Menu 5 − Motor Control

Mode: Open‑Loop

The frequency reference is given by Final Frequency Reference (03.001) but this may be altered if slip compensation is enabled or a current limit is active. Internally the reference has a resolution of better than 0.001Hz. Output Frequency (05.001) is a measurement of the actual drive output frequency with a resolution limited by the time window over which it is measured, and so it may not round exactly to the expected higher resolution reference value which may be defined by the ramp output in 0.1Hz units and the frequency offset in 0.01Hz units, or the precision reference in 0.001Hz units. 

Output Voltage (05.002) is the r.m.s. line to line voltage at the a.c. terminals of the converter.

Output Power (05.003) is the power flowing via the a.c. terminals of the drive. The power is derived as the dot product of the output voltage and current vectors, and so this is correct even if the motor parameters are incorrect and the motor model does not align the reference frame with the flux axis of a motor in RFC-A mode. A positive value of power indicates power flowing from the drive to motor.

If Enable Frequency Slaving (03.013) = 0 (normal operation) the frequency used to derive the Motor Rpm (05.004) is the Post Ramp Reference (02.001). If Enable Frequency Slaving (03.013) = 1 (frequency slaving operation) then Final Frequency Reference (03.001) is used.

Motor Rpm (05.004) = 60 x Frequency / Pole pairs

where
Pole pairs = the numeric value of Number Of Motor Poles (05.011) (i.e. 3 for a 6 pole motor)

For frequency slaving operation there will be an error due to the motor slip frequency (if relevant). For normal operation the result will be reasonably accurate provided that the slip compensation has been set up correctly.

D.c. Bus Voltage (05.005) is the measured voltage at the d.c. terminals of the converter.

Rated Frequency (05.006) and Rated Voltage (05.009) define the frequency to voltage characteristic applied to the motor. See Open-loop Control Mode (05.014) for more details. Rated Frequency (05.006), Rated Speed (05.008) and Number Of Motor Poles (05.011) are used to calculate the rated slip of the motor for slip compensation.

Rated slip (Hz) = Rated Frequency (05.006) - (Pole pairs x Rated Speed (05.008) / 60)

where:
Pole pairs = the numeric value of Number Of Motor Poles (05.011) (i.e. 3 for a 6 pole motor)

If slip compensation is required Rated Speed (05.008) should be set to the motor nameplate value, which should give the correct compensation for a hot motor provide the nameplate value is correct. Slip compensation can be used throughout the speed range of the motor, i.e. below base speed and in the flux weakening region, to correct the motor speed to minimise the change of speed with load. Rated Speed (05.008) can be set to a value that is higher than synchronous speed to deliberately introduce speed droop, which can be used to aid load sharing with mechanically coupled motors.

Slip compensation is disabled under the following conditions:

1. Rated Speed (05.008) = 0
2. Rated Speed (05.008) = Rated Frequency (05.006) x 60 / Pole pairs, i.e. synchronous speed.
3. Enable Slip Compensation (05.027) = 0

It should be noted that Rated Speed (05.008) is used to calculate the rotor time constant of the motor which is then used to determine the flux build-up time at the start of the catch a spinning motor algorithm. If spinning start is required (i.e. Catch A Spinning Motor (06.009) ≥ 1) then Rated Speed (05.008) should be set up correctly for the motor. If slip compensation is not required it can be disabled by setting Enable Slip Compensation (05.027) to 0.

It is possible to use the drive as a power supply with independent control of output voltage and frequency. The frequency is controlled with the normal frequency reference system and the voltage is controlled with Rated Voltage (05.009). To do this Rated Frequency (05.006) must be set to zero. However, Rated Frequency (05.006) is used in calculating the rated slip above rated frequency which would result in a very high slip frequency. To avoid this slip compensation is disabled if Rated Frequency (05.006) = 0.

Rated Current (05.007) is used as follows:

See Rated Frequency (05.006).

