Speed Control Motors/Brushless Motors Structure and Control Method of AC Speed Control Motors

Structure

AC speed control motors are AC motors with a tachogenerator (speed detection sensor) installed on the back side.

Control Method

The basic block diagrams and outline of the control methods are shown below.
AC speed control motors employ a closed-loop speed control method.

[1]

The speed setting voltage is supplied via a speed potentiometer.

[2]

The motor’s rotation speed is detected by a tachogenerator and the difference in speed signal voltage is output.

[3]

The difference between the speed setting voltage and speed signal voltage is output.

[4]

A voltage determined by the output from the comparator is supplied to the motor so that it will reach the set voltage.

Tachogenerator Generated Voltage Characteristics

The tachogenerator directly coupled to the speed control motor generates AC voltage roughly proportional to the motor speed, as shown in the figure below. Therefore, if an AC voltmeter is connected between the tachogenerator leads, the rotation speed of the motor can be calculated. (For the AC voltmeter, use a unit whose internal impedance is 200 kΩ or more.)