nc33/MultiSpan_SQUAD huggingface dataset:
Electric_motor

Application of electric motors revolutionized industry. Industrial processes were no longer limited by power transmission using line shafts, belts, compressed air or hydraulic pressure. Instead every machine could be equipped with its own electric motor, providing easy control at the point of use, and improving power transmission efficiency. Electric motors applied in agriculture eliminated human and animal muscle power from such tasks as handling grain or pumping water. Household uses of electric motors reduced heavy labor in the home and made higher standards of convenience, comfort and safety possible. Today, electric motors stand for more than half of the electric energy consumption in the US.

Question: How much energy do electric motors use in the modern United States?
Answer: more than half

Question: In what domain did electric motors reduce reliance on humans and animals?
Answer: agriculture

Question: Besides shafts, belts and compressed air, what did electric motors reduce the need for?
Answer: hydraulic pressure

Question: Where did electric motors improve convenience and safety?
Answer: in the home
Electric_motor

A commutator is a mechanism used to switch the input of most DC machines and certain AC machines consisting of slip ring segments insulated from each other and from the electric motor's shaft. The motor's armature current is supplied through the stationary brushes in contact with the revolving commutator, which causes required current reversal and applies power to the machine in an optimal manner as the rotor rotates from pole to pole. In absence of such current reversal, the motor would brake to a stop. In light of significant advances in the past few decades due to improved technologies in electronic controller, sensorless control, induction motor, and permanent magnet motor fields, electromechanically commutated motors are increasingly being displaced by externally commutated induction and permanent-magnet motors.

Question: What would a rotor do without current reversal?
Answer: brake to a stop

Question: What switches the input of most DC motors?
Answer: commutator

Question: From where is current to the motor supplied?
Answer: stationary brushes

Question: What two motor types are ascendant today?
Answer: externally commutated induction and permanent-magnet
Electric_motor

A commutated DC motor has a set of rotating windings wound on an armature mounted on a rotating shaft. The shaft also carries the commutator, a long-lasting rotary electrical switch that periodically reverses the flow of current in the rotor windings as the shaft rotates. Thus, every brushed DC motor has AC flowing through its rotating windings. Current flows through one or more pairs of brushes that bear on the commutator; the brushes connect an external source of electric power to the rotating armature.

Question: What does the communicator do?
Answer: reverses the flow of current in the rotor windings

Question: What type of current is featured in brushed DC motors?
Answer: AC

Question: Where do brushes get their power?
Answer: external

Question: What is the main feature of the shaft in a commutated DC motor?
Answer: rotating
Electric_motor

The rotating armature consists of one or more coils of wire wound around a laminated, magnetically "soft" ferromagnetic core. Current from the brushes flows through the commutator and one winding of the armature, making it a temporary magnet (an electromagnet). The magnetic field produced by the armature interacts with a stationary magnetic field produced by either PMs or another winding a field coil, as part of the motor frame. The force between the two magnetic fields tends to rotate the motor shaft. The commutator switches power to the coils as the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field, so that the rotor never stops (like a compass needle does), but rather keeps rotating as long as power is applied.

Question: Current flowing to create a temporary magent is called what?
Answer: electromagnet

Question: What rotates the motor shaft?
Answer: force between the two magnetic fields

Question: What element of the motor keeps the poles from alligning?
Answer: commutator

Question: What non-motor device demonstrates why a commutator is needed?
Answer: compass
Electric_motor

Many of the limitations of the classic commutator DC motor are due to the need for brushes to press against the commutator. This creates friction. Sparks are created by the brushes making and breaking circuits through the rotor coils as the brushes cross the insulating gaps between commutator sections. Depending on the commutator design, this may include the brushes shorting together adjacent sections – and hence coil ends – momentarily while crossing the gaps. Furthermore, the inductance of the rotor coils causes the voltage across each to rise when its circuit is opened, increasing the sparking of the brushes. This sparking limits the maximum speed of the machine, as too-rapid sparking will overheat, erode, or even melt the commutator. The current density per unit area of the brushes, in combination with their resistivity, limits the output of the motor. The making and breaking of electric contact also generates electrical noise; sparking generates RFI. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance (on larger motors) or replacement (on small motors). The commutator assembly on a large motor is a costly element, requiring precision assembly of many parts. On small motors, the commutator is usually permanently integrated into the rotor, so replacing it usually requires replacing the whole rotor.

Question: How is RFI generated?
Answer: sparking

Question: What do brushes need to be in contact with?
Answer: commutator

Question: What is created by contact between parts of the motor?
Answer: friction

Question: What does sparking limit?
Answer: maximum speed

Question: What motor components most need replacement and maintenance? 
Answer: Brushes
Electric_motor

Large brushes are desired for a larger brush contact area to maximize motor output, but small brushes are desired for low mass to maximize the speed at which the motor can run without the brushes excessively bouncing and sparking. (Small brushes are also desirable for lower cost.) Stiffer brush springs can also be used to make brushes of a given mass work at a higher speed, but at the cost of greater friction losses (lower efficiency) and accelerated brush and commutator wear. Therefore, DC motor brush design entails a trade-off between output power, speed, and efficiency/wear.

