Concrete vibrator

Claim 1

A concrete vibrator comprising, a motor, an eccentric pendulum, driven by the motor for effectively reducing a drop in speed of rotation of the eccentric pendulum upon immersion into a fresh concrete mass, a cable for supplying electric power to said motor, said motor having a permanent magnet rotor and three-phase stator windings, a driving circuit disposed remote from said motor and pendulum provided in the cable to drive the motor by electric power and comprising components in the cable remote from the motor and pendulum sufficiently to preclude vibrations of the vibrator affecting said components, said driven circuit components comprising, a frequency changer of a semiconductor inverter coupled to said motor, a differential amplifier, a variable speed setting circuit for generating reference pulses, a speed control circuit and a commutation control circuit connected for controlling the frequency changer to control a vibration frequency of the concrete vibrator with pulse-modulated commutation signals, a start-stop switch disposed in said cable near to the motor and pendulum connected to the inverter and the commutation control circuit for starting and stopping the vibrator, and a neutral point of the stator windings effective to feed a neutral point potential as an input to the differential amplifier; the neutral point potential occurring at an artificial neutral point at a stator winding input being fed as another input of the differential amplifier so that a detected voltage is obtained at an output of the differential amplifier as third harmonic voltage components of the three-phase stator windings, the differential amplifier being connected so that the detected voltage is compared with the reference pulses in the speed control circuit to obtain an error signal for influencing the degree of modulation of the pulse-modulated commutation signals at the commutation control circuit to reduce said error signal to zero.