SIMOREG 6RA70 Siemens DC Master Variable Speed DC Drive
Design & Mode of Operation
Closed-Loop Functions in Armature Circuit

Design and Mode of Operation6RA70 DC Master

Closed-loop functions in armature circuit

Speed setpoint

The source for the speed setpoint and additional setpoints can be freely selected through parameter settings, I.e. the setpoint source can be programmed as:

  • Analog values 0 to +10 V, 0 to +20 mA, 4 to 20 mA
  • Integrated motorized potentiometer
  • Binectors with functions:
    Fixed setpoint, inch, crawl
  • Serial interfaces on basic unit
  • Supplementary boards

The normalization is such that 100 % setpoint (product of main setpoint and additional setpoints) corresponds to the maximum motor speed.

The speed setpoint can be limited to a minimum or maximum value by means of a parameter setting or connector. Furthermore, “adding points” are included in the software to allow, for example, additional setpoints to be injected before or after the ramp-function generator. The “Setpoint enable” function can be selected with a binector. After smoothing by a parameterizable filter (PT1 element), the total setpoint is transferred to the setpoint input of the speed controller. The ramp-function generator is effective at the same time.

Actual speed value

One of four sources can be selected as the actual speed signal.

  • Analog tachometer
    The voltage of the tacho-generator at maximum speed can be between 8 and 270 V. The voltage/maximum speed normalization is set in a parameter.
  • Pulse encoder
    The type of pulse encoder, the number of marks per revolution and the maximum speed are set via parameters. The evaluation electronics are capable of processing encoder signals (symmetrical: With additional inverted track or asymmetrical: Referred to ground) up to a maximum differential voltage of 27 V.
    The rated voltage range (5 V or 15 V) for the encoder is set in a parameter. With a rated voltage of 15 V, the SIMOREG converter can supply the voltage for the pulse encoder. 5 V encoders require an external supply. The pulse encoder is evaluated on the basis of three tracks, i.e. track 1, track 2 and zero marker. Pulse encoders without a zero marker may also be installed. The zero marker allows an actual position to be acquired. The maximum frequency of the encoder signals must not exceed 300 kHz. Pulse encoders with at least 1 024 pulses per revolution are recommended (to ensure smooth running at low speeds).
  • Operation without tachometer and with closed-loop EMF control 
    No actual-value sensor is needed if the closed-loop EMF control function is employed. Instead, the converter output voltage is measured in the SIMOREG. The measured armature voltage is compensated by the internal voltage drop in the motor (I*R compensation). The degree of compensation is automatically determined during the current controller optimization run. The accuracy of this control method is determined by the temperature-dependent change in resistance in the motor armature circuit and equals approximately 5 %. In order to achieve greater accuracy, it is advisable to repeat the current controller optimization run when the motor is warm. Closed-loop EMF control can be employed if the accuracy requirements are not particularly high, if there is no possibility of installing an encoder and if the motor is operated in the armature voltage control range.
    Caution: The drive cannot be operated in EMF-dependent field- weakening mode when this control method is employed.
  • Freely selectable actual speed signal
    Any connector number can be selected as the actual speed signal for this operating mode. This setting is selected in most cases if the actual speed sensor is implemented on a technological supplementary board. Before the actual speed value is transferred to the speed controller, it can be smoothed by means of a parameterizable smoothing (PT1 element) and two adjustable band filters. The band filters are mostly used in order to filter out resonant frequencies caused by mechanical resonance. The resonant frequency and filter quality can be selected.

Ramp-function generator

The ramp-function generator converts the specified setpoint after a step change into a setpoint signal that changes constantly over time. Ramp-up and ramp-down times can be set independently of one another. The ramp-function generator also features a lower and upper transition rounding (jerk limitation) which take effect at the beginning and end of the ramp time respectively.

All time settings for the ramp-function generator are mutually independent.

3 parameter sets are provided for the ramp-function generator times. These can be selected via binary selectable inputs or a serial interface (via binectors). The generator parameters can be switched over while the drive is in operation. The value of parameter set 1 can also be weighted multiplicatively via a connector (in order to change generator data by means of a connector). When ramp-function generator time settings of zero are entered, the speed setpoint is applied directly to the speed controller.

