Parameter
structure
Keypad and
display
Parameter x.00
Parameter
description format
Advanced parameter
descriptions
Macros
Serial comms
protocol
Electronic
nameplate
Performance
RFC mode
414
Unidrive SP Advanced User Guide
www.controltechniques.com Issue Number: 10
10 Rotor Flux Control (RFC) mode
10.1 Introduction
Rotor Flux Control (RFC) mode is a sub mode of closed loop vector
mode on the drive. Instead of using a position feedback device the drive
uses a position estimator to determine the position of the flux in the
motor. The drive uses motor currents, motor voltages and key motor
parameters in a robust model of the motor to estimate flux position, flux
magnitude and motor speed. The control system is exactly the same as
the system used with closed loop vector with position feedback except
that in RFC mode the feedback is derived from a position estimator
instead of an actual feedback device. The position estimator gives an
effective resolution of 16384 lines per revolution.
RFC mode has two major benefits.
1. RFC mode can eliminate instability when operating a large motor on
light load and at low frequencies, for example in large fan
applications.
2. With RFC mode it is possible to switch between closed loop vector
mode with a position feedback device and RFC mode while running
without interruption. This is useful for high-speed application that
requires accurate position control at low speeds but also requires
operation at speeds above the range of the feedback device.
In the Unidrive SP Advanced User Guide RFC mode is also sometimes
referred to as closed-loop vector mode without position feedback.
10.2 Setting up the RFC mode
This section describes how to set up the drive to run in RFC mode.
Unidrive SP software V01.10.00 or later should be used for RFC mode.
It is recommended that the lowest possible switching frequency is used,
preferably 3kHz. This is because as the switching frequency is
increased, the effect of the IGBT switching dead times is greater, which
reduces stability. This is more of an issue on larger drives where the
IGBT dead times are longer.
To set up the Unidrive SP in RFC mode follow the instructions below.
10.3 Further Tuning
The position estimator used in RFC mode has a filter with a time
constant of 4ms which will reduce the possible bandwidth of the speed
controller compared with the bandwidth possible when position feedback
is used. This means that the speed loop integral gain will need to be
lower than the value normally used when a feedback device used. With
Unidrive SP software version V01.10.00 onwards, the default of the
speed loop I gain has been decreased from 1.00 to 0.10 and the P gain
increased from 0.0100 to 0.0300 to help in allowing RFC mode to work
well from the default parameter settings. The speed loop gains should
be modified to obtain the best possible performance for the application,
but it is unlikely that the integral gain can be increased much above 0.50.
A filter with a 4ms time constant is always present on the output of the
speed estimator, but this filter may be extended by setting Pr 3.42 as
follows: 0 = 4ms, 1 = 8ms, 2 = 16ms, 3 = 32ms, 4 = 64ms, 5 = 128ms.
The output of the speed estimator can include some ripple, which
increases as the drive passes into field weakening and the filter can be
used to remove this ripple. This is particularly useful when using
standard ramp or spinning start with a low friction high inertia load, and
can prevent over voltage trips when the drive has no braking resistor.
When operating above rated speed it may be necessary to include
further filtering of the current demand (Pr 4.12 set to a value between 1.0
and 5.0ms) to achieve stable operation.
10.4 Other Considerations
If the drive encoder lines per rev in Pr 3.34 is set to a value that is not a
power of 2 and the drive encoder type in Pr 3.38 is set to any type of
SINCOS encoder, Pr 3.24 is forced to zero and RFC mode is not
available. This is because the extra processing time required to support
the feedback device would not allow enough time for the rotor flux
control algorithm to be executed. It should be noted that if RFC mode is
active, that the current controller sample rate used with 6kHz and 12kHz
switching frequencies is reduced from 12kHz to 6kHz (i.e. the current
controller sample time is increased from 83us to 167us). Also operation
at 4kHz, 8kHz or 16kHz switching frequency is not possible, and if these
frequencies are selected the actual switching frequency is switched
down to the next lower frequency. Pr 5.37 displays the actual switching
frequency being used.
Action
Detail
1
Set the drive into closed loop
vector mode
Set Pr
xx.00
to 1253 (EUR defaults) or 1254 (US defaults), set Pr
11.31
(or Pr
0.48
) to CL VECt, and perform
a drive reset.
2
Select RFC mode
Set Pr
3.24
to 1 or 3
3
Disable the encoder wire break
trip
Set Pr
3.40
to 0
4
Enter the motor nameplate
values
Ensure all the following motor map parameters are entered into the drive using the motor nameplate.
•
Motor rated frequency in Pr
5.06
•
Motor rated current in Pr
5.07
•
Motor rated speed in Pr
5.08
•
Motor rated voltage in Pr
5.09
5
Select or deselect catch a
spinning motor mode
If catch a spinning motor mode is not required then set Pr
6.09
to 0.
If catch a spinning motor mode is required then leave Pr
6.09
at the default of 1, but depending on the size of
the motor the value in Pr
5.40
may need to be adjusted. Pr
5.40
defines a scaling function used by the
algorithm that detects the speed of the motor. The default value of Pr
5.40
is 1 which is suitable for small
motors (<4kW). For larger motors the value in Pr
5.40
will need to be increased. Approximate values of Pr
5.40
for different motor sizes are as follows, 2 for 11kW, 3 for 55kW and 5 for 150kW.
If the value of Pr
5.40
is too large the motor may accelerate from standstill when the drive is enabled. If the
value of this parameter is too small the drive will detect the motor speed as zero even if the motor is spinning.
6
Autotune
It is highly recommended that a rotating autotune is performed (Pr
5.12
set to 2). If a rotating autotune is not
possible then a static autotune must be performed (Pr
5.12
set to 1) and the nameplate value of power factor
entered into Pr
5.10
Summary of Contents for unidrive sp
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