What are questions about electric motors
Günter Pauli GmbH
In our blog series on the subject of electric motor know-how, we have already covered several special topics relating to the maintenance, repair and diagnosis of electric motors in the past few months. Today we want to devote ourselves to the topic very fundamentally and summarize everything about electric motors. How does an electric motor work? What types of electric motors are there? Which types are used where? And what are the advantages and disadvantages of an electric motor compared to combustion engines? We answer these and many other fundamental questions in this article.Maintenance questions? Get advice now!
The history of the electric motor
The development of the electric motor goes back to a discovery made by the Danish scientist Hans Christian Oersted in 1820. He found that electric current has a magnetic effect and thus laid the foundation for further electromagnetic research in the following years. It took 18 years before an electric motor was used in practice for the first time: Hermann Jacobi equipped a paddle boat in St. Petersburg with an electric drive.
The invention of the dynamo by Werner von Siemens (patented in 1866) made it possible to produce electricity on a large scale. When power plants were built and power grids laid in many places at the end of the 19th century, there was the decisive impetus for the triumph of electric motors. The commercial breakthrough came after the invention of the asynchronous motor by Mikhail Ossipowitsch Doliwo-Dobrowolski. This motor was operated with three-phase current, which, unlike direct current, could be transported over long distances with little loss.
As a result, electric motors replaced steam engines in many industrial companies. In Germany, electrical engineering is seen as a cornerstone of the second industrial revolution. But this also changed a lot in the everyday life of the citizens. Electricity made it possible for telephones to replace telegraphy. Carriages of horses were replaced by electric trams. And electric lights shone in the houses.
In the electric motor, electrical energy is converted into mechanical energy. One makes use of the phenomenon of magnetism: As we know, the same poles repel and different poles attract each other. With electric current it is possible to make a non-magnetically charged part magnetic. And the polarity can also be influenced, depending on the direction in which the current flows. In a simple electric motor there is a fixed magnetic part (stator) and a moving part (rotor) that is made magnetic by electricity. If two positive poles are facing each other due to the electrical charge, they repel each other and the moving part of the electric motor rotates. The current direction changes automatically with every half turn. This ensures that the machine is permanently in motion and does not stop at dead center.
How can this movement now be used?
Quite simply: the rotor, i.e. the moving part, is permanently installed on an axle. This rotates together with the rotor, and whatever is connected to the axle also rotates. This mechanical energy can be used for many different purposes. For example, with a small motor, the axis could operate a fan. Or, with a very large engine, the axle could set the wheels of a locomotive in motion. The speed of the motor can be controlled by the amount of energy supplied. And as with any other motor, a number of add-on parts can of course be used, which open up even more possibilities for optimal use of the mechanical energy. The following YouTube video explains how an electric motor works in a very understandable way.
Video: This is how an electric motor works
Construction: An electric motor consists of these parts
A few technical terms on the subject of electric motors have already been mentioned above. In this chapter we want to take a closer look at the individual components of the electric motor.
The stationary part of the electric motor is called the stator. Depending on the type of motor, either a permanent magnet or an electromagnet is used. Most motors have the stator on the outside and connected to the housing. But there are also motors in which the immovable part is on the inside and the rotor rotates around the stator. In this case one speaks of an outrunner.
The moving part of the electric motor is called the rotor, armature or runner. In most cases, the rotor consists of an axle and a coil of lacquered copper wire through which the current flows and turns the rotor into an electromagnet.
Anchor is often used as a synonym for the rotor, but in a narrower sense describes the iron core of the rotor around which the coils are wound.
The commutator takes its name from the Latin word commutare (= to swap) and is responsible for changing the direction of the current. It is therefore often referred to as a commutator. The magnetic field of the electromagnet changes with the direction of the current. This is necessary so that the engine does not stop. In many cases, the commutator is a metal disc that is divided into two isolated segments and that rotates with the axis of the motor. The power supply is usually provided by carbon brushes that are pressed against the commutator. After half a revolution of the motor, the power supply is briefly interrupted, then the current flows in reverse through the coil. The following animation illustrates this principle:
Source: MichaelFrey (own work) [CC BY-SA 3.0], via Wikimedia Commons
No electric motor works without electromagnets. Therefore, every electric motor must have a power source that turns the actually non-magnetic rotor into an electromagnet.
