Schneider Electric ALTIVAR 58 TRX - Manual (Page 59)

Altivar ® 58 TRX AC Drives
Assignment of Analog Inputs (AIx)
59
09/2003 © 2000–2003 Schneider Electric All Rights Reserved
Encoder Speed Feedback
The Encoder Speed Feedback function can be used to improve the speed regulation based on the
encoder feedback signal. It is intended for use in applications in which the load on the motor is changing
but accurate speed regulation is critical to the process. The speed feedback input to the drive controller
is 24 Vdc. A choice of two different types of encoders can be configured as speed feedback: a NPN
sensor type, or a quadrature type encoder.
The NPN sensor type is for use in applications in which the sensor is detecting teeth on a wheel. The
24 Vdc supply on the option card can be used to power the NPN sensor. The quadrature encoders are
usually mounted to the motor shaft. An external 24 Vdc power supply for the encoder is recommended.
Selection of the encoder is critical for obtaining desired speed regulation.
The NPN sensor type encoders can improve the speed regulation from ± 1% to ± 0.5% of motor rated
speed. The quadrature type encoder can improve the speed regulation to ± 0.02% of rated motor speed
with a 1024 pulse count encoder.
The maximum signal frequency input with this option card is 33 kHz. Higher pulse count encoders
provide greater accuracy. The pulse count must be configured in the drive controller. Use the following
to assist in selecting an encoder.
1. Determine the maximum allowable pulse count (pulses per revolution, line count, encoder
resolution) with the following formula.
2. When selecting an incremental encoder: use an encoder with a pulse count closest to, but not
greater than, the result of the calculation in step 1. This will result in the highest precision.
3. When selecting a NPN sensor, choose a device that will limit the pulse count to the value determined
in step 1. Mechanical play in the toothed wheel or other device will degrade the resulting precision.
4. The maximum pulse frequency of the sensor must not be exceeded. Generally, it is this parameter
that limits precision. As an example, the pulse count of a sensor with a maximum frequency of
2000 Hz on a motor with a top speed of 1800 RPM is:
Example
• Motor nominal RPM =1800@ 60 Hz.
• Drive controller maximum frequency = 63 Hz.
• Motor nominal RPM @ 63 Hz = 1800 x (63/60) = 1890
• Maximum pulse count = 33,000 x 60 / 1890 = 1047
• Any pulse count of 1047 or less will work. The higher the pulse count, the higher the feedback
resolution. The nearest standard encoder pulse count less than the above calculation is 1024.
Examples of the speed feedback function are shown below.
Example 1: Use of Incremental Speed Feedback in one
operating direction with an inductive sensor or
photoelectric detector. This provides simplified regulation,
but is less accurate at low speed. Example 2: Use of Incremental Speed Feedback in two
operating directions with an incremental encoder. This
provides more accurate regulation than a NPN sensor.
Max. allowable pulse count
33,000 (max. signal frequency)X60 (seconds/minute)
Motor RPM @ drive controller maximum frequency
---------------------------------------------------------------------------------------------------------------------------------------------- =
66 Pulses per revolution
2000 (max. signal frequency)X60 (seconds/minute)
1800 (Motor RPM @ drive controller maximum frequency)
---------------------------------------------------------------------------------------------------------------------------------------------------------- =
A A – B B – COM + 24
– +
NPN Sensor
Option Card
IncrSpdEx1.eps
A
A
A –
A –
B
B
B –
B –
COM
–
+
0
+ 24 V
Incremental encoder
Option Card
external 24 VDC
power supply
IncrSpdEx2.eps
NOTE: Unlike the
previous assignments,
which may be
assigned to either the
analog or digital I/O
extension cards, the
following assignments
can only be assigned
to the A+,A–,B+,B–
terminals on the digital
I/O extension card.