Electric motors appeared a long time ago, but a great interest in them arose when they became an alternative to internal combustion engines. Of particular interest is the issue of motor efficiency, which is one of its main characteristics.
Each system has a coefficient of performance that characterizes the effectiveness of its work as a whole. That is, it determines how well the system or device delivers or converts energy. The value of the efficiency does not have a value, and most often it is presented as a percentage or number from zero to one.
Efficiency parameters in electric motors
The main task of an electric motor is to convert electrical energy into mechanical energy. Efficiency determines the effectiveness of this function. The efficiency formula of the electric motor is as follows:
In this formula, p1 is the summed up electric power, p2 is the useful mechanical power that is generated directly by the engine. The electric power is determined by the formula: p1 = UI (voltage times the current strength), and the value of the mechanical power by the formula P = A / t (work to unit time ratio). This is how the calculation of the efficiency of the electric motor looks. However, this is its simplest part. Depending on the purpose of the engine and the scope of its application, the calculation will differ and take into account many other parameters. In fact, the motor efficiency formula includes a lot more variables. The simplest example was given above.
Lower efficiency
The mechanical efficiency of the electric motor must be taken into account when choosing a motor. A very large role is played by losses that are associated with engine heating, reduced power, and reactive currents. Most often, the drop in efficiency is associated with the release of heat, which naturally occurs during engine operation. The reasons for the release of heat can be different: the engine can heat up during friction, as well as for electrical and even magnetic reasons. As the simplest example, we can cite the situation when 1,000 rubles were spent on electric energy, and work was done on 700 rubles. In this case, the efficiency will be equal to 70%.
To cool electric motors, fans are used that drive air through the created gaps. Depending on the class of engines, heating can be carried out to a certain temperature. For example, Class A engines can heat up to 85-90 degrees, Class B engines up to 110 degrees. In the event that the temperature exceeds the permissible limit, this may indicate a stator short circuit.
The average efficiency of electric motors
It is worth noting that the efficiency of the DC motor (and AC too) varies depending on the load:
- At idle, the efficiency is 0%.
- With a load of 25%, the efficiency is 83%.
- At a load of 50%, the efficiency is 87%.
- With a load of 75%, the efficiency is 88%.
- At a load of 100%, the efficiency is 87%.
One of the reasons for the drop in efficiency is the asymmetry of the currents when a different voltage is applied to each of the three phases. If, for example, the first phase has a voltage of 410 V, the second - 403 V, and the third - 390 V, then the average value will be 401 V. The asymmetry in this case will be equal to the difference between the maximum and minimum voltage in the phases (410 -390), that is, 20 V. The formula for the efficiency of the electric motor for calculating losses will have the form in our situation: 20/401 * 100 = 4.98%. This means that we lose 5% efficiency during operation due to the voltage difference in the phases.
Total loss and drop in efficiency
There are a lot of negative factors that affect the drop in the efficiency of the electric motor. There are certain techniques to determine them. For example, you can determine if there is a gap through which power is partially transmitted from the network to the stator and then to the rotor.
Losses in the starter also take place, and they consist of several values. First of all, it can be losses related to eddy currents and magnetization reversal of stator cores.
If the motor is asynchronous, then there are additional losses due to the teeth in the rotor and stator. Also, eddy currents may occur in individual engine components. All this in total reduces the efficiency of the electric motor by 0.5%. Asynchronous motors take into account all losses that may occur during operation. Therefore, the range of efficiency can vary from 80 to 90%.
Car engines
The history of the development of electric motors begins with the discovery of the law of electromagnetic induction. According to him, the induction current always moves in such a way as to counteract the cause that causes it. It was this theory that formed the basis for the creation of the first electric motor.
Modern models are based on the same principle, however, they are radically different from the first copies. Electric motors have become much more powerful, more compact, but most importantly - their efficiency has increased significantly. We already wrote above about the efficiency of the electric motor, and compared to an internal combustion engine, this is an amazing result. For example, the maximum efficiency of an internal combustion engine reaches 45%.
The advantages of an electric motor
High efficiency is the main advantage of such a motor. And if the internal combustion engine spends more than 50% of the energy for heating, then in the electric motor a small part of the energy is spent on heating.
The second advantage is light weight and compact size. For example, Yasa Motors created a motor with a weight of only 25 kg. It is capable of delivering 650 Nm, which is a very decent result. Also, such motors are durable, do not need a gearbox. Many owners of electric cars talk about the efficiency of electric motors, which is logical to some extent. Indeed, during operation, the electric motor does not emit any combustion products. However, many drivers forget that coal, gas, or enriched uranium must be used to generate electricity. All these elements pollute the environment, so the environmental friendliness of electric motors is a very controversial issue. Yes, they do not pollute the air during operation. Power plants do this for them in the production of electricity.
