Menu 5 − Motor Control

Mode: RFC‑A

The output frequency is not controlled directly, but the Output Frequency (05.001) is a measurement of the frequency applied to the motor.

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.

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

Set this to the motor name plate rated frequency e.g. 50Hz or 60Hz.

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 which is used by the motor control algorithm. An incorrect estimate of rated slip has the following effects:

1. Reduced efficiency
2. Reduction of maximum torque available from the motor
3. Reduced transient performance
4. Inaccurate control of absolute torque in torque control modes
5. The drive will produce rated flux in the motor in the shortest possible time when it is enabled. Incorrect parameter values will affect the flux build-up time.

The rated speed on the motor nameplate is normally the value for a hot motor, however, some adjustment may be required when the drive is commissioned if this is inaccurate. Either a fixed value can be entered for Rated Speed (05.008) or the optimisation system within the drive may be used to automatically adjust the Rated Speed (05.008). It should be noted that the optimisation system does not operate when sensorless RFC-A mode is used (i.e. Sensorless Mode Active (03.078) = 1).

Rated Current (05.007) is used as follows:

See Rated Frequency (05.006).

The Rated Voltage (05.009) is the maximum continuous voltage that is applied to the motor. Normally this should be set to the motor nameplate value. If the drive is supplied through its own diode rectifier the maximum possible output voltage is just below the supply voltage level, and so the output voltage will not reach Rated Voltage (05.009) if this is equal to or above the supply voltage. If high transient performance is required at higher speeds then Rated Voltage (05.009) should be set to 95% of the minimum d.c. link voltage divided by √2 to allow some headroom for the drive to control the motor current. If the drive is fed through its own diode rectifier the minimum d.c. link voltage is approximately supply voltage x √2.

In some cases it may be necessary to set the Rated Voltage (05.009) to a value other than the motor nameplate value. If this is the case the Rated Frequency (05.006) and Rated Speed (05.008) should be set up as follows:

K = Rated Voltage (05.009) / motor rated voltage

Rated Frequency (05.006) = motor rated frequency x K

Rated Speed (05.008) = motor rated speed + [(K - 1) x motor rated frequency x 60 / (number of motor poles / 2)]

The Rated Voltage (05.009), Rated Frequency (05.006) and Number Of Motor Poles (05.011) are used during the auto-tuning process to determine the flux level required in the motor for normal operation. Therefore if the Rated Voltage (05.009) is set to a value other than the nameplate value and the above adjustment is not applied the motor may be under or over-fluxed

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.

* The units relate to the numeric value of the parameter and not the text string as shown on the keypad.

The numeric value in Number Of Motor Poles (05.011) should be set to the number of motor pole pairs (i.e. number of motor poles / 2). The text strings associated with Number Of Motor Poles (05.011) show the number of motor poles (i.e. the parameter value x 2). If a linear position feedback device is used Number Of Motor Poles (05.011) should be set to 1 (2 Poles).

If Number Of Motor Poles (05.011) = 0 (Automatic) the number of motor poles are calculated automatically as given below.

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

During an autotune when position feedback is being used, the drive will check to ensure that the combination of motor poles and position feedback resolution have been set up correctly, and will produce an Autotune 7 trip if this is not the case. The Autotune 7 trip will not occur if Number Of Motor Poles (05.011) ≥ 6 (i.e. 12 poles).

The following describes how an auto-tune test can be initiated and normal operation can be resumed after the test for RFC-A 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 RFC-A 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 completed 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). If Enable Stator Compensation (05.049) = 1 then Stator Base Temperature (05.048) is made equal to Stator Temperature (05.046).
2. Stator Resistance (05.017) and Transient Inductance (05.024) are used to set up Current Controller Kp Gain (04.013) and Current Controller Ki Gain (04.014). This is only performed once during the test, and so the user can make further adjustments to the current controller gains if required.

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

1. Auto-tune 1 test 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 up to 40 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.

If Flux Optimisation Select (05.013) = 0 the rated level of flux is used in the motor when flux weakening is not active. If Flux Optimisation Select (05.013) = 1 the flux is reduced so that the Id, Magnetising Current (04.017) is approximately equal to the Iq, Torque Producing Current (04.002) to optimise copper losses and reduce iron losses in the motor under low load conditions. The flux can be reduced from the rated level down to the level defined by Flux Optimisation Minimum (05.014) percent of the rated level. This feature is not available with sensorless mode (i.e. when Sensorless Mode Active (03.078) = 1).