Brushes, which are often made of graphite, supply the rotor with electricity via the commutator.
A capacitor stores energy and releases it in a targeted manner. Many electric motors have operating capacitors that ensure that the motor starts, turns in the right direction and develops its power evenly. Very large machines can also have a starting capacitor. The capacitors are often attached to the outside of the motor housing. They are comparatively cheap wearing parts and are often the cause when the electric motor does not start. More information on this can be found in our blog post Electric motor troubleshooting: Checking the motor capacitor.
There are a number of different types of electric motors. To list all types would go beyond our blog at this point. We would like to briefly introduce at least the most important types of electric motors here. E-motors are roughly divided into two classes: DC motors and three-phase or AC motors.
DC motor (also called commutator motor)
DC motors are - as the name suggests - operated with direct current. They are therefore dependent on the commutator already mentioned above in order to function. This mechanical inverter ensures that the polarity of the current is automatically reversed with every half revolution of the axis.
In the case of direct current motors, a distinction is again made between two subclasses: the permanently excited and the electrically excited motors. In a permanently excited DC motor, the stator is a permanent magnet and only the rotor is an electromagnet. This type of construction is used, for example, in ventilators or car starters.
In an electrically excited DC motor, both main components are electromagnets. A distinction is again made here between series-wound or main-wound motors on the one hand and shunt-wound motors on the other. The difference: In a shunt motor, the stator and rotor each have their own power source. A main circuit motor has only one power source. These machines can also be operated with alternating current and are therefore also known as universal motors. Electrically excited DC motors are used in many household appliances.
AC and three-phase motors
It gets a little more complicated with a three-phase motor. A three-phase motor is operated with three-phase alternating current, which is also known as three-phase current or colloquially as high-voltage current. The stator of the motors consists of three coils, each of which is fed by a phase voltage of the three-phase current. The three phases of the alternating current are each shifted in phase by 120 degrees. The magnetic field, which is offset by a third, ensures that the rotor rotates.
A distinction is also made between two subclasses for three-phase motors: synchronous and asynchronous machines. In asynchronous machines, the frequency of the rotor follows that of the magnetic field. In synchronous machines, the frequency of the rotor and the magnetic field are identical. What sounds like a small detail makes a huge difference in practice. Synchronous motors are significantly more efficient and can achieve an efficiency of up to 90%. However, they are also more expensive to manufacture and require a lot of maintenance to operate. That is why they are rarely used, for example in compressors, in ship propulsion systems or in chippers. The use as a generator is somewhat more common - in other words, in the opposite case, in which mechanical energy is to be converted into electrical energy.
Quite different with asynchronous motors: This type of motor is cheap and is considered to be very robust (see also our blog post on testing three-phase asynchronous machines). It is therefore no coincidence that three-phase asynchronous machines are by far the most widely used electric motors. It is estimated that around 80% of all energy consumed by electric motors worldwide is accounted for by asynchronous motors. They are used in almost all areas of industry, for example as drives in machine tools, fans, pumps or conveyor belts.
There are also other sub-categories for three-phase machines that we will not go into here. The following diagram summarizes the most important types.
Source: wdwd [CC BY-SA 4.0], via Wikimedia Commons
Areas of application
The areas of application for electric motors are extremely diverse. Hardly any modern achievement would be conceivable without electric motors. The range goes from the small fan that cools the computer or provides fresh air in the car, to the washing machine, to industrial engines or marine engines with many megawatts of power. We would like to present some examples from the industry and mobility sectors in the following chapters.
Use in industry
The areas of application for electric motors in industry are divided into twelve categories. They all have different requirements - which is why different types of motors are used.
- Drives for conveyor belts (robust and reliable - continuous operation).
- Drives for vehicles for material transport (high precision).
- Drives for cranes, construction hoists or other lifting devices.
- Positioning drives with which, for example, individual components can be transported to the destination.
- Drives for industrial robots.
- Synchronous drives, for example for rolling or printing.
- Drives for winding and unwinding, for example sheet steel or paper (special requirement here: the speed must be constantly adapted to the size of the roll).
- Cycle drives for cross cutters or flying saws (the material moves during the cut).
- Drives with non-uniform movement, such as those used in punching, for example.
- Drives for forming processes, for example presses.
- Tool drives, for example drilling, milling or grinding.
- Drives for fans or pumps.
