Synchronous rotor

The synchronous rotor carries the rotating independently excited field windings of the machine. For good control response, these must be fed with DC current via slip rings and these must be correctly sized for the low speeds and stall duties required by a hoist. The synchronous motor rotor will comprise of wound poles, pole bolting system, damper bars, slip rings/brush-less excitation and interconnections. 

In the late 1980s Farrel Inc. developed new rotor designs, too. They also began to use the same speed for each rotor. Traditionally, different speeds had been used for the two rotors. These machines were called synchronous rotor designs. They are described in the patents... 

Another concept is brushless excitation, in which an ac generator (exciter) is direc tfy coupled to or mounted on the motor shaft. The ac exciter has a stator field and an ac rotor armature which is directly connected to a static controllable rectifier on the motor rotor (or a shaft-mounted drum). Static control elements (to sense synchronizing speed, phase angle, etc.) are also rotor-mounted, as is the field discharge resistor. Changing the exciter field adjusts the motor field current without the necessity of brushes or slip rings. Brushless excitation is suitable for use in hazardous atmospheres, where conventional brush-type motors must have protective brush and slip-ring enclosures. 

In the preceding discussion of multispeed ac motors note that only induction motors are considered. These have no discrete physical rotor poles, so that only the stator-pole configuration need be modified to change speed. To operate multispeed, a synchronous motor would require a distinct rotor structure for each speed. Thus multispeed is practical only for squirrel-cage induction motors. 

Synchronous-motor rotor frequency can be detected because the rotor field circuit is available. Special control schemes have been devised which take into account both speed and induced rotor current in providing locked-rotor and accelerating protection. 

The magnetic field rotates at a synchronous speed, so it should also rotate the rotor. But this is not so in an induction motor. During start-up, the rate of cutting of llux is the maximum and so is the induced e.m.f. in the rotor circuit. It diminishes with motor speed due to the reduced relative speed between the rotor and the stator flux. At a synchronous speed, there is no linkage of flux and thus no induced e.m.f. in the rotor circuit, consequently the torque developed is zero. 

This is why an induction motor ceases to run at synchronous speed. The rotor, however, adjusts its speed, N such that the induced e.m.f. in the rotor conductors is Just enough to produce a torque that can counter-balance the mechanical load and the rotor losses, including frictional losses. The difference in the two speeds is known as slip. S, in r.p.m. and is expressed in terms of percentage of synchronous speed, i.e. 

The power transferred by the stator to the rotor, P, also known as air gap power at synchronous speed, can be expressed in kW by ... 

It is therefore necessary to take precautions during the test to avoid a excessive temperature rise and consequent damage to the windings. For wound rotor motors, speed-torque and speed-current tests may be taken between synchronous speed and the speed at which the maximum torque occurs. 

If the field excitation is also lost, the generator will run as an induction motor again driving the primer mover as above. As an induction motor, it will now operate at less than the synchronous speed and cause slip frequency current and slip losses in the rotor circuit, which may overheat the rotor and damage it, see also Section. 1.3 and equation (1.9). A reverse power relay under such a condition will disconnect the generator from the mains and protect the machine. 

If a large induction motor is switched on such a system it is possible that its rotor may lock up at the sub-synchronous speed and keep running at higher slips. This situation is also undesirable, as it would cause higher slip losses in addition to higher stator current and overvoltage across the series capacitors. 

Reactive control is also possible through synchronous condensers. As they rotate, the rotor stores kinetic energy which tends to absorb sudden Huctuations in the supply system, such as sudden loadings. They are. however, sluggish in operation and very expensive compared to thyristor controls. Their rotating masses add inertia, contribute to the transient oscillations and add to the fault level of the system. All these factors render them less suitable for such applications. Their application is therefore gradually disappearing. 

Critical speeds correspond to the natural frequencies of the gears and the rotor bearings support system. A determination of the critical speed is made by knowing the natural frequency of the system and the forcing function. Typical forcing functions are caused by rotor unbalance, oil filters, misalignment, and a synchronous whirl. 

