Menu 4 − Torque and current control

Mode: RFC‑A

RFC-A and RFC-S modes

The diagram below is common to both RFC-A and RFC-S modes.

The drive can have a heavy duty rating intended for applications where high overload current may be required under transient conditions, or it can have a normal duty rating where a lower level of overload current is required. The duty rating is selected automatically by the drive based on the setting of Rated Current (05.007). The Maximum Heavy Duty Rating (11.032) and Maximum Rated Current (11.060) are fixed for each drive size and the table below shows the possible duty ratings that can be selected depending on the levels of these parameters.

The different duty ratings modify the motor protection characteristic (see Motor Thermal Time Constant 1 (04.015)), and also change the maximum settings for the current limits.

In a drive that contains multiple power modules Full Scale Current Kc (11.061) is the full scale current of an individual module multiplied by the number of modules. Maximum Heavy Duty Rating (11.032) and Maximum Rated Current (11.060) are the value for an individual module multiplied by the number of modules.

RFC-A mode

The torque reference is provided by the speed controller. During supply loss or when standard ramp mode is selected and the motor is regenerating it is possible that the torque producing current reference may be provided by the d.c. bus voltage controller as shown above. The torque reference is converted to the torque producing current reference based on the level of motor flux and then current limits are applied. For more details of the current limit system and the calculation of the variable maximums that limit the current limit user parameters (VM_MOTOR1_CURRENT_LIMIT) see the "Drive Limits" document included in this parameter reference guide. 

RFC-S mode

The torque reference is provided by the speed controller. During supply loss or when standard ramp mode is selected and the motor is regenerating it is possible that the torque producing current reference may be provided by the d.c. bus voltage controller as shown above. The torque reference is converted into the final current reference and can include the effect of changes in magnet flux and then the current limits are applied. For more details of the current limit system and the calculation of the variable maximums that limit the current limit user parameters (VM_MOTOR1_CURRENT_LIMIT) see the "Drive Limits" document included in this parameter reference guide. The final current reference defines the current magnitude that should be applied to the motor which is split into d and q axis components depending on the Rated Torque Angle (05.089). Either the d or q axis current may be further modified to limit the motor voltage when flux weakening is required.

Instantaneous level of fundamental current at the drive output terminals in r.m.s. amps.

Iq, Torque Producing Current (04.002) is the instantaneous level of torque producing current scaled so that it represents the r.m.s. level of current under steady state conditions. Iq, Torque Producing Current (04.002) is proportional to the torque produced by the motor provided field weakening is not active. For field weakening operation the Iq, Torque Producing Current (04.002) is boosted for a given level of torque to compensate for the reduction in the motor flux.

The Speed Controller Output is converted into the Final Current Reference (04.004) by applying a torque to current conversion and by applying the Final Current Limit (04.018). The torque to current conversion is applied as follows:

Current reference = Speed Controller Output x Motor Rated Flux / Motor Flux

It is possible to disable, or change the method for the flux compensation using Flux Compensation (05.028). This may be required, for example to prevent motor instability when an inaccurate value for Rated Speed (05.008) is being used.

The Motoring Current Limit (04.005) limits the current when the motor is being accelerated away from standstill. The Regenerating Current Limit (04.006) limits the current when the motor is being decelerated towards standstill. If the Symmetrical Current Limit (04.007) is below the Motoring Current Limit (04.005) then it is used instead of the Motoring Current Limit (04.005). If the Symmetrical Current Limit (04.007) is below the Regenerating Current Limit (04.006) then it is used instead of the Regenerating Current Limit (04.006).

The maximum possible current limit (VM_MOTOR1_CURRENT_LIMIT [MAX]) varies between drive sizes with default parameters loaded. For some drive sizes the default value may be reduced below the value given by the parameter range limiting.

See Motoring Current Limit (04.005).

See Motoring Current Limit (04.005).

.

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. The time constant used is dependent on Speed Controller Gain Select (03.016) so that different time constants can be used with different gains.

If Low-pass Filter Cut-off Frequency (04.050) is set to a value of 10Hz or greater then it overrides the settings of Current Reference Filter 1 Time Constant (04.012).