The efficiency of electric motors in industry
In the past few decades, the idea of environmental protection has not only led to ever higher demands on the efficiency of electrical appliances in the private sector. The low-voltage three-phase asynchronous motors - usually used in industry - also have their own efficiency classes. What is a scale from G to A (best) for refrigerators is the scale from IE1 to IE4 (best) for low-voltage three-phase asynchronous motors in the power range from 0.75 kW to 375 kW. Motors with the standard efficiency of class IE1 may only be sold to a limited extent since 2011.
Even if electric cars and e-bikes have only been on everyone's lips for a few years - the history of electric mobility goes back to the 19th century. It quickly caught on in trains and trams. With other modes of transport such as cars and bicycles, it took more than 100 years before technology really made its breakthrough.
In the car
When the automobile was still in its infancy, the question of which type of drive would one day establish itself as the standard was by no means decided in favor of the internal combustion engine as quickly as one might think today. Maschinenfabrik A. Flocken built the first four-wheeled passenger car with electric drive in Germany in 1888. In 1900, 40% of all cars in the US were steam-powered, 38% were electric, and only 22% were gasoline. But the electric car soon became a niche product. The sensitive batteries and the much shorter range were major disadvantages of the electric car, which spoke in favor of the internal combustion engine. The electric motor was only used for add-on parts, for example for interior ventilation or the starter.
New developments in batteries and progress in environmental protection ensured that the topic came back on the agenda in the 1990s. But it was only the US company Tesla, which presented its roadster in 2006, that finally brought dynamism to the electric car market. Today almost all major manufacturers have pure electric cars or at least plug-in hybrids on offer.
Which electric motor is used in the car?
There are various concepts that can be used to drive an electric car, for example converter-controlled synchronous and asynchronous motors, but also DC motors. The BMW i3, for example, has a 170 hp hybrid synchronous motor in the rear. A 115 HP permanent magnet synchronous machine does its job in the VW e-Golf. The small car Renault Zoe has a separately excited three-phase synchronous motor with a maximum output of 92 hp.
Unlike in private transport, the advantages of electric motors were recognized early on in public transport. Because rails and overhead lines can be electrified, trains, subways and trams do not have the disadvantage of short range. Because electricity is always available for railways, capricious batteries are not required. The great potential of electric motors in trains became clear at the beginning of the 20th century. In 1903, experimental electric railcars from Siemens and AEG both cracked the 200 km / h top speed. The world speed record for rail vehicles is now more than 570 km / h, set by an experimental vehicle based on TGV.
Which engines drive the ICE?
Different drive concepts and motors are also used in the various series of the InterCity Express operated by Deutsche Bahn. For example, a 200-meter-long high-speed train of the type ICE 3 has 16 traction motors that are distributed across the entire train. A single one of these three-phase asynchronous motors has a maximum output of 500 kilowatts. The entire train has an output of 8,000 kilowatts, that is more than 10800 hp.
In the e-bike
The first bicycles with electric assistance already existed in the 19th century. Ebikes and pedelecs have only really come into fashion in the recent past. The permanent further development, especially in the area of battery technology, is making bicycles with electric motors an attractive means of transport for more and more people. The market share in 2016 was around 15% - and the trend is clearly increasing.
There are different drive concepts for bicycles with an electric motor. Strictly speaking, an e-bike is a bike in which the motor is active even without the driver's involvement. In contrast to this, the motor on the pedelec is only active when the driver is pedaling - the motor only provides assistance. In common parlance, however, “Ebike” is often used for both types. The motor is either in the wheels (hub motor) or is installed centrally on the wheel (middle motor). At least on the type of electric motor used, the industry is now largely in agreement. Almost only permanent magnet excited DC motors without sliding contacts are used.
Ships that only run with electric motors are still the absolute exception. The world's first all-electric ferry was presented in Norway in 2013. Anyone who thinks that almost all modern ships run exclusively with internal combustion engines is wrong. The integrated electric drives are also common. Here, electric motors are responsible for the actual propulsion, while diesel or gas-powered generators generate the necessary electricity. Prominent representatives of this category are the transatlantic liners Queen Elisabeth 2 and Queen Mary 2. The Queen Elisabeth 2 was converted to a diesel-electric drive in 1986, with two electric motors each delivering 44 megawatts of power. The Queen Mary 2 has four electric motors and has a total machine output of 86 megawatts. The electricity is generated by four diesel engines and two gas turbines with a total output of 126 megawatts.
