Electric machine
Encyclopedia
The academic study of electric machines is the universal study of electric motor
s and electric generators. By the classic definition, electric machine is synonymous with electric motor or electric generator, all of which are electro-mechanical energy converters: converting electricity to mechanical power (i.e., electric motor) or mechanical power to electricity (i.e., electric generator). The movement involved in the mechanical power can be rotating or linear.
Although transformer
s do not contain any moving parts
they are also included in the family of electric machines because they utilise electromagnetic phenomena.
Electric machines (i.e., electric motors) consume approximately 60% of all electricity produced. Electric machines (i.e., electric generators) produce virtually all electricity consumed. Electric machines have become so ubiquitous that they are virtually overlooked as an integral component of the entire electricity infrastructure. Developing ever more efficient electric machine technology and influencing their use are crucial to any global conservation, green energy, or alternative energy
strategy.
or if it is built into the motor in the form of a commutator
. If the controller is included as a part of the machine all machines can be powered by both AC and DC current, although some machines will need a more advanced controller then others. Classification is complicated by the possibilities of combining physical principles when constructing an electrical machine. It can e.g. be possible to run a brushed machine as a reluctance machine (without using the rotor coils) if the rotor iron has the correct shape.
Generally all electric machines can be turned inside out, so rotor and stator exchange places. All rotating electric machines have an equivalent linear electric machine where stator moves along a straight line instead of rotating. The opposite—linear to rotary dual—is not always the case. Motors and generators can be designed with or without iron to improve the path of the magnetic field (teeth to reduce the air gap is a common example) and with and without permanent magnets (PM), with different pole number etc., but still belong to different classes of machines. Electric machines can be synchronous meaning that the magnetic field set up by the stator coils rotates with the same speed as the rotor; or asynchronous, meaning that there is a speed difference. PM machines and reluctance machines are always synchronous. Brushed machines with rotor windings can be synchronous when the rotor is supplied with DC or AC with same frequency as stator or asynchronous when stator and rotor are supplied with AC with different frequencies. Induction machines are usually asynchronous, but can be synchronous, if there are superconductors in the rotor windings.
. Still the magnetic field created by modern PMs (Neodymium magnet
s) is stronger, which means that PM machines have a better torque/volume and torque/weight ratio than machines with rotor coils under continuous operation. This may change with introduction of superconductors in rotor.
Since the permanent magnets in a PM machine already introduce considerable magnetic reluctance, then the reluctance in the air gap and coils are less important. This gives considerable freedom when designing PM machines.
It is usually possible to overload electric machines for a short time until the current in the coils heats parts of the machine to a temperature which cause damage. PM machines can in less degree be subjected to such overload because too high current in the coils can create a magnetic field strong enough to demagnetise the magnets.
There is iron (usually laminated steel cores made of sheet metal
) between the rotor coils and teeth of iron between the stator coils in addition to black iron behind the stator coils. The gap between rotor and stator is also made as small as possible. All this is done to minimize magnetic reluctance of the magnetic circuit
which the magnetic field
created by the rotor coils travels through, something which is important for optimizing these kind of machines.
Large brushed machines which are run with DC to the stator windings at synchronous speed are the most common generator in power plants, because they also supply reactive power to the grid, because they can be started by the turbine and because the machine in this system can generate power at constant speed without a controller. This type of machine is often referred to in the literature as a synchronous machine.
This machine can also be run by connecting the stator coils to the grid, and supplying the rotor coils with AC from a inverter. The advantage is that it is possible to control rotating speed of the machine with a fractionally rated inverter. When run this way the machine is known as a brushed double feed "induction" machine. "Induction" is misleading because there is no useful current in the machine which is sett up by induction.
) has short circuited rotor coils where a current is set up and maintained by induction
. This requires that the rotor rotates at other than synchronous speed, so that the rotor coils are subjected to a varying magnetic field created by the stator coils. An induction machine is an asynchronous machine.
Induction eliminates the need for brushes which is usually a weak part in an electric machine. It also allows designs which make it very easy to manufacture the rotor. A metal cylinder will work as rotor, but to improve efficiency a "squirrel cage" rotor or a rotor with closed windings is usually used. The speed of asynchronous induction machines will decrease with increased load because a larger speed difference between stator and rotor is necessary to set up sufficient rotor current and rotor magnetic field. Asynchronous induction machines can be made so they start and run without any means of control if connected to an AC grid, but the starting torque is low.
A special case would be an induction machine with superconductors in the rotor. The current in the superconductors will be set up by induction, but the rotor will run at synchronous speed because there will be no need for a speed difference between the magnetic field in stator and speed of rotor to maintain the rotor current.
Another special case would be the brushless double fed induction machine, which has a double set of coils in the stator. Since it has two moving magnetic fields in the stator, it gives no meaning to talk about synchronous or asynchronous speed.