Speed controller

The speed controller compares the speed setpoint and actual value and, if these two quantities deviate, applies a corresponding current set point to the current controller (operating principle: Closed-loop speed control with subordinate current controller). The speed controller is a PI controller with additional selectable 0 component. A switchable speed droop can also be parameterized. All controller characteristics can be set independently of one another. The value of Kp (gain) can be adapted as the function of a connector signal (external or internal).

The P gain of the speed controller can be adapted as a function of actual speed, actual current, setpoint/actual value deviation or winding diameter.
To achieve a better dynamic response in the speed control loop, a feedforward control function can be applied by, for example, adding a torque setpoint quantity after the controller as a function of friction or drive moment of inertia. The friction and moment of inertia compensation values can be calculated in an automatic optimization run.

The output quantity of the speed controller directly after enabling can be set via a parameter.

Depending on how parameters are set, the speed controller can be bypassed and the converter operated under torque or current control. Furthermore, it is possible to switch between closed-loop speed control/closed-loop torque control in operation by means of selection function “Master/slave switch-over”. The function can be selected as a binary assignable-function terminal or a serial interface. The torque setpoint is applied by means of a selectable connector and can thus be supplied by an analog assignable-function terminal or a serial interface.

In “slave drive” operation (under torque or current control), a limiting controller is active. Here, the limiting controller can intervene on the basis of an adjustable, parameterized speed limit in order to prevent the drive from accelerating too far. In this case, the drive is limited to an adjustable speed deviation.

Torque limitation

Dependlng on parameterization, the speed controller output acts as either the torque setpoint or current setpoint. In closed-loop torque control mode, the speed controller output is weighted with machine flux F and then transferred as a current setpolnt to the current limitation. Torque-control mode is mostly used in conjunction field weakening so that the maximum motor torque can be limited Independently of speed.

The following functions are available:

  • Independent setting of positlve and negative torque limits via parameters.
  • Switchover of torque limit via binector as a function of a parameterizable changeover speed.
  • Free input of torque limit by means of a connector, e.g. via analog input or serial interface.

The lowest input quantity is always applied as the current torque limit. Additional torque setpoints can be added after the torque limit.

Current limitation

The purpose of the current limitation set after the torque limit is to protect the converter and motor. The lowest input quantity is always applied as the current limit.

The following current limit values can be set:

  • Independent setting of positive and negative current limits via parameters (setting of maximum motor current).
  • Free input of current limit via a connector, e.g. from an analog input or serial interface.
  • Separate setting of current limit via parameters for shutdown and fast stop.
  • Speed-dependent current limitation: Parameters can be set to implement an automatically triggered, speed-dependent reduction in the current limitation at high speeds (commutation limit curve of motor).
  • I2t monitoring of power section: The temperature of the thyristors is calculated for all current values. When the thyristor limit temperature is reached, the converter current is either reduced to rated DC current or the converter shut down with fault message, depending on how the appropriate response parameter is set. This function is provided to protect the thyristors.

Current controller

The current controller is a PI controller with mutually independent P gain and reset time settings. The P or I component can also be deactivated (to obtain pure P controller or pure I controller). The actual current is acquired on the three-phase AC side by means of current transformers and applied to the current controller after A/D conversion via a burden and rectifying circuit. The resolution is 10 bits for converter rated current. The current limiting output is applied as the current setpoint.

The current controller output transfers the firing angle to the gating unit, the feedforward control function acts in parallel.

Feedforward control

The feedforward control function in the current control loop improves the dynamic response of the control, allowing rise times of between 6 and 9 ms to be achieved in the current controlloop. The feedforward control operates as a function of the current setpoint and motor EMF and ensures that the necessary firing angle is transferred speedily to the gating unit, in both intermittent and continuous DC operation or when the torque direction is reversed.

Auto-reversing module

The auto-reversing module (only on converters for fourquadrant drives) acts in conjunction with the current control loop to define the logical sequence of all processes required to reverse the torque direction. One torque direction can be disabled by a parameter setting if necessary.

Gating unit

The gating unit generates the gate pulses for the power section thyristors in synchronism with the line voltage. Synchronization is implemented independently of the rotating field and electronics supply and is measured on the power section. The gating pulse position timing is determined by the output values of the current controller and feedforward control. The firing angle setting limit can be set in a parameter.

The gating unit is automatically adjusted to the connected line frequency within a frequency range of 45 to 65 Hz.

 

Siemens Energy & Automation
SIMOREG DC Master Base Drive Operating Instructions