Digitax HD M753 Control User Guide
93
Issue Number: 3
8.5
High speed operation
8.5.1
Encoder feedback limits
The maximum encoder frequency should be prevented from exceeding
500 kHz. In RFC-A and RFC-S modes the maximum speed that can be
entered in to the speed reference clamps (Pr
00.002
and Pr
00.001
) can
be limited by the drive. This is defined by the following (subject to an
absolute maximum of 33,000 rpm):
Where:
ELPR is the equivalent encoder lines per revolution and is the
number of lines that would be produced by a quadrature encoder.
•
Quadrature encoder ELPR = number of lines per revolution
•
F and D encoder ELPR = number of lines per revolution / 2
•
SINCOS encoder ELPR = number of sine waves per revolution
This maximum speed limit is defined by the device selected with the
speed feedback selector (Pr
03.026
), and the ELPR set for the position
feedback device. In RFC-A mode it is possible to disable this limit via
Pr
03.024
, so that the drive can be switched between operation with and
without feedback when the speed becomes too high for the feedback
device.
8.5.2
Field weakening (constant power) operation
(Open loop and RFC-A mode only)
The drive can be used to run an induction machine above synchronous
speed into the constant power region. The speed continues to increase
and the available shaft torque reduces. The characteristics below show
the torque and output voltage characteristics as the speed is increased
above the rated value.
Figure 8-2 Torque and rated voltage against speed
Care must be taken to ensure the torque available above base speed is
sufficient for the application to run satisfactorily. The saturation
breakpoint parameters (Pr
05.029
, Pr
05.030
, Pr
05.062
and Pr
05.063
)
found during the autotune in RFC-A mode ensure the magnetizing
current is reduced in the correct proportion for the specific motor.
(In open loop mode the magnetizing current is not actively controlled).
8.5.3
Permanent magnet motor high speed
operation
High speed servo mode is enabled by setting Pr
05.022
=1. Care must
be taken when using this mode with permanent magnet motor to avoid
damaging the drive. The voltage produced by the permanent magnet
motor magnets is proportional to speed. For high speed operation the
drive must apply currents to the motor to counter-act the flux produced
by the magnets. It is possible to operate the motor at very high speeds
that would give a very high motor terminal voltage, but this voltage is
prevented by the action of the drive.
If however, the drive is disabled (or tripped) when the motor voltages
would be higher than the rating of the drive without the currents to
counter-act the flux from the magnets, it is possible to damage the drive.
If high speed mode is enabled the motor speed must be limited to the
levels given in the table below unless an additional hardware protection
system is used to limit the voltages applied to the drive output terminals
to a safe level.
Ke is the ratio between r.m.s. line to line voltage produced by the motor
and the speed in V/1000 rpm. Care must also be taken not to de-
magnetize the motor. The motor manufacturer should always be
consulted before using this mode.
By default, high speed operation is disabled (Pr
05.022
= 0).
It is also possible to enable high speed operation and allow the drive
to automatically limit the motor speed to the levels specified in the
table and generate an
Overspeed.1
trip if the level is exceeded
(Pr
05.022
= -1).
8.5.4
Switching frequency
Ideally a minimum ratio of 12:1 should be maintained between the
switching frequency and the output frequency. This ensures the number
of switchings per cycle is sufficient to ensure the output waveform quality
is maintained at a minimum level. If this is not possible, quasi-square
switching should be enabled (Pr
05.020
=1). The output waveform will
be quasi square above base speed ensuring a symmetrical output
waveform, which results in a better quality output than would otherwise
result.
8.5.5
Maximum speed / frequency
In all operating modes (Open loop, RFC-A and RFC-S) the maximum
output frequency is limited to 550 Hz. However, in RFC-S mode the
speed is also limited by the voltage constant (Ke) of the motor. Ke is a
specific constant for the servo motor being used. It can normally be
found on the motor data sheet in V/k rpm (volts per 1,000 rpm).
Maximum speed limit (rpm) =
500 kHz x 60
ELPR
=
3.0 x 10
7
ELPR
Rated
voltage
Torque
Speed
Speed
Rated speed
Drive voltage
rating
Maximum motor
speed (rpm)
Maximum safe line to line
voltage at the motor
terminals (V rms)
200
400 x 1000 / (Ke x
√
2)
400 /
√
2
400
800 x 1000 / (Ke x
√
2)
800 /
√
2
575
955 x 1000 / (Ke x
√
2)
955 /
√
2
690
1145 x 1000 / (Ke x
√
2)
1145 /
√
2
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