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158 HVAC Drive H300
Issue Number: 3
Pr 00.040 {05.064} RFC Low Speed Mode /
Pr 00.041 {05.071} Low Speed Sensorless Mode Current
(0) Injection mode
For low speed sensorless operation with signal injection (RFC Low Speed Mode (05.064) = 0) it is necessary to have a ratio of Lq/Ld = 1.1. Even if
a motor has a larger ratio on no load, this ratio normally reduces as the q axis current is increased from zero. Low Speed Sensorless Mode Current
(05.071) should be set at a level that is lower than the point where the inductance ratio falls to 1.1. The value of this parameter is used to define the
drive current limits when signal injection is active and prevent loss of control of the motor.
(1) Non-salient mode
For low speed sensorless operation for non-salient motors (RFC Low Speed Mode (05.064) = 1) this defines a current applied in the d axis to aid
starting. For most motors and application requiring up to 60 % torque on starting the default value is suitable. However the level of current may need
to be increased to make the motor start.
(2) Current
This method, which applies a rotating current vector at the frequency defined by the speed reference, can be used with any motor with no saliency
or moderate saliency. It should only be used with motors where more of the torque is produced in conjunction with the magnet flux rather than from
saliency torque. This mode does not provide the same level of control at low speed as injection mode, but is easier to set up and more flexible than
"Non-salient" mode. The following should be considered:
1. A current specified by Low Speed Sensorless Mode Current (05.071) is applied when low speed mode is active. This current should be
sufficient to start the motor with the highest expected load. If the motor has some saliency with no-load applied, and a suitable saturation
characteristic, the drive can detect the rotor position and apply the current at the correct angle to avoid starting transient. If the motor is non-
salient as defined by the conditions for Inductance trip then the drive will not attempt to detect the rotor position and the current will be applied
at an arbitrary angle. This could cause a starting transient if the level of current applied is high, and so Low Speed Sensorless Mode Current
(05.071) should not be set to a higher level than necessary. To minimise the movement as a result of applying the current, it is increased over
the period defined by Sensorless Mode Current Ramp (05.063) in the form of a squared characteristic (i.e. it is increased with a low rate of
change at the beginning and the rate of change is gradually increased).
2. As the level of current when low speed mode is active is not dependent on the applied load, but is as defined by Low Speed Sensorless Mode
Current (05.071), and so the motor may become too hot if low speed mode is active for a prolonged period of time.
3. Generally Low Speed Sensorless Mode Current (05.071) should be set to a level higher than the expected maximum load, and can be set to a
much higher level than the load if the saliency and saturation characteristic allow the position of the rotor to be detected on starting. However,
Low Speed Sensorless Mode Current (05.071) should be matched more closely to the expected load under the following conditions: the load
inertia is high compared to the motor interia, or there is very little damping/loss in the load system, or where the q axis inductance of the motor
changes significantly with load.
(3) Current no test
The "Current" method is used, but no attempt is made to determine the position of the rotor before applying the current. This can be selected for
example, if the motor does not have a suitable saturation characteristic to allow the rotor position to be determined during starting, or if faster
starting is required. The initial current vector angle will be at an arbitary position with respect to the actual rotor position. As the vector sweeps round
it must make the rotor start to rotate. If the ramp rate is too high the rotor may not keep up with the current vector and the motor may not start. If this
is the case then the ramp rate should be reduced and/or the current used to start the motor should be increased.
Pr 04.012 Current Reference Filter 1 Time Constant
Current Reference Filter 1 Time Constant (04.012) defines the time constant of a first order filter that can be applied to the Final Current Reference
(04.004). The filter is provided to reduce acoustic noise and vibration produced as a result of position feedback quantisation. The filter introduces a
lag in the speed controller loop, and so the speed controller gains may need to be reduced to maintain stability as the filter time constant is
increased.
Pr 04.013 / Pr 04.014 Current Loop Gains
The current loop gains proportional (Kp) and integral (Ki) gains control the response of the current loop to a change in current (torque) demand. The
default values give satisfactory operation with most motors. However, for optimal performance in dynamic applications it may be necessary to
change the gains to improve the performance. The proportional gain (Pr 04.013) is the most critical value in controlling the performance. The values
for the current loop gains can be calculated by performing a stationary or rotating autotune (see Autotune Pr 00.024, earlier in this table) the drive
measures the Stator Resistance (05.017) and Transient Inductance (05.024) of the motor and calculates the current loop gains.
This will give a step response with minimum overshoot after a step change of current reference. The proportional gain can be increased by a factor
of 1.5 giving a similar increase in bandwidth; however, this gives a step response with approximately 12.5 % overshoot. The equation for the integral
gain gives a conservative value. In some applications where it is necessary for the reference frame used by the drive to dynamically follow the flux
very closely the integral gain may need to have a significantly higher value.
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