In electrical engineering, an electric machine is a general term for a machine that makes use of electromagnetic forces and their interactions with voltages, currents, and movement, such as motors and generators. They are electromechanical energy converters, converting between electricity and motion. The moving parts in a machine can be rotating (rotating machines) or linear (linear machines).
Electric machines, in the form of synchronous and induction generators, produce about 95% of all electric power on Earth (as of early 2020s). In the form of electric motors, they consume approximately 60% of all electric power produced. Electric machines were developed in the mid 19th century and since have become a significant component of electric infrastructure. Developing more efficient electric machine technology is crucial to global conservation, green energy, and alternative energy strategy.
While transformers are occasionally called "static electric machines", they do not have moving parts and are more accurately described as electrical devices "closely related" to electrical machines.
History
The basis for electric machines date back to the early 19th century, and the research and experiments of Michael Faraday in the relationship of electricity and magnetism. One of the first demonstrations of an electric machine was in 1821, with a free-hanging wire dipped into a pool of mercury, on which a permanent magnet (PM) was placed. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a close circular magnetic field around the wire. While primitive compared to modern electric machines, this experiment showed the ability to convert electric energy to motion.
Improvements to electric machines continued throughout the 19th century, however as this predated the existence of an electric power system, they struggled to gain viability and acceptance. Near the end of the 19th century, when the first electric grids came into existence, electric machines grew into more applications. Of note, the invention of the dynamo by Werner von Siemens in 1867 and invention of the first practical DC motor by Frank Sprague in 1886.Robbins, Michael (26 October 2012). "The Early Years of Electric Traction". The Journal of Transport History. 21 (1): 92–101. .
As electric power systems moved from DC to AC during the war of currents, so did electric machines. While alternators began to replace dynamos, AC motors proved more difficult. It was not until Nikola Tesla's invention of the induction motor that AC motors began to replace DC motors in significant quantities.W. Bernard Carlson, Tesla - Inventor of the Electrical Age, Princeton University Press · 2015
Operating principle The main operating principles of electric machines take advantage of the relationship between electricity and magnetism, specifically that changes in one can create changes in the other.Fleisch, D. (2008). A Student's Guide to Maxwell's Equations. United Kingdom: Cambridge University Press. For example, moving a bar magnet around a wire to induce a voltage across it, or running current through a wire in a magnetic field to generate a force.
This is largely based on Maxwell's equations and can be analytically and mathematically complex. However, most electric machines are governed by the same 4 principles:
Lorentz force, a force generated due to current flowing in a magnetic field Faraday's law of induction, a voltage induced due to movement within a magnetic field Kirchhoff's voltage law (KVL), the sum of voltages around a loop is zero Newton's laws of motion, an applied force on an object is equal to its mass by its acceleration As current flows within a magnetic field, a force is induced that causes movement. With this movement also within the magnetic field, a voltage is induced in the machine. This induced voltage affects the current in the machine, which in turn affects the force and speed, and ultimately the induced voltage again. This feedback tends to drive the machine to an equilibrium so that the electrical energy and mechanical energy are matched (plus losses). With proper orientation of magnetic fields, wires, voltages, and currents, an electric machine can convert electric energy (electricity) to mechanical energy (motion) and vice-versa.
Electric machines typically separate their moving and non-moving portions and identify them uniquely. In rotating machines, the stationary portion is called the stator, while the rotating portion is the rotor. The stator and rotor may have windings (wire wound around them) to carry the current on the electrical side and/or to help create the magnetic field. The current-carrying winding is called the armature winding while the magnetic field winding is called the field winding. All rotating machines have armature windings, but not all machines have field windings: the magnetic field can be created by a permanent magnet or an electromagnet created by the field winding. The armature winding and field winding (if applicable) can be on either the stator or rotor, depending on the machine design, however they are rarely on the same part.
Characteristics of electric machines While electric machines have their differences, they share many traits, and are often grouped by some part of their construction or intended operation. Below are some of the characteristics common to most electric machines.
Motors and generators If an electric machine converts mechanical energy into electrical energy, it is referred to as a generator, while machines that convert electricity to motion are called motors.
Generators that produce alternating current (AC) are called alternators, while direct current (DC) generators are called dynamos. Motors are referred to as pumps when their motion is used to move a fluid, such as water.
Theoretically, most electric machines can be used as either a generator or a motor, however in practice it is common for machines to be specialized to one or the other.