Question: What are large brushes favorable for?
Answer: motor output

Question: Whate are small brushes favorable for?
Answer: speed

Question: What type of brushes are cheapest?
Answer: small

Question: How can the speed of larger brushes be increased?
Answer: Stiffer brush springs

Question: Besides wear, what is a downside of stiff brush springs?
Answer: lower efficiency
Electric_motor

A PM motor does not have a field winding on the stator frame, instead relying on PMs to provide the magnetic field against which the rotor field interacts to produce torque. Compensating windings in series with the armature may be used on large motors to improve commutation under load. Because this field is fixed, it cannot be adjusted for speed control. PM fields (stators) are convenient in miniature motors to eliminate the power consumption of the field winding. Most larger DC motors are of the "dynamo" type, which have stator windings. Historically, PMs could not be made to retain high flux if they were disassembled; field windings were more practical to obtain the needed amount of flux. However, large PMs are costly, as well as dangerous and difficult to assemble; this favors wound fields for large machines.

Question: What can a PM motor not be adjusted for?
Answer: speed control

Question: What is another name for PM fields?
Answer: stators

Question: What design feature does a PM motor lack?
Answer: field winding

Question: What do field windings provide?
Answer: flux
Electric_motor

To minimize overall weight and size, miniature PM motors may use high energy magnets made with neodymium or other strategic elements; most such are neodymium-iron-boron alloy. With their higher flux density, electric machines with high-energy PMs are at least competitive with all optimally designed singly-fed synchronous and induction electric machines. Miniature motors resemble the structure in the illustration, except that they have at least three rotor poles (to ensure starting, regardless of rotor position) and their outer housing is a steel tube that magnetically links the exteriors of the curved field magnets.

Question: Why do miniature PM motors use high energy magnets?
Answer: To minimize overall weight and size

Question: What are high energy magnets typically made of?
Answer: neodymium-iron-boron alloy

Question: What does the outer housing of a miniature motor resemble?
Answer: steel tube

Question: How do PMs rate against other types of electric motors?
Answer: least competitive
Electric_motor

Operating at normal power line frequencies, universal motors are often found in a range less than 1000 watts. Universal motors also formed the basis of the traditional railway traction motor in electric railways. In this application, the use of AC to power a motor originally designed to run on DC would lead to efficiency losses due to eddy current heating of their magnetic components, particularly the motor field pole-pieces that, for DC, would have used solid (un-laminated) iron and they are now rarely used.

Question: What is the typical maximum range of universal motors?
Answer: 1000 watts

Question: What type of motor was used in trailway traction applications?
Answer: universal

Question: What is the result of using AC power on a DC motor?
Answer: efficiency losses

Question: What were universal motor pole pieces constructed from?
Answer: solid (un-laminated) iron
Electric_motor

An advantage of the universal motor is that AC supplies may be used on motors which have some characteristics more common in DC motors, specifically high starting torque and very compact design if high running speeds are used. The negative aspect is the maintenance and short life problems caused by the commutator. Such motors are used in devices such as food mixers and power tools which are used only intermittently, and often have high starting-torque demands. Multiple taps on the field coil provide (imprecise) stepped speed control. Household blenders that advertise many speeds frequently combine a field coil with several taps and a diode that can be inserted in series with the motor (causing the motor to run on half-wave rectified AC). Universal motors also lend themselves to electronic speed control and, as such, are an ideal choice for devices like domestic washing machines. The motor can be used to agitate the drum (both forwards and in reverse) by switching the field winding with respect to the armature.

Question: What kind of motor can employ both AC and DC?
Answer: universal

Question: What component of a universal motor is most likely to fail?
Answer: commutator

Question: When is torque most important in universal motor applications?
Answer: starting

Question: Switching the field winding in washing machines causes the drum to agitate how?
Answer: forwards and in reverse

Question: How is stepped speed control achieved?
Answer: Multiple taps on the field coil provide
Electric_motor

Whereas SCIMs cannot turn a shaft faster than allowed by the power line frequency, universal motors can run at much higher speeds. This makes them useful for appliances such as blenders, vacuum cleaners, and hair dryers where high speed and light weight are desirable. They are also commonly used in portable power tools, such as drills, sanders, circular and jig saws, where the motor's characteristics work well. Many vacuum cleaner and weed trimmer motors exceed 10,000 rpm, while many similar miniature grinders exceed 30,000 rpm.

