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Electric Motors
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271. Does the monorail at Disneyland and the metro in D.C. run on the idea of direct current motors? Since they reverse directions? Is it like plugging the train in backwards?
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Those trains probably run on AC motors, because then they can use transformers to transfer power between circuits. Most likely, these trains use induction motors. To reverse the direction of the train, the engineer reverses the direction in which magnetic poles in the motors' stators circle the motors' rotors. When the poles reverse directions, the rotor has to reverse its direction, too, so that it chases those poles around in a circle.

720. How does an electric motor work? - BR
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An electric motor uses the attractive and repulsive forces between magnetic poles to twist a rotating object (the rotor) around in a circle. Both the rotor and the stationary structure (the stator) are magnetic and their magnetic poles are initially arranged so that the rotor must turn in a particular direction in order to bring its north poles closer to the stator's south poles and vice versa. The rotor thus experiences a twist (what physicists call a torque) and it undergoes an angular acceleration—it begins to rotate. But the magnets of the rotor and stator aren't all permanent magnets. At least some of the magnets are electromagnets. In a typical motor, these electromagnets are designed so that their poles change just as the rotor's north poles have reached the stator's south poles. After the poles change, the rotor finds itself having to continue turning in order to bring its north poles closer to the stator's south poles and it continues to experience a twist in the same direction. The rotor continues to spin in this fashion, always trying to bring its north poles close to the south poles of the stator and its south poles close to the north poles of the stator, but always frustrated by a reversal of the poles just as that goal is in sight.

839. What is the difference between a single-phase electric motor and a three phase motor? Does that make one of them more efficient, better, or longer lasting than the other? — EJ, Houston, TX
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To keep the center component or "rotor" of an electric motor spinning, the magnetic poles of the electromagnets surrounding the rotor must rotate around it. That way, the rotor will be perpetually chasing the rotating magnetic poles. With single-phase electric power, producing that rotating magnetic environment isn't easy. Many single-phase motors use capacitors to provide time-delayed electric power to some of their electromagnets. These electromagnets then produce magnetic poles that turn on and off at times that are delayed relative to the poles of the other electromagnets. The result is magnetic poles that seem to rotate around the rotor and that start it turning. While the capacitor is often unnecessary once the rotor has reached its normal operating speed, the starting process is clearly rather complicated in a single phase motor.

In a three phase motor, the complicated time structure of the currents flowing through the three power wires makes it easy to produce the required rotating magnetic environment. With the electromagnets surrounding the rotor powered by three-phase electricity, the motor turns easily and without any starting capacitor. In general, three phase motors start more easily and are somewhat more energy efficient during operation than single phase motors.


892. I would like to know a little more about the ac slip ring motor and its uses, particularly in elevators. - M
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A normal induction motor uses a set of stationary electromagnets to produce a magnetic field that seems to rotate rapidly around the motor's rotating central component—its "rotor." The rotor consists of a cylindrical aluminum metal cage and the rotating magnetic field causes currents to flow in the cage so that it becomes magnetic. The nature of the magnetism in the rotor causes it to be dragged along with the rotating magnetic fields around it and it begins to turn with those fields. When you first turn on the induction motor, the stationary rotor leaps into rotation as it tries to follow the spinning magnetic fields. That sudden start is acceptable for many applications, but you wouldn't want it in an elevator—the sudden starting of the elevator car that would accompany the sudden starting of its motor would throw the occupants to the floor. Instead, the aluminum cage in the rotor is replaced by a group of wires that are connected by way of metal ring (the "slip rings") and some stationary conductive brushes to some components outside the rotor. During the starting process, the currents that are induced in the rotor's wires are limited by the components outside the rotor. The rotor starts spinning gradually and gracefully. When the rotor has reached full speed, the brushes are retracted from the slip rings and the slip rings are shorted together so that the rotor behaves like the aluminum cage of a normal induction motor.

897. If increasing the power demand on a generator that is turning at a steady rate simply increases the torque needed to keep that generator turning, why do brownouts occur?
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As long as the generator continues to turn steadily, it will produce its normal voltage rise and the frequency of its alternating current won't change. When the homes powered by the generator draw more current, then the generator simply becomes more difficult to turn and the steam turbine that spins it has to exert more torque on it. But suppose that the turbine can't exert any more torque on the generator. In that case, the power company can either shut down the generator or it can reduce the strength of the generator's rotating magnet. This rotating magnet is actually an electromagnet and its strength determines the voltage rise across the generator. During a period of excessive current demand, the power company may choose to weaken the rotating electromagnet to prevent the steam turbine from becoming overloaded. When they weaken the electromagnet, the generator becomes easier to spin but it produces less voltage. The electricity leaving the generator still has the right frequency alternating current, but it voltage is somewhat lower than normal and the light bulbs it powers glow relatively dimly—a brown-out.