AC vs DC Electric machines can be connected to either an AC or DC electrical system, with the AC being either single phase or three phase. With rare exceptions, such as universal motors, machines cannot switch between electric systems.Marsh, L. W. (1943). A Study of the Universal Motor. (n.p.): University of Cincinnati. AC machines are largely either synchronous generators or induction motors.
A DC machine is somewhat of a misnomer, as all DC machines use alternating voltages and currents to operate.Direct Current Machines. (2007). India: Laxmi Publications Pvt Limited. Most DC machines include a commutator, which allows the armature windings within the DC machine to periodically change their connections to the DC electrical system as the machine rotates, effectively alternating the direction of voltages and currents within the machine, but keeping DC voltages and currents on the electrical side.
Brushed vs brushless If an electric machine has an electric circuit on its rotor, it needs a means to power the circuit even while the rotor is rotating. One method of doing this is to attach metallic brushes to the stator and have them held under tension against the rotor.Brushes for Electrical Machines. (1963). United States: American Standards Association. These brushes are then energized on the stationary stator side, and transfer electricity to the moving rotor. The part of the rotor that contacts the brushes are called slip rings, and are designed to withstand both the electricity being passed through them and the mechanical wear of continuously spinning against the brushes. The brushes are generally made of carbon, for its strength and conductivity. Brushes wear down and need replacing throughout the life of the machine.
Another technique to power the electric circuit on the rotor is through electromagnetic induction. As the rotor is already moving, it meets one of the main requirements of induction (varying magnetic field), and can be adapted to have a magnetic field induced into it. This technique is very common for induction motors, but is also used in bushless synchronous machines.Krishnan, R. (2017). Permanent Magnet Synchronous and Brushless DC Motor Drives. United States: CRC Press.
If the winding on the rotor is a field winding, its purpose it to act as an electromagnet and generate a magnetic field that rotates. This can be replaced with a permanent magnet, removing the need for brushes or slip rings and simplifies the design of the machine. Large permanent magnetics are expensive and do not always allow for a machine to act as both a motor and generator, so PM machines tend to be limited to small power motors.Gieras, J. F. (2002). Permanent Magnet Motor Technology: Design and Applications, Second Edition,. United States: Taylor & Francis.
Speed and torque Electric motors convert electricity to motion, and are able to move increasing larger mechanical loads by drawing more electrical energy. This comes at a cost: with the Lorenz Force defining the speed of the machine, if the force has to overcome a larger mechanical load, the speed of the machine slows down. In rotating motors, the forces are viewed as torques, and this behavior is referred to as the speed-torque curve of the machine. Electric motors denote speed in terms of revolutions per minute (RPM).
The shape of the speed-torque curve depends on the design of the motor. In DC motors, the speed-torque curve is linear, with maximum torque occurring with zero speed (stall torque) and maximum speed occurring at zero torque (no-load speed). In AC motors, the torque-speed curve is a more complex shape, beginning at the starting torque associated with the locked-rotor current at no speed, gradually increasing with speed until peaking at the breakdown torque, and finally rapidly falling to zero at the no-load (max) speed. The exact shape of the curve depends on the AC motor design.
Synchronous vs asynchronous In AC electric machines, one magnetic field rotates around the machine due to the electrical system connections, while the other magnetic field rotates due to the rotor's physical motion. If these two magnetic fields rotate at the same speed, the machine is said to be a synchronous machine, and operates at synchronous speed. If the magnetic fields rotate at different speeds the machine is asynchronous, with a speed either above or below synchronous speed. If the rotors field is slower than the stator field, the machine acts as a motor, if it is faster it acts as a generator. Asynchronous machines cannot operate at synchronous speeds. Another common name for asynchronous machines is induction machines.
DC machines are not classified as either synchronous or asynchronous, as the magnetic fields do not rotate. The magnetic field from the field winding (or PM) is on the stator and is stationary. The armature winding is on the rotor and rotates, but has its polarity reversed by commutation. The DC system also lacks a frequency to compare the speed to.
Common machines While there are many different types of electric machines, a few different machine configurations account for the most common electric machines.
Synchronous generator
A synchronous generator is a synchronous machine with a prime mover attached to its rotor, which is driven by a steam or gas turbine. A synchronous generator typically has a three phase armature winding, and generators AC power. The rotor's field winding is typically excited through brushes and slip rings, however brushless machines are possible through either PM or an exciter circuit consisting of AC induction from stator to rotor and a rectifier on the rotor to provide DC power. They range in sizes from a few kilowatts at residential voltages up to 500 MW and greater at voltages above 20,000 V. Synchronous generators are the most common form of traditional generation for the AC power system.
Induction motor