Motors Synchronous

Photometric Moisture Analysis TTis analyzer reqiiires a light source, a filter wheel rotated by a synchronous motor, a sample cell, a detector to measure the light transmitted, and associated electronics. Water has two absorption bands in the near infrared region at 1400 and 1900 nm. This analyzer can measure moisture in liquid or gaseous samples at levels from 5 ppm up to 100 percent, depending on other chemical species in the sample. Response time is less than 1 s, and samples can be run up to 300°C and 400 psig. 

Compressors up to around 75 kW (100 hp) usually have a single center-throw crank, as illustrated in Fig. 10-83. In larger sizes compressors are commonly of duplex construction with cranks on each end of the shaft (see Fig. 10-87). Some large synchronous motor-driven units are of four-corner construction i.e., they are of doubleduplex construction with two connecting rods from each of the two crank throws (see Fig. 10-88). Steam-driven compressors have one or more steam cylinders connected directly by piston rod or tie rods to the gas-cyhnder piston or crosshead. 

Field current is an important control element. It controls not only the power factor but also the pullout torque (the load at which the motor pulls out of synchronism). For example, field forcing can prevent pullout on anticipated high transient loads or voltage dips. Loads with known high transient torques are driven freqiiently with 80 percent power-factor synchronous motors. The needed additional field supplies both additional pullout torque and power-factor correc tion for the power system. When high pullout torque is required, the leading power-factor machine is often less expensive than a unity-power-factor motor with the same torque capabihty. 

Synchronous speeds are calculated by Eq. (29-10). Speeds above the limits given are obtained through step-up gears large high-speed centrifugal compressors are examples. Two-pole (3600 r/min at 60 Hz) synchronous motors can be built but are uneconomical in comparison with geared drives. 

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. 

Reduced-Voltage Starting Reduced-voltage starting is used to reduce system voltage dip. Voltage dips must be limited otherwise, they may drop other motors off the line, cause synchronous motors on the system to pull out of step, or cause objectionable lamp flicker. 

When a generator is designed for a leading p.f. (in the underexcitation mode) it can operate as both a synchronous motor and a synchronous condenser. The machine is now self-starting and does not require a prime mover. 

As a synchronous motor The machine is run primarily to drive a mechanical load and is operated at the synchronous speed and at unity p.f. The efficiency is now better than that of an induction motor. Except in assisting the system by consuming power at unity p.f., it does not help the system to improve its p.f. 

After replacing these large induction motors with as many oversized synchronous motors, while the active load at 9325 kW remains the same, the reactive load of induction motors at 785 kVAr will be eliminated and instead a leading reactive load of 5415 kVAr will be added. The net compensation therefore will be... 

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. 

G will generate an excess power compared to Gy. Therefore while G will operate as a generator, Gy. receiving power from G, will operate as a synchronous motor. Since G is overloaded compared to Cy. it will tend to retard, and Gy, receiving power from G, will tend to accelerate. The net effect would be that both generators will tend to synchronize on their own once again. 

If i is slower and falls behind vectorially, then G, will operate as a synchronous motor and receive reactive power /"Z, from the infinite bus (Figure 16.23) since = , + / Z,. 

G, slower and E, falling behind (Operating as a synchronous motor)... 

A reverse power relay (RPR) (Relay Code 32) This is meant for both active and reactive powers. If the incoming machine is slow, it will operate as a synchronous motor and draw power from the system rather than feed it, not a desirable situation. The relay will isolate the machine before it causes an overloading of the existing source. 

For determining the off potentials of cathodically protected pipelines, time relays are built into the cathodic protection station to intermpt the protection current synchronously with neighboring protection stations for 3 s every 30 s. The synchronous on and off switching of the protection stations is achieved with a synchronous motor activated by a cam-operated switch. The synchronization of the protection station is achieved as follows a time switch is built into the first protection station. An interruption of the protection current is detectable at the next protection station as a change in the pipe/soil potential. Since the switching time is known, the time switch of the second protection station can be activated synchronously. The switching of further protection stations can be synchronized in the same manner. 

Wright, J., A Practical Solution to Transient Torsional Vibration in Synchronous Motor Drive Systems, American Society of Mechanical Engineers, Pub. 75-DE-15. 

For many years it was common practice to give standard open motors a 115% service factor rating that is, the motor would operate at a safe temperature at 15% overload. This has changed for large motors, which are closely tailored to specific applications. Large motors, as used here, include synchronous motors and all induction motors with 16 poles or more (450 rpm at 60Hz). 

The practice of using 1.0 service factor induction motors would be consistent with that generally followed in selecting hp requirements of synchronous motors. 

Modern technology has reduced the size of motors, increased their expected life and improved their resistance to dirt and corrosion. Other important developments of the last 30 years are brushless excitation for synchronous motors and new two-speed, single-winding, induction motors. 

Synchronous and induction motors cannot always be compared on an equal speed basis. In geared applications such as high-speed centrifugal compressor drives (above 3,600 rpm), the most economical induction moior speed is usually 1,800 rpm. The most economical synchronous motor speed for the same application might be 900 or 1,200 rpm, depend-... 

A 5.000 hp break point has been used rather arbitrarily, as larger motors are built, ranging in size to 30,000 hp. Generally as the size increases, the synchronous motor becomes more competitive. However, final selection is not only dictated by the driver economics above, but includes the power system as well. 

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. 

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