Electric motors

Summary: Electric motors are machines that convert electrical energy into motion, and this motion is normally rotational energy. Electric motors come in many shapes, speeds, and sizes and can use direct current (DC) or alternating current (AC) as their power source.

History does not seem to have recorded the exact person who invented the electric motor. There are many claims, and many who contributed. In 1834, Moritz Jacobi of Russia built a working battery-powered electric motor, which a few years later he installed on a boat that was propelled by it. About the same time, Robert Davidson of Scotland built a working electric carriage. In the United States, Vermont blacksmith Thomas Davenport built a small-scale electric motor for a toy-sized electric car in 1835.

All these inventors seem to owe the roots of their inventions to Michael Faraday, who pioneered electromagnetic rotation and induction a few years earlier than Jacobi, Davidson, and Davenport. All early electric motors were powered by direct current (DC). Alternating current (AC) and the AC motor came along later: In 1887, Nikola Tesla invented the first AC induction motor, and he was later granted a US patent on this type of electric motor.

All electric motors have some common parts. A good way to start learning about a motor’s parts is to learn which parts are stationary and which parts move. The moving parts are collectively called the rotor or armature. On an induction AC motor, the rotor’s main components include a hard metal shaft, which has electromagnetic bars of metal mounted to it in the shape of a cylinder. The ends of the rotor bars are all connected in what is often referred to as a shorting ring. The rotor shaft is supported by bearings of many different sizes and types. The stationary parts also have electromagnetic poles mounted in a cylinder shape. There is always an air gap between the rotor and the stator. The rotor’s electromagnet has two poles, which are pushed when the electromagnets in the stator alternate polarity.

Normally, the poles on an electric motor are made from aluminum or copper wire wound in loops that form the electric coil. Increasing the number of loops in the coil increases the strength of the magnetism and therefore increases the strength or horsepower rating of the electric motor. This push of magnetism rotates the rotor at the frequency of the AC power if the stator has two poles. Increasing the number of poles in the motor’s stator slows the speed of the rotor. The rotor speed on an AC induction motor is proportional to the number of poles in the stator. Since the same-polarity magnetic poles repel each other, and different polarity magnetic poles attract each other, this repulsion and attraction causes the rotor to turn.

DC motors work the same way but have an additional component, called a commutator. The commutator is normally a split ring that applies the incoming DC power to the poles, but the split in the ring changes the polarity of the DC poles, simulating AC power. Since the power on a DC motor must be applied to this ring, the power is normally applied through brushes that touch the commutator ring but allow the commutator to slide against the rotor, allowing the armature to rotate. Modern brushes are usually solid pieces of electrically conductive carbon that are spring-loaded to keep good contact with the commutator ring. The number of poles of a DC motor does not determine speed, as in an AC motor. DC motors change speed with changes in voltage or amperage supplied.

The turns of wire that make stator poles into electromagnets have a coating of insulating material on these wires to prevent the wires from shorting against each other. The quality and temperature rating of this insulation coating are major factors in determining the life of an electric motor. The National Electrical Manufacturers Association (NEMA) has a rating system that is commonly used to grade the quality of motor insulation. NEMA grades are A, B, F, and H, and the later the letter is in the alphabet, the higher it is rated for temperature and longer life. Motor casings are also graded, and the main grade distinction is whether casing is open or totally enclosed. Open casings are not sealed; dust and foreign objects can get in this type of motor. Totally sealed motors also have subclasses, including explosion-proof and waterproof. Motors have a service factor that gives the amount of amperage over the full load rating the motor can run with no adverse effect. Most motors have a cooling fan, mounted either externally or internally, that is driven by the motor’s shaft. Induction AC motors can be single-phase or polyphase. Higher-horsepower AC induction motors are usually powered by three-phase polyphase power. The three-phase AC power allows the motor to be smaller and lighter than a single-phase AC motor. While NEMA standards continued to be used in the United States into the 2020s, worldwide, many nations began using electric motors that adhered to standards set by the International Electrotechnical Commission (IEC).

Synchronous motors operate at a constant speed and can be used for many special applications, including turning metering devices for precision delivery of bulk material feed and even step motors for robotics. There are two main types of synchronous motors: the reluctance type and the permanent magnet type. In the permanent magnet type, the rotor is a permanent magnet, instead of the magnetism being generated by an electromagnetic field. In the reluctance type, the rotor’s magnetic field is locked to the stator field and is not as efficient as the permanent magnet synchronous motor.

Use

Electric motors are widely used both in industry and in home use. Electric motors drive everything from windshield wipers on passenger cars to pumps that move millions of gallons of water. For stationary machines that are connected to the electric grid, electric motors have significant advantages. Most industrial pumps, fans, and conveyors are driven by electric motors.

The motors are very efficient in converting electrical potential energy into rotating energy and are almost always more efficient than fossil-fuel-powered machines. Most motors that run on fossil fuels create exhaust gases that are hazardous to humans, so electric motors can be used indoors with little or no ventilation, since even when totally enclosed indoors, the motors give off no fumes. Fuel tanks are not needed, another significant advantage. The bearing and lubrication systems for electric motors are usually less complex than those needed for fossil-fuel-powered drivers, yet another advantage.

Electric motors can be any size, from smaller than a thimble to several tons rated at several thousand horsepower. Electric motors provide smooth torque and speed control for vehicles. Most modern locomotives are diesel electric, whereby a diesel engine powers a generator that in turn powers electric motors that drive the wheels of the locomotive. Hybrid automobiles, in which the primary drive is an electric motor and the gasoline engines provides secondary propulsion and power to charge the battery, have been growing in use since the late 1990s. Electric motors are also another technology on the rise in the twenty-first century. According to the US Department of Energy, more than 3.3 million electric vehicles (EVs) were on US roads in 2023. That was up from 1.3 million in 2021. The state of California led the way with 1.17 million EVs on the road in 2023. Electric motors are relatively safe; if a motor experiences an electrical failure, the power would be automatically removed by correctly installed fuses, electrical breakers, and electric thermal overload protection relays. These protective devices can be sized so amperage over the full-load amperage (FLA) of the motor is detected and the electric current and voltage are isolated from the machine before severe damage occurs. Motors can also have safety devices that detect high temperatures in the bearings or windings of the motor. Higher cost motors can even have vibration detectors that can emit alarms or isolate the motor if it reaches a predetermined high level of vibration.

In short, electric motors are adaptable, safe, reliable, and efficient drivers for most machinery. Electric motors can be powered by the public electric grid service, battery power, or local generators and so can be used in both rural and urban areas. Electric motors are even used in some spacecraft and artificial satellites to operate remote arms and position telecommunications antennas, since these motors can be powered by solar cells in space.

Bibliography

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Malinowski, John. "Differences Between NEMA and IEC Electric Motors." Plant Engineering, 17 Sept. 2021, www.plantengineering.com/articles/differences-between-nema-and-iec-electric-motors/. Accessed 30 July 2024.

"Maps and Data—Electric Vehicle Registrations by State." US Department of Energy, 2024, afdc.energy.gov/data. Accessed 30 July 2024.

Mazur, Glen, and Thomas E. Proctor. Troubleshooting Electric Motors. Homewood, IL: American Technical, 2005.

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