908. Is it possible to mechanically connect two motors of equal speeds and powers to provide twice as much power as a single motor? — EG, Torrance, CA
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As long as they're both AC induction motors, I don't see any reason why not. While induction motors would turn synchronously with the power line if they had absolutely no load, they naturally lag slightly behind in normal situations. While a line synchronous AC motor would turn at 1800 or 3600 rpm, depending on how it's wired, a typical induction motor turns at 1725 or 3450 rpm. The more you load an induction motor, the slower it turns and the more torque it exerts on that load. By coupling two induction motors together mechanically, you'll make them turn at the same rate. Since the torque each motor exerts on the load depends on rotation speed, they'll both contribute equally to the task and will together provide twice the power of a single motor.

I wouldn't try this with any kind of motor that doesn't have such a clear relationship between rotational speed and power output. If you join two mismatched motors with one another, one may end up doing all the work and the other motor might effectively become a generator rather than a motor!


948. How do electric/magnetic linear drives work?
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Linear electric motors are very much like rotary electric motors—they use the forces between magnetic poles to push one object relative to another. But while a rotary motor uses these forces to twist a rotor around in a circle, a linear motor uses these forces to push a carriage along a track. Both the carriage and the track must contain magnets and at least some of these magnets must be electromagnets that can be turned on and off, or reversed. By timing the operations of the electromagnets properly, the linear motor pushes or pulls the carriage along the track smoothly and continuously.

1108. How does an electromagnetic doorbell work? — SH, Sault Ste. Marie, Ontario
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When you press the button of an electromagnetic doorbell, you complete a circuit that includes a source of electric power (typically a low voltage transformer) and a hollow coil of wire. Once the circuit is complete, current begins to flow through it and the coil of wire becomes magnetic. Extending outward from one end of the coil of wire is an iron rod. When this the coil of wire—also called a solenoid—becomes magnetic, so does the iron rod. The iron rod becomes magnetic in such a way that it's attracted toward and into the solenoid, and it accelerates toward the solenoid. The attractive force diminishes once the rod is all the way inside the solenoid, but the rod then has momentum and it keeps on going out the other side of the solenoid. It travels so far out of the solenoid that it strikes a bell on the far side—the doorbell! The rod rebounds from the bell and reverses is motion. It has traveled so far out the other side of the solenoid that it's attracted back in the opposite direction. The rod overshoots the solenoid again and, in some doorbells, strikes a second bell having a somewhat different pitch from the first bell. After this back and forth motion, the rod usually settles down in the middle of the solenoid and doesn't move again until you stop pushing the button. Once you release the button, the current in the circuit vanishes and the solenoid and the rod stop being magnetic. A weak spring then pulls the rod back to its original position at one end of the solenoid.

1140. How does a fan motor work? — JM, Toronto, Ontario
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A fan motor is an induction motor, with an aluminum rotor that spins inside a framework of stationary electromagnets. Aluminum is not a magnetic metal and it only becomes magnetic when an electric current flows through it. In the fan, currents are induced in the aluminum rotor by the action of the electromagnets. Each of these electromagnets carries an alternating current that it receives from the power line and its magnetic poles fluctuate back and forth as the direction of current through it fluctuates back and forth. These electromagnets are arranged and operated so that their magnetic poles seem to rotate around the aluminum rotor. These moving/changing magnetic poles induce currents in the aluminum rotor, making that rotor magnetic, and the rotor is dragged along with the rotating magnetic poles around it. After a few moments of starting, the spinning rotor almost keeps up with the rotating magnetic poles. The different speed settings of the fan correspond to different arrangements of the electromagnets, making the poles rotate around the aluminum rotor at different rates.

1237. In a three-phase induction motor, there is a rotating magnetic field in the stator, which induces a rotating magnetic field in the rotor. Those two magnetic fields will interact together to make the rotor turn. Is the interaction attractive or repulsive? — G
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The magnetic interaction between the stator and the rotor is repulsive—the rotor is pushed around in a circle by the stator's magnetic field; it is not pulled. To see why this is so, imagine unwrapping the curved motor so that instead of having a magnetic field that circles around a circular metal rotor you have a magnet (or magnetic field) that moves along a flat metal plate. As you move this magnet across the plate, it will induce electric currents in that plate and the plate will develop magnetic poles that are reversed from those of the moving magnet-the two will repel one another. That choice of pole orientation is the only one consistent with energy conservation and is recognized formally in "Lenz's Law". For reasons having to do with resistive energy loss and heating, the repulsive forces in front of and behind the moving magnet don't cancel perfectly, leading to a magnetic drag force between the moving magnet and the stationary plate. This drag force tends to push the plate along with the moving magnet. In the induction motor, that same magnetic drag force tends to push the rotor around with the rotating magnetic field of the stator. In all of these cases, the forces involved are repulsive-pushes not pulls.

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