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162 HVAC Drive H300
Issue Number: 3
8.4 High speed operation
8.4.1 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. Figure 8-2 shows 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.
8.4.2 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 tables and
generate an Overspeed. 1 trip if the levels are exceeded (Pr 05.022 = -1)
8.4.3 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).
8.4.4 Quasi-Square wave (open-loop only)
The maximum output voltage level of the drive is normally limited to an equivalent of the drive input voltage minus voltage drops within the drive (the
drive will also retain a few percent of the voltage in order to maintain current control). If the motor rated voltage is set at the same level as the supply
voltage, some pulse deletion will occur as the drive output voltage approaches the rated voltage level. If Pr 05.020 (Quasi-square wave enable) is set
to 1 the modulator will allow over modulation, so that as the output frequency increases beyond the rated frequency the voltage continues to increase
above the rated voltage. The modulation depth will increase beyond unity; first producing trapezoidal and then quasi-square waveforms.
This can be used for example:
• To obtain high output frequencies with a low switching frequency which would not be possible with space vector modulation limited to unity
modulation depth,
or
• In order to maintain a higher output voltage with a low supply voltage.
The disadvantage is that the machine current will be distorted as the modulation depth increases above unity, and will contain a significant amount of
low order odd harmonics of the fundamental output frequency. The additional low order harmonics cause increased losses and heating in the motor.
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
Rated
voltage
Torque
Speed
Speed
Rated speed
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