Rated Frequency (05.006) and Rated Voltage (05.009) define the frequency to voltage characteristic applied to the motor. See Open-loop Control Mode (05.014) for more details.

Open-loop Control Mode (05.014) not equal to 6 (Sync Motor):

Rated Power Factor (05.010) is the true power factor of the motor under rated conditions, i.e. the cosine of the angle between the motor voltage and current. If Stator Inductance (05.025) is set to a non-zero value then the stator inductance is used to calculate the rated magnetising current for the motor and the rated power factor can be calculated by the drive. Therefore if Stator Inductance (05.025) is non-zero Rated Power Factor (05.010) is continuously set to the calculated value of rated power factor by the drive. If Stator Inductance (05.025) is set to zero then Rated Power Factor (05.010) is used to estimate the rated magnetising current which is an approximation and not as accurate. Stator Inductance (05.025) can be measured by the drive during auto-tuning and this is the preferred option, however, if it is not possible to obtain the value for Stator Inductance (05.025) then Rated Power Factor (05.010) should be set to the motor nameplate value.

Open-loop Control Mode (05.014) equal to 6 (Sync Motor):

Rated Power Factor (05.010) is the true power factor of the motor under rated conditions, i.e. the cosine of the angle between the motor voltage and current. This is used to calculate the real component of rated current and is used to define the current limits and the motor damping (see Synchronous Motor Damping (05.065)).

The default setting of this parameter is "Automatic" which uses the following fomula to work out the correct setting: 

Pole pairs = 60 x Rated Frequency (05.006) / Rated Speed (05.008) rounded down to the nearest integer.

When setting Number Of Motor Poles (05.011) via the keypad or Connect, it must be set to the number of motor poles e.g. for a 1500 rpm motor the number of poles is 4.

When setting Number Of Motor Poles (05.011) via communications, it must be set to the number of motor pole pairs, i.e. number of motor poles / 2, e.g. for a 1500 rpm motor the number of motor pole pairs is 2.

* The units relate to the numeric value of the parameter and not the text string.

The following describes how an auto-tune test can be initiated and normal operation can be resumed after the test for Open-loop mode:

1. An auto-tune test cannot be initiated if the drive is tripped or the drive inverter is active, i.e. Drive Healthy (10.001) = 0 or Drive Active (10.002) = 1. The inverter can be made inactive by ensuring that the Final drive enable is inactive, or the Final drive run is inactive and Hold Zero Speed (06.008) = 0.
2. An auto-tune test is initiated by setting Auto-tune (05.012) to a non-zero value and making the Final drive enable and the Final drive run active.
3. All tests that move the motor will move the motor in the forward direction if Reverse Select (01.012) = 0 or the reverse direction if Reverse Select (01.012) = 1. 
4. If the auto-tune sequence is completed successfully the Final drive enable is set to the inactive state and Auto-tune (05.012) is set to zero. The Final drive enable can only be set to the active state again by removing the enable and reapplying it. The enable can be removed by setting Drive Enable (06.015) = 0, or by setting bit 0 of the Control Word (06.042) to 0 provided Control Word Enable (06.043) = 1, or by making Hardware Enable (06.029) = 0.
5. If a trip occurs during the auto-tune sequence the drive will go into the trip state and Auto-tune (05.012) is set to zero. As in 4. above the enable must be removed and re-applied before the drive can be restarted after the trip has been reset. However, care should be taken because if the auto-tune was not completed the drive parameters that should have been measured and set up will still have their original values. 
6. If the Final drive enable is made active, the Final drive run is inactive and Hold Zero Speed (06.008) = 1 the drive would normally be in the Stop state (i.e. the inverter is active, but the frequency or speed reference is 0).

The following describes the effects of the auto-tune test on the drive parameters for Open-loop mode:

1. All auto-tune tests rely on the motor being stationary when the test is initiated to give accurate results.
2. When each stage of the test is complete the results written to the appropriate parameters and these parameters saved in the drive non-volatile memory. If Parameter Cloning (11.042) is set to 3 or 4 the parameters are also written to a non-volatile media card fitted in the drive.

