Spring applied brakes with DC coils can be operated from low voltage supplies, for example 12, 24, 36 and 48V DC. Switchgear should be sized for the relevant current. However many applications utilise an AC supply with a rectifier that converts to DC voltage. When switching the brake on the AC side of the rectifier, brake engagement times are extended by a factor of 3-4 over catalogue values.
The simplest form of connection to a motor, in parallel with the rectifier and brake coil, further extends the engagement time. This is because the motor which is already switched off but still turning continues to excite the brake. With falling loads such as hoists, lifts and cranes, it is ESSENTIAL to switch the brake coil on the DC side of the supply. A spark suppressor is required to prevent inductive voltages from damaging the brake coil or rectifier. Reducing the brake torque also prolongs brake engagement times.
The disengagement time is not influenced by AC or DC switching. It can only be shortened by over-excitation of the coil, for example by using a force voltage rectifier.
Three types of Rectifier are available
- Full wave rectifier which gives a ratio of 1.1 between AC input and DC output, i.e. 230V supply gives 209V DC. Rectifiers are chosen to match available brake stator voltages which are up to a maximum of 250V DC. Optionally the rectifiers can include spark suppression and terminals for DC switching.
- Half wave rectifiers which give a ratio of 2.2 between supply and output, for example when using a 400V supply and a 180Vbrake coil. Spark suppression and DC switching are optional.
- Forced voltage rectifiers which use a combination of half and full wave rectification to over-voltage the brake coil for a short period and so speed brake release. Forced voltage rectifiers can alternatively be used to achieve a lower holding voltage which reduces energy consumption and speeds brake engagement.