45 Key Concepts and Principles in Electrical Engineering: From Transformers to Electric Motors

introduction

Welcome to this comprehensive exploration of key concepts and principles in the field of electrical engineering. This article is designed to provide a deep dive into the fascinating world of transformers and motors, two critical components that power our modern world.

Whether you're an electrical engineering student seeking to solidify your understanding, a seasoned professional looking to refresh your knowledge, or simply an enthusiast with a keen interest in learning how these devices work, this article is for you.

We will journey through the intricacies of single-phase transformers, DC motors, asynchronous motors, synchronous generators, and more. Each section is carefully crafted to explain complex principles in an accessible manner, making the world of electrical engineering more approachable.

Moreover, this article also serves as a guide for those seeking to source high-quality electric motors from professional manufacturers in China, a country renowned for its advanced manufacturing capabilities in the electrical engineering sector.

So, whether you're here to learn, to review, or to find a reliable manufacturer, we hope this article will serve as a valuable resource. Let's embark on this enlightening journey together.

45 Key Concepts and Principles in Electrical Engineering

1. The current of a single-phase transformer under no-load conditions is not in phase with the main magnetic flux, and there is a phase angle difference aFe due to the presence of iron loss current. The no-load current is a peak waveform because it contains a large third harmonic.
2. The armature winding of a DC motor also carries an alternating current. However, the excitation winding carries a direct current. The excitation modes of a DC motor include separate excitation, parallel excitation, series excitation, and compound excitation.
3. The back electromotive force expression of a DC motor is E =CE F n; the electromagnetic torque expression is Tem =CTFI.
4. The number of parallel branches in a DC motor is always in pairs. However, the number of parallel branches in an AC winding is not necessarily the same.
5. In a DC motor, the elements of a single-layer winding are connected in series in a way that one is stacked on top of the other. Whether it is a single-wave winding or a single-layer winding, the commutator connects all the elements in series to form a single closed loop.
6. An asynchronous motor is also known as an induction motor because the rotor current of the asynchronous motor is generated through electromagnetic induction.

1. When an asynchronous motor starts with reduced voltage, the starting torque decreases, and the starting torque and the square of the starting current of the winding decrease proportionally.
2. When the amplitude and frequency of the primary side voltage are constant, the saturation degree of the transformer core is basically unchanged, and the excitation reactance is also basically unchanged.
3. The short-circuit characteristic of a synchronous generator is a straight line. When a three-phase symmetrical short circuit occurs, the magnetic circuit is unsaturated; during a three-phase symmetrical steady-state short circuit, the short-circuit circuit is a pure demagnetizing direct-axis component.
4. The current in the excitation winding of a synchronous motor is a direct current. The excitation methods mainly include excitation generator excitation, static rectifier excitation, rotating rectifier excitation, etc.
5. There are no even harmonics in the three-phase synthetic magnetomotive force; when a symmetrical three-phase winding passes a symmetrical three-phase current, there are no magnetic harmonics that are multiples of 3 in its synthetic magnetomotive force.

1. Three-phase transformers generally hope that one side is connected in a delta shape or that one side is grounded in the middle. Because the winding connection of the three-phase transformer hopes to have a path for the third harmonic current.
2. When a symmetrical three-phase winding passes a symmetrical three-phase current, the fifth harmonic in its synthetic magnetomotive force is reversed; the seventh harmonic is forward.
3. The mechanical characteristics of the series-excited DC motor are relatively soft. The mechanical characteristics of the separately excited DC motor are relatively hard.
4. Transformer short-circuit tests can measure the leakage impedance of the transformer winding; while no-load tests can measure the excitation impedance parameters of the winding.
5. The transformation ratio of the transformer is equal to the turns ratio of the primary winding to the secondary winding. The transformation ratio of a single-phase transformer can also be expressed as the ratio of the rated voltages of the primary and secondary sides.
6. When normally excited, the power factor of the synchronous generator is equal to 1; keeping the output active power unchanged, when the excitation current is less than normal excitation (under-excitation), the nature of the direct-axis armature reaction is magnetizing; keeping the output active power unchanged, when the excitation current is greater than normal excitation (over-excitation), the nature of the direct-axis armature reaction is demagnetizing.

1. In a DC motor, iron loss mainly exists in the rotor iron core (armature iron core) because the stator iron core magnetic field is basically unchanged.
2. In a DC motor, the first pitch y1 is equal to the number of slots between the first and second sides of the element. The composite pitch y is equal to the number of slots between the upper element edges of the two elements connected in series.
3. In a DC motor, when saturation is not considered, the characteristic of the cross-axis armature reaction is to offset the position of the zero magnetic field, but the magnetic flux per pole does not change. When the brush is on the geometric neutral line, the armature reaction is cross-magnetic.
4. In a DC motor, the component that converts external DC power into internal AC power is the commutator. The function of the commutator is to convert DC to AC (or vice versa).
5. In a synchronous motor, when the magnetic flux F0 of the stator winding interlinking excitation is at its maximum value, the back electromotive force E0 reaches its minimum value, when F0 reaches zero, E0 reaches its maximum value, the phase relationship between F0 and E0 is F0 leads E0 by 90o. And the relationship between E0 and F0 is expressed as: E0 = 4.44 f N kN1F0.
6. In a motor, the leakage magnetic flux refers to the magnetic flux that only interlinks with the winding itself, and the back electromotive force it generates can often be equivalent to a leakage reactance drop (or negative reactance drop).