The table below shows the parameters required for motor control indicating which should be set by the user and which can be measured with an auto-tune test.

1: Basic
This test measures the basic control parameters without moving the motor.

1. A stationary test is performed to measure Stator Resistance (05.017) and Transient Inductance (05.024).

2: Improved
This test measures the parameters for improved performance by rotating the motor.

1. Auto-tune test 1 is performed.
2. A rotating test is performed in which the motor is accelerated with the currently selected ramps up to a frequency of Rated Frequency (05.006) x 2/3, and the frequency is maintained at that level for 4 seconds. Stator Inductance (05.025) is measured and this value is used in conjunction with other motor parameters to calculate Rated Power Factor (05.010). The motor should be unloaded for this test.

The table below shows the trips that can occur during an auto-tune test:

Low load power saving is intended for applications where power loss should be kept to a minimum under low load conditions, but dynamic performance is not important. The reduction in power loss under low load conditions is achieved by increasing the rated frequency used to derive the frequency to voltage characteristic of the drive with reduced load. If Low Load Power Saving (05.013) = 0 then Rated Frequency (05.006) is used directly to define the output voltage characteristic. If Low Load Power Saving (05.013) = 1 then a modified value of rated frequency is used when |Percentage Load (04.020)| < 70%.

rated frequency = Rated Frequency (05.006) x [2 - |Percentage Load (04.020)| / 70.0%]

For higher load levels Rated Frequency (05.006) is used directly.

Open-loop Control Mode (05.014) defines the drive output mode as given below. It should be noted that the maximum output voltage of the drive is limited to a level just below D.c. Bus Voltage (05.005) / √2. Therefore, if the drive is being supplied via its own rectifier input stage the output voltage is limited to a level just below that of the supply voltage. The output voltage is limited to Rated Voltage (05.009) or the maximum possible output voltage whichever is the lowest. (If Quasi-square Enable (05.020) = 1 the maximum possible output voltage can be increased. See Quasi-square Enable (05.020) for details.)

0: Ur S (Resistance compensation, stator resistance measured at each start)
Resistance compensation is a form of stator flux oriented sensorless motor control. A linear frequency to voltage characteristic is used where the drive output voltage is increased from 0V to Rated Voltage (05.009) as the |Output Frequency (05.001)| increases from 0Hz to Rated Frequency (05.006). When the |Output Frequency (05.001)| is above Rated Frequency (05.006) the output voltage is limited to Rated Voltage (05.009). Vector based stator resistance compensation is applied below Rated Frequency (05.006)/4 and then this is tapered out from Rated Frequency (05.006)/4 to Rated Frequency (05.006)/2. This method controls the flux level correctly in the motor in the steady state provided the correct value of Stator Resistance (05.017) is used.

The Stator Resistance (05.017) is measured each time the drive is started. This test can only be done with a stationary motor where the flux has decayed to zero. Therefore this mode should only be used if the motor is guaranteed to be stationary each time the drive is enabled. To ensure that the measurement is not carried out before the flux has decayed, there is a period of one second after the inverter has been disabled during which the test is not carried out if the drive is re-started. The Stator Resistance (05.017) is not automatically saved in non-volatile memory after each test. If the resistance is higher than the maximum value allowed for the drive size, because the drive has a higher power rating than the motor for example, the drive does not trip but uses the maximum value allowed. This will enable the drive to run the motor, although the torque at low speeds will be less than could be achieved if the drive and motor are more closely matched. The maximum resistance value allowed is (VM_DC_VOLTAGE / √2) / Full Scale Current Kc (11.061).

1: Ur (Resistance compensation with no stator resistance measurement)
Resistance compensation is used as in Ur S mode, but the Stator Resistance (05.017) is not measured.

2: Fixed (Fixed boost with linear characteristic)
A fixed frequency to voltage characteristic is used as shown below where the voltage at 0Hz is defined by Low Frequency Voltage Boost (05.015).

