Squirrel cage induction motors speed control

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

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. 

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. 

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. 

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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