Hitachi SP18VA, Technical Data And Service Manual

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8. FUNCTION AND OPERATION OF THE CONTROL CIRCUIT

8-1. Control Circuit (Block Diagram)

8-2. Function of Each Block

(1) Rotation speed detector:

Generates a frequency signal proportionate to the actual rotation speed of the motor.  A magnetic sensor in the

controller monitors changes in the magnetic flux from a magnet, and generates a frequency signal.

(2) F-V converter:

Converts the frequency signal produced by the rotation speed detector into DC voltage.

(3) Standard voltage circuit:

Generates a standard voltage which determines the no-load rotation speed of the motor and serves as a

reference in maintaining constant rotation speed even when there are fluctuations in the load.

(4) Ramp generator:

Generates the voltage which determines the acceleration of the motor.  The soft start function when the motor

is turned on is achieved through a gradual increase of the voltage generated by this circuit.  When fully

charged, the voltage of the ramp generator becomes equal to the standard voltage (above).

(5) Differential amplifier:

Compares the voltage from the ramp generator voltages received from the F-V converter (which is

proportionate to the actual motor rotation speed), and amplifies the result.

This circuit functions as follows to maintain a constant rotation speed:

Motor rotation speed decreases (or increases)

frequency signal decreases

(or increases)

output voltage of the F-V converter decreases (or increases)

output voltage of the differential amplifier increases (or decreases).

Rotation speed
detector

Phase control circuit

Load current control circuit

Load current detecting circuit

Ramp generator

Differential
amplifier

Variable resistor

F-V converter

Standard voltage
circuit

Soft start and constant speed control circuit

Triac

Motor

Switch

AC power source