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Technical Documents - Documentos Técnicos: Electric Motors

DC Electric Motors

Because an automobile has a good supply of dc from the storage battery, there are many dc motors for such tasks as raising and lowering the windows, and many motor-like actuators for things like automatically regulating the air intake. Much larger dc motors are also used in industrial machines, elevators, and other activities involving a lot of starting and stopping, and changing of speed.

A simplified dc electric motor is diagrammed in Fig. 1. A coil "stator" is stationary, and a steel magnet "rotor" can rotate around the axle signified by a black dot. The half-circle is a copper metal "commutator," so the electromagnet can be turned on in order to attract the "north" pole of the iron permanent magnet, which then swings up to the position shown by the dashed lines. In order to prevent this pole from tending to stop when it gets to that position, the conductive graphite "brushes" signified by the small white circles will lose contact with the commutator.

Fig. 1 - A dc motor, with a mechanical commutator switch.

The coil then loses its magnetism, and the rotor can continue spinning by inertia, until it comes around again to a position where it will be attracted when the commutator regains contact to the brushes.

The reader probably knows already that real motors have more coils and poles, and more complex commutators. There is more than one pulse of attraction, and sometimes the current is reversed to also cause repulsion. The coil almost always has a small soft iron core, and the permanent magnet is a large piece of hard steel or ferrite (iron oxide and barium oxide ceramic). Because the permanent magnet is heavier than the coil, it is usually the stationary part, not the way it is shown here.

However, this diagram communicates the main ideas. Other types of dc motors can have coils in both the rotor and stator magnets, which are able to provide more total magnetism than permanent magnets alone.

The stator coil can be wired in series with the rotor coil, or in parallel (called "shunt" wiring). The main features of each type are listed in the following table.

 Table 1 - DC Motor Types Advantages Disadvantages Permanent Magnets Variable speed Heavy magnets Series Coils High starting torque Speed varies Shunt Coils Constant speed Low starting torque

An electric generator can be just like a motor, but a steam engine or water turbine causes the rotation and electricity is taken out, as most readers know. A dc generator requires a commutator, which tends to wear out quickly when extremely high currents are involved. An ac generator, often called an "alternator," does not need a commutator, so that type is used in modern automobiles.

If a dc motor is slowed down too much by a mechanical load, it will allow excessive current to flow through the rotor coil. The reason is that the inductance of the coil does not offer much impedance to the flow of very low frequency ac current (almost "dc"), when the commutator is not changing the direction quickly. This might overheat the motor and possibly damage it.

AC Electric Motors

Almost all modern electric power for homes and factories is ac, mostly because itsvoltage can easily be "stepped up" by transformers, sent long distances with very little power loss, and then "stepped down" to 120V or 208V, etc., nearer to the user. The reason for such small power loss is that the heat loss is mainly

{Power lost in the wires:} P = I2   (2)

Therefore decreasing the current will decrease the power a great deal. Because of the equation

{Power to local transformer and motor:} P = VI            (3)

increasing the voltage (to about 400,000 volts in long distance power lines nowadays) allows the current to be decreased by more than a factor of 1,000. Thus the heat loss during transmission is decreased by more than a factor of a million, compared to transmission at a few hundred volts (eqn. 2). Although the ac can be rectified to dc by the user, most electric motors operate on ac, because rectification always involves some equipment and also some power loss.

Repulsion Motors

It should be noted that, while an ac electromagnet will attract a piece of previously unmagnetised iron by inducing the opposite magnetic pole, a piece of nonmagnetic copper or aluminum will be repelled. Electricity has to first be induced in copper, and this generates its own magnetic field, which repels the original field. (Each turn of the extremely powerful magnets used in nuclear reactor experiments tends to repel each other turn, and such magnets would explode if they were not constructed with mechanically strong materials.)

If an ac motor has coils in both stator and rotor, it can be made with a commutator and will operate very much like the dc example described above. This is called a "repulsion motor," because attraction is not the main force used to get it started rotating, although both attraction and repulsion are usually involved in operation after it has come up to full speed. Sometimes these motors have extra windings, and some even use rectified dc to aid in starting or in speed variation.