Current Controller Kp Gain (04.013) and Current Controller Ki Gain (04.014) are the proportional and integral gains of the current controllers. It is possible to use the current controller in standard mode (Current Controller Mode (04.030) = 0) or high performance mode (Current Controller Mode (04.030) = 1). The set up method for the current controller gains is described separately for each of these modes below. It should be noted that when an auto-tune is performed that measures the Transient Inductance (05.024) and Stator Resistance (05.017) the Current Controller Kp Gain (04.013) and Current Controller Ki Gain (04.014) are automatically set to the levels defined in the description for standard mode even if high performance mode is selected. These gains will give good performance in standard mode and produce moderate acoustic noise due to position feedback quantisation with a standard incremental encoder. These represent the maximum levels that are likely to be used with this mode in most applications. For high performance mode it is recommended that a high resolution position feedback device is used or else the acoustic noise due to position feedback quantisation is likely to be excessive. In high performance mode the proportional gain can be increased to a higher level as given in the description of this mode.

Standard mode
Standard mode can be used to give good current control dynamic performance and is compatible with the performance of Unidrive SP. The current controller gains can either be set using auto-tuning (see Auto-tune (05.012)) or the values can be set up manually by the user. The calculations given below are those used by the auto-tuning system and should give good performance without excessive overshoot.

The proportional gain, Current Controller Kp Gain (04.013), is the most critical value in controlling the performance of the current controllers. The required value can be calculated as

Current Controller Kp Gain (04.013) = (L / T) x (I_fs / V_fs) x (256 / 5)

where:

T is the sample time of the current controllers. The drive compensates for any change of sample time, and so it should be assumed that the sample time is equivalent to the base value of 167μs.

L is the motor inductance. For a servo motor this is half the phase to phase inductance that is normally specified by the manufacturer. For an induction motor this is the per phase transient inductance (σLs). The inductance for either of these motors can be taken from the manufacturers data or it can be obtained from the value stored in the Transient Inductance (05.024) after auto-tuning.

I_fs is the peak full scale current feedback, i.e. full scale current x √2. The r.m.s. full scale current is given by Full Scale Current Kc (11.061), and so I_fs = Full Scale Current Kc (11.061) x √2.

V_fs is the maximum d.c. bus voltage.

Therefore:

Current Controller Kp Gain (04.013) = (L / 167μs) x (Kc x √2/ V_fs) x (256 / 5) = K x L x Kc

Where K = [√2 / (V_fs x 167μs)] x (256 / 5)

There is one value of the scaling factor K for each drive voltage rating as shown in the table below.

The integral gain, Current Controller Ki Gain (04.014), is less critical. A suggested value which matches the zero with the pole caused by the electrical time constant of the motor and ensures that the integral term does not contribute to current overshoot is given by

Current Controller Ki Gain (04.014) = Current Controller Kp Gain (04.013) x 256 x T / τ_m

Where τ_m is the motor time constant (L / R). R is the per phase stator resistance of the motor (i.e. half the resistance measured between two phases).

Therefore:

Current Controller Ki Gain (04.014) = (K x L x Kc) x 256 x 167μs x R / L = 0.0427 x K x R x Kc

The above equations give the gain values that should give a good response with minimal overshoot. If required the gains can be adjusted to modify the performance as follows:

1. Current Controller Ki Gain (04.014) can be increased to improve the performance of the current controllers by reducing the effects of inverter non-linearity. These effects become more significant with higher switching frequency. These effects will be more significant for drives with higher current ratings and higher voltage ratings. If Current Controller Ki Gain (04.014) is increased by a factor of 4 it is possible to get up to 10% overshoot in response to a step change of current reference. For high performance applications, it is recommended that Current Controller Ki Gain (04.014) is increased by a factor of 4 from the auto-tuned values. As the inverter non-linearity is worse with higher switching frequencies it is may be necessary to increase Current Controller Ki Gain (04.014) by a factor of 8 for operation with 16kHz switching frequency.
2. It is possible to increase Current Controller Kp Gain (04.013) to reduce the response time of the current controllers. If Current Controller Kp Gain (04.013) is increased by a factor of 1.5 then the response to a step change of reference will give 12.5% overshoot. It is recommended that Current Controller Ki Gain (04.014) is increased in preference to Current Controller Kp Gain (04.013).

As already stated, the drive compensates for changes of switching frequency and the sampling method used by the controller. The table below shows the adjustment applied to the proportional and integral gains.

The amount of acoustic noise produced in the motor from position feedback quantisation is related to the resolution of the position feedback and the product of the speed controller and current controller proportional gains. The values in this table can be used in conjunction with the speed controller loop proportional gain to assess the amount of acoustic noise that is likely to be produced.

High performance mode
High performance mode gives fast closed-loop dynamic performance as though the proportional gain has been set to the maximum value defined below. This is the maximum value that should be used to prevent excessive over-shoot or instability. It should be noted that this is 5 times the maximum value used for standard mode. 

