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September 23, 2020
By Katherine Reiss
Electric Motors
Electric Motors
The main purpose of an electric motor is to convert electrical energy to mechanical energy.[1] Electric motors are used in a number of applications, some of which will be discussed below. There are two types of electric motors that will be explored here. The first are called Permanent Magnet DC (PMDC) motors and the second are called Alternating Current (AC) motors. The internal mechanics as well as the similarities and differences of these electric motors will be discussed.
Permanent Magnet DC (PMDC) Motors
Permanent Magnet DC (PMDC) Motors
The first type of electric motor that will be explored is the Permanent Magnet DC Motor or PMDC Motor. In a PMDC motor, there are two types of magnets - an armature magent and a field magnet. The field magnet is generally attached to a steel cylinder and surrounds the armature magnet. The field magnet alternates its North and South ends in the orientation seen in Figure 1 to the right.[2] This orientation creates a magnetic field in the open area in the interior of these magnets. An armature magnet is placed in the middle of these surrounding field magnets. When a voltage is supplied by a power source then a current travels through the armature magnet. When this happens, a magnetic field is created around the armature magnet. Essentially, this field and that of the permanent magnets interact in such a way that causes a back and forth rotation of the armature magnet.[1] When placing two magnets next to each other, with the North ends facing, we know that the magnets will repel. When the magnets are placed next to eachother with opposite poles facing one another, they will attract. This constant attracting and repelling is essentially what is causing the armature magnet to rotate.
Figure 1: Field magnet orientation[2]
Figure 2: The purpose of a commutator[1]
Because current is unidirectional, or only flows in one direciton, the armature magnet will experience a back and forth motion, instead of a complete rotational motion. To combat this movement, a commutator is used to change the direction of the current so that the armature magnet rotates completely.[1]
Figure 2 to the left outlines this process. You can see that the commutator changes the direction of the current which allows a clockwise rotation of the internal magnet.
The armature magnet is attached to an axle in the interior of the motor. [1] Therefore, the spinning of the motor causes the rotational motion of the axle. This is the transition of electrical to mechanical energy that the motor is made to accomplish.
Alternating Current (AC) Motors
Alternating Current (AC) Motors
The next electric motor that we will explore is called an alternating current motor or an AC motor. As described above, the interaction of the two magnetic fields causes a back and forth rotational motion of the armature on the interior of the permanent field magnets of the PMDC motors. As discussed, a commutator is used to combat this back and forth motion to create a complete rotational motion by reversing the current. AC motors combat this issue in a different way. [1] Different than a PMDC motor, the current does not travel through the internal magnet of an AC motor. Instead, the electricity travels through the surrounding magnets. [3] The current is increasing and decreasing as it travels through the surrounding magnets because of the properties of an alternating current. This causes a magnetic field to form on the inside of these surrounding magnets. In turn, this magnetic field creates a current that has its own field, directly surrounding and rotating around the rotor, which rotates also in an attempt to combat this motion. [3] Figure 3 and 4 below show the shifting magnetic field of the surrounding magnets which ultimately results in the rotational motion of the internal rotor. Once again, the electric motor has managed to turn the electrical energy into mechanical energy.
Figure 3: One orientation of the magnetic field [3]
Figure 4: Another orientation of the magnetic field [3]
PMDC vs. AC Applications
PMDC vs. AC Applications
There are a number of applications of both these motor types. PMDC motors are generally used in smaller devices and applications. This might include small toys, for example. AC motors are used to power large items - this could include a house, large engines, or turbines. [3] A number of changes could be made to the motor that might increase or decrease the power it is able to supply. For example, a PMDC motor can supply more power if the two magnets are closer together and less power if these magnets are further apart.[1] This is just one way the power supply could increase or decrease for electric motors.
References
References
[1] “How Do Electric Motors Work?,” Explain that Stuff, accessed September 23, 2020, http://www.explainthatstuff.com/electricmotors.html.
[2] Electrical4U, “Permanent Magnet DC Motor (PMDC Motor) – How Do They Work?,” https://www.electrical4u.com/, accessed September 23, 2020, https://www.electrical4u.com/permanent-magnet-dc-motor-or-pmdc-motor/.
[3] “AC Induction Motors | How AC Motors Work,” Explain that Stuff, accessed September 23, 2020, http://www.explainthatstuff.com/induction-motors.html.
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