What are the differences between spring-operated and permanent magnet brakes? How are they influenced by free-wheeling diode and spark suppressors?
• The spring-operated brake normally opens at approx. 20 V and closes at approx. 3 V (values depend on size and spring type)
• A permanent magnet brake opens at approx. 20 V and closes at approx. 16 V ≥ this may cause the brake to close in case of voltage fluctuations
• Free-wheeling diodes and spark suppressors protect the switching contact in case of switching on the DC side (for 24V brakes)
• If free-wheeling diodes are used, inductive voltage peaks are decreased. This is not the case with spark suppressors ≥ 6 to 10 times of the brake closing time if a free-wheeling diode is used (compared to spark suppressors)
• If permanent magnet brakes are used the immense time difference is not really perceptible (closes at 16 V) - unlike spring-operated brakes, as the voltage decrease to 3 V takes considerably longer.
This means that the closing time of permanent magnet brake and spring-operated brake is different if free-wheeling diodes are used. Therefore, spark suppressors should be preferred together with spring-operated brakes (in the Lenze brakes catalog a spark suppressor for 24 V is defined - one type for all brake sizes).
Can, if frequency inverters (e. g. 8200 vector or 8400) are applied in DC-bus operation, several internal brake choppers be used in parallel in order to reduce surplus regenerative energy?
No. In a DC-bus operation with several drives only one internal brake chopper can be used to reduce regenerative energy, since - due to the hardware tolerances - it is not ensured that all brake choppers are switched on simultaneously (the internal brake choppers cannot be synchronised). Consequently, all the regenerative energy would be reduced by one of the brake resistors only. The other resistors would not be activated. Further brake resistors would only be activated, when the DC-bus voltage would continue to rise due to an insufficient peak power of a single brake-resistor. This constellation would also lead to an uneven spread of the power loss to the single brake resistors.
If one single internal brake chopper is not sufficient, an external brace chopper should be used, which could also be synchronised with further external brake choppers. This ensures simultaneous activation of all brake choppers. This makes sure that the power loss would be spread evenly to all brake resistors.