Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 55)

DESCRIPTION OF OPERATION 5-37
1557-UM050F-EN-P –June 2013
Tachometer/Encoder Option
The optional tachometer/encoder provides two significant enhancements to the drive control:
1. Provides an accurate measurement of motor speed at all times
2. Extends closed loop speed and torque control down to nearly zero speed.
A pulse tachometer, also called a pulse generator or incremental encoder, produces a pulse train output
with a frequency proportional to shaft speed. The optional Tachometer Feedback board provides
optically isolated inputs for up to four two-phase (quadrature) pulse tachs. Parameter Tach Select
determines which one of the four tach inputs is used. Single-phase tachs can be used if reverse rotation
never occurs. Parameter Tach Type specifies which type is used. The Tach Feedback board contains
counters that count the number of tach pulses produced during a specified time interval. Dividing the
number of tach pulses by the sampling period yields the tach output frequency, from which the shaft
speed can be calculated using the tach pulses per revolution (ppr) specified by parameter Tach
Pulse/Rev and the number of motor poles. The shaft speed is measured six times per cycle for stator
frequencies above 10 Hz, and at a fixed rate of 60 times per second for lower frequencies. This is a lower
sample rate than used by most low voltage drives. The tach resolution determines the minimum motor
speed that can be measured. If high starting torque or very low speed operation is required then a high
resolution such as 1024 or 2048 ppr must be provided. Otherwise, a low resolution such as 240 or 360
ppr is adequate.
In order to control motor flux and torque, the machine converter gating must be synchronized with the
rotor flux. Over most of the speed range, the machine converter gating is synchronized to the rotor flux
signal reconstructed from the stator voltage and current feedback by analog hardware (refer to p. 5-18).
Because the rotor flux is measured, this method is called direct vector control. However, the analog flux
signal is not usable for stator frequencies less than about 5 Hz. To control flux and torque at low speeds,
the 1557 drive switches to indirect vector control, which is widely used in low voltage drives. With indirect
control the position of the rotor flux is not directly measured but is indirectly predicted by adding the
calculated slip angle to the measured rotor angle. The rotor angle is obtained by integrating the output of
the pulse tach (the zero position is arbitrary). The slip frequency required to provide the desired flux and
torque is calculated from the d-axis and q-axis current commands and the rated slip frequency. The slip
frequency is integrated to get the slip angle and added to the measured rotor angle to obtain the flux
angle. Indirect control can be used at any speed but its weakness is that the calculated slip is sensitive to
errors in the motor parameters. Errors in slip frequency increase the coupling between flux and torque
which adversely affects the stability of the flux control. Since large motors generally have lower
magnetizing current and lower slip than small motors they are more sensitive to parameter errors (i.e. a
small error in slip produces a large error in torque and flux).
Because of its salient pole construction, the position of the rotor flux in a synchronous machine is not
arbitrary but is determined by the physical position of the rotor. A synchronous machine therefore
requires an absolute position encoder instead of an incremental encoder for indirect vector control. The
encoder must also be aligned with the direct axis of the rotor. To avoid having to physically align the
encoder, an offset angle specified by parameter Encoder Offset is added to the encoder output to
compensate for the difference between the encoder zero and the direct axis of the rotor. Parameter
Encoder Reverse is provided to reverse the encoder rotation in software if it does not match the rotation
of the motor.
The encoder output must be Gray code and not normal binary. In most cases the encoder will actually
have a 10 or 12-bit parallel output, but the Tach Feedback board can only accommodate 8 bits. Which 8
bits are used is determined by the number of motor poles and is shown on the electrical drawings. There
is no parameter to specify the encoder resolution; it is inferred from the number of motor poles.