Cage rotor induction motor

Most chemical-plant-size compressors are electrically driven [43]. Moore [25] discusses the characteristics of squirrel-cage induction and synchronous electrical motors. Wound rotor induction motors have not been used for compressor drives. For 370 to 4500 kW (500 to 6,000 hp), the induction motors are the first choice. The squirrel-cage induction motor is the most coimnonly used driver in the process industries from 1/8 to 1,5000 hp (0.0932 to 1,120 kW [25]. From 15,000 hp (149 to 11,200 kw) the synchronous motor could be used [25]. If the compressor is operated at 7,500, 11,000, and 23,000 rad/s (1,200, 1,800 and 3,600 rpm), no step-up gears are required. The least costly speed for an induction motor is 1,000 rad/s (1800 rpm) so that this speed is usually selected. Step-up gears are used to obtain higher speeds. 

AC methods include standard squirrel-cage induction motors, wound rotor induction motors, synchronous motors and commutator motors. Speed variation is obtained by the control of applied voltage to the stator or the control of current and voltage in the rotor by external circuit connections. 

Various standard types AC motors are used e.g. squirrel-cage and wound rotor induction motors, variable speed commutator AC motors. 

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. 

This is a vital relationship, which reveals that during start-up and until such speed, the reactance of the motor windings / 2> the rotor current will also remain almost the same as the starting current and will fall only at near the rated speed. (Refer to the current curves in Figures 1.5(a) and (b)). The initial inrush current in a squirrel cage induction motor is very high. In a slip-ring motor, however, it can be controlled to a desired level. (Refer to Section 5.2.1.)... 

When the motor first starts there is a sudden surge of current into the stator winding (inrush current). This is usually 6 to 6.5 times the amps the motor sees when it is running. This causes similar surge in the squirrel cage rotor hars, so great care must he taken to size all the hars, WMC uses copper hars, correctly. This magnetic circuit in the rotor induces a current thus, the name induction motors. 

Induction Motors. An induction motor is an alternating-current motor in which a primary winding on one member (usually the stator) is connected to the power source and a polyphase secondary winding or a squirrel-cage secondary winding on the other member (usually the rotor) carries induced current. There are two types ... 

Repulsion-Start Induction Motor. A repulsion-start induction motor is a single-pliasc motor having the same windings as a repulsion motor, but at a predetermined speed the rotor winding is short circuited or otherwise connected to give the equivalent of a squirrel-cage winding. This type of motor starts as a repulsion motor but operates as an induction motor w ith constant-speed characteristics. 

Repulsion-Induction Motor. A repulsion-induction motor is a form of repulsion motor that has a squirrel-cage winding in the rotor in addition to the repulsion motor w inding. A motor of this type may have either a constant-speed (see MG 1-1.30) or varying-speed (.see MG 1-1.31) characteristic. 

Asynchronous motors (squirrel cage induction motors, the rotor-stator part of slipring induction motors) and the rotor-stator part of synchronous motors are within the scope of increased safety due to the omission of sparks and arcs assuming an appropriate construction of rotor and windings. Tire standards IEC 60079-7 and EN 50019 contain mechanical and thermal requirements that follow this aim. 

Figure 6.96 Double-rotor cage induction motor, flameproof, shaft and stator housing water cooled.

Rotating electrical machines shall be tested at standstill and running with 0.9X 1.0X their rated speed. The rotor causes a certain turbulence of the test gas, resulting mainly in a considerably increased dp/dt value [2] and enhanced peak pressure. Cage induction motors may be fed from outside with an adjustable voltage and frequency via a frequency convertor, motors with sliprings or commutators need an auxiliary drive, e.g. a small cage induction motor powered by said frequency convertor. 

For a squirrel cage induction motor none of the mutual and self-inductances are functions of the rotor position. 

Squirrel Cage Motors - This is another name for an induction motor. The motors consist of a rotor inside a stator. The rotor has laminated, thin flat steel discs, stacked with channels along the length. If the casting composed of bars and attached end rings were viewed without the laminations the casting would appear similar to a squirrel cage. 

The cage induction motor rotor is simple and robust. 

There are two types of induction motor rotor - the wound rotor and the cage rotor. The cage rotor consists of a laminated cylinder of silicon steel with copper or aluminium bars slotted in holes around the circumference and short-circuited at each end of the cylinder, as shown in Fig. 2.50. In small motors the rotor is cast in aluminium. Better starting and quieter running are achieved if the bars are slightly skewed. This type of rotor is extremely robust and since there are no... 

A cage rotor is used on single-phase a.c. motors, the turning force being produced in the way described previously for three-phase induction motors and shown in Fig. 2.49. Because both windings carry currents which are out of phase with each other, the motor is known as a split-phase motor. The phase... 

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