Dc Motor Principle of Operation Essay

Published: 2020-04-22 08:25:56
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Principles of operation In any electric motor, operation is based on simple electromagnetism. A current-carrying conductor generates a magnetic field; when this is then placed in an external magnetic field, it will experience a force proportional to the current in the conductor, and to the strength of the external magnetic field. As you are well aware of from playing with magnets as a kid, opposite (North and South) polarities attract, while like polarities (North and North, South and South) repel.

The internal configuration of a DC motor is designed to harness the magnetic interaction between a current-carrying conductor and an external magnetic field to generate rotational motion. Lets start by looking at a simple 2-pole DC electric motor (here red represents a magnet or winding with a North polarization, while green represents a magnet or winding with a South polarization). Every DC motor has six basic parts ” axle, rotor (a. k. a. , armature), stator, commutator, field magnet(s), and brushes.

In most common DC motors (and all that BEAMers will see), the external magnetic field is produced by high-strength permanent magnets1. The stator is the stationary part of the motor ” this includes the motor casing, as well as two or more permanent magnet pole pieces. The rotor (together with the axle and attached commutator) rotate with respect to the stator. The rotor consists of windings (generally on a core), the windings being electrically connected to the commutator. The above diagram shows a common motor layout ” with the rotor inside the stator (field) magnets.

The geometry of the brushes, commutator contacts, and rotor windings are such that when power is applied, the polarities of the energized winding and the stator magnet(s) are misaligned, and the rotor will rotate until it is almost aligned with the stators field magnets. As the rotor reaches alignment, the brushes move to the next commutator contacts, and energize the next winding. Given our example two-pole motor, the rotation reverses the direction of current through the rotor winding, leading to a flip of the rotors magnetic field, driving it to continue rotating.

In real life, though, DC motors will always have more than two poles (three is a very common number). In particular, this avoids dead spots in the commutator. You can imagine how with our example two-pole motor, if the rotor is exactly at the middle of its rotation (perfectly aligned with the field magnets), it will get stuck there. Meanwhile, with a two-pole motor, there is a moment where the commutator shorts out the power supply (i. e. , both brushes touch both commutator contacts simultaneously).

This would be bad for the power supply, waste energy, and damage motor components as well. Yet another disadvantage of such a simple motor is that it would exhibit a high amount of torque ripple (the amount of torque it could produce is cyclic with the position of the rotor). So since most small DC motors are of a three-pole design, lets tinker with the workings of one via an interactive animation (JavaScript required): Youll notice a few things from this ” namely, one pole is fully energized at a time (but two others are partially energized).

As each brush transitions from one commutator contact to the next, one coils field will rapidly collapse, as the next coils field will rapidly charge up (this occurs within a few microsecond). Well see more about the effects of this later, but in the meantime you can see that this is a direct result of the coil windings series wiring: Theres probably no better way to see how an average DC motor is put together, than by just opening one up. Unfortunately this is tedious work, as well as requiring the destruction of a perfectly good motor.

Luckily for you, Ive gone ahead and done this in your stead. The guts of a disassembled Mabuchi FF-030-PN motor (the same model that Solarbotics sells) are available for you to see here (on 10 lines / cm graph paper). This is a basic 3-pole DC motor, with 2 brushes and three commutator contacts. The use of an iron core armature (as in the Mabuchi, above) is quite common, and has a number of advantages2. First off, the iron core provides a strong, rigid support for the windings ” a particularly important consideration for high-torque motors.

The core also conducts heat away from the rotor windings, allowing the motor to be driven harder than might otherwise be the case. Iron core construction is also relatively inexpensive compared with other construction types. But iron core construction also has several disadvantages. The iron armature has a relatively high inertia which limits motor acceleration. This construction also results in high winding inductances which limit brush and commutator life. In small motors, an alternative design is often used which features a coreless armature winding.

This design depends upon the coil wire itself for structural integrity. As a result, the armature is hollow, and the permanent magnet can be mounted inside the rotor coil. Coreless DC motors have much lower armature inductance than iron-core motors of comparable size, extending brush and commutator life. Diagram courtesy of MicroMo The coreless design also allows manufacturers to build smaller motors; meanwhile, due to the lack of iron in their rotors, coreless motors are somewhat prone to overheating.

As a result, this design is generally used just in small, low-power motors. BEAMers will most often see coreless DC motors in the form of pager motors. Again, disassembling a coreless motor can be instructive ” in this case, my hapless victim was a cheap pager vibrator motor. The guts of this disassembled motor are available for you to see here (on 10 lines / cm graph paper). This is (or more accurately, was) a 3-pole coreless DC motor. I disembowel em so you dont have to¦

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