Three-phase motors

Eig. 10. EuU-load efficiencies and wholesale prices (in 1988 U.S. dollars) of (-----) standard and (—) energy efficient (EEM) three-phase motors (13). 

With a view to achieve yet more standardization in motor design, NEMA Standard MG-1 has also recommended the maximum locked rotor current of single-speed three-phase motors for the various rotor designs A, B, C, and D, for various recommended torque values. These have been derived for a 415 V a.c. system and are shown in Table 2.2. 

Safety Standards (enclosures) for construction and guide for selection, installation and use of rotating machines Sound level prediction for insulated rotating electrical machines Energy management guide for selection and use of three phase motors Polyphase induction motors for power generating stations... 

Energy management guide for selection and use of three phase motors... 

On a three-phase motor where the load is pulsating the Power Ftictor may be checked by the following formula, obtained from independent watttiieter readings ... 

Siemens, High-Voltage Three-Phase Motors, Catalogue M-2 Supplement, 1975, Siemens West Germany. 

Frequency converters constructed for use with three-phase motors less than 4 kW can therefore be delivered with a single-phase supply from the electrical mains. For greater power requirements, it is most common to use a three-phase power supply. Through switches and potentiometers or the programming module of the frequency converter, the important parameters of electrical motors can be adjusted as needed. 

Circuits that carry AC current employing two, three, or more sinusoidal potentials are C2 ed polyphase circuits. Polyphase circuits provide for more efficient generation and transmission of power than single-phase circuits. Power in a three- (or more) phase circuit is constant rather than pulsating like the single-phase circuit. As a result, three-phase motors operate more efficiently than single-phase motors. 

Single-Phase Operation. If single-phase power only is available, it is advisable to consider the use of single-phase to three-phase converters and three-phase motors. This avoids the use of large single-phase capacitor start motors, which are relatively expensive and contain a starting switch that could be a source of trouble due to failure or to the presence of flammable gas in the vicinity of the well,... 

A model 6R Bico vertical grinder, equipped with a 220/240V three phase motor, was used in all studies. The grinding system was modified to incorporate a custom-designed hopper, hopper cover, sample-cup holder and cleaning jets, which were all co-ordinated by the Zymate II central controller. 

A process motor in most American plants will be a three-phase motor, and operate at 3600 rpm or, less commonly, 1800 rpm. In European plants or facilities in South America once owned by European corporations, motors run at 3000 or 1500 rpm. At home in the United States, our motors are the two-phase variety, and operate at 1800 rpm. 

There is an important difference between a two-phase and a three-phase motor. The three-phase motor can also be used as an electrical power generator. The two-phase motor, without modification, cannot. The difference is important, in that I once used this fact to avoid tripping off a critical process pump. 

Figure 24.1 is a sketch of a centrifugal pump, driven by a three-phase motor, with a turbine helper. This particular pump was charging a light gas oil stream to a high-pressure hydrocracker. The pump was operating quite close to its design conditions of... 

This extra shaft work will be converted to electrical power. That is, the three-phase motor will turn into an electrical power generator. It will begin to export electricity into the grid. A two-phase motor does not have this sort of capability. 

If fuses are used in the power distribution system, three-phase motor starters should have solid-state overload devices that include singlephase protection. 

The synchronous drive motor (45 MW) at 3000 rpm showed a trouble-free operational behaviour. The same applieds to the asynchronous three-phase motor (4.5 MW) for the start-up. 

The three-phase windings of an a.c. generator may be star connected or delta connected as shown in Fig. 2.54. The important relationship between phase and line currents and voltages is also shown. The square root of 3 (Vs) is simply a constant for three-phase circuits, and has a value of 1.732. The delta connection is used for electrical power transmission because only three conductors are required. Delta connection is also used to connect the windings of most three-phase motors because the phase windings are perfectly balanced and, therefore, do not require a neutral connection. 

The horsepower of three-phase motors ranges from Vi to 2500 or more. Starting current required is low to medium, about 5 to 7 times fuU-load current. 

Three-phase motors can be easily reversed electrically, making them useful for applications involving control of direction of rotation or remote positioning. Different combinations of speed-torque characteristics are also available so that motor performance can be specifically matched to an application. 

Power supply will be either single- or three-phase. A single-phase motor of the proper voltage can be used on a three-phase system if properly connected. But a three-phase motor cannot be used on single-phase supply. Three-phase motors generally cost less, perform better, and last longer than single-phase motors of the same size. 

---