Current Controller Kp Gain (04.013) = (L / T) x (I_fs / V_fs) x 256 = K x L x Kc x 5

The closed-loop dynamic performance defines the response of the current controllers to a change of current reference. This response cannot be changed by modifying Current Controller Kp Gain (04.013), however the ability of the current controllers to reject voltage disturbances is affected by Current Controller Kp Gain (04.013). Normally the auto-tuned value (which is one fifth of the maximum recommended value) will give good rejection of voltage disturbances, but the proportional gain can be increased up to the maximum value to improve this. It should be noted that the higher closed-loop response of the controllers means that encoder position quantisation will cause significant acoustic noise in the motor unless a high resolution encoder is used. Increasing Current Controller Kp Gain (04.013) also increases acoustic noise due to noise on the current feedback. High performance mode uses the measured motor resistance and inductance, and so it is recommended that these are obtained with auto-tuning using test 1 or 2.

The integral gain provides a trim on the currents, and generally the auto-tuned value should be sufficient, however, this may be increased if required.

The drive compensates for changes of switching frequency used by the controller. The table below shows the adjustment applied to the proportional and integral gains.

See Current Controller Kp Gain (04.013).

A dual time constant thermal model is provided that can be used to estimate the motor temperature as a percentage of its maximum allowed temperature. The input to the model is the Current Magnitude (04.001). It should be noted that if the parameters that have been added in addition to those in Unidrive SP are left at their default values the model is a simple single time constant model as provided in Unidrive SP.

Percentage Losses
The losses in the motor are calculated as a percentage value.

Percentage Losses = 100% x [Load Related Losses + Iron Losses]

where:
Load Related Losses = (1 - K_fe) x (I / (K₁ x I_Rated))²
Iron Losses = K_fe x (w / w_Rated)^1.6

where:
I = Current Magnitude (04.001)
I_Rated = Rated Current (05.007)
K_fe = Rated Iron Losses As Percentage Of Losses (04.039) / 100%

The iron losses are relatively low in motors that have a rated frequency of 60Hz or less, and so the motor could be modelled based on load related losses alone. This can be done by setting K_fe to zero. In motors where iron losses are significant, K_fe defines the proportion of losses that are iron losses under rated conditions (i.e. rated current and rated frequency). For example if the iron losses are 30% of losses and other losses are 70% of losses under rated conditions Rated Iron Losses As Percentage Of Losses (04.039) should be set to 30%.

The value of K₁ defines the continuous allowable motor overload as a proportion of the Rated Current (05.007) before the Motor Protection Accumulator (04.019) reaches 100%. The value of K₁ can be used to model reduced cooling at low speeds and to allow the motor to operate under rated conditions with a small margin to prevent spurious trips. K₁ is defined in more detail later.

Motor Protection Accumulator
So far the steady state motor losses have been defined, but the motor model must estimate the temperature within the motor under dynamically changing conditions, and so the Motor Protection Accumulator (04.019) is given by the following equation.

T = Percentage Losses x [(1 − K₂) (1 − e^-t/τ1) + K₂ (1 − e^-t/τ2)]

where
T = Motor Protection Accumulator (04.019)
K₂ = Motor Thermal Time Constant 2 Scaling (04.038) / 100%
τ1 = Motor Thermal Time Constant 1 (04.015)
τ2 = Motor Thermal Time Constant 2 (04.037)

[(1 − K₂) (1 − e^-t/τ1) + K₂ (1 − e^-t/τ2)] gives the effects of the thermal time constants in the motor. K₂ defines the ratio of the contribution to the Motor Protection Accumulator (04.019) value from each of the time constants. If K₂ is set to its default value of 0 then only Motor Thermal Time Constant 1 (04.015) is included and the model will give the temperature of the main mass of the motor body. To give better protection to the motor, the model can be used to model a particular point in the motor, for example the stator windings. This can be done by including an additional shorter time constant representing the thermal impedance between the windings and the main mass of the motor body which can be modelled with Motor Thermal Time Constant 2 (04.037).

Reduced cooling with lower speed
If Rated Current (05.007) ≤ heavy duty rating then K₁ is defined as shown below. If Low Speed Thermal Protection Mode (04.025) = 0 the characteristic is intended for a motor which can operate at rated current over the whole speed range. Induction motors with this type of characteristic normally have forced cooling. If Low Speed Thermal Protection Mode (04.025) = 1 the characteristic is intended for motors where the cooling effect of motor fan reduces with reduced motor speed below half of rated speed. The maximum value for K₁ is 1.05, so that above the knee of the characteristics the motor can operate continuously up to 105% of rated current.

