Three phase machines

Power input to a three-phase machine may be measured by two single-phase wattmeters, connected as in the two-w attmeter method. (Section 11.4.3). Alternatively i single polyphase wattmeter may be used. 

Six models, available as two- and three-phase machines, with variable specifications Decanter sales... 

Rotating electrical machines, Starting performance of single-speed three-phase cage induction motors for voltages up to and including 660 V 8789/1996 BS EN 60034-12/1996 ... 

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... 

The insulated core transformer is another direct electron beam machine. It is similar to a three-phase power transformer in that a core and coils are used. One insulated core transformer will drive three accelerator tubes, which allows great flexibility and good power utilization. The Linac (linear accelerator) is an indirect electron beam ma-... 

There are three phases to the development of a problem solution on a digital computer. First, the problem must be stated and the mathematical methods for its solution described. Second, the logical structure of the problem must be analyzed from the viewpoint of the calculator. Third, the program of actual machine instructions, in its prescribed coded form, must be written and tested. 

Synchronous AC machines have a stator similar to the asynchronous machines, which has usually a three-phase stator winding. Whereas there are two types of rotor structures round cylindrical rotor and salient pole rotor as illustrated schematically in Fig. 5.6. The rotor field is generated by permanent magnets or a DC current winding, as reported in the same figure. In the first case they are called permanent-magnet synchronous AC machines. 

AC electric drives require more sophisticated converters when they are supplied with DC sources, because electric machines requires periodic voltage and current waves with a variable frequency depending on the load requirements. In Fig. 5.8, the scheme of an example of three-phase induction motor driven by a pulse-width-modulated inverter is reported. In this scheme a three-phase bridge connection with six power modules is shown to form the so-called inverter. Each power module can be composed by a number of power switches connected in parallel to carry higher currents. Across each power switch (IGBT) a parallel diode is connected to provide a return path for the phase current when the power module is switched off. 

The test bench for the power train characterization is realized by using a 120 kW dynamic electric brake composed by a three-phase asynchronous machine... 

Some rule-of-thumb methods are often stated in the purchasing specifications of the motor-machine unit, see for example Table 1.11, which applies to low voltage three-phase induction motors. 

The stator, also called the armature, carries the three-phase AC winding. The rotor, also called the field, carries the DC excitation or field winding. The field winding therefore rotates at the shaft speed and sets up the main magnetic flux in the machine. 

If a three-phase sub-system is unearthed and a line-to-casing fault occurs, then the two healthy lines will have their voltage-to-casing raised by a factor of /3. Normally the insulation of machines and cables can withstand this increase for a long period of time without harm. It is good practice to specify that the insulation systems of transformers, motors, generators and cables should be able to withstand an overvoltage of this type continuously. 

DC methods mostly use shunt or compound wound motors. Occasionally series wound motors are used when high torque at low speeds is required. These machines are fed with DC voltage derived from a three-phase AC source using a thyristor converter. The thyristor converter rectifies the AC into DC but with control over the magnitude of the average DC voltage. Thyristors are also called silicon controlled rectifiers . 

The derived reactances are those most frequently used to specify synchronous generators and motors. They are the synchronous, transient and sub-transient reactances in the d and g-axes. The most convenient method of deriving these is from the application of a three-phase short circuit at the terminals of the unloaded machine, whether it be a generator or a motor. For a motor the testing procedure is more complicated as described in sub-section 5 of Reference 23. The factory tests. The g-axis are usually taken as their design values because the necessary factory tests are more difficult to perform. The tests are described in for example IEEE standard 112 and BS4296. 

In a multi-machine network the generators and motors should be considered in relation to the source impedance to which they are connected. This impedance will also be dependent upon the location and type of disturbance e.g. near to a generator, remote from a generator, three-phase fault, line-to-ground fault, change in the state of the load such as starting a large motor direct-on-line. 

Application of a three-phase short circuit to the terminals of an unloaded induction motor is not a practical factory test, especially for a large high-voltage motor, because the motor can only be excited at its stator windings from the power supply. A three-phase short circuit at or near the stator terminals can occur in practice e.g. damaged supply cable, damage in the cable terminal box. The parameters of the stator and rotor windings can be obtained from other factory tests. However, the derived reactance can be defined in the same manner as those for the synchronous machine, but with... 

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