3: Ur Auto (Resistance compensation, stator resistance measured on first start)
Resistance compensation is used as in Ur S mode, but the Stator Resistance (05.017) is only measured once when the drive is first enabled. After the test has been completed successfully the mode is changed to Ur mode and Stator Resistance (05.017) is saved to non-volatile memory. If Parameter Cloning (11.042) is set to 3 or 4 the Stator Resistance (05.017) is also written to a non-volatile media card fitted in the drive. If the test fails because the resistance is too high the mode is changed to Ur mode and the maximum allowed value is used for the stator resistance.

4: Ur I (Resistance compensation, stator resistance measured at power-up)
Resistance compensation is used as in Ur S mode, but the Stator Resistance (05.017) is only measured when the drive is enabled for the first time after each power-up.

5: Square (Fixed boost with square characteristic)
A fixed square frequency to voltage characteristic is used as shown below. When the |Output Frequency (05.001)| is below Rated Frequency (05.006) the Output Voltage (05.002) is given by:

Output Voltage (05.002) = Low Frequency Voltage Boost (05.015) + [Rated Voltage (05.009) - Low Frequency Voltage Boost (05.015)] x (Output Frequency (05.001) / Rated Frequency (05.006))²]

6: Sync Motor (Synchronous motor control)
This selects a mode that can be used with synchronous reluctance motors. The control method cannot provide the same level of robust control and stability that can be achieved with induction motors but can be used for simple low performance applications. The following should be noted about this control method.

1. If the motor is loaded too much it is possible for it to "pole slip" which means that it loses synchronisation with the supply from the drive. The drive current limit should be set up so that the drive output frequency is reduced before the load level is sufficient for the motor to pole slip. If this is not the case, particularly above rated speed when flux weakening is active, it is possible for the motor to partially stall and run at half synchronous speed. The current limit operates on the magnitude of the motor current and only reduces the speed towards zero for a motoring load, and so only Motoring Current Limit (04.005) and Symmetrical Current Limit (04.007) affect the current limit. A current limit is not provided for overhauling loads. Above rated speed the current limit is reduced because the available torque is reduced. The default reduction of the current limit above rated speed may be too conservative, and so this characteristic can be modified with Synchronous Motor Flux Weakening Limit (05.067) to allow the motor to produce more torque. 
2. Particularly if the motor is operating above rated speed and the current limit becomes active when standard ramp is being used an over-voltage trip may occur because standard ramp is overridden by the current limit. If this occurs either the load should be reduced, or fast ramps and a braking resistor should be used.
3. To avoid pole slipping with a load applied to the motor the V to F characteristic must be set up so that the rated voltage is reached before the frequency reaches rated frequency. The gradient is controlled by Synchronous Motor VF Characteristic Slope Boost (05.066) where a value of 0% gives rated voltage at rated frequency. The default value of 20%, which boosts the voltage characteristic by 20%, is suitable for most motors. Increasing the boost will allow higher torque to be produced but increases the current with no load.
4. Synchronous reluctance motors have no damper windings, and so the drive must provide damping or else the motor will oscillate. The damping is adjusted with Synchronous Motor Damping (05.065) which defines the dynamic change of frequency for rated motor power. This does not give a droop of frequency with applied load but only affects the motor frequency under transient conditions.
5. Fixed-boost is automatically selected when Open-loop Control Mode (05.014) is set to 6. The boost is controlled with Low Frequency Voltage Boost (05.015).
6. If more efficient operation is required when operating with light loads Low Load Power Saving (05.013) should be set to reduce the no-load current. If dynamic v to f is used it may be necessary to increase Synchronous Motor Damping (05.065) for the motor to be stable at low speeds. 
7. Auto-tuning is not necessary for synchronous reluctance motor control, and it is not important for non-zero values to be set in Maximum Deadtime Compensation (05.059)  or Current At Maximum Deadtime Compensation (05.060). However, the following parameters should be set up correctly from the motor nameplate: Rated Frequency (05.006), Rated Current (05.007), Rated Voltage (05.009) and Rated Speed (05.008). Number Of Motor Poles (05.011) can either be left at "Auto" for a 4 pole motor, or set to the number of poles for the motor.
8. Supply loss ride through and injection braking are not provided in this mode.
9. Catch a spinning motor can be enabled or disabled as required with Catch A Spinning Motor (06.009).
10. It is difficult to predict the torque based on the motor current because the torque depends on the motor inductance which varies with current. To obtain an indication of torque (motor load) the rated motor torque should be set in Rated Torque (04.041) and Percentage Torque (04.026) should be used to give the torque or load indication. Auto-tuning is not provided to measure the stator resistance of the motor, but the calculation of torque can be improved by putting an estimate for the stator resistance in Stator Resistance (05.017).
11. The default gains are suitable for most motors, however the following adjustments are possible: Spin Start Boost (05.040) adjusts the current controller gain for spin-start and standard ramp control. This may need to be increased if spin-start fail to work or standard ramp control is unstable especially above rated speed; Voltage Controller Gain (05.031) adjusts the d.c. link voltage controller gain used for standard ramp control.