If Rated Current (05.007) > heavy duty rating then K₁ is defined as shown below. Two different characteristics are provided, but in both cases the motor performance is limited at lower speeds and the permissible overload is reduced from 105% to 101%.

Time for Motor Protection Accumulator to reach 100%
Assuming a single time constant model is being used (i.e. Motor Thermal Time Constant 2 Scaling (04.038), the time for the Motor Protection Accumulator (04.019) to change from its initial value to 100% is given by the following equation:

Time to reach 100.0% = -τ1 x ln[(1 − C₁) / (C₀ − C₁)]

C₀ represents the conditions that have persisted for long enough for the Motor Protection Accumulator (04.019) to reach a steady state value. If the motor current and speed are I₀ and w₀ then,

C₀ = [(1 - K_fe) x (I₀ / (K₁ x I_Rated))²] + [K_fe x (w₀ / w_Rated)^1.6]

C₁ represents the conditions that begin at the start of the time being calculated. If the motor current and speed are by I₁ and w₁ then,

C₁ = [(1 - K_fe) x (I₁ / (K₁ x I_Rated))²] + [K_fe x (w₁ / w_Rated)^1.6]

Example 1: The effect of iron losses are not modelled (K_fe = 0), Motor Thermal Time Constant 1 (04.015) = 89s, the initial current is zero, Rated Current (05.007) ≤ heavy duty rating and the new level of current is 1.5 x Rated Current (05.007).

C₀ = 0
C₁ = [1.5 / (1.05 x 1.0)]² = 2.041

Time to reach 100.0% = -89 x ln(1 − 1/C₁) = -89 x ln(1 − 1/2.041) = 60s

This is the default setting for Open-loop and RFC-A modes allowing an induction motor to run at 150% rated current for 60s from cold.

Example 2: The effect of iron losses are not modelled (K_fe = 0), Motor Thermal Time Constant 1 (04.015) = 89s, the initial current is Rated Current (05.007), Rated Current (05.007) ≤ heavy duty rating and the new level of current is 1.5 x Rated Current (05.007).

C0 = [1.0 / (1.05 x 1.0)]² = 0.907
C1 = [1.5 / (1.05 x 1.0)]² = 2.041

Time to reach 100.0% = -89 x ln((1 − C1) / (C0 − C1)) = -89 x ln[(1 − 2.041) / (0.907 − 2.041)] = 7.6s

This is the default setting for Open-loop and RFC-A modes allowing an induction motor to run at 150% rated current for 7.6s after running under rated conditions for a significant period of time.

Motor Protection Accumulator Reset
The initial value in the Motor Protection Accumulator (04.019) at power-up is defined by Motor Protection Accumulator Power-up Value (04.036) as given in the table below.

The Motor Protection Accumulator (04.019) is reset under the following conditions:

1. Motor Thermal Time Constant 1 (04.015) is set to 0.0. Note that this is not possible in the standard product as the minimum parameter value is 1.0.
2. Rated Current (05.007) is modified.
3. Thermal Protection Mode (04.016) is modified.

Motor Protection Accumulator Warning
If Percentage Losses > 100% then eventually the Motor Protection Accumulator (04.019) will reach 100% causing the drive to trip or the current limits to be reduced. If this is the case and Motor Protection Accumulator (04.019) > 75.0% then [Motor Overload] alarm indication is given and Motor Overload Alarm (10.017) is set to one.

Thermal Protection Mode (04.016) defines the action taken by the drive when Motor Protection Accumulator (04.019) reaches 100% and/or Percentage Of Drive Thermal Trip Level (07.036) exceeds 90%. The actions for each mode are given in the table below.

The current limit is derived from the current limit parameters (i.e. Motoring Current Limit (04.005), etc.) depending on the set-up and conditions. The current limit can be further limited by current limit on motor overload and/or drive temperature monitoring as shown below to give the Final Current Limit (04.018).

Current limiting on motor overload
When the Motor Protection Accumulator (04.019) reaches 100.0% the current limit is limited to (K₁ – 0.05) x 100.0%. This limitation is removed when the Motor Protection Accumulator (04.019) falls below 95.0%. (K₁ is defined in the description of Motor Thermal Time Constant 1 (04.015).)

Drive thermal monitoring current limiting
If Percentage Of Drive Thermal Trip Level (07.036) exceeds 90% the current limit is modified as follows:

Final Current Limit (04.018) = Current limit x (100% - Percentage Of Drive Thermal Trip Level (07.036)) / 10%

If both of the above attempt to reduce the final current limit the lowest calculated value of current limit is used.