) has no windings in rotor, only a ferromagnetic material shaped so that "electromagnets" in stator can "grab" the teeth in rotor and move it a little. The electromagnets are then turned off, while another sett of electromagnets is turned on to move stator further. Another name is step motor, and it is suited for low speed and accurate position control. Reluctance machines can be supplied with PMs in stator to improve performance. The “electromagnet” is then “turned of” by sending a negative current in the coil. When the current is positive the magnet and the current cooperate to create a stronger magnetic field which will improve the reluctance machine’s maximum torque without increasing the currents maximum absolute value.
) torque is created by attraction or repulsion of electric charge in rotor and stator.
) are true DC machines where current is supplied to a spinning wheel through brushes. The wheel is inserted in a magnetic field, and torque is created as the current travels from the edge to the centre of the wheel through the magnetic field.
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s and electric generators. By the classic definition, electric machine is synonymous with electric motor or electric generator, all of which are electro-mechanical energy converters: converting electricity to mechanical power (i.e., electric motor) or mechanical power to electricity (i.e., electric generator). The movement involved in the mechanical power can be rotating or linear.
Although transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s do not contain any moving parts
Moving parts
The moving parts of a machine are those parts of it that move. Machines comprise both moving and fixed parts. The moving parts have controlled and constrained motions....
they are also included in the family of electric machines because they utilise electromagnetic phenomena.
Electric machines (i.e., electric motors) consume approximately 60% of all electricity produced. Electric machines (i.e., electric generators) produce virtually all electricity consumed. Electric machines have become so ubiquitous that they are virtually overlooked as an integral component of the entire electricity infrastructure. Developing ever more efficient electric machine technology and influencing their use are crucial to any global conservation, green energy, or alternative energy
Alternative energy
Alternative energy is an umbrella term that refers to any source of usable energy intended to replace fuel sources without the undesired consequences of the replaced fuels....
strategy.
Classifications
When classifying electric machines (motors and generators) it is reasonable to start with physical principle for converting electric energy to mechanical energy. It is important to distinguish between the machine and the controller regardless of the controller being a separate inverterInverter (electrical)
An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
or if it is built into the motor in the form of a commutator
Commutator (electric)
A commutator is a rotary electrical switch in certain types of electric motors or electrical generators that periodically reverses the current direction between the rotor and the external circuit. In a motor, it applies power to the best location on the rotor, and in a generator, picks off power...
. If the controller is included as a part of the machine all machines can be powered by both AC and DC current, although some machines will need a more advanced controller then others. Classification is complicated by the possibilities of combining physical principles when constructing an electrical machine. It can e.g. be possible to run a brushed machine as a reluctance machine (without using the rotor coils) if the rotor iron has the correct shape.
Generally all electric machines can be turned inside out, so rotor and stator exchange places. All rotating electric machines have an equivalent linear electric machine where stator moves along a straight line instead of rotating. The opposite—linear to rotary dual—is not always the case. Motors and generators can be designed with or without iron to improve the path of the magnetic field (teeth to reduce the air gap is a common example) and with and without permanent magnets (PM), with different pole number etc., but still belong to different classes of machines. Electric machines can be synchronous meaning that the magnetic field set up by the stator coils rotates with the same speed as the rotor; or asynchronous, meaning that there is a speed difference. PM machines and reluctance machines are always synchronous. Brushed machines with rotor windings can be synchronous when the rotor is supplied with DC or AC with same frequency as stator or asynchronous when stator and rotor are supplied with AC with different frequencies. Induction machines are usually asynchronous, but can be synchronous, if there are superconductors in the rotor windings.
Electromagnetic-rotor machines
Electromagnetic-rotor machines are machines having some kind of electric current in the rotor which creates a magnetic field which interacts with the stator windings. The rotor current can be the internal current in a permanent magnet (PM machine), a current supplied to the rotor through brushes (Brushed machine) or a current set up in closed rotor windings by a varying magnetic field (Induction machine).Permanent magnet machines
PM machines have permanent magnets in the rotor which set up a magnetic field. The magnetomotive force in a PM (caused by orbiting electrons with aligned spin) is generally much higher than what is possible in a copper coil. The copper coil can, however, be filled with a ferromagnetic material, which gives the coil much lower magnetic reluctanceMagnetic reluctance
Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is analogous to resistance in an electrical circuit, but rather than dissipating magnetic energy it stores magnetic energy...
. Still the magnetic field created by modern PMs (Neodymium magnet
Neodymium magnet
A neodymium magnet , the most widely-used type of rare-earth magnet, is a permanent magnet made from an alloy of neodymium, iron, and boron to form the Nd2Fe14B tetragonal crystalline structure. Developed in 1982 by General Motors and Sumitomo Special Metals, neodymium magnets are the strongest...
s) is stronger, which means that PM machines have a better torque/volume and torque/weight ratio than machines with rotor coils under continuous operation. This may change with introduction of superconductors in rotor.