Electric motor vs. internal combustion engine
There are many areas of life in which electric motors have largely or even completely prevailed over internal combustion engines. Is that time soon to come when it comes to mobility too? With Volvo, the first well-known car manufacturer has already announced that it intends to rely entirely on electric motors in the medium term. So it's high time to compare the advantages and disadvantages of electric motors and combustion engines.
Simpler structure: An internal combustion engine is much more complicated than a comparable electric motor. While a typical car engine today consists of around 1400 individual parts, a comparable electric motor can get by with just 1000 individual parts. The less complex an engine is, the less susceptible it is to failure and the more cost-effective it is to maintain.
Light weight: An electric motor is significantly lighter than an internal combustion engine. With the same output, a gasoline engine is about four times as heavy. However, this only applies to the motor itself; the advantage is negated by the heavy batteries (see below: energy density).
Performance development: Clear advantage for the electric motor. An electric motor can call up its full torque shortly after starting. An internal combustion engine usually needs a certain speed in order to achieve the maximum torque - in order to come as close as possible to this value during operation, the speed is adjusted via a gearbox. In most cases, a gearbox is not required for an electric motor.
Less noise: In addition to the advantage that no pollutants are emitted when operating an electric car (which, however, are already generated during the production of electricity), the electric motor has the advantage that it is significantly quieter.
Low energy density: Perhaps the biggest disadvantage of the electric motor is the low energy density of the batteries. The power storage systems are nowhere near the energy density of gasoline or diesel. Therefore, many batteries are required to enable a range that comes at least close to that of an internal combustion engine. The batteries are heavy - the advantage of the low weight is a thing of the past.
Weak infrastructure: When it comes to refueling, the electric car is currently still losing out. The infrastructure is being continuously expanded - but if all cars were only to fill up with electricity overnight, absolute chaos would be programmed. Because the charging process for an electric car - fast charger or not - takes significantly longer than refueling with 50 liters of petrol or diesel, significantly more charging stations would probably be required than there are petrol pumps today. Otherwise, the supply would not be guaranteed, at least at typical travel times (start of vacation).
Heating is about the range: Actually, it is an advantage that there is hardly any waste heat when operating an electric motor (see below: Energy balance): In winter, however, there are also downsides. Because the engine does not heat up the interior, electricity has to be used for heating. The use of heating therefore reduces the range of the electric car.
Efficiency and ecological balance: The energy balance of electric cars is significantly better than that of a car with a combustion engine - despite the complex production of the battery cells. Viewed over its entire life cycle, an e-car emits at least 30% less CO2. At first glance, the electric motor has a huge advantage: 90% of the energy used is converted into motion - in the case of a combustion engine it is only 30%, the rest is lost here through heat and friction. However, two things should not be forgotten: an electric motor is only as environmentally friendly as the source from which the electricity comes. If it comes from coal-fired power plants, the electric motor loses. The more clean or renewable energy sources are used, the better the electric motor will of course perform. A lot of energy is also required in the production of the batteries. And here, too, the type of energy source plays a decisive role in how the eco-balance of the electric car turns out.
Diagnosis and repair
After having dealt with many general aspects of the electric motor in the previous chapters, we now turn to a topic in which Günter Pauli GmbH has specialized for decades: fault diagnosis and repair of electric motors. Since we have already discussed this topic in detail in previous blog posts, we do not want to go into detail here, but only refer to our previous articles. If you need tips on troubleshooting electric motors, you can find them in our articles on the topics "The electric motor won't start" or "Electric motor troubleshooting: Checking the capacitor". We will go into the special features of a three-phase motor here: "Checking the three-phase motor".
Professional maintenance of electric motors is important so that a defect does not occur in the first place. We have also published a blog article on the subject of maintenance of electric motors: DC motor maintenance - step by step. However, regular maintenance is rarely recommended nowadays, because intact components are often replaced and the costs are unnecessarily high as a result. The experts at Günter Pauli GmbH recommend condition-based or ideally predictive maintenance. We explain the differences between the various maintenance strategies here.
Optimize your maintenance!
Would you like to improve maintenance in your industrial company and reduce costs? We would be happy to inform you! It is best to arrange a non-binding consultation appointment today.contact now
Tags: asynchronous three-phase motor, three-phase motor, electric motors
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