This system has the effect of reducing the current limit to zero at the point where the drive should be tripped because its thermal monitoring has reached a trip threshold. This is intended to limit the load on the drive to prevent it from tripping when supplying a load that increases with speed and does not include rapid transients.

Id, Magnetising Current (04.017) is the instantaneous level of magnetising current scaled so that it represents the r.m.s. level of magnetising current under steady state conditions.

Final Current Limit (04.018) is the current limit level that is applied to the current controller when Current Limit Active (10.009) = On(1).

See Motor Thermal Time Constant 1 (04.015).

Percentage Load (04.020) gives the Iq, Torque Producing Current (04.002) as a percentage of the rated torque producing current for the motor.

With synchronous reluctance or high saliency permenant motors 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.

If Current Feedback Filter Disable (04.021) = 0 a 4ms filter is applied to the current feedback components measured by the drive to be used in Iq, Torque Producing Current (04.002) and Id, Magnetising Current (04.017). This filter removes ripple components associated with the PWM switching. If Current Feedback Filter Disable (04.021) = 1, the filter is disabled and the user parameters are based on the current components sampled every 250us.   

User Current Maximum Scaling (04.024) defines the variable maximum/minimums VM_USER_CURRENT and VM_USER_CURRENT_HIGH_RES which are applied to Percentage Load (04.020). This is useful when routing these parameters to an analog output as it allows the full scale output value to be defined by the user.

The maximum value (VM_TORQUE_CURRENT_UNIPOLAR [MAX]) varies between drive sizes with default parameters loaded. For some drive sizes the default value may be reduced below the value given by the parameter range limiting.

See Motor Thermal Time Constant 1 (04.015).

The shaft torque of the motor is estimated by the drive and Percentage Torque (04.026) gives this torque as a percentage of the expected torque defined by Rated Torque (04.041). The default value for Rated Torque (04.041) is zero which disables this feature so that Percentage Torque (04.026) is always zero. To enable the torque estimation system Rated Torque (04.041) should be set to the expected torque from the motor under rated conditions. 

If Low Load Detection Level (04.027) is set to 0.0 the low load detection system is disabled, otherwise the low load detection system is enabled. The low load detection system is provided so that loss of load can be detected and action taken. So that the detector can be used with fan and pump type loads, where the load is relatively light at low motor speed, the detector is only active when the output frequency or speed is above the level defined by Low Load Detection Speed/Frequency Threshold (04.028). The detector is also only enabled when the motor is at the required speed (i.e. not accelerating or decelerating), and so it is only active when At Speed (10.006) = 1. Once the detector is active, the low load condition is detected when the Percentage Load (04.020) falls below the threshold defined by Low Load Detection Level (04.027). Therefore the condition for detecting low load is given by,

At Speed (10.006) = 1 AND |Speed Feedback (03.002)| > Low Load Detection Speed/Frequency Threshold (04.028) AND |Percentage Load (04.020)| < Low Load Detection Level (04.027)

The diagram below shows a typical fan type load and the shaded areas define where low load is detected.

Enable Trip On Low Load (04.029) defines the action taken when low load is detected. If Enable Trip On Low Load (04.029) = 0 a Low Load warning is displayed and Low Load Detected Alarm (10.062) is set to one. If Enable Trip On Low Load (04.029) = 1 no warning is given, but a Low Load trip is initiated.

See Low Load Detection Level (04.027).

See Low Load Detection Level (04.027).

See Motor Thermal Time Constant 1 (04.015).

See Motor Thermal Time Constant 1 (04.015).

See Motor Thermal Time Constant 1 (04.015).

See Motor Thermal Time Constant 1 (04.015).

See Percentage Torque (04.026).

See Percentage Torque (04.026).

Magnetising Current Limit (04.049) defines the maximum level of magnetising current used as a percentage of Rated Current (05.007). The magnetising current is normally at the rated level for the motor, but may increase up to this limit when the drive is enabled to raise the flux in the motor as fast as possible. The magnetising current can also be increased above the rated level when the motor is decelerated rapidly from the flux weakening range. The default value for Magnetising Current Limit (04.049) is normally suitable, but may be decreased if required. The maximum level of magnetising current will not be decreased below the rated level for the motor how ever low the value in Magnetising Current Limit (04.049).

The current reference low-pass filter is normally set up using its time constant with Current Reference Filter 1 Time Constant (04.012). If this parameter is used the resolution of the cut-off frequency is poor with shorter time constants. If Low-pass Filter Cut-off Frequency (04.050) is less than 10Hz it has no effect. If it is set to 10Hz or above it overrides all other setting parameters and defines the cut-off frequency in Hz.