1. The rotor of an asynchronous motor has two types: squirrel-cage and wound rotor.
2. The slip rate s of an asynchronous motor is defined as: the ratio of the difference between the synchronous speed and the rotor speed to the synchronous speed. When the asynchronous motor works in the motor state, the range of its slip rate s is 1>s>0.
3. The Tem-s curve of the relationship between the electromagnetic torque Tem and the slip rate s of an asynchronous motor has three key points, which are the starting point (s = 1), the maximum electromagnetic torque point (s=sm), and the synchronous point (s=0). When the rotor resistance of the asynchronous motor changes, the characteristics of its maximum electromagnetic torque Tem and the slip rate sm are: the size remains unchanged, and the position of s changes.
4. The asynchronous motor must absorb reactive power of lagging nature from the grid for excitation.
5. When an AC current is passed through a coil group, its magnetomotive force has a pulsating nature as time changes. A single coil passes an AC current, and its magnetomotive force also has a pulsating nature as time changes.
6. When a synchronous generator is connected to the grid, it requires that its three-phase terminal voltage has the same: frequency, amplitude, waveform, phase sequence (and phase) as the grid three-phase voltage.

1. The rotor of a synchronous motor has two types: salient pole and cylindrical.
2. The equivalent phase number of the squirrel cage rotor is equal to its slot number, and the equivalent turns per phase is 1/2.
3. For a symmetrical three-phase AC winding, when a symmetrical three-phase AC current is passed, its fundamental wave synthetic magnetomotive force is a circular rotating magnetomotive force, and its rotation direction is from the leading phase winding axis to the lagging phase axis, and then to the next lagging phase axis.
4. There are two connection methods for the three-phase windings of a three-phase transformer: star and delta; the magnetic circuit has two structures: core type and shell type.
5. The six odd connection group numbers of the three-phase transformer are 1, 3, 5, 7, 9, 11. And the six even connection group numbers are 0, 2, 4, 6, 8, 10.
6. In an AC winding, the number of slots per pole per phase q =q = Z/2p/m (assuming the number of slots is Z, the number of pole pairs is p, and the number of phases is m). In AC windings, both 120o phase bands and 60o phase bands are used. The basic wave winding coefficient and back electromotive force of the 60o phase band are higher.

1. The symmetrical component method can be used to analyze the asymmetrical operation of transformers and synchronous motors. The premise of its application is that the system is linear, so the superposition principle can be applied to decompose the asymmetrical three-phase electrical system into positive sequence, negative sequence, zero sequence and other three groups of symmetrical three-phase systems.
2. The calculation formula of the short pitch coefficient is ky1 = sin(p/2×y1/t), and its physical meaning is the discount (or reduction coefficient) given by the short pitch to the back electromotive force (or magnetomotive force) compared with the whole pitch. The calculation formula of the distribution coefficient is kq1 = sin(qa1 /2 ) / q / sin(a1 / 2), and its physical meaning is the reduction coefficient (or discount) of the back electromotive force (or magnetomotive force) relative to the concentrated situation when q coils are successively different by a1 electrical angle.
3. The current transformer is used to measure current, and its secondary side cannot be open. The voltage transformer is used to measure voltage, and its secondary side cannot be short-circuited.
4. The motor is a device that converts mechanical energy into electrical energy (or vice versa), or changes one AC voltage level to another AC voltage level. From the perspective of energy conversion, motors can be divided into three categories: transformers, motors, and generators.
5. The calculation formula of the slot pitch electrical angle a1 is a1 = p×360o/Z. It can be seen that the slot pitch electrical angle a1 is p times the slot pitch mechanical angle am.
6. The principle of transformer winding conversion is: before and after the conversion, ensure that the magnetomotive force of the winding is unchanged, and ensure that the active and reactive power of the winding is unchanged.

1. The characteristic of the efficiency of the transformer is that there is a maximum value, which is reached when the variable loss is equal to the constant loss.
2. The no-load test of the transformer is usually performed by applying voltage and measurement on the low-voltage side. The short-circuit test of the transformer is usually performed by applying voltage and measurement on the high-voltage side.
3. When the transformer is operated in parallel, the condition for no circulating current at no load is: the same transformation ratio and the same connection group number.
4. When the transformer is operated in parallel, the principle of load distribution is: the per-unit value of the transformer load current is inversely proportional to the per-unit value of the short-circuit impedance. The condition for the capacity of the transformer to be fully utilized during parallel operation is: the per-unit values of the short-circuit impedance should be equal, and their impedance angles should also be equal.

Conclusion

Understanding the principles and concepts of electrical engineering, particularly those related to transformers and motors, is crucial for anyone involved in the field. This knowledge not only aids in the comprehension of how these devices function but also assists in troubleshooting and optimizing their performance.

If you're looking for professional electric motor manufacturers in China, it's important to consider companies with a strong reputation for quality and innovation. China is home to numerous manufacturers that are renowned for their high-quality products and cutting-edge technology in the field of electrical engineering. Always ensure to conduct thorough research and consider factors such as the manufacturer's experience, certifications, product range, and customer reviews before making a decision.

Remember, the right manufacturer will not only provide you with the best products but also offer comprehensive after-sales service and technical support. As a top professional electric motor manfuacturer - Dongchun motor will be a good choice for you. This will ensure that you get the most out of your investment and that your electrical systems operate efficiently and effectively.