Some factory motors of such complexity are still being used. However, most modern electric motors are no longer designed with "hybrid" ac plus dc coils or with commutators for repulsion, because simpler designs have been improved to the point where they are competitive.

Induction Motors

The use of ac has an advantage in that no commutator is needed. The ac magnetic field of the coil (almost always with an iron core) will induce an ac current in any nearby conductor, so a copper or aluminum bar can be the rotor, as in Fig. 2.

Fig. 2 An ac induction motor (synchronous), with a solid armature

Very small motors such as this are often used in electric power meters, which the electricity supplier installs in a house or business to determine how much money to charge the user each month. This motor is not powerful, and it needs something else to get it started, which is not shown in the diagram.

Once it gets going, the rotor has to come around to the correct position for repulsion, just in time for the next ac cycle to occur. This is one of the principles of ac induction motors, and the speeds are usually "synchronous" with the 60 Hz ac. That is, the revolutions per minute (rpm) is 60 times per minute, divided by some factor that is dictated by the design, although there is a few percent of "slippage" behind an exactly synchronous speed. (Experiments with controlling the speed of an ac motor will be described in the next chapter.)

There are advantages to using an electrically conductive coil for the rotor, as shown in Fig. 3. This can provide much higher starting torque, because of the multiple turns in the winding.

Fig.3 - An ac motor with a wire-wound armature.

Sometimes the incoming electricity is sent to the rotor coil, rather than the stator, in which case there is a metal ring in the middle of the rotor for making contact to the incoming wires, similar to a commutator. Such a ring is continuous, not divided into sectors like a commutator, and it is called a "slip ring" (not shown in these diagrams). Slip rings are also used in ac generators ("alternators").

Another way to have multiple conductors (but all wired in parallel as in Fig. 4) is called a "squirrel cage," which it really does resemble. It does not need any slip rings, because all the rotor current is induced.

Fig. 4  - An ac motor with a squirrel cage armature.

In order to have a synchronous motor run at slow speed, as in electric clocks, the rotor can have many poles, which is illustrated in Fig. 5. Clocks can have hundreds of tiny poles and turn at only a few rpm when driven by 60 Hz ac (3600 cycles per minute).

Fig. 5 - A motor with a six-pole armature, 600 r.p.m. with 60 Hz ac.

Ordinary two-wire ac is called "single phase" electricity. Induction motors without commutators would have to be started mechanically (for example, by hand), if no special motor design were used with that kind of electric power source. Therefore some clever tricks are built into most motors to aid in starting.

Methods for Motor Starting

An extra coil, usually quite small, can be arranged a few angular degrees from the main running coil. A fairly large capacitor is put in series with the coil, with its reactance being greater than the inductive reactance of the coil. Therefore the phase of this current "leads" that of the strongly inductive main coil, which "lags" the current in the other one. When starting, the rotor gets a little bit of temporary pull that is biased in one particular direction, and then the main coil keeps it going. Without that bias, it would just stand still, being pulled radially from the axle toward the stator, but not in any rotating direction. In a complex manner, the force vectors end up with the rotor starting toward the capacitor and its small coil. Once it has started, the rotor continues in that direction in a synchronous manner (Fig. 6). With a heavy mechanical load, the motor can continue to run, even if it falls far behind synchronous speed.

Fig. 6  - An ac motor with a capacitor starter.

"Capacitor start" motors, listed in the table, run better without the extra little coil, once they are going. Therefore, some have centrifugal switches to disconnect the capacitor after they are up to a high speed. Small ones for such applications as cooling fans can keep the capacitor in the circuit continuously, since they do not use much total energy, so a small percentage that is wasted is not important.

However, capacitors tend to become faulty sooner than coils, so these motors require more maintenance than some other designs.

A simple and reliable starting method, used by most modern fan motors and other low torque types is the "shaded pole" configuration. In Fig. 7, it can be seen that a small part of the metal or ferrite pole has a single turn of heavy copper wire around it. This acts like the short-circuited secondary coil of a transformer. The inductance at that edge of the pole is decreased, similar to the effect of a small capacitor. The force vectors are quite complex.

