Hitachi SP18VA, Technical Data And Service Manual

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(6) Load current detecting circuit:

Detects the load current by means of a low resistance resistor which is in series with  the motor  and the triac.

(7) Load current control circuit:

When the detected load is excessive (overload), this circuit sends a command (control signal) to reduce power

to the motor.

(8) Phase control circuit:

Generates the trigger signals necessary to turn the triac on, and delivers them to the gate of the triac.  The

conductive angle of the triac is determined by the timing of the generated trigger signals.

When starting the tool: Controlled by the soft start circuit, the phase control circuit generates trigger signals

     to gradually increase the conductive angle of the triac.

Constant speed operation: When there are variations in the rotation speed, the phase control circuit

 generates trigger signals to keep the rotation speed as close as possible to the

 present rotation speed determined by the standard voltage circuit.  In short, when

 the load increases and causes the rotation speed to decrease, the conductive

 angle of the triac is increases, and the voltage applied to the motor is increased.

 In this way, the rotation speed is kept as close as possible to the standard

 rotation speed.

8-3. Motor Characteristics and Voltage Applied to the Motor

Phase control voltage waveform

Conductive
angle

(When electronic control
not applied)

Rotation speed
determined by
the standard
voltage circuit

As the load inceases,
the voltage applied to the
motor increases.

Soft start characteristics
when starting the tool

Voltage applied to
the motor increases
gradually

Switch ON

Time

Rotation speed

Dial
6

Rotation speed

0   No-load

Load

Dial

Dial