Constant synchronous speed

The main turbine of a power station is an important special case, since it will be running at essentially constant, synchronous speed as a result of its directly coupled alternator being connected to an electricity supply grid. Hence the blade speed will be constant at cl ... 

A constant field rotating at synchronous speed Ns Figure 1.4 Production ol magnetic field in a 3-0 winding... 

The motor/generator is modeled as an energy source/sink limited by its specific design capabilities. As a generator, it will absorb all excess power available while holding speed constant (synchronous) as long as it is electrically connected to the utility grid. 

The synchronous motor is a constant-speed machine. Unlike the induction motor which inherendy has slip from losses, the synchronous motor uses an excitation system to continually keep the rotor in synchronous speed with current flowing through the stator. Within its designed torque characteristics, it will operate at synchronous speed regardless of load variations. 

It should be noted that H is a function of the synchronous speed of the machine. If the speed should vary over a wide range then the variation of H with speed should be included in the mathematical simulation. For small excursions in speed about the synchronous speed, the error in using a constant value of H is negligible. This point is discussed in Reference 11. 

The rotor speed cannot reach the same speed as that of the stator field, otherwise there would be no induced emfs and currents in the rotor, and no torque would be developed. Consequently when the rotor speed is near to the synchronous speed the torque begins to decrease rapidly until it matches that of the load and rotational friction and windage losses. When this balance is achieved the speed will remain constant. 

In the steady state the transformation of the three-phase currents and voltages into their d and q axis equivalents, when the rotor is rotating at the synchronous speed, causes them to become constant values. The magnitude of these constant values is equal to the peak value of their corresponding rms values in the phase windings. This is because the transformations have been made with a synchronous reference frame. 

Wound-rotor motors may be used as either constant-speed or adjustable-speed motors. With full load, the speed may be reduced by as much as 50 percent of synchronous speed for certain fixed loads such as fans or compressors. These motors are frequently used when high locked-rotor and accelerating torque with low starting current are required. They are also used where heavy or delicate loads must be accelerated gradually and smoothly, as in hoists and elevators. A variety of solid-state control systems are available for use in the rotor circuit of wound-rotor motors. 

Synchronous Alternating-Current Motors These motors run in exact clock synchronism with the power system. For most modern power systems, these are truly constant-speed motors. 

With the help of bridle no. I driven by motors M and M4, the uncoiler section speed is controlled by monitoring the tension of the travelling sheet and hence maintaining constant speed of the sheet in the uncoiler section. The tensile difference of T tind Ti determines the speed of the uncoiler. Speed and tension of the sheet must remain constant for absolute synchronization between the uncoiler process and the recoiler sections. 

It is, however, recommended for better control and machine utilization that when the load s demand is for constant-speed operation, this must be met through separate synchronous motors at unity p.f. and the p.f. must be improved separately through synchronous condensers with variable field excitation. 

Steam turbine performance is modeled using a standard steam flow versus horsepower map and valve position versus steam flow. The turbine inlet valve(s) is positioned by the governor system to maintain constant speed (or another parameter when synchronized). 

The driver is a prime mover capable of developing the required torque at a constant speed or over a range of speeds. The driver s energy source can be either electrical or mechanical. Electrical energy is used by motors, either of the induction or synchronous type, while the mechanical covers a multitude of sources. It may be a fuel, as in internal or external combustion engines, or it may be a gas, such as steam or process gas used in a turbine or expander. 

A rule of thumb that was used in the past for constant speed applications wii.s to consider the selection of a synchronous motor where the application horsepower was larger than the speed. This, of course, was only an approximation and tended to favor the selection of a synchronous motor and would be considered too severe by current standards. However, the rule can aid in the selection of the motor type by giving some insight as to when the synchronous might be chosen. For example, applications ol several hp per rpm often offer a distinct advantage of the synchronous over the induction motor. In fact, at the lowest speeds, larger sizes and highest hp/rpm ratios may be the only choice. 

A major hurdle to greater efficiency is the constant-speed nature of induction and synchronous motors. Nevertheless, considerable advances have been made in improving motor speed controls that essentially better optimize the motor speed to the task at hand, resulting in substantial energy savings, decreased wear of the mechanical components, and usually increased productivity from the user. 

For squirrel-cage motors, slip ranges to 5% of the synchronous or constant speed. For variahle-frequency motor drives, see Reference 89. 

This is a very good motor for direct connection to certain loads, particularly where constant speed is required. NEMA defines it as a synchronous machine which transforms electrical power from an alternating-current system into mechanical power. It usually has direct-current field excitation by a separately driven direct-current generator or one directly connected to the motor. This motor remains synchronous with the supply frequency and is not affected by the load. Proper application requires consideration of the following ... 

Pull-in torque For a synchronous motor, this is the maximum constant torque under which the motor will pull its connected inertia load into synchronism, at rated voltage and frequency, when its field excitation is applied. The speed to which a synchronous motor will bring its load depends on the power required to drive it, and whether the motor can pull the load into step from this speed depends on the inertia of the revolving parts. So, the pull-in torque cannot be determined without having the Wk as well as the torque of the load. 

Synchronous motors are made in speeds from 1800 (two-pole) to 150 rpm (48-pole). They operate at constant speed without slip, an important characteristic in some applications. Their efficiencies are 1-2.5% higher than that of induction motors, the higher value at the lower speeds. They are the obvious choice to drive large low speed reciprocating compressors requiring speeds below 600 rpm. They are not suitable when severe fluctuations in torque are encountered. Direct current excitation must be provided, and the costs of control equipment are higher than for the induction types. Consequently, synchronous motors are not used under 50 HP or so. 

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