The future of VSE motors, Amin Almasi
The cost of the rotating machinery trains could amount to between 30 and 60 percent of the total cost of many plants.
It is the general trend of major operation companies to replace the gas turbine and steam turbine drivers (and other drivers) with the variable-speed electric motors due to better reliability and efficiency.
The variable-speed electric motors, using variable-speeddrives (VSD), are becoming more popular. To avoid any problem with the variablespeed electric motors, the VSD should provide an output with the following qualities: Voltage and current with good phase balance and minimal DC component. Limited harmonic content. These two factors can minimize circulating current losses and minimize torque pulsations. The characteristics and advantages of the electric VSDs should be considered as an important element of control that provide flexible and safe operation, instead of the more common concept found in literature that see the VSD only as an energy saving device.
Induction electric motors
An induction electric motor is a type of AC machine where power is supplied to the rotor by means of electromagnetic induction. These electric motors are widely used in industrial drives, because they are rugged, simple and reliable.
Their speed is determined by the frequency of the supply current to electric motors. The most common type is the squirrel cage electric motor, this term is sometimes used for induction motors generally.
The stator of an induction motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimise the distribution of the magnetic field, the windings are distributed in the slots around the stator, with the magnetic field having the same number of north and south poles.
The main components of a conventional induction motor rotor are usually the shaft, the core (could be constructed of laminated steel punching), the conductor bars, and the conductive end-rings.
While this type of construction is relatively simple and robust, for a highspeed and high-power density application, the centrifugal and thermal stresses require rigorous reliability studies of each of motor components. A fabricated rotor cage is usually the only option in high-speed, high-power applications given the strength levels required from the end-ring and the bar materials. A synchronous electric motor is an AC motor in which the shaft rotating speed is synchronized with the frequency of the AC current. The synchronous electric motors contain electromagnets on the stator of the motor that create a magnetic field, which rotates in time with the oscillations of the supply current.
The rotor turns in step with this field, at the same rate.
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In other words, the electric motor does not rely on ‘slip' under usual operating conditions, and as a result produces torque at the synchronous speed.
Synchronous motors can be contrasted with the induction motors, which should slip in order to produce torque. The speed of the synchronous motor is determined by the number of magnetic poles, as well as the frequency of the power supply to the motor.
The motors are available in highhorsepower direct-current excited industrial sizes. In large sizes, the synchronous motor provides two important functions; first, it offers a relatively high efficiency; second, it can operate at leading or unity power factor and thereby provide the power-factor correction.
Previously, there were only two major types of synchronous motors; ‘nonexcited' and ‘direct-current excited', which had no self-starting capability to reach synchronism. Advances in independent brushless excitation control of the rotor winding set have offered a new type of synchronous motor.
The ‘brushless wound-rotor electric motor' is the most suitable type of synchronous motor with all the theoretical qualities of the synchronous motor, and the wound-rotor motor combined.
Features include the power factor correction, the highest power density, the highest potential torque density, relatively low cost electronic controller, the highest efficiency, and others. Brushless synchronous type electric motors are commonly used for the high power level applications.
Output ratings of two-pole synchronous motors are theoretically unlimited in terms of power, since generators of this type have been built for above 100MW with many successful references. Many of these generators are above 200MW.
However, the shaft speed operating ranges of such electrical machines are limited for reasons, such as: The centrifugal forces acting on different rotating components and their supports. In other words, the mechanical strength of the rotor or the supporting component materials, in
• general, can become a limiting factor. The length of the rotor (the bearing span) is limited to cope with the lateral vibrations and rotordynamics issues. The brushless exciter, which should supply full field power from standstill to rated speed, is usually solidly flanged to the non-drive end of the electric motor and sometimes has its own bearing.
This rotor assembly is known as the “three-bearing rotor”, often used for highspeed electric motors. This can result in a well-defined (in terms of lateral vibration behaviour) rotor system with three bearings and a controlled rotordynamics response.
Three bearings electric motors offer relatively high stiffness and excellent dynamic behaviour.
For relatively low speeds, sleeve bearings are usually employed. Sometimes, there is a requirement for better stability and additional damping of tilting-pad bearings for relatively highspeeds.
Usually, advanced tilting-pad bearings should be used; particularly for speeds above 2000 rpm, tilting-pad bearings are always preferred.