See Flux Optimisation Select (05.013) for details.

The default value of 1% is suitable for most Pump systems where the torque required to start is relatively small. For a waste water system where ragging or a build up of material inside the pump is likely, an increased percentage can be used to get the pump started e.g. 2% or 3%.

During auto-tune test 2 the drive uses the Open-loop mode control strategy with fixed voltage boost. Low Frequency Voltage Boost (05.015) is used to define the level of low voltage boost used during the test.

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.

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.

The output voltage is limited to the voltage limit defined either by the maximum possible drive output voltage or Rated Voltage (05.009) whichever is lowest by the outer voltage controller shown below. If the voltage limit is not active then the output of the voltage controller is clamped to give rated flux in the motor. An inner flux controller is provided to control the magnetising current which is limited by Magnetising Current Limit (04.049). Using an inner flux controller gives faster rate of rise of flux on enable because a magnetising current higher than rated magnetising current can be used to force the flux to increase. It also gives faster reduction of flux as the motor accelerates into flux weakening because negative flux producing current can be used to reduce the flux.

Default set-up
If Flux Control Gain (05.027) is set to the default value of 1.0 the system that controls the flux and motor voltage is automatically set up based on the motor parameters to give stable operation. The closed-loop transfer function of the flux controller approximates to a first order time constant of 25ms, and the closed-loop transfer function of the voltage controller approximates to a first order time constant of 50ms. The automatic set-up of the flux controller can result is very high gains if the motor has a very long rotor time constant (i.e. greater than 0.5s), and this can result in noise on the magnetising current. To avoid this the gain is limited, and so the response time of the flux and voltage controllers will reduce proportionally when the rotor time constant exceeds 0.5s. For example if the rotor time constant is 1s then the response time of the controllers will be doubled (i.e. flux control 50ms and voltage control 100ms).

Improved voltage control
The integral gain of the voltage controller is set to a conservative value to ensure the system is always stable. If Flux Control Gain (05.027) is changed from the default value of 1.0 in the range from 0.1 to 10.0 the integral gain of the voltage controller is multiplied by Flux Control Gain (05.027). This is most useful to increase the integral gain of the voltage controller, so that the motor voltage is more tightly controlled during fast acceleration into the flux weakening region. The only gain that is affected is the integral gain of the voltage controller and the maximum limit on the flux controller gains described in the default set-up still applies.

Reduced noise on the magnetising current
The maximum limit on the flux controller gains should prevent excessive noise on the magnetising current when the motor has a long rotor time constant. However, if Flux Control Gain (05.027) is reduced below -1.0 the time constants of the flux controller and voltage closed-loop transfer function are extended. For example a value of -2.0 will extend these to 50ms and 100ms respectively. It should be noted that if the motor has a long rotor time constant, smaller negative values may not have an effect because the maximum limits on the flux controller gain may already be extending the time constants.

The settings for Flux Control Gain (05.027) are summarised below.

In all cases the controller gains are limited with a rotor time constant that is longer than 0.5s.

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.

Percentage Flux (05.034) gives an indication of the flux level in the motor where a value of 100% is equivalent to the rated flux level for the motor.

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).

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. This algorithm is not used if position feedback is being used, and so in these applications Spin Start Boost (05.040) has no effect. 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.

When the motor voltage controller limit is active, the voltage applied to the motor is always limited by Rated Voltage (05.009). When Voltage Headroom (05.041) is set to its default value of zero the output voltage of the inverter is also limited to a level equivalent to full modulation, which is the supply voltage minus voltage drops within the inverter itself. Depending on the relative values of the supply voltage and Rated Voltage (05.009) there may be some headroom between the rated voltage limit and the maximum possible voltage from the inverter to allow the current control system to give good dynamic performance. In some applications it is useful to enforce some headroom between the maximum allowed motor voltage and the inherent limit imposed by the inverter. If the supply voltage is known this can be done by setting Rated Voltage (05.009) to a suitable value below the supply voltage level, however, it is more convenient to set Rated Voltage (05.009) to the actual rated voltage of the motor, and to use Voltage Headroom (05.041) to enforce the voltage headroom. This parameter can be used to increase the headroom between the maximum modulation limit and the maximum motor voltage from zero up to 20% of the maximum modulation limit. For example, if the supply voltage is 400V then a value of 10% will give a voltage headroom of approximately 40V.

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.