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An electromagnet consists of a wire coil wrapped around a core material connected to a power source that provides electrical current. Due to its construction an electromagnet can produce a magnetic field. When the core is made from metal the electromagnet can produce a much stronger magnetic field than one with a non-metallic core. The strength of the magnetic field can also be increased by adding more turns to the wire around the core or by increasing the amount of current flowing through the wire. In summary, electromagnetism occurs when electrons flow through a wire wrapped around a core. An example of an electromagnet can be seen in Figure 1.
Magnetic induction occurs when a conductor is moved through a magnetic field resulting in an induced voltage. The induced voltage will cause electrons to flow in one direction. "The polarity of the induced voltage is determined by the polarity of the magnetic field in relation to the direction of movement."(1. Pg.18) In other words, the poles of the induced current will shift as the magnet cuts through the lines of flux as seen in Figure 2. There are three factors that determine the amount of voltage in the conductor; the number of turns of the wire, the strength of the magnetic flied, and the speed of the cutting action. As long as one of these three factors increases, so will the strength of the magnetic field. Without electromagnetism there would be no magnetic induction. Magnetic induction is a product of electromagnetism. It takes the properties of electromagnetism to produce the cutting action of flux lines.
A Direct Current (DC) motor is a machine that converts electrical energy into mechanical energy. There are three basic types of DC motors Series, Shunt and Compound. These three types of electric motors all use magnetic induction to produce torque and convert electrical energy into mechanical energy. There are three factors that affect induced voltage in a DC motor. They include the speed of the cutting action of the conductor, the length of the conductor wire and the strength of the magnetic field. Most DC motors are also similar in that they are constructed with an armature, brushes, field windings and a commutator Figure 3. While these 3 types of DC motors share the characteristics mentioned above, they also differ in how they function. DC motors are characterized by the connection of the field windings in relation to the armature. In the following sections, analyze how DC motors differ.
Figure 3: The parts of a DC Motor
DC Series Motors
Figure 4: Schematic diagram of a DC series motor.
A DC Series Motor has field windings connected in series with the armature Figure 4. The field windings are usually made of thick copper wire. They offer low resistance and can therefore carry a large amount of current. This provides a very high start-up torque but, no speed control Figure 5. Therefore, DC Series motors should be connected to a load for operation. The current passing through the field winding is proportional to the armature current. If one increases so will the other along with the mechanical load on the shaft. As the current in the motor increases, the magnetic flux also increases and will reduce the speed of the motor due to the large amount of torque produced. When saturation is reached by the coils they produce a strongest magnetic field possible. This provides strong enough torque to produce maximum power. It should be noted that upon rotation the armature produces counter electromotive force or CEMF. CEMF is a voltage with polarity opposite to that of the power supply. It results in a slight decrease of the supply voltage and current. As the speed of the motor increases, current and torque will decrease. This type of motor uses torque and current to keep a load in motion. They can be used for pitch systems in some wind turbine generators.
Figure 5: The relationship between series motor speed and the armature current.
DC Shunt Motors
Figure 6: Electrical diagram of a DC shunt motor connected in parallel with the armature.
A DC Shunt Motor has the field windings connected in parallel with the armature Figure 6. The DC shunt motor offers excellent speed regulation. It is sometimes referred to as a constant speed motor because at full rpm, its speed remains fairly constant. Unlike the heavy field windings in the series DC motor, shunt windings are made of small-gauge wire with a large number of turns in the coil. These types of windings are not made to withstand a large amount of current. They do however offer high resistance and can create a strong magnetic field. This high resistance also gives the shunt motor a very low starting torque making a smaller shaft load optimal during operation. Since these types of motors operate with small loads, the armature current does not need to be very strong. DC shunt motors also produce CEMF that reduces current in the armature by a slight amount, but allows the motor to maintain rotation at faster speeds. The armature shaft slows down when load is added and produces less CEMF. When this occurs, current flow and torque increase Figure 7. This is what allows the motor to resume its rpm with an increased load. Speed in a DC shunt motor can be controlled by changing the current flow to either the shunt field or the armature. Current to the shunt field can be influenced by rheostat connected in series with it. When voltage to the armature is constant, increasing the current in the shunt field slows down the rotor speed. On the contrary, decreasing the current in the shunt field speeds it up. In relation to the changes in field current the armature must provide enough CEMF to continue driving the load. The speed of the shunt motor can also be controlled by changing the amount of voltage applied to the armature. However, slowing the motor by reducing voltage makes the motor operate below its rated voltage. Since torque and armature current in a DC shunt motor are directly proportional the motor will not reach its full potential torque. Regulating the speed of the motor in this manner can lead to overheating. This type of motor is useful for applications that require speed regulations such as ceiling fans.
