Speed control squirrel cage motors

Occasionally d.c. motors were used in the 1980s in order to provide an infinitely-variable control of the auger shaft speeds, but since then frequency-controlled squirrel-cage motors have become the standard. 

Speed control in slip-ring motors has been discussed in the previous chapter. Squirrel cage motors have limitations ill their speed control in view of their fixed rotor parameters. Speed variation, in fixed steps, however, is possible in such motors if the stator is wound for multipolcs and such motors arc known as pole changing motors. Up to four different speeds can be achieved in such motors economically, in combinations of 2/4, 4/6. 4/8, 6/8, 6/12, 2/4/6. 4/6/8. 2/4/6/12 and 4/6/8/12 poles etc, or any other similar combination. For limitation in the motor size and tlux distribution, winding sets of more than two are not recommended. I he two windings can be arranged for two. three oi (maximum) four different speeds. 

The above methods provide speed variation in steps, as in squirrel cage motors or in two machines or more, as in frequency converters, and cannot be u.sed for a process line, which requires frequent precise speed controls. Until a few years ago there was no other option with all such applications and they had to be fitted with d.c. motors only. D.C. motors possess the remarkable ability of precise speed control through their separate armature and field controls. In d.c. motors the speed... 

Before thyristors and power transistors were introduced for AC to DC and AC to DC to AC converter systems, there were a number of special designs of AC motors that gave better performance than standard squirrel-cage motors. These motors required connections to the rotor windings. They had better speed control, superior torque versus speed characteristics and some methods were energy efficient. However, they were more complicated and hence more expensive. 

The shim-rod motors are three-phase 220-volt 1/6-hp squirrel-cage motors, of 1800 rpm rating. Start, stop, and reverse are the only control actions called for. Reversal is accomplished. by phase, interchange. This simple control picture is, however, slightly complicated by two considerations the desire to have a finer speed control than is obtained by simple on-off switching, and concern as to the possible safety implications of transition from three-phase to single-phase operation by loss (open circuit) of one power wire. 

Unhke squirrel-cage induction motors, wound-rotor motors have controllable speed and torque. Their application is considerably different from squirrel-cage motors because of the accessibility of the rotor circuit. Various performance characteristics can be obtained by inserting different resistances in the rotor circuit. 

One method of controlling the speed of ac motors is with a pulse-width-modulated (PWM) variable-frequency drive (VFD). In the simplest terms, the VFD converts ac line voltage into variable voltage and frequency, which is then used to control the speed of ac squirrel-cage motors. As a controller, start/stop, hand-off-auto, and various other control schemes may be applied. 

The most popiilar form of motor speed control for adjustable-speed pumping is the voltage-controlled pulse-width-modulated (PWM) frequency synthesizer and AC squirrel-cage induction motor combination. The flexibility of apphcation of the PWM motor drive and its 90 percent- - electric efficiency along with the proven ruggedness of the traditional AC induction motor makes this combination popular. 

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.)... 

The most recent development in the starting of squirrel-cage induction motors is the introduction of the electronic soft-start. This principle has been derived from variable-frequency speed controllers using switched Thyristor or power transistor bridges. The supply sine wave is chopped so that a reduced voltage and frequency is applied to the motor.These are gradually increased so that the motor speed rises in a controlled manner, with the starting current limited to any chosen value. 

Good range. Relatively good low speed efficiency. Automatic speed control easy Low cost. Can use squirrel cage induction motor. Automatic control easy... 

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. 

Three-Phase Induction Wound-Rotor Motors. Wound-rotor motors offer more speed and torque control than squirrel-cage induction motors, as well as the major advantage of accessibility of the rotor circuit. Performance characteristics can be varied merely by inserting different values of resistance in the rotor circuit. 

The BOR-60 pumps are driven by standard motors with stepless speed control. For the BN-350 pumps special squirrel-cage induction motors with two speeds (250 and 1000 rpm) are used. For the BN-600 pumps controllable synchronous motors are used coupled to synchronous-rectifier drivers with phase rotors. 

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