AC Voltage Switching versus DC Voltage Switching of a Rectifier
PRECIMA spring loaded (Safety/Fail-safe) brakes are operated with direct current. This can be obtained by:
- Direct Supply
Battery, DC mains etc.
- Half Wave or Full Wave (Bridge) Rectifiers
Half Wave Rectifier – DC output voltage is 45% of the applied input AC voltage.
Full Wave Rectifier – DC output voltage is 90% of the applied input AC voltage.
- Fast Excitation Rectifier
Fast Excitation Rectifier is designed to switch from an initial full-wave mode to a final half-wave mode.
If supplied via rectifier devices, the circuit can be interrupted on AC side or, respectively, on DC side.
AC Voltage Switching
Circuit Interruption of the magnetic brake coil happens at the AC current side of the rectifier. During voltage switch-off, the current conducts further via the rectifier diodes and decrease slowly. Simultaneously, the magnetic field also decreases slowly which results in gradual rise of the braking torque. The energy from the magnetic coil is unloaded over the rectifier’s diodes which act as recovery diodes during switching off.
Against the DC voltage switching, AC voltage switching has approximately 4~5 times longer switching time.
When switch-off time is irrelevant, AC voltage switching should be the preference as there is no high voltage spike generated which shorten the life of the switching contacts.
DC Voltage Switching
Circuit Interruption of the magnetic brake coil happens at the DC current side of the rectifier; between the rectifier and the coil. The magnetic field decreases very rapidly which results in immediate rise of the braking torque. However, the energy from the magnetic coil is discharged over the opening of the switching contacts as ‘switching-off spark’ causing heavy contact usage.
When using DC voltage switching, only use contact that is capable of switching inductive loads with contact opening of at least 3mm and be rated IEC AC3.