The default value for this parameter depends on the frame size of the drive as follows:

• 3.0% up to frame size 06 drives,
• 2.0% for frame size 07 and frame size 08 drives
• 1.0% for larger sizes

See Open-loop Control Mode (05.014).

The Stator Resistance (05.017), Transient Inductance (05.024) and Stator Inductance (05.025) are derived from the star connected per phase equivalent circuit of an induction motor shown below.

The steady state parameters are converted to equivalent transient model parameters:

R_s = R₁

L_m = L_m

L_s = L₁ + L_m

L_r = L₂ + L_m

σL_s = L_s - (L_m² / L_r)

The equivalent drive parameters are:

Stator Resistance (05.017) = R_s

Transient Inductance (05.024) = σL_s

Stator Inductance (05.025) = L_s

Stator Resistance (05.017) is used as described in the table below.

Although the maximum value of this parameter is 1000ohms the maximum resistance is limited to (VM_DC_VOLTAGE / √2) / Full Scale Current Kc (11.061). If the parameter is set to a higher value a Resistance.001 trip is initiated.

Maximum Switching Frequency (05.018) should be set to the required PWM switching frequency. The drive inverter will operate at this frequency unless the inverter temperature becomes too hot. Under these conditions the drive will reduce the switching frequency in an attempt to avoid tripping  (see Auto-switching Frequency Change (05.035) ). The actual switching frequency is shown in Switching Frequency (05.037). The switching frequency has a direct effect on the sample rate for the current controllers (see Current Controller Kp Gain (04.013)). All other control tasks are at a fixed rate.

If High Stability Space Vector Modulation (05.019) = 0 standard space vector modulation is used to derive the inverter control signals. If High Stability Space Vector Modulation (05.019) = 1 a modified form of space vector modulation is selected which can give the following advantages:

1. With some induction motors instability can occur when operating below Rated Frequency (05.006) / 2 especially with light loads and high switching frequency. The preferred method to solve this problem is to use RFC-A mode without position feedback. However, if Open-loop mode is required, the instability can sometimes be reduced by selecting high stability space vector modulation.
2. As the output voltage approaches the maximum available from the drive pulse deletion occurs. This can cause unstable operation with a lightly or fully loaded motor. High stability space vector modulation will reduce this effect.

The disadvantages of using high stability space vector modulation are:

1. Increased acoustic noise from the motor.
2. A reduction in the thermal capability of the drive at low output frequencies.

If Quasi-square Enable (05.020)  = 0 then the output of the drive is limited to a modulation index of unity, which limits the output voltage to a level just below D.c. Bus Voltage (05.005) / √2. Therefore, if the drive is being supplied via its own rectifier input stage the output voltage is limited to a level just below that of the supply voltage or Rated Voltage (05.009) whichever is lower.