Question: At around what RMP do weed trimmer motors operate?
Answer: 10,000

Question: At around what RMP do miniature grinders operate?
Answer: 30,000

Question: What type of motor can universal motors operate much faster than?
Answer: SCIMs

Question: What attributes make universal motors desireable for consumer applications like hair dryers?
Answer: high speed and light weight
Electric_motor

Currents induced into this winding provide the rotor magnetic field. The shape of the rotor bars determines the speed-torque characteristics. At low speeds, the current induced in the squirrel cage is nearly at line frequency and tends to be in the outer parts of the rotor cage. As the motor accelerates, the slip frequency becomes lower, and more current is in the interior of the winding. By shaping the bars to change the resistance of the winding portions in the interior and outer parts of the cage, effectively a variable resistance is inserted in the rotor circuit. However, the majority of such motors have uniform bars.

Question: What determines the properties of speed and torque?
Answer: shape of the rotor bars

Question: At high speed, where is the most current?
Answer: interior of the winding

Question: Is the current frequency higher or lower at high speed?
Answer: lower

Question: Changing the shapes of the bars creates what?
Answer: variable resistance

Question: Where can variable resistance be created?
Answer: rotor circuit
Electric_motor

In a WRIM, the rotor winding is made of many turns of insulated wire and is connected to slip rings on the motor shaft. An external resistor or other control devices can be connected in the rotor circuit. Resistors allow control of the motor speed, although significant power is dissipated in the external resistance. A converter can be fed from the rotor circuit and return the slip-frequency power that would otherwise be wasted back into the power system through an inverter or separate motor-generator.

Question: Where might a resistor be attached?
Answer: rotor circuit

Question: What can a resistor control?
Answer: motor speed

Question: How might wasted power be salvaged?
Answer: inverter or separate motor-generator

Question: From where may a converter be fed?
Answer: rotor circuit
Electric_motor

When used with a load that has a torque curve that increases with speed, the motor will operate at the speed where the torque developed by the motor is equal to the load torque. Reducing the load will cause the motor to speed up, and increasing the load will cause the motor to slow down until the load and motor torque are equal. Operated in this manner, the slip losses are dissipated in the secondary resistors and can be very significant. The speed regulation and net efficiency is also very poor.

Question: What will a motor do if the load is reduced?
Answer: speed up

Question: What will a motor do if the load is increased?
Answer: slow down

Question: Where are slip losses dissipated?
Answer: secondary resistors

Question: How efficient is such a design?
Answer: very poor
Electric_motor

A common application of a torque motor would be the supply- and take-up reel motors in a tape drive. In this application, driven from a low voltage, the characteristics of these motors allow a relatively constant light tension to be applied to the tape whether or not the capstan is feeding tape past the tape heads. Driven from a higher voltage, (and so delivering a higher torque), the torque motors can also achieve fast-forward and rewind operation without requiring any additional mechanics such as gears or clutches. In the computer gaming world, torque motors are used in force feedback steering wheels.

Question: How are torque motors used in computer gaming?
Answer: feedback steering wheels

Question: A torque motor at low voltage provides what?
Answer: constant light tension

Question: A torque motor at high voltage provides what?
Answer: higher torque

Question: In a tape drive, what is not needed if a torque motor is used?
Answer: gears or clutches
Electric_motor

Another common application is the control of the throttle of an internal combustion engine in conjunction with an electronic governor. In this usage, the motor works against a return spring to move the throttle in accordance with the output of the governor. The latter monitors engine speed by counting electrical pulses from the ignition system or from a magnetic pickup and, depending on the speed, makes small adjustments to the amount of current applied to the motor. If the engine starts to slow down relative to the desired speed, the current will be increased, the motor will develop more torque, pulling against the return spring and opening the throttle. Should the engine run too fast, the governor will reduce the current being applied to the motor, causing the return spring to pull back and close the throttle.

Question: What element of a combustion engine does a governor monitor?
Answer: electrical pulses

Question: What is a governor connected to?
Answer: return spring

Question: What kinds of adjustments would a governor make?
Answer: small

Question: What would a governor do to a too-fast running engine?
Answer: reduce the current being applied to the motor
Electric_motor

A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the AC and resulting magnetic field which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor, which must slip to produce torque. One type of synchronous motor is like an induction motor except the rotor is excited by a DC field. Slip rings and brushes are used to conduct current to the rotor. The rotor poles connect to each other and move at the same speed hence the name synchronous motor. Another type, for low load torque, has flats ground onto a conventional squirrel-cage rotor to create discrete poles. Yet another, such as made by Hammond for its pre-World War II clocks, and in the older Hammond organs, has no rotor windings and discrete poles. It is not self-starting. The clock requires manual starting by a small knob on the back, while the older Hammond organs had an auxiliary starting motor connected by a spring-loaded manually operated switch.