In screw compressors of the dry type, the rotors are synchronized by timing gears. Because the male rotor, with a conventional profile, absorbs about 90% of the power transmitted to the compressor, only 10% of the... 

Today s standard motor enclosure for indoor applications is the open, drip-proof enclosure for induction and high-speed synchronous motors. For large motors, open, drip-proof construction is available up to about 20,000 hp and is used for squirrel-cage, synchronous, and wound-rotor motors. 

Semiconductor control modules gate the thyristors, which switch cm rent to the motor field at the optimum motor speed and precise phase angle. This assures synchronizing with minimum system disturbance. On pull-out, the discharge resistor is reapplied and excitation is removed k> provide protection to the rotor winding, shaft, and external electrical system. The control resynchronizes the motor after the cause of pull-out i.n removed, if sufficient torque is available. The field is automatically applied if the motor synchronizes on reluctance torque. The control is calibrated at the factory and no field adjustment is required. The opti-... 

The determination of the first bending critical speed is well established however, there is also concern with regard to the rotor support system s sensitivity to exciting forces. These come from unbalance and/or gas dynamic forces arising during operation in service. Operation with dirty corrosive gas will soon cause rotor unbalance. The rotor dynamics verification test is concerned with synchronous excitaticm, namely unbalance. The test must also verify that the separation margins are to specification. 

Another potential problem is due to rotor instability caused by gas dynamic forces. The frequency of this occurrence is non-synchronous. This has been described as aerodynamic forces set up within an impeller when the rotational axis is not coincident with the geometric axis. The verification of a compressor train requires a test at full pressure and speed. Aerodynamic cross-coupling, the interaction of the rotor mechanically with the gas flow in the compressor, can be predicted. A caution flag should be raised at this point because the full-pressure full-speed tests as normally conducted are not Class IASME performance tests. This means the staging probably is mismatched and can lead to other problems [22], It might also be appropriate to caution the reader this test is expensive. 

Molecular weight, effect on centrifugal sizing, 159 Mollier charts, 27 Monitoring system, 356 Motor, 146 enclosure, 260 equations, 267 insulation, 257 locked rotor torque, 270 selection, 270 service factor, 262 starting characteristics, 270 starting time, 273, 274 synchronous vs induction, 265 variable frequency drives, 27/, 280 voltage, 258 Motors... 

Examination of the drive ratios shows that if both motors have ISOOrpm synchronous speeds that the pony motor is made to operate near 3600rpm when the big motor is operating. This is typically not a problem as to rotor balance or bearing duty because manufacturers make 3600 rpm versions of these same motors. Nevertheless, the duty should be checked and if this is a problem, the small motor can be changed to a 900 rpm model and the synchronous speeds of each motor will not be exceeded. 

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. 

The rotating magnetic field in the stator travels around the stator at what is called synchronous speed. By grouping stator coils together in what is called poles, the motor rotor can be designed to turn at a certain speed (revolutions per minute/rpm). On an induction motor the number of poles cannot be seen or counted without the drawings. 

Torque is the turning effort developed by the motor or the resistance to turning exerted by the load. Usually torque is expressed in ft-lb however, the usual expression is as a percentage of the full load torque. Synchronous motors usually offer several types of torque. Starting or breakaway (called locked rotor) torque is developed at the instant of starting, see Figure 14-12. 

Type of motor (cage, wound-rotor, synchronous, ordc). 

Polyphase Motors. Alternating-current polyphase motors are of the squirrel-cage, wound-rotor, or synchronous types. 

Screw compressors have two rotors with interlocking lobes and act as positive-displacement compressors (see Figure 44.11). This type of compressor is designed for baseload, or steady state, operation and is subject to extreme instability should either the inlet or discharge conditions change. Two helical gears mounted on the outboard ends of the male and female shafts synchronize the two rotor lobes. 

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