Figure 7: Armature current vs. armature speed for a shunt motor.
DC Compound Motors
Fig 8: Schematic diagrams of: (a) cumulative compound motor, (b) differential compound motor, (c) interpole compound motor.
The DC compound motor combines the torque characteristics of a series motor with the speed adjustment characteristics of a shunt motor. Its series field windings are connected in series with the armature and its shunt field windings are connected in parallel with the armature Figure 8. Having both characteristics makes the DC compound motor quite versatile and very common in industrial settings. Compound motors can be connected in two distinct ways. In a short shunt connection, the shunt field is connected in parallel with only the armature. In a long shunt connection, the shunt field is connected in parallel with the series field as well as the armature Figure 8.
There are three types of DC compound motors Cumulative, Differential and Interpole. For the purpose of this report the Interpole compound DC motor will not be discussed. The series and shunt windings in a cumulative compound motor have magnetic fields with the same polarity. Due to this connection the strength of the magnetic fields can be added together. In other words, they are "cumulative" and thus provide the armature with a strong magnetic field. To put it simply, due to the connection the series windings bolster the magnetic field of the shunt windings resulting in a strong magnetic field. During startup the armature is at a standstill and no CEMF is being produced, giving the motor very high torque characteristics. This allows the DC compound motor to start when connected to a load and continue to operate with slight load deviations. When the load increases the CEMF decreases due to a reduction in the armature speed. Since the armature is cutting lines of flux in both the shunt and series fields this reduction in CEMF in small. Current in the armature and series field will increase with an increased load and produce more torque.
The series and shunt windings in a differential compound motor have magnetic fields of opposite polarity making the magnetic field produced by the armature relatively weak. This gives the motor low starting torque characteristics. When the load increases the rpm of the motor will decrease quickly but, the armature and series field current will increase. CEMF will decrease slightly because the shunt and series have opposite polarity. With this type of connection the magnetic field of the series winding opposes the magnetic field of the shunt winding. If a large load is added to a differential compound motor the strength of the series field will overtake the shunt field and the motor will reverse rotation. Differential compound motors do not suitable for most applications.
The Use of DC Motors in Wind-Turbine Technology
Other than the DC motors used by GE in the pitch system of some of their WTG's, I had an extremely difficult time finding anyone who uses them in wind turbine technology. Even the GE pitch motors seem to be somewhat of a mystery. I assume that this is because the industry itself tends to be secretive when it comes to patents and new technology. However, I did find a patent application filed by GE in an attempt to patent the DC motor for use on their pitch drives. This is application states, "In at least one known wind turbine, a control system pitches one or more blades to adjust an operation of the wind turbine. The pitch control system includes a motor that rotatably drives the blades to a desired pitch angle to adjust an amount of wind energy captured by the blades. Known pitch control systems typically use a direct current (DC) motor that has a series field winding to pitch the blades." http://www.patentstorm.us/patents/7942631/description.html. One of the DC motor that we looked at in class came directly form a GE pitch system. It was a 4.1 kW, Class H1 series motor with 1930 rmp speed rating.
I also found that DC motors are widely used for residential and commercial applications rather than for utility scale power production. Some people who are seeking to build their own wind turbines for home use prefer the use of a DC motor over a generator because it tends to be a cheaper option. They are scavenging motor from old treadmills, steppers and standard permanent magnet motor in an attempt to produce 10W - 40W of power. A few sites included information on how to select the right kind of motor. They suggested 1 to 3 HP in areas of low wind and up to 10 HP in areas of high wind. It was also mentioned to pick a motor that can handle the size of the rotor and one that produces at least 30% more power than what is needed. There are a plethora of do it yourself sites, forums and blogs. A lot of them gave me a chuckle because some of the people on them don't seem to know what they're doing. The following link was informative and somewhat more useful than a lot of the information that is out there. One site listed some of the advantages of using a DC motor for constructing a wind turbine in laymen's terms. They included the fact that DC electricity is good for storing in a battery system, the fact that DC motors are cheap and this, "You'll want a motor that has a higher voltage, higher current, and a lower rpm. This will allow you to generate much more power at a lower speed (rpm). The advantage of this is that running your wind generator at slower speeds lets it last longer. There is much less wear and tear at slower speed than higher ones." http://www.zimbio.com/Wind+power/articles/77/DC+Permanent+magnet+motors+building+wind+turbine
I found this to be a very useful breakdown of what to look for when picking a DC motor to build a wind turbine. It seems that DC motors are far more useful for residential and commercial use in the wind energy technology. Other than GE using them in pitch systems, they are not very prominent in the utility scale sector of wind energy.