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Motors
In order to understand how a motor works, you must understand magnetism and electromagnetism. We know that if you place one permanent magnet near another, the north pole of one magnet will be attracted to the south pole of the other. Also, if 2 south poles are placed together, they will repel each other.
In electricity we can produce a magnet by wrapping a wire around an iron core. When electricity is passed thru the wire, electromagnetism is produced in the wire and concentrated in the iron core. This magnet is exactly the same as a permanent one except when the electricity is turned off the magnetism stops. Also, if the direction of the electricity is reversed, the poles of the electromagnet are reversed. So, how do we make a motor out of this? We can use this attraction and repel to rotate an armature.
Looking at the pics below, there is a field magnet mounted to surround an armature. The armature rotates inside the field magnet. Note the pic on the left. the field magnet is a permanent magnet. North will always be north and south will always be south. The armature magnet (which is an electromagnet) is aligned north to north and south to south. These will repel each other. So the armature will try to push away from the field. As the armature rotates, note on the image on the right. As the north is repelled, the south pole approaches the north field pole. They are attracted to each other so the armature continues rotating. As the south pole (north on the bottom), gets closer to the opposite pole, the attraction increases. Once the south approaches north, the armature will stop with south and north aligned. Here is where the design gets interesting. The power going to the armature is reversed. This reverses the magnetism in the armature. So now north becomes south and south becomes north. The armature and the field are now repelling each other as they were when we started. So with each 180 degree rotation, the armature polarity reverses and the motor continues to run. All electric motors use this principal of attraction and repelling of magnetic poles. This very simple principal becomes quite complex as different types of motors are discussed.
In electricity we can produce a magnet by wrapping a wire around an iron core. When electricity is passed thru the wire, electromagnetism is produced in the wire and concentrated in the iron core. This magnet is exactly the same as a permanent one except when the electricity is turned off the magnetism stops. Also, if the direction of the electricity is reversed, the poles of the electromagnet are reversed. So, how do we make a motor out of this? We can use this attraction and repel to rotate an armature.
Looking at the pics below, there is a field magnet mounted to surround an armature. The armature rotates inside the field magnet. Note the pic on the left. the field magnet is a permanent magnet. North will always be north and south will always be south. The armature magnet (which is an electromagnet) is aligned north to north and south to south. These will repel each other. So the armature will try to push away from the field. As the armature rotates, note on the image on the right. As the north is repelled, the south pole approaches the north field pole. They are attracted to each other so the armature continues rotating. As the south pole (north on the bottom), gets closer to the opposite pole, the attraction increases. Once the south approaches north, the armature will stop with south and north aligned. Here is where the design gets interesting. The power going to the armature is reversed. This reverses the magnetism in the armature. So now north becomes south and south becomes north. The armature and the field are now repelling each other as they were when we started. So with each 180 degree rotation, the armature polarity reverses and the motor continues to run. All electric motors use this principal of attraction and repelling of magnetic poles. This very simple principal becomes quite complex as different types of motors are discussed.
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So, what causes the armature to reverse polarity? For this we use a commutator. Note below is an armature. The commutator is on the right. Brushes are mounted against the commutator to pass power to the windings of the armature. Note there are many sections to the commutator. Each of these sections are connected to a winding around a pole of the commutator. The field can be a permanent magnet or an electromagnet.
In the illustrations above, there are only 2 magnetic poles. On the illustration below, there are many more poles to give more even torque to the motor.
This is a basic motor design that has been in use for many years. It is still used in high starting torque motors such as portable drills.
In the illustrations above, there are only 2 magnetic poles. On the illustration below, there are many more poles to give more even torque to the motor.
This is a basic motor design that has been in use for many years. It is still used in high starting torque motors such as portable drills.
Below is a video explaining how the small brush motor works.
Induction motors
All electric motors use this principal of attraction and repelling of magnetic poles. This very simple principal becomes quite complex as different types of motors are discussed.
Most of the motors used in HVAC applications are induction motors. These motors are quite different from brush motors. There is no electrical connection to the rotor. Power is fed only to the field (stator) windings. Below is a pic of a stator. This type of motor can only run on alternating current. Because alternating current changes polarity 120 (100 in some countries) times a second, the electromagnets of the field windings build up their magnetic fields and collapse those fields each time the power reverses. The collapsing field crosses the wires (you can see the wires of the field below) and induces in any wire it crosses a voltage.
Most of the motors used in HVAC applications are induction motors. These motors are quite different from brush motors. There is no electrical connection to the rotor. Power is fed only to the field (stator) windings. Below is a pic of a stator. This type of motor can only run on alternating current. Because alternating current changes polarity 120 (100 in some countries) times a second, the electromagnets of the field windings build up their magnetic fields and collapse those fields each time the power reverses. The collapsing field crosses the wires (you can see the wires of the field below) and induces in any wire it crosses a voltage.
The rotor is the rotating part of the induction motor. Below is a rotor. It is a combination of iron plates (seen below in the center of the rotor). These plates are surrounded by aluminum that separates the iron plates and are effectively wires that are connected on the ends. The collapsing fields of the stator are very close to the rotor and voltage is induced into the rotor. This makes the rotor a magnet quite similar to the armature of the brush motor. Instead of brushes in contact with a commutator to switch the poles of the magnet, the reversing of poles of AC current provide the switching. This type of motor reduces the parts necessary to make it run and eliminates arcing brushes that are suseptical to wear and could cause fire from the sparks. In an ideal world, this motor could run indefinitely if the bearings do not fail.
Split phase motors
Single phase motors are all some form of split phase.
All split phase motors have 2 windings. The run winding is the workhorse winding. It is the one than takes the running load. It is on all the time. However, in single phase, the run winding will not start the motor. Once the motor is turning, the run winding will keep the motor turning. To start the motor we have give the motor we have to give it a "kick". In order to do this we use a second winding. This is called a start winding. The run winding has more turns on the stator and is a larger diameter than the start winding. The start winding is smaller diameter and less number of windings and is placed in a different position on the stator. This creates magnetism that is somewhat displaced from the run winding magnetism. That magnetism gives the kick to the rotor motor to get it started. Once the motor starts rotating, the run winding and the induced magnetism of the rotor becomes a rotating field that follows the switching polarity of the a/c current.