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The academic study of electric machines is the universal study of electric motors and electric generators. By the classic definition, electric machine is synonymous with electric motor or electric generator, all of which are electromechanical 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.
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.
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 inverter 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 than others. Classification is complicated by the possibilities of combining physical principles when constructing an electrical machine. It can, for example, 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 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 reluctance. Still the magnetic field created by modern PMs (Neodymium magnets) 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 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) 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 an 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 set up by induction.
Induction machines have 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.
Reluctance machines have 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 set 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.
In electrostatic machines, torque is created by attraction or repulsion of electric charge in rotor and stator.
Homopolar machines 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 machine systems
For optimized or practical operation of electric machines, today's electric machine systems are complemented with electronic control.
Comparing electric machine systems
Comparing the cost-performance between electric machine systems of different classification or from different manufacturers is difficult without a critical baseline of metrics. For equitable comparison of efficiency, cost, torque, and power between electric machine systems, the comparison should be matched with the same voltage and speed at a given frequency of excitation; or instead, the additional cost, efficiency, and real-estate of the transmission for instance for coupling to the speed of the application, the transformer for matching voltage, the frequency converter to match excitation frequency, etc., should be included.
Other parameters that should always be considered in any revealing comparison: • Duty Cycle – although directly related to cost, efficiency, and power density, duty cycle gives meaning to application applicability; • Peak Torque Potential – the closeness of peak torque potential to continuous torque indicates the safe margin of the design; • Cost, Efficiency, and Real-estate of the Electronic Controller – unless integrated and included in the specifications of the electric machine system, the electronic controller, which is required for practical system operation, should always be included. • Utilization of the Magnetic Core and frame assembly – As an example: with the wound-rotor doubly fed electric machine as the only exception, rotor assemblies, which consume nearly half the volume of the electric machine, passively participate in the energy conversion process and are under-utilized.
- Flanagan. Handbook of Transformer Design and Applications, Chap. 1 p1.