My friend H, who regularly undertakes field projects, sent me a WeChat message: "Hey buddy, my diesel generator used to power several welding machines without a problem, but now it can't even power one! What's wrong with it? I'm so worried!" Ms. Can replied, "Check the excitation circuit first; maybe that screw has come loose." The next morning, right after I finished washing up, another WeChat message from H arrived . It turned out that Ms. Can's prediction was correct: the screw securing the main excitation circuit terminal had fallen off. After replacing it with a new screw, everything was fine !
If the excitation is abnormal, the motor will break down at any moment.
The resultant magnetic flux is established by the resultant magnetomotive force of all windings of the motor. In a traditional DC motor, the effective magnetomotive force is mostly generated by the excitation winding; in a transformer, the net excitation is provided by either the primary or secondary winding, or each provides a portion, and the situation is similar for AC motors.
Providing excitation for an AC motor has a significant impact on the motor's operational economy and reliability. The perplexing issue raised by friend H—excitation failure—clearly illustrates that a properly functioning excitation system is a necessary prerequisite for ensuring the normal operation of the motor.
Power factor of AC motor
Excitation is closely related to the reactive power of an AC motor, and providing reactive power incurs a certain cost. Therefore, the power factor of an AC motor during operation is an important indicator for measuring its economic efficiency. The adverse effects of a low power factor on system operation are mainly threefold:
● Generators, transformers, and transmission equipment are rated in kVA (capacity) rather than kW (active power) because their losses and heat generation are primarily determined by the product of voltage and current, and are independent of the power factor. The physical size and cost of AC equipment are roughly proportional to its rated capacity. Therefore, the investment required for generators, transformers, and transmission equipment to provide a certain amount of useful active power is roughly inversely proportional to the power factor.
● A low power factor means that there will be greater current and I²R losses in power generation and transmission equipment.
● A low power factor will reduce voltage regulation.
Reactive power and magnetic flux
The relationship between reactive power and the magnetic flux makes it easy to see the factors affecting the reactive power required by an electric motor. Like any other electromagnetic device, the resultant magnetic flux required for an electric motor to operate must be established by the magnetizing component of the current. Whether the magnetizing current flows in the stator winding or the rotor winding, the magnetic circuit and the basic energy conversion process are identical.
● In a transformer, it makes no fundamental difference which winding carries the excitation current to establish magnetic flux. In some cases, each winding provides partial excitation. If all or part of the magnetizing current is provided by an AC winding, then the input of that winding must include lagging reactive power, because the magnetizing current lags the voltage by 90°. Effectively, this lagging reactive power establishes magnetic flux in the motor.
● In an induction motor, the only possible source of excitation is the stator input. Therefore, induction motors must operate with a lagging power factor, which is very low under no-load conditions and increases to 85% to 90% or higher under full load. This improvement in power factor is due to the increased active power required as the load increases.
● For a synchronous motor, there may be two excitation sources: alternating current in the armature winding or direct current in the field winding. If the current in the field winding is just sufficient to provide the required magnetomotive force, then the armature winding does not require a magnetizing current component or reactive power, and the motor operates at a unity power factor of 1. If the excitation current decreases, resulting in underexcitation of the motor, the insufficient magnetomotive force must be compensated by the armature winding, and the motor will operate at a lagging power factor. If the excitation current increases, resulting in overexcitation of the motor, the excess magnetomotive force must be balanced by the armature winding, and a leading component will appear in the armature current, thus the motor will operate at a leading power factor.
Transformers, induction motors and synchronous motors
Because magnetizing current must be supplied to inductive loads such as transformers and induction motors, and overexcited synchronous motors possess the ability to provide lagging current, this is a significant advantage and economically advantageous. In effect, overexcited synchronous motors act as generators producing lagging reactive power, thus eliminating the need for the power source required to supply this reactive component. Therefore, they can serve the same function as compensating capacitors. Sometimes, unloaded synchronous motors are also connected to the power system solely for adjusting the power factor or controlling reactive power. Such synchronous motors are typically called synchronous compensators, and in larger sizes, they are more economical than static capacitors.
Steam turbine generator excitation system
As the achievable capacity of steam turbine generators continues to increase, providing them with DC excitation current (reaching 1000A or higher in larger units) becomes increasingly difficult. A common excitation power source is a DC generator driven coaxially with the main generator, whose output excites the excitation windings of the AC generator via brushes and slip rings. Alternatively, a conventional coaxially driven AC generator can be used as the main exciter to excite the main generator. This exciter has a stationary armature and rotating excitation windings, with a frequency of 180Hz or 240Hz. Its output is fed to a stationary solid-state rectifier, whose output excites the steam turbine generator via brushes and slip rings.
Rotary rectifier and brushless excitation
Slip rings, commutators, and brushes inevitably involve cooling and maintenance issues. Many modern excitation systems avoid these problems by minimizing the use of sliding contacts and brushes. For example, some excitation systems use coaxially driven AC generators, but the exciter's field winding is stationary while its AC armature winding rotates with the shaft. Using a rotating rectifier, DC excitation can be applied directly to the main generator's field winding without passing through slip rings.