Since the permanent magnets in a PM machine already introduce considerable magnetic reluctance, then the reluctance in the air gap and coils are less important. This gives considerable freedom when designing PM machines.
It is usually possible to overload electric machines for a short time until the current in the coils heats parts of the machine to a temperature which cause damage. PM machines can in less degree be subjected to such overload because too high current in the coils can create a magnetic field strong enough to demagnetise the magnets.
Brushed machines
Brushed machines are machines where the rotor coil is supplied with current through brushes in much the same way as current is supplied to the car in an electric slot car track. More durable brushes can be made of graphite or liquid metal. It is even possible to eliminate the brushes in a "brushed machine" by using a part of rotor and stator as a transformer which transfer current without creating torque. Brushes must not be confused with a commutator. The difference is that the brushes only transfer electric current to a moving rotor while a commutator also provide switching of the current direction.There is iron (usually laminated steel cores made of sheet metal
Sheet metal
Sheet metal is simply metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. Countless everyday objects are constructed of the material...
) between the rotor coils and teeth of iron between the stator coils in addition to black iron behind the stator coils. The gap between rotor and stator is also made as small as possible. All this is done to minimize magnetic reluctance of the magnetic circuit
Magnetic circuit
A magnetic circuit is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron, although there may be air gaps or other...
which the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
created by the rotor coils travels through, something which is important for optimizing these kind of machines.
Large brushed machines which are run with DC to the stator windings at synchronous speed are the most common generator in power plants, because they also supply reactive power to the grid, because they can be started by the turbine and because the machine in this system can generate power at constant speed without a controller. This type of machine is often referred to in the literature as a synchronous machine.
This machine can also be run by connecting the stator coils to the grid, and supplying the rotor coils with AC from a inverter. The advantage is that it is possible to control rotating speed of the machine with a fractionally rated inverter. When run this way the machine is known as a brushed double feed "induction" machine. "Induction" is misleading because there is no useful current in the machine which is sett up by induction.
Induction machines
Induction machines (Induction motorInduction motor
An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...
) has short circuited rotor coils where a current is set up and maintained by induction
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
. This requires that the rotor rotates at other than synchronous speed, so that the rotor coils are subjected to a varying magnetic field created by the stator coils. An induction machine is an asynchronous machine.
Induction eliminates the need for brushes which is usually a weak part in an electric machine. It also allows designs which make it very easy to manufacture the rotor. A metal cylinder will work as rotor, but to improve efficiency a "squirrel cage" rotor or a rotor with closed windings is usually used. The speed of asynchronous induction machines will decrease with increased load because a larger speed difference between stator and rotor is necessary to set up sufficient rotor current and rotor magnetic field. Asynchronous induction machines can be made so they start and run without any means of control if connected to an AC grid, but the starting torque is low.
A special case would be an induction machine with superconductors in the rotor. The current in the superconductors will be set up by induction, but the rotor will run at synchronous speed because there will be no need for a speed difference between the magnetic field in stator and speed of rotor to maintain the rotor current.
Another special case would be the brushless double fed induction machine, which has a double set of coils in the stator. Since it has two moving magnetic fields in the stator, it gives no meaning to talk about synchronous or asynchronous speed.
Reluctance machines
Reluctance machines (Reluctance motorReluctance motor
A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance.A reluctance motor, in its various incarnations, may be known as a:...
) has no windings in rotor, only a ferromagnetic material shaped so that "electromagnets" in stator can "grab" the teeth in rotor and move it a little. The electromagnets are then turned off, while another sett of electromagnets is turned on to move stator further. Another name is step motor, and it is suited for low speed and accurate position control. Reluctance machines can be supplied with PMs in stator to improve performance. The “electromagnet” is then “turned of” by sending a negative current in the coil. When the current is positive the magnet and the current cooperate to create a stronger magnetic field which will improve the reluctance machine’s maximum torque without increasing the currents maximum absolute value.
Electrostatic machines
In electrostatic machines (Electrostatic motorElectrostatic motor
An electrostatic motor or capacitor motor is a type of electric motor based on the attraction and repulsion of electric charge.Often, electrostatic motors are the dual of conventional coil-based motors. They typically require a high voltage power supply, although very small motors employ lower...
) torque is created by attraction or repulsion of electric charge in rotor and stator.
Homopolar machines
Homopolar machines (Homopolar motorHomopolar motor
A homopolar motor is an electric motor that works without the need for a commutator, by rotating along a fixed axis that is parallel to the external magnetic field produced by a permanent magnet. The name homopolar indicates that the electrical polarity of the motor does not change...
) are true DC machines where current is supplied to a spinning wheel through brushes. The wheel is inserted in a magnetic field, and torque is created as the current travels from the edge to the centre of the wheel through the magnetic field.