American Electricians Handbook by Terrell Croft

Author:Terrell Croft
Language: eng
Format: epub
Tags: -
Publisher: McGraw-Hill Education
Published: 2013-07-03T16:00:00+00:00

FIGURE 7.132 AC static, stepless thyristor control for crane ac wound-rotor motors. [Square D, Schneider Electric]

Five pairs of thyristor assemblies are used in the motor primary circuit to provide static, stepless control of the phase sequence and magnitude of motor voltage. The power-limit stop is arranged to disconnect the motor and directly open the brake circuit when the switch trips. Balanced three-phase motor power for lowering out of the tripped limit stop is also provided. A fixed resistor in each phase of the motor secondary is used to alter the basic speed-torque characteristic for speed-regulated operation. The application of this type of control is shown in Fig. 7.132.

Reduced speed is obtained by adjusting the degree of conduction of the power thyristors. The solid-state control modules produce properly synchronized firing pulses to turn on the thyristors in response to a speed-control signal.

The master-switch inductor unit produces an ac output voltage which is proportional to the amount of deflection of the master-switch handle from the off point. The signal converter changes this ac command signal to a dc signal. A permanent-magnet type of tachometer generator, mechanically coupled to a rotating unit of the hoist drive, provides the feedback signal used to obtain speed regulation. The signal comparator module compares these signals. Its output speed-control signal is used to actuate the thyristor control modules, which energize either the hoist or lower thyristors to produce proper motor operation.

The tachometer continuity module is used to apply an ac voltage to the tachometer circuit and its collectors to ensure continuity. This ac voltage operates the undervoltage relay. Overspeed protection is provided by a mechanical centrifugal overspeed switch in the tachometer assembly.

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