Improving the efficiency of electric motors
Electric motors have some disadvantages that adversely affect work efficiency. This is a weak starting torque, high starting current and inconsistency of the mechanical moment of the shaft with the mechanical load. This leads to the fact that the efficiency of the device is reduced.
To increase efficiency, they try to provide engine load up to 75% and higher and increase power factors. There are also special devices for regulating the frequency of the supplied current and voltage, which also leads to increased efficiency and increased efficiency.
One of the most popular devices for increasing the efficiency of an electric motor is a soft starter, which limits the rate of increase in inrush current. It is also appropriate to use frequency converters to change the speed of rotation of the motor by changing the frequency of the voltage. This leads to a reduction in energy consumption and provides a smooth start of the engine, high accuracy of adjustment. The starting torque also increases, and at variable load the rotation speed is stabilized. As a result, the efficiency of the electric motor increases.
Maximum motor efficiency
Depending on the type of construction, the efficiency in electric motors can vary from 10 to 99%. It all depends on what kind of engine it will be. For example, the efficiency of a piston pump electric motor is 70-90%. The final result depends on the manufacturer, the structure of the device, etc. The same can be said about the efficiency of the crane electric motor. If it is 90%, then this means that 90% of the consumed electricity will be used for mechanical work, the remaining 10% will be used to heat parts. Nevertheless, there are the most successful models of electric motors, the efficiency of which approaches 100%, but is not equal to this value.
Is efficiency over 100% possible?
It's no secret that electric motors, whose efficiency exceeds 100%, cannot exist in nature, as this contradicts the basic law on energy conservation. The fact is that energy cannot come from nowhere and disappear in the same way. Any engine needs a source of energy: gasoline, electricity. However, gasoline is not eternal, as is electricity, because their reserves have to be replenished. But if there was a source of energy that did not need replenishment, then it would be entirely possible to create a motor with an efficiency of more than 100%. Russian inventing Vladimir Chernyshov showed a description of the engine, which is based on a permanent magnet, and its efficiency, according to the inventor himself, is more than 100%.
Hydroelectric power station as an example of a perpetual motion machine
For example, take a hydroelectric power station, where energy is generated by falling from a large height of water. Water rotates a turbine, and it produces electricity. Drop of water is carried out under the influence of gravity of the Earth. And although the work on generating electricity is being completed, the gravity of the Earth does not become weaker, that is, the force of gravity does not decrease. Then the water evaporates under the influence of sunlight and again enters the reservoir. This completes the cycle. As a result, electricity was generated, and the costs of its production were resumed.
Of course, we can say that the sun does not last forever, it is, but it will last a couple of billions of years. As for gravity, it constantly does work, drawing moisture from the atmosphere. Generally speaking, a hydroelectric power station is an engine that converts mechanical energy into electrical energy, and its efficiency is more than 100%. This makes it clear that you should not stop looking for ways to create an electric motor, the efficiency of which can be more than 100%. After all, not only gravity can be used as an inexhaustible source of energy.
Permanent magnets as energy sources for motors
The second interesting source is a permanent magnet, which does not receive energy from anywhere, and the magnetic field is not consumed even when the work is done. For example, if a magnet attracts something to itself, it will do the job, and its magnetic field will not become weaker. They have tried more than once to use this property to create the so-called perpetual motion machine, but so far nothing more or less normal has come of it. Any mechanism will wear out sooner or later, but the source itself, which is a permanent magnet, is practically eternal.
However, there are experts who claim that over time, permanent magnets lose their strength as a result of aging. This is not true, but even if it were true, then it would be possible to return it to life with just one electromagnetic pulse. The engine, which would require recharging once every 10-20 years, although it cannot claim to be an eternal one, is very close to this.
There have already been many attempts to create a perpetual motion machine based on permanent magnets. So far there have been no successful solutions, unfortunately. But given the fact that there is a demand for such engines (it simply cannot but be), it is quite possible that in the near future we will see something that will come very close to the model of a perpetual motor that will run on renewable energy.
Conclusion
The efficiency of the electric motor is the most important parameter that determines the efficiency of a particular motor. The higher the efficiency, the better the motor. In an engine with an efficiency of 95%, almost all the energy expended is spent on performing work and only 5% is spent unnecessarily (for example, for heating spare parts). Modern diesel engines can achieve an efficiency of 45%, and this is considered a cool result. The efficiency of gasoline engines is even less.