If Quasi-square Enable (05.020) = 1 then the limit imposed by Rated Voltage (05.009) is ignored and the modulation index is allowed to increase beyond unity. As the output voltage increases the output voltage changes through trapezoidal waveforms to a quasi-square wave output. This mode is useful where the ratio between the switching frequency and the fundamental output frequency is less than 12, as it removes any sub-harmonic effects that occur. However, it does produce significant multiple odd harmonics of the output frequency in the output currents which cause acoustic noise, torque ripple and motor heating.

See Stator Resistance (05.017).

Transient Inductance (05.024)  is used as described in the table below.

See Stator Resistance (05.017).

Stator Inductance (05.025) is used as described in the table below.

See Rated Frequency (05.006).

Voltage Controller Gain (05.031) can be used to modify the proportional gain of the d.c. link voltage controller used for standard ramp and supply loss control.

The inverter can be damaged if the temperature is too high. The inverter can also be damaged or the lifetime of the power devices reduced, if the temperature ripple of the devices is too high. Auto-switching Frequency Change (05.035) defines the action taken if the drive inverter becomes too hot or the temperature ripple becomes too high. The inverter temperature is provided in Inverter Temperature (07.034).

Enabled:
If the inverter becomes too hot or the ripple temperature is higher than the level defined by Maximum Inverter Temperature Ripple (05.039) the switching frequency is reduced in an attempt to prevent tripping.

Disabled:
The switching frequency is not reduced, and so the drive will trip on OHt Inverter.100 if the inverter is too hot or the temperature ripple is too high.

No Ripple Detect:
The switching frequency is only reduced if the inverter temperature is too high. If the temperature ripple exceeds the level defined by Maximum Inverter Temperature Ripple (05.039) then the switching frequency is not reduced and the drive will trip on OHt Inverter.100.

The switching frequency is changed in steps defined by Auto-switching Frequency Step Size (05.036). For example with a switching frequency of 16kHz and a step size of two, the frequency will be reduced to 8kHz, then 4kHz etc. Minimum Switching Frequency (05.038) defines the minimum switching frequency that the system will attempt to use. If the switching frequency needs to switch to a lower level, then the drive will trip on OHt Inverter.100. If Minimum Switching Frequency (05.038) is changed the new value will only become active when Switching Frequency (05.037) is at or above the minimum value.

See Auto-switching Frequency Change (05.035).

Shows the actual inverter switching frequency after the auto-change function.

See Auto-switching Frequency Change (05.035).

Maximum Inverter Temperature Ripple (05.039) defines the maximum inverter temperature ripple which the drive uses to manage the inverter stage temperature. On reaching the maximum inverter temperature ripple the drive will reduce its switching frequency using Maximum Switching Frequency (05.018), Minimum Switching Frequency (05.038) and Auto-switching Frequency Step Size (05.036) where allowable. If the drive is unable to reduce its switching frequency and the temperature continues to exceed the maximum inverter temperature ripple a OHt Inverter.100 trip will be generated. See Auto-switching Frequency Change (05.035).

Open-loop Control Mode (05.014) not equal to 6:

Spin Start Boost (05.040) is used by the algorithm that detects the speed of a spinning motor when the drive is enabled and Catch A Spinning Motor (06.009) ≥ 1. For most motors Spin Start Boost (05.040) does not need to be changed from the default value, but for some larger motors Spin Start Boost (05.040) may need to be increased. If Spin Start Boost (05.040) is too small the drive will detect zero speed whatever the speed of the motor, and if Spin Start Boost (05.040) is too large the motor may accelerate away from standstill when the drive is enabled.

Open-loop Control Mode (05.014) equal to 6:

If Open-loop Control Mode (05.014) is set to 6 (Sync Motor) then this parameter controls the current loop proportional gain used during motor speed detection. The default value of 1.0 should be suitable for most motors, but can be used to adjust the gain if the motor inductance is very high (increased value required) or low (reduced value required).

