Why synchronous motor has a constant speed
Lexicon> Letter E> Electric motor
Definition: a motor that works with electrical energy
More general terms: engine
More specific terms: DC motor, AC motor, series motor, permanent magnet motor, linear motor, synchronous motor, asynchronous motor, squirrel cage motor
English: electric motor
Categories: electrical energy, vehicles
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: 04/26/2010; last change: 03/14/2020
A Electric motor is a motor working with electrical energy, i. H. a machine that converts electrical energy into mechanical energy. In the vast majority of cases, this is done with the help of magnetic forces, which are created by the flow of current in metallic conductors (or sometimes in superconducting materials), and not by electrical forces.
Types of electric motors
For electric motors there is a very large range of different designs with correspondingly different properties. Here are just a few of the most basic distinctions:
- Most electric motors drive a rotating shaft, but there are also Linear motors.
- Some electric motors (especially smaller ones) contain permanent magnets (→permanently excited motors) and electromagnets, others only electromagnets. The electrical excitation costs additional energy and tends to provide weaker magnetic fields, but permanent magnets are expensive for large motors.
- Some electric motors work with direct current (DC motor), others with alternating current or three-phase current. In the latter case it can either be alternating current with a fixed frequency (mostly for one or a few fixed speeds) or with a variable frequency (for variable speeds). With some AC motors, the oscillations of the AC or three-phase current must be precisely synchronized with the movement of the motor axis (Synchronous motors), with others (Asynchronous motors), on the other hand, there is a slight slip. In both cases there is a close relationship between the speed and the mains frequency, which is also influenced by the number of poles in the motor. But there are also series motors in which the speed is not linked to the frequency at all.
- In order to supply electromagnets on the rotating part (rotor) with electricity, sliding contacts are required, which are often divided into many smaller contacts and as commutator (i.e. to periodically reverse the direction of the current). Often carbon brushes then make the contact; they are wearing parts. However, there are more durable ones brushless motorsthat do not require any sliding contacts, because the rotor only contains permanent magnets or because it is a Squirrel cage motor acts.
Some types like Synchronous motors are only more powerful and efficient through development Power electronics become widely applicable. This comes e.g. B. benefit the development of hybrid drives and electric cars.
Requirements for electric motors
The large number of types of electric motors is explained by the fact that very different requirements are made depending on the application:
- There are electric motors with a wide variety of drive powers between well below one watt and hundreds of megawatts.
- Motors are operated at very different speeds and torques, which are either roughly constant or (possibly even strongly) variable during operation.
- Depending on the operating mode and construction, the efficiency can vary greatly: from over 98% for large motors to less than 50% for cheaply built or poorly operated small motors.
- The design decides whether a motor can work with direct current, alternating current or three-phase current and whether it can be operated with a constant operating voltage and frequency, or whether voltage and frequency z. B. the speed must be adjusted.
- Conversely, many electric motors can also serve as a generator, i.e. H. they can deliver electrical energy when they are mechanically driven. This is z. B. used for recuperation (regenerative braking) in electric vehicles.
- Other aspects are the service life, the tolerance to temporary overload, the reactive power requirement (with AC motors) as well as the noise development and smooth running.
The applications for electric motors are extremely diverse:
- Various vehicles can be powered electrically, e.g. B. Electric locomotives, electric cars, forklifts. If electrical energy cannot be supplied via an overhead contact line (as is common on trains), a storage device must be carried along, such as a rechargeable battery.
- Many stationary machines and devices contain electric motors, e.g. B. Production machines, refrigerators, heat pumps, circulation pumps for central heating systems and various other pumps.
- Suitable electric motors with small dimensions, low weight and low costs are available for numerous small and very small drive applications.
Electric motors in heating and circulation pumps, fans in ventilation systems and compressor motors in refrigerators and freezers and electric heat pumps are particularly important for the electrical energy demand in the household.
Typical advantages and disadvantages of electric drives
Compared to internal combustion engines and other heat engines, electric motors have some typical advantages:Larger electric motors in particular can be extremely efficient - even at partial loads.
- Very high efficiencies are possible, especially with larger motors. With outputs in the megawatt range, 98-99% is often achieved, at least in the optimal speed range. However, the system efficiency is not necessarily high if the power generation in the power plant is lossy.
- A high degree of efficiency is often possible over a wide range of drive powers and speeds, and an electric motor can be switched on and off at any time without any problems. For example, this allows electric cars to be operated economically, especially in city traffic, where combustion engines are usually very uneconomical.
- Another efficiency advantage results in vehicles if braking energy can be recovered (Recuperation) to e.g. B. to recharge an accumulator or to feed electricity back into a network (as usual with trains). Most of the time, the motor itself then serves as a generator. Even if the generated electrical energy cannot be recovered, electrical braking can be advantageous because it enables a reliable and low-wear solution.
- The design is very compact, especially for smaller capacities.
- During operation, there are no exhaust gases (except in a heat engine, if this is used to generate electrical energy) and usually little noise. Some electric motors work almost silently.
- The lifespan of electric motors is usually long and usually little or no maintenance is required.
- Electric motors do not need an increased operating temperature, so do not have the cold start problems of internal combustion engines.
- Unlike internal combustion engines, electric motors do not require any air supply - at most cooling air, which can, however, be circulated if necessary. This is advantageous e.g. B. for submarines.
Probably the only significant disadvantage of electric motors is that they require high quality electrical energy. This is a problem in particular in the case of mobile applications such as electric cars, since the carrying along of stores for electrical energy has considerable disadvantages compared to fuels as stores of chemical energy. Rechargeable batteries can store far less energy per kilogram than fuel, and fuel cells have so far also had various disadvantages such as the need for expensive materials and a very limited selection of fuels.
Electric motors can be very energy efficient, but the existing technical efficiency potential is often not exploited. Especially with motors that are used continuously, this often leads to an unnecessarily high consumption of electrical energy.
Some common problems are:Some motors waste a lot of energy. An inappropriately chosen application (e.g. unnecessarily high drive speed) can also cause this.
- Motors with unfavorable outdated designs (e.g. shaded pole motors) are still in use in many places. Modern motors (e.g. permanent magnet motors) can often save more than 50% of the energy.
- Motors with an unnecessarily high power for the application are selected. The drive speed can also be selected to be higher than necessary.
- Motors are operated at a constant speed instead of the speed being adjusted as required. This is particularly harmful, for example, with heating circulation pumps that struggle against closed thermostatic valves in warm weather (build up unnecessarily high pressure) and then even consume more power than in normal heating mode.
Modern, highly efficient engines often have additional advantages, e.g. B. a lower reactive power requirement, a quieter run and a longer service life. Some advantages of highly efficient motors result from a more complex design of the motor itself (e.g. with more copper or with strong permanent magnets), others from additional facilities such as e.g. B. efficient converters and controls.
In the absence of information, such a motor is often not used even in industrial applications, where a better motor could often amortize the additional costs within a few years (i.e. a fraction of the service life). Efforts are underway by government and non-governmental institutions to fill such information gaps. One means of doing this is to use internationally uniform definitions as far as possible Efficiency classes for engines such as B. The IE3 star, IE2 star and IE1 star classes, which have been harmonized since 2008.
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See also: electrical energy, mechanical energy, motor, electric car
as well as other articles in the categories of electrical energy, vehicles
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