Fig. 7 - An ac motor with a shaded pole starter.

THREE PHASE AC MOTORS

The easiest and generally best way to start ac motors is to have the magnetic field of the stator be rotating. Then any conductive rotor design will be self-starting. This is one of the advantages of three-wire, three-phase electricity.

If the generator at the power station has three coils arranged as in Fig. 8, and there are three wires going from it to a motor that has three stator coils wired the same way, then the magnetic field in the motor will rotate at the same speed as the generator's rotor is turning. The voltage at each coil will reach a maximum value and then go down 60 times a second, just like ordinary two-wire ac (50 times a second in some other countries). The next coil will have the same ac in it, but one third of a cycle later, and the last coil will then get further-delayed ac. Much electric power is generated and transmitted this way, because of its great value for driving motors without needing capacitors, shorted turns, or commutators.

The original high voltage generator (480V or so), and the power company's"substation" transformer primary and secondary coils, can each be wired as in Fig. 8 (called "Y" or "Wye" or "star" wiring).

Alternatively, any of them can be wired as in Fig. 9 ("delta" wiring). A transformer primary can be wye and the secondary delta, or vice versa, or both can be the same type.

It is quite common for large electric motors and heaters to run on 208 volts. This can be obtained from substation transformer 208V secondary windings of either type (wye, or delta), if the coil has the right number of turns. It can also be obtained from 240V secondaries.

Fig. 8 - Three phase transformer coils with Y ("wye" or "star") wiring.

When 120 volts is used for U.S. house or office wiring, it can be taken from the middle of the transformer's secondary windings, as shown in Fig. 20.8. Either a fourth wire can be used (for heavy currents), or else a very good ground is necessary, but neither are desirable for long distances.

Instead of that, 240 volts can be sent through the wires that go along the streets, either up on "telephone poles" or down in buried cables. Sometimes none of these are grounded. A local "step-down" transformer (not shown in the diagrams), usually only about a block away from the house or office building, has a center tap in its 240 V secondary that produces a pair of 120 volt circuits, with the center tap wire being shared. This center tap is the "neutral," and it is also grounded locally.

For 120 volt service, sometimes one of the wire pairs is used for part of a building, with the other pair being used for another part of the same building, splitting the total load so that neither pair of wires has to pass too much current.

Another alternative for 120 volt service is to take it from a center-tapped delta type transformer, as shown in Fig. 9. However, where heavy usage of the 120 volts is likely, the other wires might then be unable to supply their full voltages, due to an "unbalancing" effect. Also, their phases (timing) might become altered and wrong for starting large motors. Electricity customers can discuss these alternatives with local power company engineers and find out what windings are available in the nearest substation. Sometimes a factory or large office building can buy its own substation and/or local center-tapped transformers and then only take high voltage inputs from the power company, usually at decreased cost per watt-hour.

Fig. 9 - A higher voltage transformer with center-tapped delta wiring.

Looking at Fig. 9, if the power company's generator is rotating clockwise (as shown in the figure), then let us consider the point in time when the upper-left coil's ac voltage is at its maximum value. At that particular time, the bottom (horizontal) coil voltage will be one third of a cycle "out of phase."

Mathematically, it works out that because of this out-of-phase situation, the voltage between the top of the triangle (as shown here) and the center tap of the bottom coil gets 32 volts subtracted from the 240 volts, so 208 volts is available across those two terminals. (These are RMS values — see index if necessary.) This is one of the reasons why 208 volts is often used for electric motors and heaters.

 Table 2 - AC Motor Types. Advantages Disadvantages Motor Winding Solid Armature Inexpensive Inefficient Wire Wound High starting torque Expensive Squirrel Cage High running torque Low starting torque Multipole Slow (for clocks, etc.) Low torque Starting Method, Single Phase Capacitor High starting torque Less reliable Shaded Pole Simple and reliable Low starting torque Repulsion High starting torque Needs commutator Starting Method, Three Phase Wye If close, 120V to ground If far, 120V grounding Constant motor torque Delta Three voltages available Easily unbalanced Constant horsepower

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