Question: How much slip does a synchronous motor have?
Answer: zero

Question: How does an induction motor produce torque?
Answer: slip

Question: Whithout rotor windings, how must a Hammond clock be started?
Answer: manually

Question: How can a synchronous motor be made more like an induction motor?
Answer: DC field

Question: How is current brought to the rotor in a synchronous motor?
Answer: Slip rings and brushes
Electric_motor

Finally, hysteresis synchronous motors typically are (essentially) two-phase motors with a phase-shifting capacitor for one phase. They start like induction motors, but when slip rate decreases sufficiently, the rotor (a smooth cylinder) becomes temporarily magnetized. Its distributed poles make it act like a PMSM. The rotor material, like that of a common nail, will stay magnetized, but can also be demagnetized with little difficulty. Once running, the rotor poles stay in place; they do not drift.

Question: What do hysteresis synchronous motors' rotors do when slip rate decreases?
Answer: becomes temporarily magnetized

Question: When in operation, what do hysteresis synchronous motors poles do?
Answer: stay in place

Question: What do hysteresis synchronous motors resemble at startup?
Answer: induction motors

Question: How can a hysteresis synchronous motor's rotor be described?
Answer: a smooth cylinder

Question: What do the distributed poles of a hysteresis synchronous motor resemble?
Answer: PMSM
Electric_motor

Doubly fed electric motors have two independent multiphase winding sets, which contribute active (i.e., working) power to the energy conversion process, with at least one of the winding sets electronically controlled for variable speed operation. Two independent multiphase winding sets (i.e., dual armature) are the maximum provided in a single package without topology duplication. Doubly-fed electric motors are machines with an effective constant torque speed range that is twice synchronous speed for a given frequency of excitation. This is twice the constant torque speed range as singly-fed electric machines, which have only one active winding set.

Question: What type of motor has two independent multiphase winding sets?
Answer: Doubly fed electric motors

Question: What advanage do independent multiphase winding sets have?
Answer: contribute active (i.e., working) power to the energy conversion process

Question: What is one of the independent multiphase winding sets configured for?
Answer: variable speed operation

Question: Doubly fed motors have what advantage over single fed?
Answer: twice the constant torque speed range
Electric_motor

Nothing in the principle of any of the motors described above requires that the iron (steel) portions of the rotor actually rotate. If the soft magnetic material of the rotor is made in the form of a cylinder, then (except for the effect of hysteresis) torque is exerted only on the windings of the electromagnets. Taking advantage of this fact is the coreless or ironless DC motor, a specialized form of a PM DC motor. Optimized for rapid acceleration, these motors have a rotor that is constructed without any iron core. The rotor can take the form of a winding-filled cylinder, or a self-supporting structure comprising only the magnet wire and the bonding material. The rotor can fit inside the stator magnets; a magnetically soft stationary cylinder inside the rotor provides a return path for the stator magnetic flux. A second arrangement has the rotor winding basket surrounding the stator magnets. In that design, the rotor fits inside a magnetically soft cylinder that can serve as the housing for the motor, and likewise provides a return path for the flux.

Question: A coreless DC motor is distinguished by what?
Answer: rapid acceleration

Question: What is absent in a coreless DC motor?
Answer: iron core

Question: What forms can the rotor take?
Answer: winding-filled cylinder, or a self-supporting structure
Electric_motor

Because the rotor is much lighter in weight (mass) than a conventional rotor formed from copper windings on steel laminations, the rotor can accelerate much more rapidly, often achieving a mechanical time constant under one ms. This is especially true if the windings use aluminum rather than the heavier copper. But because there is no metal mass in the rotor to act as a heat sink, even small coreless motors must often be cooled by forced air. Overheating might be an issue for coreless DC motor designs.

Question: What is a likely problem of coreless DC motors?
Answer: Overheating

Question: How does a coreless rotor compare to traditional rotors in terms of weight?
Answer: lighter

Question: What advantage does a coreless rotor have over traditional variants?
Answer: accelerate much more rapidly

Question: The lack of a metal mass in the core causes what function to be absent?
Answer: heat sink

Question: How is a coreless motor cooled?
Answer: forced air
Electric_motor

These motors were originally invented to drive the capstan(s) of magnetic tape drives in the burgeoning computer industry, where minimal time to reach operating speed and minimal stopping distance were critical. Pancake motors are still widely used in high-performance servo-controlled systems, robotic systems, industrial automation and medical devices. Due to the variety of constructions now available, the technology is used in applications from high temperature military to low cost pump and basic servos.

Question: What was the original purpose for pancake motors?
Answer: drive the capstan(s) of magnetic tape drives

Question: What are advantages of pancake motors?
Answer: minimal time to reach operating speed and minimal stopping distance

Question: For what type of military applications are pancake motors currently used?
Answer: high temperature

Question: What type of servo-controlled systems use pancake motors?
Answer: high-performance