If Reverse Output Phase Sequence (05.042) = 0 the output phase sequence is U-V-W when Output Frequency (05.001) is positive and W-V-U when Output Frequency (05.001) is negative. If Reverse Output Phase Sequence (05.042) = 1 the output phase sequence is reversed so that the phase sequence in W-V-U for positive frequencies and U-V-W for negative frequencies.

The following parameters are used to adjust the control system when the drive is operating in Open-loop mode and controlling a synchronous reluctance motor (i.e. Open-loop Control Mode (05.014) = 6):

Synchronous Motor Damping (05.065)
Synchronous Motor VF Characteristic Slope Boost (05.066)
Synchronous Motor Dynamic V to F Threshold (05.068)
Synchronous Motor Flux Weakening Limit (05.067)

Most synchronous reluctance motors have no damper windings, and so the drive must provide damping or else the motor will oscillate. The damping is adjusted with Synchronous Motor Damping (05.065) which defines the dynamic change of frequency for rated motor power. This does not give a droop of frequency with applied load but only affects the motor frequency under transient conditions. The default value is suitable for most applications but the level of damping can be increased by increasing Synchronous Motor Damping (05.065) if necessary. 

To control a synchronous reluctance motor the V to F characteristic must be steeper so that the rated voltage is reached before the frequency reaches rated frequency to ensure that the motor does not go unstable or stall during speed changes or when load is applied. The gradient is controlled by Synchronous Motor VF Characteristic Slope Boost (05.066) where a value of 0% gives rated voltage at rated frequency. The default value of 20%, which boosts the voltage characteristic by 20%, is suitable for most motors. Increasing the boost will allow higher torque to be produced but increases the current with no load.

When operating a synchronous reluctance motor with V to F (Open-loop) control the voltage must be high enough to allow the motor to operate at maximum load but this causes significant "magnetising current" with light loads. To reduce this current dynamic V to F control can be enabled (see Low Load Power Saving (05.013)) however the reduction of voltage at lower speed can make the motor unstable particularly when transient loads are applied. To avoid this dynamic V to F control does not become active until the frequency exceeds the level defined by Synchronous Motor Dynamic V to F Threshold (05.068). The default value should prevent stability problems when dynamic V to F control is enabled provided the voltage boost (Low Frequency Voltage Boost (05.015)) is sufficient for the application. If required the threshold can be reduced to extend the range for dynamic V to F control or increased to avoid stability problems.      

It is important that a synchronous motor does not pole slip and lose synchronisation during fast acceleration or when a transient load is applied. It is possible, particularly when flux weakening is active, for the motor to run at half synchronous speed if a high transient load is applied and synchronisation loss occurs. To prevent synchronisation loss during acceleration sufficiently long ramp rates should be used. When a transient load is applied the current limit system, which limits the motor current magnitude, attempts to modify the drive output frequency to prevent synchronisation loss. It is important that the current limit is set to a low enough level to ensure that the maximum torque of the motor is not exceeded. When flux weakening is active the current limit is reduced in proportion with 1/Final Frequency Reference (03.001). This may be too conservative and reduce the maximum torque more than required. The current limit can be increased independently from the normal current limit settings with Synchronous Motor Flux Weakening Limit (05.067). The diagram below shows the effect of Synchronous Motor Flux Weakening Limit (05.067) on the current limits.

This parameter only applies when the drive is operating in Open-loop mode and controlling a synchronous reluctance motor (i.e. Open-loop Control Mode (05.014) = 6). 

See Synchronous Motor Damping (05.065).

This parameter only applies when the drive is operating in Open-loop mode and controlling a synchronous reluctance motor (i.e. Open-loop Control Mode (05.014) = 6). 

See Synchronous Motor Damping (05.065).

This parameter only applies when the drive is operating in Open-loop mode and controlling a synchronous reluctance motor (i.e. Open-loop Control Mode (05.014) = 6). 

See Synchronous Motor Damping (05.065).