2.3.3 Calculating the unit of rotation speed
The tachometer is able to receive an open collector type input signal of maximum 10KHz and displays the unit as
Hz on the screen of the tachometer. Therefore, if you wish the rotation speed to be displayed in rpm, you have to
convert Hz into rpm by an equation and fill the obtained value into
of the tachometer to display the
rotation speed in rpm on the screen.
The conversion equation:
Frequency (Hz) × Pre-scale = Rotation Speed (rpm)
Pre-scale = 60/N
N※ = Number of pulses per revolution (ppr)
Example 1: Connecting Delta servo drive with a servo motor and measure the current rotation speed by CTA
The built in encoder in Delta servo motor is 10,000ppr with four phases, OA, OB,
OA
and
OB
. Every phase is
2,500ppr.
Pre-scale = 60/2,500 = 0.024
At this time, the servo inputs 240rpm, equivalent to the tachometer receiving 10KHz currently.
Frequency and rotation speed of CTA connected to Delta servo drive:
Frequency
Rotation speed
10KHz 240rpm
5KHz 120rpm
1KHz 24rpm
200Hz 4.8rpm
Note: OA and OB of Delta ASD servo motor A and B series only support differential signals. OZ (10,000ppr) supports
open collector.
Example 2: Measuring the current rotation speed of the AC fan motor through a sensor
Figure 1: 1 revolution of the fan equals 1 pulse; therefore
Pre-scale = 60/N = 60/1 = 60
At this time, the sensor detects the speed and sends it back to tachometer. Assume the value displayed on
the screen is 2,700rpm,
F × 60/1 = 2,700
F = 45Hz
Figure 2: 1 revolution of the fan equals 3 pulses; therefore
Pre-scale = 60/N = 60/3 = 20
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