Ballast calculation for single-axis drives

This documentation refers to single-axis drives. Further calculations are required for multi-axis drives and coupled axes.

Reaction of a servomotor during operation:

• In the acceleration phase, the servomotor draws energy from the DC link.
• In the braking phase, the servomotor (generator operation) feeds energy into the DC link.

The feedback causes an increase in the DC link voltage, which is limited by the maximum permissible DC link voltage of the servo drive (by a brake resistor). This limitation serves to protect the device. The excess energy is converted into heat in the brake resistor.

The power that is fed back can be calculated from the available kinetic energy. The result is offset pro rata against the total cycle time. The additional energy (E_c) is not taken into account. It serves as a reserve.

Example:

Default values

• 600 V nominal voltage (DC link) at 400 V_AC supply voltage
• 850 V switch-on point of the brake chopper^*1.
• Formula

E_c[Ws] = 0.5 x C_{DC link} (850 V² - 600 V²)

Special case with an EL72xx servo terminal:

• 48 V nominal voltage (DC link)
• 55 V switch-on point of the brake chopper^*1.

Formula

Ec [Ws] = 0.5 x CDC link (55 V² - 48 V²)

*1: Below this threshold, the energy is stored in the DC link capacitors.

Additional storable energy of an AX5000 servo drive at 400 V_AC (EL72xx at 48 V_DC) supply voltage

Calculation of the load moment of inertia: For an exact determination of the inertia reduced to the motor shaft, please use the Beckhoff TC3 Motion Designer. You can find it under: www.beckhoff.com .