Abstract: This paper introduces the importance of regenerative resistors and proposes that selecting a suitable regenerative resistor is an important task in building a servo system.
1. Introduction
Regenerative resistors play a crucial role in servo systems with feedback characteristics. An unsuitable regenerative resistor may cause the system to overload and shut down, leading to resistor failure. Once the protection of the regenerative resistor is lost, the bus capacitor and power inverter module will be damaged next. Therefore, selecting a suitable regenerative resistor is an important task in building a servo system.
Since there are different discharge strategies, i.e., many different calculation methods, this paper proposes a more engineering-oriented estimation method, ignoring some complex calculations. Different velocity planning and discharge strategies correspond to different calculation methods. Here, to simplify the calculation, we make the following assumptions:
1. Assume that the deceleration is constant during the deceleration process, the mechanical friction force remains unchanged, and the current remains unchanged.
2. Deceleration: At the start of deceleration, the bus voltage is [value missing], Uclamp triggers discharge, and discharge stops at URegenStop.
3. Assuming that the deceleration process discharges only once, the regenerative resistor will continue to discharge once it is triggered.
Calculation using a single axis as an example
2.1 Busbar Capacity Calculation
Where: Ec—energy that the bus can further absorb beyond the normal voltage.
Unom—Bus operating voltage
Uclamp—Bus safety voltage, the threshold for initiating discharge.
Taking the ACS integrated drive and control controller SPiiPlusCMba/hp as an example:
If there is no need to select a regenerative resistor, then there is no need to select one; otherwise, a regenerative resistor is required.
2.6 Determination of Regenerative Resistance Parameters
If EM > Ec, the parameters of the regenerative resistor need to be calculated. Due to the closed-loop effect of the feedback discharge module, the bus voltage is maintained at URegenStop after the deceleration ends. Therefore, during the entire deceleration process, from the start of deceleration to the end of deceleration, the added energy storage of the bus is E∆.
Where URegenStop is the voltage threshold at which the regenerative circuit stops operating (Unom is the total energy fed back to the bus from the start of deceleration to the bus being fully charged).
Where: DEC—deceleration; tstartRegen—time from the start of deceleration to triggering discharge; vstartRegen—speed at which the regenerative resistor discharges.
Substituting tstartRegen into the equation and solving the quadratic equation, we can obtain vstartRegen, which represents the operating time of the regenerative resistor:
Energy consumed by the regenerative resistor during the entire deceleration process:
When activating the regenerative resistor, it is essential to ensure that the bus voltage no longer increases, based on the power balance principle:
We can obtain the reference parameters for the resistor:
The peak power of the resistor is:
Average power:
Where tAverage is the average deceleration period.
Calculation method for control scheme of multi-axis common bus
The most severe scenario involves simultaneous deceleration of multiple axles, and the calculation method is as follows:
4 Other possible effects
Impact of velocity planning: T-programming, S-programming, and even higher-order programming can affect the feedback process. Calculations for higher-order velocity planning are extremely complex, and generally, the feedback curve will be smoother than that of T-programming, but the difference is not significant. For more accurate estimation, the best approach is to construct a model using auxiliary software for simulation, such as Simulink simulation.
