist. We will look at the most basic type.  As previously stated, AC current alternates back and forth; the current does not flow in one constant direction as it does in DC. In North America, this cycles 60 times per second.

 The AC motor works on the same principle as the DC motor, like charges repel causing the motor to spin. If the motor seen to the right was used it would spin at around 30 RPS because the current is cycling at 60 Hz.

 There are many other types of AC motors, these are three examples of 3 phase motors

Building a basic DC motor  Starting about 3 inches from the end of the wire, wrap it 7 about times around a toilet paper tube or a large marker. Remove the tube (you don't need it any more). Cut the wire, leaving a 3 inch tail opposite the original starting point.

 Wrap the two tails around the coil so that the coil is held together and the two tails extend perpendicular to the coil. See illustration left:  Note: Be sure to center the two tails on either side of the coil. Balance is important. You might need to put a drop of glue where the tail meets the coil to prevent slipping

 On one tail, use fine sandpaper to completely remove the insulation from the wire. Leave about 1/4" of insulation on the end and where the wire meets to coil. On the other tail, lay the coil down flat and lightly sand off the insulation from the top half of the wire only. Again, leave 1/4" of full insulation on the end and where the wire meets the coil.

 Bend the two paper clips into the following shape:  Use the rubber band to hold the loop ends to the terminals of the "D" Cell battery:

 Place the coil in the cradle formed by the right ends of the paper clips. You may have to give it a gentle push to get it started, but it should begin to spin rapidly. If it doesn't spin, check to make sure that all of the insulation has been remove d from the wire ends. If it spins erratically, make sure that the tails on the coil are centered on the sides of the coil. Note that the motor is "in phase" only when it is held horizontally (as shown in the drawing).